Fachhochschule Aachen
Studiengang: Maschinenbau
Studienrichtung: Energie- und Umweltschutztechnik,
Kerntechnik
Kraft-Wärme-Kälte-Kopplung mit Mikroturbinen
Technische und wirtschaftliche Besonderheiten für diverse Lösungskonzepte
IN ENGLISCHER SPRACHE VERFASST
Combined heat, cold and power generation with microturbines
Technical and economical features for various solution concepts
Diplomarbeit von Erik Hooft
Juli 2001
5
Diese Diplomarbeit wurde betreut von:
Herrn Prof. Dr.-Ing Klaus Peter Dielmann
Herrn MSc Stefan Ernebrant
Diese Arbeit wurde von mir selbständig angefertigt und verfasst.
Es sind keine anderen als die angegebenen Quellen
und Hilfsmittel benutzt worden.
Jülich, den 9. Juli 2001
6
Abstract
ABSTRACT
The Turbec T100 microturbine is a CHP system with 100 kW electrical power and 156
kW thermal power. The economical application of this installation for on-site energy
production is set by market conditions. The installation should be operated for a
minimum amount of hours per year to attain a good economy. Here, the utilization of heat
in a heat driven chiller during summer time enables an increase of operating hours next to
the heating period in winter. Aim of this investigation is to determine the best installation
variant and the minimum required amount of chilling hours to make the additional invest
for the chiller equipment profitable.
After the investigation of the different chiller technologies the absorption chiller was
found out to be the most efficient of heat driven chillers on the market. All possible
connections for the different types of absorption chillers with the microturbine were then
investigated and installation models enabled a validation of the different interconnections.
One single-stage and one multi-stage chiller type was selected for further investigation
based on their efficiency and power range.
The energy dynamics of different buildings types were investigated and an office
building was selected as reference object for a energy concept. The two installation
concepts with the different absorption chillers were modeled to supply in the energy need
of the office building and brought to comparison with a conventional solution with grid
connection, electric chiller and boiler.
The cost comparison between the three installation concepts led to the selection of the hot
water-fired single-stage absorption chiller as solution with a cost benefit, whereas the
yearly energy cost of the multi-stage chiller concept are the highest of all three.
A direct cost comparison between the hot water-fired absorption chiller and the
compressor chiller resulted in the
critical amount of operating hours where the
absorption chiller starts to have a cost benefit to the compressor chiller. This comparison
was also made for the microturbine with the conventional solution (grid and boiler).
3
Abstract
The hereby determined critical 3700 operating hours of the microturbine (CHP) was then
used for the calculation of the liquidation (3,5 years) and the return of invest (14,1%) of
the T100. Based on this economy the additional invest for the absorption chiller was
validated. This means that an amount of 3000 chilling hours was determined where the
return of invest and the liquidation period is the same as for the T100 alone.
4
Table of contents
TABLE OF CONTENTS
1
INTRODUCTION AND SCOPE OF TASK .....................................................................................8
2
TECHNOLOGY ..................................................................................................................................9
2.1
HEAT AND POWER .........................................................................................................................9
2.2
HEAT, COLD AND POWER .............................................................................................................10
2.3
MICROTURBINE ...........................................................................................................................10
2.3.1
Thermodynamics....................................................................................................................11
2.3.2
Operation...............................................................................................................................11
2.3.3
Emissions ...............................................................................................................................12
2.4
CHILLERS ....................................................................................................................................14
2.4.1
Heat-driven chillers other than absorption ...........................................................................14
2.4.1.1
2.4.1.2
2.4.2
Compressor chiller ................................................................................................................15
2.4.2.1
2.4.2.2
2.4.3
principle ..................................................................................................................................... 15
performance ............................................................................................................................... 16
Absorption chiller ..................................................................................................................17
2.4.3.1
2.4.3.2
2.4.3.3
2.4.3.4
2.4.3.5
2.4.3.6
2.4.3.7
3
adsorption chiller........................................................................................................................ 14
desiccant cooler.......................................................................................................................... 15
working fluids ............................................................................................................................ 17
principle ..................................................................................................................................... 18
cooling cycle .............................................................................................................................. 19
heat balance and coefficient of performance.............................................................................. 21
single-stage and double-stage chillers ........................................................................................ 23
operation .................................................................................................................................... 27
miscellaneous............................................................................................................................. 28
INSTALLATION MODELING .......................................................................................................30
3.1
INSTALLATION COMPONENTS ......................................................................................................30
3.1.1
Microturbine..........................................................................................................................30
3.1.2
Absorption chiller ..................................................................................................................30
3.1.3
Cooling tower ........................................................................................................................30
3.1.4
Miscellaneous ........................................................................................................................31
3.2
SINGLE-STAGE CHILLER CONCEPTS .............................................................................................31
3.2.1
Concept 1: single-stage hot water-fired ................................................................................32
3.2.1.1
3.2.1.2
3.2.2
performance ............................................................................................................................... 33
cooling tower ............................................................................................................................. 34
Concept 2: single-stage exhaust gas-fired .............................................................................34
3.2.2.1
3.2.2.2
performance ............................................................................................................................... 35
cooling tower ............................................................................................................................. 36
3.3
MULTI-STAGE CHILLER CONCEPTS ..............................................................................................37
3.3.1
Concept 3: double-stage natural gas-fired ............................................................................37
3.3.1.1
3.3.2
3.3.2.1
3.3.3
performance ............................................................................................................................... 39
Concept 5: unrecuperated microturbine................................................................................41
3.3.3.1
3.4
performance ............................................................................................................................... 38
Concept 4: multi-stage exhaust gas-fired ..............................................................................39
performance ............................................................................................................................... 41
CONCEPT REVIEW ........................................................................................................................42
3.4.1.1
3.4.1.2
single-stage hot water-fired........................................................................................................ 43
multi-stage exhaust gas-fired ..................................................................................................... 43
5
Table of contents
4
INSTALLATION CONCEPTS AND ECONOMICS.....................................................................44
4.1
APPLICATIONS .............................................................................................................................44
4.1.1
Heat, cold and electricity.......................................................................................................44
4.1.2
Residential buildings .............................................................................................................45
4.1.3
Office buildings......................................................................................................................46
4.1.4
Hotels.....................................................................................................................................46
4.1.5
Super markets ........................................................................................................................46
4.1.6
Department stores..................................................................................................................47
4.2
TECHNICAL REVIEW ....................................................................................................................47
4.2.1
Hot water-fired absorption chiller.........................................................................................48
4.2.2
Multi-stage absorption chiller ...............................................................................................50
4.2.3
Electric chiller .......................................................................................................................51
4.2.4
Concept review ......................................................................................................................52
4.3
ECONOMICAL REVIEW .................................................................................................................54
4.3.1
Basic economical data...........................................................................................................54
4.3.2
Concept review ......................................................................................................................56
4.3.3
Cost comparison calculation .................................................................................................58
4.3.3.1
4.3.3.2
4.3.4
Liquidation and Return of invest ...........................................................................................60
4.3.4.1
4.3.4.2
4.3.5
4.3.6
cost comparison chillers............................................................................................................. 58
cost comparison microturbine .................................................................................................... 59
liquidation microturbine............................................................................................................. 61
liquidation of microturbine and absorption chiller ..................................................................... 62
Sensitivity analysis for a good economy ................................................................................63
Decision criteria ....................................................................................................................63
5
CONCLUSIONS AND PROSPECTS ..............................................................................................65
6
LITERATURE ...................................................................................................................................66
7
APPENDIX.........................................................................................................................................68
6
Used nomenclature and abbreviations
USED NOMENCLATURE AND ABBREVIATIONS
The following nomenclature and abbreviations are used in this thesis.
AC
bar (a)
bar(g)
Btu
CFC
CHCP
CHP
CO
COP
dB(A)
DC
Factor Zcold
Factor Zheat
gpm/ton
GWP
HC
HCFC
HFC
HHV
HVAC
KWcold
KWel
KWpe
KWth
NOx
ODP
Rpm
RT
ηtot
ηel
°C
°F
Alternating current
Pressure (atmosphere)
Pressure (gauge)
British thermal unit (1,055 kJ)
Chlorofluorcarbons
Combined heat, cold and power
Combined heat and power
Carbon oxide
Coefficient of performance
Noise level
Direct current
Cold-to-power ratio
Heat-to-power ratio
Gallons per minute/ refrigeration tonnage
Global warming potential
Hydro carbons
Hydrochlorofluorocarbons
Hydrofluorocarbons
Higher heating value
Heating, ventilation and air conditioning
Chilling power
Electrical power
Fuel power (primary energy)
Thermal power
Nitrogen oxide
Ozone depleting potential
Rounds per minute
Refrigeration tonnage (1 RT= 3,516 kW)
Overall efficiency
Electrical efficiency
Degrees Celsius
Degrees Fahrenheit
7
Introduction and scope of task
1 INTRODUCTION AND SCOPE OF TASK
To achieve good economy with microturbines in distributed power, high running hours
must be attained and the use of both heat and power guaranteed. This motivates the use of
waste heat for chilling during summer to increase the amount of operating hours.
Therefore knowledge is required about the possibilities for combined production of heat,
cold and power or even rather continuous cold production to cover a cooling demand
throughout the year.
Aim of this thesis is the presentation of installation concepts for combined heat, cold and
power generation with microturbines. The effect on the efficiency by type of connection
of the chiller, and the feasibility for certain applications as a result of flexibility in control
of different installation concepts are investigated. An economical review will present the
decision criteria for the possible technical solutions. Heat driven chillers are mainly used
for air conditioning applications and the type for production of refrigeration temperatures
(below 6°C) will therefore not be investigated in this thesis.
The thesis is build up as follows. The properties of heat driven chillers and the relation/
interaction of the operation parameters of this type of cold production are extensively
presented in chapter 2 together with the basics of microturbine technology. By looking at
efficiency and installation properties a selection of feasible chillers are made based on the
accumulated knowledge. Theoretical modeling of installation concepts with overall
efficiency and simplicity of the interconnection as main criteria then leads to feasible
solutions in chapter 3. With a presentation of performance data and specific properties of
these concepts in chapter 4 a comparison is made with conventional types of chillers on
the market. Technical as well as economical details for various applications with different
user behaviors are compared and brought to discussion. The main results and conclusions
of this thesis are presented in chapter 5 together with points for further investigation and
possible required technical development.
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Technology
2 TECHNOLOGY
2.1 Heat and power
Combined heat and power (CHP) is one of the most efficient tools used to attain the
Kyoto goals. Compared to the reduction of air pollutants, there are no economic
technologies available to remove carbon dioxides from the exhaust gases. The reduction
of carbon dioxide emissions can therefore only be achieved with higher efficiencies in the
energy sector, and the use of less carbon-containing and carbonless fuels.
The heat of the energy cycle in a CHP system is recovered, which increases the overall
efficiency and reduces the use of primary energy. Efficient CHP systems therefore benefit
from several tax reductions and funding programs aid in the technical development of this
technology. Good economies, however depend on site-specific parameters and high
running hours is mostly always required. The use of both forms of energy, electricity and
heat must be guaranteed to attain the overall efficiency, which is the predefined condition
for tax credits. The recovered heat can be utilized for comfort heating during the heating
period in winter as shown in figure 2-1 together with the demand for power and cold
during the year.
Energy demand during the year
Demand [%]
100
80
60
Heat
Power
Cold
40
20
0
Jan
Feb
Mar
Apr
Maj
Jun
Jul
Aug Sep
Okt
Nov Dec
Month
Figure 2-1: Heat, cold and power demand during the year
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2.2 Heat, cold and power
During the summer months, where no heating is required, the rejected heat would have to
be wasted since the facility has a more or less constant electricity requirement that has to
be generated. To maintain the overall efficiency, this heat can be utilized in a heat-driven
chiller for comfort cooling. The energy system then realizes a combined production of
heat, cold and power, commonly known as combined heat, cold and power (CHCP).
Other than in space conditioning of buildings various industrial processes have an
immediate use for heat or cold and cogeneration systems can in these cases provide in the
energy need throughout the year. The market for building energy systems is however
found out to be the biggest and the investigation were thus mainly targeted on comfort
cooling rather than on industrial processes. To understand the possibilities and technical
constrains of cold production with microturbines the individual technologies are
described in the rest of this chapter.
2.3 Microturbine
The core of the microturbine unit can be seen as a turbo charger with centrifugal
compressor and radial turbine. A permanent magnet is mounted on the single shaft, which
is the only moving part of the core engine. The electrical generator, combustion chamber
and the recuperator, used for transferring heat from the exhaust gas to the combustion air,
complete the basic system shown in figure 2. The simplicity of the microturbine enables a
long lifetime of machine components and maintenance-free operation over long periods.
1. Generator
2. Inlet air
3. Combustion chamber
4. Air to recuperator
5. Compressor
6. Turbine
7. Recuperator
8. Exhaust gas heat
9. Boiler
10. Exhaust gas outlet
11. Hot water outlet
12. Water inlet
Figure 2-2: Schematics of a microturbine
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2.3.1 Thermodynamics
In the compressor ambient air is compressed to approx. 4.5 bar (a). With this low
pressure ratio the air has a lower temperature after compression than the hot gases at
turbine outlet. Heat can therefore be transferred from the exhaust gases to the compressed
air in the recuperator attached to the microturbine. The preheated compressed air is then
mixed with fuel and burned in the combustion chamber. The hot gases leave the
combustion chamber at a temperature of 950°C and are expanded through the turbine
wheel. The pressure decreases to close to atmospheric pressure and the heat drops to
approx. 650°C at turbine outlet. The exhaust gas rejects most of its heat to the
compressed combustion air before leaving the recuperator at a temperature of 270°C. The
use of the hot exhaust gases to preheat the compressed air prior to combustion in the
recuperator greatly increases the electrical efficiency to a level of 30%, previously
unattainable by small turbines.
The electric power is generated with the rotating four-pole permanent magnet on the rotor
at nominal speed of 70.000 rpm. The electrical generator produces a high frequent AC
that is first rectified to DC and then converted to a three-phase AC in the power
electronics. The generator also works as electric starter to bring the microturbine in
operation.
2.3.2 Operation
The power electronics in the microturbine unit works as an electric gearing to always
produce a three-phase AC at constant frequency. This enables to change speed to control
the microturbine. The mass flow through the system changes, where industrial gas
turbines always operate at nominal speed and control the output of the turbine with the
turbine inlet temperature. The turbine inlet temperature of the microturbine is however
kept constant over a wide range of the part load, which results in a good part load
performance because it operates in its thermodynamic optimum.
Next to power generation heat can be recovered from exhaust gases leaving the
recuperator. The temperature level of 270°C enables usage for steam and hot water
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production. The exhaust gas flow as only waste heat source can technically be easily used
compared to the different heat sources at various temperature levels (oil, cooling water,
exhaust) with reciprocating engines. The exhaust gas can be used to a temperature as low
as 55°C without the risk of condensations and corrosion. The high air-to-fuel ratio results
in a rest oxygen of approx. 17% and the hot gases can be used as preheated combustion
air for high-temperature burners.
The total cooling requirement of the machine components is realized by ventilation with
ambient air provided to the unit. The cooling of the high-speed generator and power
electronics is either realized with air-cooling or a closed cooling water system. In the case
where oil bearings are used rather than air or magnetic bearings the lubrication of the oil
film bearings is realized in an enclosed system where the oil temperature of approx. 50°C
results in low oil consumption.
The microturbine requires a minimum gas pressure of 6 bar (a). The unit can either be
directly connected to a high-pressure gas system, or in the case of a low-pressure fuel
system, natural gas is routed to the fuel gas compressor before being fed to the fuel
system. Other than natural gas, diesel and a variety of other liquid and gaseous fuels can
power the unit. Low-Btu units for biogas application are suitable for operation with
fluctuating gas quality and a wide range of heating values.
2.3.3 Emissions
Noise emission of a microturbine is maintained below 70 dB(A) at 1 meter by the casing
of the unit, without the risk of low frequent vibrations being transferred to the
surrounding building. Exhaust gas emissions of the microturbine are significantly low
since it is operated with a higher air-to-fuel ratio than industrial gasturbines and relative
low pressures compared to internal combustion engines. This results in a decrease of the
combustion temperature and the formation of thermal nitrogen oxide (NOx) is therefore
negligibly small. The combustion chamber is also relatively big for its power output and
the continuous isobaric combustion guarantees a burn out of all hydrocarbons and low
CO emission.
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The emission values in figure 2-3 below are presented @ 15% O2 to actually bring up a
visual comparison between the emissions and the strictest emission standards for
reciprocating engines and gas turbines in Europe. Please note that the percentage of
oxygen used to calculate emission limits is crucial. Emission values of internal
combustion engines are normally presented @ 5% O2 and the following conversion
formula is used when a figure given at 5% must be translated into 15% [1].
Factor =
(21 - 15)
= 0,375
(21 − 5)
The emission values of the engine (NOx <250mg/Nm3 @ 5% O2) are based on the leanburn engine concept. A higher air-to-fuel ratio (less fuel) is used to lower the flame
temperature and NOx emissions. This however results in a loss of electrical efficiency of
about 3% to a level of 33-34%.
Emission comparison at @ 15% O2
150
[mg/Nm3]
120
Reciprocating Engine limit
Reciprocating Engine limit
Gas turbine limit
90
60
93
Gas turbine limit
30
0
Engine
30
38
19
Microturbine
Engine
Microturbine
NOx
CO
Figure 2-3: Emissions of microturbines and reciprocating engines
A further comparison of the two different technologies brings up the possibility to install
the microturbine without special foundation and the simplicity of microturbine
technology significantly reduces maintenance and operating costs. Investment costs of the
microturbine are comparable to the gas engine with a future potential to lower the price
and increase electrical efficiency.
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2.4 Chillers
Various types of chillers were investigated, where first the use of cooling must be
categorized into two main areas. The first area is refrigeration and freezing used for
production steps and processes or storage of products. Electric chillers, especially in the
small power range mainly rule in this area where temperatures below 6°C are required.
The second and investigated area for the use of cold is comfort cooling with feeding
temperatures of air conditioning systems of above 6°C [3]. Next to the mainly used heatdriven absorption chiller in waste-heat or electricity saving applications, other heat-driven
chillers and compressor chillers were investigated to determine the best options. Steam jet
refrigeration systems were not investigated.
2.4.1 Heat-driven chillers other than absorption
2.4.1.1 adsorption chiller
Adsorption chillers work by the principle of the adsorption of the refrigerant vapor
(water) by a hygroscopic solid (i.e. silica gel) in its micropores. The core of the chiller is
build up out of two separate heat exchangers with tubes that are covered with silica gel
granulate. In one section a certain amount of water (refrigerant) is stored while the other
silica gel package is dry (regenerated). Hot water as heat source regenerates the saturated
section and the dry section is used to adsorb the refrigerant in the cooling cycle. Control
valves switch the two sections in a 10-minute cycle. These systems mainly find their
application when the heating temperature is lower than 85°C such as in solar cooling.
Figure 2-4 shows that adsorption chillers become more efficient than absorption chillers
when the heating temperature is lower than 85°C. Their standard working point is at 75°C
hot water temperature with a COP of 0,6.
COP
COP of Adsorbtion vs. absorption chilling
0,75
0,70
0,65
0,60
0,55
0,50
0,45
0,40
Working points
60
65
70
75
80
85
90
95
Hot water temperature 'C
Figure 2-4: Working points of adsorption and absorption chillers
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The adsorption chiller is available in the power range from 50 to 450 kWcold chilling
power. An adsorption chiller has more than four times the operation weight than an
absorption chiller with the same capacity and is at least double as expensive. Further
information about the construction and the cooling cycle of this type of chiller can be
found in appendix: A1: Adsorption chiller.
2.4.1.2 desiccant cooler
Desiccant systems remove moisture from the air by the adsorption principle with a
rotating wheel packed with silica gel or other desiccant. The latent heat (water vapor) is
removed, which results in a cooling effect. Through this, cooling loads can be reduced
because comfort-cooling levels are achieved in low humid conditions. People can release
their body heat easier and a higher chiller temperature set point can suffice to achieve
comfortable room conditions. The use of desiccant cooling systems increases the
efficiency of the chilling system and enables a downsize of the chiller’s capacity or an
extend of the cooling load combined with a high air quality for human comfort.
2.4.2 Compressor chiller
The compressor chiller can be used for al chilling temperatures and finds its application
in all market areas. Depending on the required chilling temperature and application
different refrigerants are used. The use of historical refrigerants for compression chilling
has already been limited due to their ozone depleting effect and global warming potential.
Further information about the future for refrigerants is given in appendix A2 together
with a comparison of their environmental impact.
2.4.2.1 principle
In a compressor chiller heat is acquired at low temperature and pressure (chilling) and the
compression of the refrigerant to a higher pressure makes it possible to reject the heat at a
higher temperature level. The refrigerant acquires heat from the system water in the
evaporator at low pressure and evaporates. This refrigerant vapor is compressed to a
high-pressure level by an electrical or differently driven compressor. The refrigerant
liquefies due to compression and rejects its heat to the cooling water in the condenser at a
high temperature level. This high-pressure refrigerant is then expanded through a throttle
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Technology
and enters the lower pressure evaporator. Flash boiling occurs since the boiling point is
much lower at lower pressure and the system is cooled down to the evaporator saturation
temperature.
C ompressor
C ooling water outlet
C old water outlet
Low
High
pressure pressure
C old water inlet
Evaporator
Condenser
C ooling water inlet
Throttle
Figure 2-5: Schematic of a liquid-cooled compressor chiller
Other than the use of cooling water in the condenser, the compressor chillers for airconditioning in the small power range are mainly air-cooled.
2.4.2.2 performance
The compressor can be reciprocating or centrifugal and both variants find application in
all power ranges. The efficiency of compressor chillers is expressed in coefficient of
performance (COP).
COP =
Qe
chilling capacity
=
power or fuel input
Qg
INDEX
Qe = Chilling capacity of evaporator
Qg = Power or fuel input for compression (generation of refrigerant)
An Electric chiller for air conditioning in the power range up to 140 tons (≤ 500 kW) has
a COP of 4.0 at full load condition. Options to make the chiller more efficient such as
multiple-stage compression and variable frequency drive for part-load operation start
from 160 tons (≥ 500 kW). The part-load COP with compressor chillers in airconditioning applications lies around 3,0 for the small power range.
16
Technology
Figure 2-15 on page 23 presents a comparison of the part-load performance of a
compressor chiller with that of two different types of absorption chillers. Next to the
electric chiller, where an electric motor drives the compressor, gas engines and other
types of drive systems are used. In the gas engine concept approx. 30% of all the energy
generated by the engine actually goes to driving the compressor [2]. Gas engine-driven
chiller products are available starting from 50 tons (≥ 175 kW), achieve a COP of approx.
2.0 [8] and have a footprint comparable to an electric chiller.
2.4.3 Absorption chiller
The absorption technology is the most commonly used for chilling in waste heat
applications on the market. In this heat-driven system a working fluid that consists of
refrigerant and absorbent is used to produce a continuous cooling effect. A detailed
description of the absorption chiller technology is given as basic knowledge for the
evaluation of different installation concepts.
2.4.3.1 working fluids
Two main types of absorption chillers are available on the market that work with
different working fluids. One works with lithium bromide with water as refrigerant
(LiBr/H2O) and finds application in the air conditioning sector since it can only produce
cold down to approx. +4ºC. The other type works with ammonia as refrigerant and has
water as absorbent (NH3/H2O) [4]. The ammonia chiller is more suitable for industrial
refrigeration (deep-freeze) because it can produce cold with temperatures as low as 60ºC. In some cases a three-compound mixture is used with either ammonia or
hydrofluorocarbons as refrigerant, but these types were not investigated.
LIBR-H2O
Absorption chillers mainly work with the working fluid LiBr/H2O. The absorbent lithium
bromide is chemical similar to common salt (NaCl) and harmless for environment and
has no ozone depleting effect. The liquid LiBr-solution has a property to absorb the water
vapor due to its chemical affinity.
NH3-H2O
Technically seen NH3/H2O can be used in all chilling applications. However, the toxic,
burnable and corrosive properties of ammonia have always limited its commercial use.
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Technology
This technology is therefore currently mainly used for industrial refrigeration (> 250
kW). Residential and light commercial gas-fired absorption chillers and heat pumps are
also available on the market.
2.4.3.2 principle
Absorption chillers use heat energy to produce a refrigerating effect. The refrigerant
vapor is not compressed to a higher pressure-level as in compressor chillers, but absorbed
by a solution with a high affinity to the refrigerant. Here, the concentration of the solution
determines the evaporation pressure of the refrigerant and therefore the chilling
temperature, because the boiling point of a fluid directly proportional is to the pressure.
At atmospheric pressure water boils at 100°C. As the pressure is lowered (vacuum) in the
chiller its boiling point goes down. At 10 mbar absolute pressure the boiling point of
water lies below 7°C thus able to absorb heat at low temperature level by evaporating.
Condenser
Evaporator
Generator
Absorber
Figure 2-6: Cooling cycle of an absorption
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The diagram above, shows the cooling cycle of an absorption chiller with the various
temperatures and pressures. The stages 1 through 6 of the cooling cycle are discussed in
the next paragraph.
2.4.3.3 cooling cycle
The absorption cycle uses a condenser and evaporator, just like a compressor chiller.
Only a thermal fluid compressor that consists of generator, absorber and small fluid
pumps replaces the large motor and compressor in an electric chiller that drives the
cooling cycle.
2
Hot water inlet 88°C
Hot water outlet 82°C
C ooling water outlet 34°C
1
Generator
Condenser
Solution heat exchanger
4
3
Solution pump
62% LiBr
C old water outlet 7°C
5
C old water inlet 12°C
Evaporator
Refrigerant pump
6
Absorber
C ooling water inlet 29,5°C
Solution pump
57% LiBr
Figure 2-7: Schematic of a single-stage absorption chiller
GENERATOR
The working fluid enters the generator (1) and hot water as a heat source concentrates the
solution by evaporation of the refrigerant (1→ 2). The concentrated solution (4) is
pumped to the absorber and the generator supplies high-pressure refrigerant to the
condenser. The generator can be seen as the pressure side of the thermal fluid
compressor, where the absorption process in the absorber provides the suction to the
evaporator.
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CONDENSER
Refrigerant vapor produced by the generator is introduced into the condenser. The
refrigerant vapor (water) condenses and rejects its heat to the cooling water flowing
through the condenser tubes (2→ 3). The high pressure is equal to the vapor pressure of
the water in the condenser (See figure 2-6 on page 13)
EVAPORATOR
Liquid refrigerant enters the evaporator from the condenser through a throttle (3→ 5). As
the refrigerant passes to the lower pressure evaporator, flash boiling occurs, cooling the
remaining liquid refrigerant to the evaporator saturation temperature. This chilled
refrigerant is continuously circulated to the evaporator spray system by the refrigerant
pump. The transfer of heat from the system water to the refrigerant then causes the
refrigerant to vaporize at low pressure
ABSORBER
The vaporized refrigerant in the evaporator (5) flows to the slightly lower pressure in the
absorber. Here, refrigerant is absorbed by the lithium bromide solution (4) because of its
high affinity for water vapor. This is the fundamental principle of the absorption process.
The temperature and concentration of the solution sprayed over the tubes determines the
pressure in the absorber. As the refrigerant vapor is absorbed by the concentrated
solution, the heat taken up in the evaporator is transferred to the cooling water.
SOLUTION AND REFRIGERANT PUMP
The dilute solution is pumped to the generator by the solution pump (6→ 1), which closes
the absorption cycle. This small pump requires only a small amount of energy since the
working fluid only has to be pumped to pressure-level that is only approx. 10 kPa higher.
The refrigerant pump continuously circulates the chilled refrigerant in the evaporator
spray system. The total power consumption for pumping is approx. 1% of the chilling
power.
SOLUTION HEAT EXCHANGER
In the solution heat exchanger the hot concentrated solution (4) preheats the dilute
solution (1) before it enters the generator. This internal heat recuperation has a great
effect on the efficiency of the chiller since it reduces the required heat for the boiling
process in the generator and the cooling water load in the absorber.
20
Technology
2.4.3.4 heat balance and coefficient of performance
Absorption chillers have heat as the driving force and the system can therefore be
evaluated in function and efficiency by looking at the heat flows, where the efficiency of
the chiller is expressed in coefficient of performance (COP) as with compressor chillers.
HEAT BALANCE
The heat balance of the absorption cycle is made on the chilling capacity of the
evaporator, heat input to the generator and the rejected heat to the cooling water in
condenser and absorber. The energy input for the small fluid pumps is taken into account
in form of a heat equivalent (1% of the cold output).
Condenser
Evaporator
Absorber
Cold
Qe
Waste heat
Qc
Generator
Heat input
Qg + Q p
Figure 2-8: Heat balance of an absorption chiller
Heat balance : heat in = heat out ⇔ Qe + Q g + Q p = Qc
21
Technology
INDEX
Qe = Chilling capacity of evaporator
Q g = Heat or fuel input to generator
Q p = Heat equivalent of pumping energy, approx. 1% of the heat input.
Qc = Heat rejected to cooling tower
COEFFICIENT OF PERFORMANCE
The coefficient of performance (COP) of absorption chillers is expressed as follows:
COP =
Qe
chilling capacity
=
Q g + Q p heat or fuel input
SEE INDEX ABOVE
The maximal achievable COP is based on the two physical properties of the working
fluid: the evaporation heat of the refrigerant and the solution heat of the refrigerant in the
solution. This means that the refrigerant should have a small specific heat to evaporate
quickly and the solution should reject as little as possible heat and be able to absorb the
refrigerant on a as high as possible temperature level. The practical attained COP of the
cooling cycle correlates with all temperatures entering and leaving the chiller. A lower
chilling temperature and a higher cooling water inlet temperature have a negative effect
on the efficiency and capacity. The influence of these temperatures is shown in figure 4
and 5 on the next page. Figure 4 presents a single-stage hot water-fired chiller and figure
5 shows the performance of a double-stage absorption chiller1. Both chillers have the
same general conditions of 6,7°C chilling water temperature and 29,4 °C cooling water
inlet temperature. These are important parameters when dimensioning a chiller and
comparing different types of chillers with each other. A closer look at figure 4 and 5
shows that a much higher COP can be achieved with a double-stage chiller than with a
single-stage chiller. This is the result of a higher temperature level of the heating source
in an additional high-pressure stage.
1
Properties of single-stage and double-stage chillers are discussed in the next paragraph
22
Technology
PERFORMANCE CURVES
1,38
140
0,72
120
1,34
120
0,70
100
1,30
100
0,68
80
1,26
80
0,66
60
1,22
60
0,64
40
1,18
40
0,62
20
1,14
20
0,60
0
1,10
6
7
8
9
10
Temperature °C
11
12
COP
Cooling water temperature vs. COP/ capacity
6
7
8
9
10
Temperature 'C
11
12
13
Cooling water temperature vs. COP/ capacity
140
140
0,73
120
1,32
120
100
1,28
100
1,25
80
1,21
60
1,18
40
20
0,71
60
0,70
40
COP
80
Capacity %
0,74
0,72
COP
0
5
13
0,69
20
1,14
0,68
0
1,11
24
26
28
30
32
34
35
Temperature °C
36
38
Figure 2-9: Single-stage chiller
0
24
40
Capacity %
5
Capacity %
COP
140
Capacity %
Chilling water temperature vs. COP/ capacity
Chiling water temperature vs. COP/ capacity
0,74
26
28
30
32
34
35
Temperature °C
36
38
40
Figure 2-10: Double-stage chiller
2.4.3.5 single-stage and double-stage chillers
Next to the required chilling water temperature and cooling water inlet temperature the
temperature level of the heat input used for the boiling process in the generator has a
great effect on the attainable COP. The heating power for an absorption chiller can be
provided by hot water, steam, natural gas or oil. The natural gas or oil-fired units are
commonly known as direct-fired chillers, while the steam or hot water driven units are
seen as indirect-fired chillers. Other hot liquid or vapor heat sources can also be used in
heat-recovery applications. This paragraph explains different type of absorption chillers
to give insight into the possibilities of their usage at different firing temperatures.
SINGLE-STAGE CHILLER
The major components for a single-stage absorption chiller include an evaporator
absorber, generator, condenser, solution heat exchanger and small fluid pumps. The
theoretical achievable COP with the working fluid LiBr/H2O is approx. 0,85 to 0,9. A
single-stage unit for air conditioning, as presented in figure 2-11 on the next page, are
mainly hot water-fired with hot water temperatures between 85°C and 95°C. These
commercial chillers reach a COP of approx. 0,7.
23
Technology
Cooling water outlet
Chilled water
Hot water or exhaust
Refrigerant (water)
Heat input
Refrigerant vapor
Diluted solution
Concentrated solution
Cold water cycle
Cooling water inlet
Cooling water
1.
2.
3.
4.
5.
6.
7.
8.
Evaporator
Absorber
Generator
Condenser
Solution HX
Solution pump
Refrigerant pump
Cooling/ heating
switch valve (closed)
Figure 2-11: Single-stage absorption chiller
Higher temperatures with the use of low-pressure steam or hot gases (i.e. exhaust
microturbine 270°C) result in a coefficient of performance of up to 0,8. The cooling/
heating switch valve (Nr. 8 in figure 2-11) shows the possibility to use the chiller as a
boiler for heating in winter in the case of exhaust gas firing. This avoids investment in an
additional boiler does however create similar wear to the unit as during chilling
operation.
DOUBLE-STAGE CHILLER
A double-stage chiller has an additional, higher temperature generator where heat in form
of hot gases from combustion or high-pressure steam (6-9 bar) is used. Additional strong
solution (high concentration) and water vapor is produced. The water vapor produced in
the high stage is condensed to drive the low-temperature (and lower pressure) generator.
The additional refrigerant produced in the high-pressure stage provides additional cooling
and this double use of higher temperature heat therefore results in a COP of up to 1,2
(See figure 2-12). An additional high-temperature solution heat exchanger is also
included in this unit for internal heat recuperation and better efficiency.
24
Technology
Hot water cycle
Cooling water
outlet
Exhaust 170°C
Fuel
Cold water
cycle
Cooling water
inlet
1.
2.
3.
4.
5.
6.
7.
8.
High stage generator
Low stage generator
Condenser
Evaporator
Absorber
High temp. solution HX
Low temp. solution HX
Hot water HX
9. Solution pump
10. Refrigerant pump
Chilled water
Hot water
Refrigerant (water)
Refrigerant vapor
Diluted solution
Concentrated solution
Cooling water
Figure 2-12: Double-stage absorption chiller
Double-stage chiller can be used in the case of firing temperatures higher than approx.
150°C. The system is more complex and more expensive, but not only the higher
coefficient of performance makes up for this. The cooling tower load is substantially
lower (approx. 40%) than with single-stage chillers. This is because the vapor coming
from the high-stage generator releases its heat during the boiling process in the low-stage
generator and enters the condenser already as condense. Therefore only the heat from the
low-stage generator is rejected to the cooling water. Figure 2-12 above shows that the
heating cycle has a separate heat exchanger in the high stage generator. Three cooling/
heating switch valves separate the main shell2 from the high-stage generator, which
means that it is not used for the heating cycle as with a single-stage chiller (See figure 211). The dual effect of chilling and heating with additional available hot tap water can
cover a buildings total heat and cold demand simultaneously.
2
The main shell is the vessel with the low stage generator, condenser, evaporator and absorber
25
Technology
MULTI-STAGE CHILLER
For the application of hot gases (i.e. microturbine exhaust) in a double-stage chiller to
attain a high efficiency, an additional generator is arranged with the basic design.
Exhaust 120°C
Hot water
Cooling water outlet
Exhaust
170°C
270°C
Cold water cycle
Fuel
Cooling water inlet
Fuel
SEE INDEX FIGURE 2-12
Figure 2-13: Multi-stage absorption chiller
The exhaust of the microturbine is divided into two separate flows. The first flow is used
as preheated combustion air that is mixed with fuel and burned in the high-stage
generator. The required air is only a small amount of the total mass flow (1 Nm3/kWhcold)
since the high-stage generator is designed for flame temperatures up to 1400°C. The
separated remaining hot gases directly fire an additional stage for free generation of
refrigerant. This stage is similar to the generator of a single-stage absorption chiller (See
figure 2-11). Figure 2-13 shows that this type of chiller uses the main shell for the heating
cycle. This is because the recovered heat from the hot gases in the additional stage cannot
be used differently. The advantage of separating the main shell during heating mode,
which reduces the wear to the unit, can therefore not be used.
26
Technology
2.4.3.6 operation
The effect of the cooling water inlet temperature and the chilling water temperature on
the COP and capacity of an absorption chiller was already presented in figure 2-9 and 210. Both figures show that a raising cooling water inlet temperature results in a reduction
of the chiller’s capacity. Full load operation is therefore normally not possible anymore
when the cooling water inlet temperature exceeds 35°C without raising the chilling water
temperature. The design of air conditioning systems and the chiller’s capacity is based on
peak conditions to guarantee a comfortable climate in a facility. Chillers therefore rarely
operate at full load and spend most of their operating time between 50% and 75% load.
PART-LOAD OPERATION
Part-load chiller operation is normally associated with reduced tower water temperatures
since less heat is rejected to the cooling water in the absorber and condenser. The cooler
tower water results in a lower solution temperature entering the generator. Here, is a
relation between the cooling water inlet temperature and the return temperature of the
heat source in the generator. As a rule of thumb, with hot water-fired absorption chillers,
an increase of the cooling water inlet temperature by 1°C results in an increase of 2,5°C
hot water return temperature. The reverse effect appears with a 1°C colder chilling
temperature. The cooling water inlet temperature is therefore an important parameter that
determines the efficiency of the heat recovery system. The efficiency can be presented in
rated COP and the COP with integrated part-load value (IPLV). The equation for the
IPLV COP in figure 14 below is presented according to ARI Standard 550/590-19983.
Absorption chillers have good part-load performance characteristics and with gas-fired
double-stage chillers the part-load efficiency is even higher than full-load. Figure 2-15
shows the part-load performance of a single-stage and double-stage absorption chiller
together with that of a compressor chiller.
3
ARI: Air-conditioning & Refrigeration Institute in the USA
27
Technology
IPLV = 0,01A + 0,42 B + 0,45C + 0,12 D
100
90
A = COP at 100%
B = COP at 75%
C = COP at 50%
D = COP at 25%
Double-stage chiller:
rated COP :1,21 and IPLV COP :1,34
Energy input %
INDEX
80
70
Compressor chiller
60
50
Single-stage
40
Double-stage
30
20
20
30
40
50
60
70
80
90
100
Percent of design load %
Figure 2-14: Integrated part-load value
Figure 2-15: Part-load performance
The absorption chiller can operate from 20% to 100% load and is controlled by the return
chilled water temperature that is measured with a temperature sensor in the piping
system. A dual effect chiller/ heater is designed with an additional sensor to measure the
heating water outlet temperature. The burner of a gas-fired absorption chiller is hereby
accordingly controlled. Part-load operation of a hot water-fired absorption chiller at 70%
of design load i.e. is operated with 65% of the hot water flow that is controlled to the
chiller mostly by means of a three-way valve. It can take up to 10-15 minutes until cold is
available when putting the chiller into operation and after shutdown there is a residual
chilling period of approx. 10 minutes. The schematic of a typical piping system of a
double-stage gas-fired absorption chiller is shown in attachment A3.
2.4.3.7 miscellaneous
Absorption chillers are designed for indoor placement with room temperatures between
5°C and 40°C. The lower temperature is to avoid the risk for the copper tubes to crack
and the upper temperature is set to assure the functionality of the electrical components
with a surrounding humidity ≤ 85%. Water-fired absorption chillers have noise emissions
of approx. 50 dB(A) at 1 meter. With gas-fired absorption chillers the sound level is
higher due to the gas burner. A unit that is designed with sound absorber has a noise level
≤ 70 dB(A) and achieves emissions ≤ 44 ppm NOx @ 5% O2. Absorption chillers have a
planned maintenance interval once a year and the life span extends up to 20 years.
28
Technology
The absorption chiller presents the heat-driven chiller with the most flexibility to be
integrated into combined heat and power systems, such as with microturbines. High
efficiencies can be achieved and a wide variety of interconnections can be realized to
create various systems that can suffice the requirements for different applications. The
various types of absorption chillers were further investigated for the different forms of
heat input and interconnections with the microturbine. Modeled installation concepts are
presented
and
validated
in
chapter
3.
29
Installation modeling
3 INSTALLATION MODELING
This chapter presents the compatibility of absorption chillers with microturbines and the
mechanical layout of different installation concepts. Feasible concepts that were modeled
are described with their specific properties such as overall efficiency and technical
constrains.
3.1
Installation components
3.1.1 Microturbine
The Turbec T100 microturbine CHP system produces 100 kWel at 30% net. electrical
efficiency according to ISO Standard conditions. The heat of the 270°C hot gases behind
the recuperator as only heat source are recovered in the heat exchanger of the standard
CHP package. The amount of recovered heat mainly depends on the return temperature of
the hot water system, where in the standard 70/90°C heating cycle 156 kWth of heat can
be recovered. The firing of an absorption chiller can be realized by the indirect use of the
heat with hot water or the direct use of the hot gases without the need of the standard heat
exchanger.
3.1.2 Absorption chiller
The performance data of the chillers is based on the standard rated conditions of 6,7°C
chilling water temperature and 29,4 °C cooling water inlet temperature according to the
ARI Standard presented in appendix A4. These standard conditions allow a valid
comparison and evaluation of the different chiller types. The power consumption of the
chiller is taken into account in this review, although it is only approx. 1% of the cooling
power. This is especially important in the cases where large sized chillers (≥ 1 MWth) are
used in the systems.
3.1.3 Cooling tower
Two types of cooling towers with wet or dry cooling cycles can be differentiated. The
choice for the right type of tower depends on the ambient air and climate conditions at
site location. Any local manufacturer can provide a cooling tower that meets the cooling
requirements of the chiller. Manufacturers of dry cooling towers give an economical
temperature difference of 5 K between ambient air temperature and the required cooling
30
Installation modeling
water temperature. This means that dry cooling towers can supply the rated 29,4°C
cooling water temperature to the chiller up to an air temperature of approx. 25°C. Wet
cooling towers are however dimensioned with the wet bulb temperature. Cooling can be
realized at a temperature difference of 2 K between the wet bulb temperature and the
rated cooling water temperature. Here, air with a temperature of 30°C and a relative
humidity of 80% i.e. resembles with a wet bulb temperature of 27°C. The system can
therefore operate in warmer climates when also cooling demand is at its peak. Adequate
water supply is of course required for the operation of a wet cooling tower. Lack of water
or high make-up water costs could therefore make the dry cooling tower a better solution
in despite of its four times higher fan power.
To estimate the power consumption and later the investment costs, wet cooling towers
designed for a wet bulb temperature of 25°C (28°C and 80% relative humidity) were
selected for the individual installation concepts. At 50% load the power consumption of
the cooling tower drops to approx. 25% of full load.
3.1.4 Miscellaneous
Next to the main installation components above, several additional components are
required for the proper operation of the installation. The pumping energy that is required
in the case of a hot water-fired chiller for example is taken into account as power
consumption of the energy system. The auxiliary systems such as controlling system as
well as temperature sensors, valves and piping are not described in detail4. To summarize;
the presented electrical efficiency of the systems takes the power consumption of the
chiller, hot water pumps (if applied), cooling water pump and the cooling tower fan into
account.
3.2 Single-stage chiller concepts
For the heat input of chillers the direct firing (natural gas or exhaust gas) and the indirect
firing (hot water or steam) where a heat exchanger is placed between the hot gases and
absorption chiller was already differentiated. The serial commercial units are mainly hot
31
Installation modeling
water-fired and the chillers of different manufacturers all achieve a similar efficiency, and
mainly only differ in maintenance concept and price. The use of exhaust gas as heat
source had already found a select application in the industrial area before it was just
recently initiated for the commercial use with microturbines.
3.2.1 Concept 1: single-stage hot water-fired
Serial manufactured water-fired chillers are designed with relative small heat exchanger
to reduce space requirement and manufacturing costs. High water flows and only a small
difference between hot water inlet and outlet temperature of approx. 5°C are thus
standard design conditions for the heat input into the generator. For the controlling of the
hot water heat input, a storage system is often inserted next to the use of a three-way
valve. This unlinks the cogeneration production, which enables a more continuous heat
recovery when heat is not required. The chiller and the heating system are in this case
fired from the storage tank. Figure 3-1 shows the installation concept of a hot water-fired
chiller for a cooling system where the chiller is fired from a hot water storage tank.
Exhaust
Single-stage water-fired
Absorption chiller
90°C/ 0,8 kg/s
34-36,3°C
3 l/s
Recovered heat
146 kW
95°C
82-85°C
90°C
<3 m3
7,3 l/s
Fuel
333 kW
Cooling tower
Heat
250 kW
COP 0,7
Gen-set
Air
29,4°C
6,7°C 12°C
30 m3/h
Power consumption 12 kW
Power
88 kW
Cold
102 kW
Figure 3-1: Installation concept 1: water-fired single-stage chiller
4
Please study the appendix A3 for the placement of sensors and valves in the piping system.
32
Installation modeling
Part of the hot water with a return temperature of 82-85°C from the chiller is fed to the
heat exchanger and approx. 146 kW is recovered from the hot gases of the microturbine.
Hot water at a temperature of approx. 95°C is fed into the storage tank, which results in a
mix temperature of the required 90°C to fire the chiller. The storage tank therefore
enables a maximum heat recovery and a load reduction of the cogeneration system when
a sufficient amount of heat is stored and less electricity is required. The three-way valve
realizes a circulation of the hot water when the hot water return temperature of the chiller
is too high. This prevents a contamination of the storage tank with a too high return
temperature that would result in bad heat recovery. A 3000-liter storage tank can fire the
chiller at full load operation during a period of 15 minutes, and can next to firing the
heating/ cooling system also supply tap water. This saves the investment for a boiler and
reduces space requirement.
3.2.1.1 performance
The hot water-fired absorption chiller has a limited coefficient of performance of 0,7 at
rated conditions and this type of connection with the standard microturbine CHP system
can provide an output of 102 kWth cold which equals 29 Refrigeration tonnage (RT5). To
minimize the start up time or for covering short peak loads the chiller can be given a
higher heat input using heat from cogeneration and additional heat from the hot water
storage tank. The power requirement of the system during chilling operation is 12 kWel,
which leaves an electrical output of 88 kWel. For the heating period 156 kWth of heat can
be recovered in a typical 70/90°C heating cycle with the current design of the CHP
package. The validation of this installation concept is given below by the calculation of
the efficiencies and the cold-to-power ratio. The figures for each installation concept are
comprised in table 3-1 where they are presented together with the overall efficiency of
the installation concept. The performances of the different installation concepts are
brought to discussion in the next chapter.
5
One ton of refrigeration RT= 3,516 kWcold
33
Installation modeling
SINGLE-STAGE HOT WATER-FIRED CHILLER
COOLING
Overall efficiency = η tot =
cold + power output 102 kWth + (100 − 12 kWel )
=
= 57 %
fuel input
333 kW pe
Electrical efficiency = η el =
88 kWel
power output
=
= 26,4 %
333 kW pe
fuel input
Cold to power ratio = factor Ζ cold =
102 kWth
cold output
=
= 1,2
power output
88 kWel
HEATING
Overall efficiency = η tot =
heat + power output 156 kWth + 100 kWel
=
= 77 %
333 kW pe
fuel input
Electrical efficiency = η el =
power output 100 kWel
=
= 30 %
333 kW pe
fuel input
Heat to power ratio = factor Ζ heat =
156 kWth
heat output
=
= 1,6
power output 100 kWel
3.2.1.2 cooling tower
The amount of rejected heat to the cooling tower is approx. 250 kW. This heat is rejected
to the environment at a low temperature level of approx. 35°C. A wet cooling tower in
this power range has an electrical consumption of 2 kWel for the fan and an evaporative
water loss of approx. 0,4 m3/h at full load operation. This results in 400 liter make-up
water per operating hour. A dry cooling tower with the same cooling water mass flow
(operating condition of the chiller) would in this case have a higher power consumption
of 7,5 kWel with more than one fan.
3.2.2 Concept 2: single-stage exhaust gas-fired
The direct use of the hot gases behind the recuperator of the microturbine is an
application that has already found its application in prototypes, but is until now not
commercially available. A higher COP is achieved with the higher firing temperature of
270°C. A lower performance (kW) is however achieved than with hot water-fired
absorption chillers since less heat is recovered from the hot gases in the single-stage
generator of the chiller.
34
Installation modeling
Single-stage exhaust gas-fired
Exhaust
0,8 kg/s
Absorption chiller
75°C
Cooling tower
34,5°C
Heat 36 kW:
120°C
Recovered heat
120 kW
270°C
Heat
215 kW
COP 0,8
Fuel
333 kW
Gen-set
Air
29,4°C
6,7°C 12°C
36 m3/h
Power consumption 8 kW
Power
92 kW
Heat
36 kW
Cold
96 kW
Figure 3-2: Installation concept 2: exhaust gas-fired single-stage chiller
The bypass system presented in figure 2 shows the possibility to control the load between
the chiller and the hot water heat exchanger. Not shown is an additional bypass for the
heat exchanger that would enable cooling only operation when no heat is required. The
maximum pressure drop behind the exhaust gas duct of the microturbine has to be taken
into account and a draft fan (0,75 kWel) between exhaust duct and generator of the chiller
could be necessary to prevent a reduction of the power output at certain installation sites.
3.2.2.1 performance
With the direct introduction of the hot gases into the generator of the absorption chiller
120 kWth of heat can be recovered. This results in a cold output of 96 kWth (27 RT) at a
COP of 0,8. The residual heat in the 120°C hot gases behind the chiller can be recovered
in a hot water heat exchanger. Here, 36 kWth is recovered with a water inlet temperature
of 70°C and the exhaust gases in this case, leave the system at a temperature of 75°C.
This type of chiller can also be operated in a heating mode. Due to internal transfer losses
it can however only produce approx. 82 kWth of heat and use the of the T100 CHPsystem for heating is far more efficient. For a comparison of the performances of heating
35
Installation modeling
with the chiller unit or with the standard hot water heat exchanger of the T100, the
calculations below can be compared with the heating performance of concept 1.
SINGLE-STAGE EXHAUST GAS-FIRED CHILLER
COOLING
η tot =
η el =
96 kWth + 36 kWth + (100 − 8 kWel )
= 67 % ; 8 kWel for cooling tower and chiller
333 kW pe
92 kWel
= 27,6%
333 kW pe
Ζ cold =
96 kWth
= 1,0
92 kWel
HEATING
η tot =
85 kWth + 36 kWth + (100 − 1,5 kWel )
= 66 % ; 1,5 kWel for chiller operation
333 kW pe
η el =
98,5 kWel
= 29,6 %
333 kW pe
Ζ heat =
96 kWth
= 1,0
98,5 kWel
Without the recovery of heat (36 kWth) behind the generator during chilling mode the
overall efficiency of the concept is also 57% as in concept 1. A closer look at the figures
above shows a bad heating performance of the chiller in heating mode compared to
heating with the T100. The operation of the chiller unit requires power (1,5 kWel) for the
working fluid pumps and a similar wear and fouling to the unit as during chilling
operation also shortens the lifetime of the chiller.
3.2.2.2 cooling tower
Due to higher firing temperatures a higher efficiency can be attained and less heat input is
required for the same cold output of the exhaust-fired chiller compared to the water-fired
type. An effect of this is that also less heat is rejected to cooling tower. For this concept
the cooling tower is designed for 215 kWth with an hourly evaporative water loss of 300
liter.
36
Installation modeling
3.3 Multi-stage chiller concepts
Efficiencies higher than 1,0 can be achieved with a high temperature generator. The highstage generator produces steam with high firing temperatures. The low-pressure steam
then drives the low-pressure generator and enters the condenser as refrigerant. This
additional refrigerant realizes a COP higher than 1,0. High-stage generators are designed
for flame temperatures up to 1400°C and the required combustion air is approx. 1
Nm3/kWhcold. The use of the microturbine exhaust can be brought into concept in two
principle ways.
3.3.1 Concept 3: double-stage natural gas-fired
The first concept presents a recuperated microturbine with a natural gas-fired absorption
chiller. The complete exhaust flow is used as feed air to the burner of the chiller. The hot
gases with residual oxygen of approx. 17% and a temperature of 270°C supplement the
fuel input.
Double-stage exhaust gas-fired
Exhaust
0,8 kg/s
75°C
Heat 75 kW:
Fuel
1237 kW
Absorption chiller Cooling tower
Recovered heat
34,5°C
984 kW
170°C
1400°C
904 kW
333 kW
270°C
Heat
2186 kW
COP 1,22
Gen-set
Air
29,4°C
6,7°C 12°C
Power
30 kW
Heat
75 kW
338 m3/h
Power consumption 70 kW
Cold
1200 kW
Figure 3-3: Installation concept 3: exhaust gas-fired double-stage chiller
37
Installation modeling
3.3.1.1 performance
This installation concept presents the combination of the T100 microturbine with an
absorption chiller in the power range of 1,2 MWcold. This type of chiller has the
possibility to produce hot water with an additional heat exchanger in the hightemperature generator (see figure 2-10). The installation of the 75 kWth hot water heat
exchanger as shown in figure 3 is therefore in this power range an option that has to be
reviewed in individual cases. Since the possibility is given for the heat recovery from the
170°C exhaust, the 75 kWth is taken into account with the calculation of the overall
efficiency. The bypass enables to adjust the amount of feed air to the chiller and gives the
possibility to operate the microturbine at full load while the chiller operates at part load or
is shut down. Heat can in this case be directly recovered from the 270°C exhaust.
DOUBLE-STAGE EXHAUST GAS-FIRED CHILLER
COOLING
η tot =
1200 kWth + 75 kWth + (100 − 70 kWel )
= 105 %
1237 kW pe
Ζ cold =
1200 kWth
= 40
30 kWel
HEATING
η tot =
1000 kWth + 75 kWth + (100 − 3 kWel )
= 90,7 %
(1163 − 204 kW pe ) + 333 kW pe
Ζ heat =
1000 kWth + 75 kWth
= 11
97 kWel
The electrical efficiency is not given since the cold-to-power ratio is 40 and the
microturbine covers the power consumption of the installation. The small amount of
excess power (30 kWel) can be used to drive system pumps that are not taken into account
in this review. The evaporative loss of the cooling tower result in a requirement of 3500
liters make-up water per hour.
38
Installation modeling
The performance data of the heating mode shows a fuel saving of 204 kWpe compared to
the 1163 kWpe of a chiller without the use of preheated combustion air. The chiller unit
attains an efficiency of approx. 85% during heating mode. The same principle way can
also be realized with an unrecuperated microturbine (See concept 5).
3.3.2 Concept 4: multi-stage exhaust gas-fired
The multi-stage chiller (described on page 21) takes part of the microturbine exhaust as
feed air to the burner that is supplemented with fuel and generates cold in the hightemperature generator with a COP of 1,2. An additional generator is aligned with the
high-stage generator for the rest of the exhaust flow. This special heat exchanger is
similar to the type of generator used in a single-stage exhaust gas-fired chiller and the
efficiency is thus limited to the COP of approx. 0,8 (concept 2).
3.3.2.1 performance
The cooling load of the multi-stage chiller can be controlled with the amount of hot gases
and natural gas fed to the burner. The burner and high-stage generator can be designed
for a specific maximum load and the special generator is then dimensioned to recover the
heat of the residual hot gases. The special generator may even be dimensioned for the
maximum mass flow for part-load operation without the use of the burner and high stage.
Multi-stage exhaust gas-fired
Exhaust
75°C
0,8 kg/s
Heat 46 kW:
Absorption chiller
120°C
Fuel
559 kW
226 kW
333 kW
170°C
1400°C
m =0,2 kg/s
270°C
m =0,6 kg/s
Cooling tower
34,5°C
Heat
708 kW
Recovered heat
246 + 90 kW
Gen-set
Air
29,4°C
6,7°C 12°C
Power
80 kW
Heat
46 kW
110 m3/h
Power consumption 20 kW
Cold
372 kW
Figure 3-4: Installation concept 4: exhaust gas-fired multi-stage chiller
39
Installation modeling
Different sizing of the special generator is possible. It can i.e. be designed to handle the
total mass flow and cover the base load of a building without the use of the high-stage
generator. The chiller would then have the same performance and efficiency as the
exhaust gas-fired single-stage chiller (1200 kWcold at COP 1,2). The installation concept
in figure 3-4 shows a chiller with 372 kWth. The gas burner is designed for approx. 0,3
m3/s, which results in 300 kWcold with a COP of 1,22 and the residual exhaust is
introduced into the special generator where 90 kW of recovered heat results in 72 kWcold.
MULTI-STAGE EXHAUST GAS-FIRED CHILLER
COOLING
η tot =
η el =
372 kWth + 46 kWth + (100 − 20 kWel )
= 89 %
226 kW pe + 333 kW pe
80 kWel
= 14,3 %
559 kW pe
Ζ cold =
372 kWth
= 4,65
80 kWel
HEATING
η tot =
315 kWth + 46 kWth + (100 − 1,5 kWel )
= 75 %
282 kW pe + 333 kW pe
η el =
98,5 kWel
= 29,6 %
333 kW pe
Ζ heat =
351 kWth
= 3,6
98,5 kWel
Without the recovery of heat behind the chiller in a hot water heat exchanger an overall
efficiency of 81% can be achieved during chilling operation. The comparison of this
installation concept with the double-stage chiller shows that the lower COP of the special
generator results in a lower overall efficiency. This is technically seen however the most
efficient way to recover heat from the microturbine exhaust for chilling. The burner and
high stage can be designed for peak load requirement and the special generator always
40
Installation modeling
recovers the residual heat for part load operation. The microturbine can in this case
operate nearly independent of the chillers load.
3.3.3 Concept 5: unrecuperated microturbine
The last installation concept presents a double-stage absorption chiller with an
unrecuperated microturbine. The electrical efficiency of the microturbine drops to
approx. 15% since the exhaust heat is not recuperated in the thermodynamic cycle. The
air at turbine outlet temperature of 650°C is fed to the burner of the chiller. Fuel is added
and the hot gases are introduced into the high stage. The high air temperature does not
allow the use in a low-stage generator and the chiller should therefore be designed for the
complete mass flow. The use of the microturbine exhaust in a hot water heat exchanger as
shown in figure 5 requires a construction that is resistant to these high temperatures.
3.3.3.1 performance
The system presents the same chiller as in installation concept 3 with the recuperated
microturbine and the cold output is therefore the same. The use of fuel is of course
somewhat different. See figure 5 and the calculations below.
Double-stage exhaust gas-fired without recuperator
Exhaust
0,8 kg/s
75°C
Heat 75 kW:
Fuel
1266 kW
Absorption chiller Cooling tower
Recovered heat
34,5°C
984 kW
170°C
1400°C
600 kW
666 kW
650°C
Heat
2186 kW
COP 1,22
Gen-set
29,4°C
6,7°C 12°C
Power
30 kW
Heat
75 kW
338 m3/h
Power consumption 70 kW
Cold
1200 kW
Figure 3-5: Installation concept 5: for a microturbine without recuperator
41
Installation modeling
DOUBLE-STAGE EXHAUST GAS-FIRED CHILLER WITHOUT RECUPERATOR
COOLING
η tot =
1200 kWth + 75 kWth + (100 − 70 kWel )
= 105 %
1237 kW pe
Ζ cold =
1200 kWth
= 40
30 kWel
HEATING
η tot =
1000 kWth + 75 kWth + (100 − 3 kWel )
= 88,7 %
(1163 − 508 kW pe ) + 666 kW pe
Ζ heat =
1000 kWth + 75 kWth
= 11
97 kWel
The performance data of this installation concept at full load does not differ greatly from
that of the exhaust gas-fired double-stage chiller with a recuperated microturbine. The
microturbine should however exactly follow the load of the chiller since the recovery of
heat from the excess hot gases is technically demanding, because the heat exchanger has
to be designed for temperatures up to 650°C. This installation concept is more suitable
for industrial applications where continuous cold is required for production processes.
3.4 Concept review
The modeling of the different chiller types resulted in the selection of two installation
concepts. The hot water-fired single-stage chiller and the exhaust gas-fired multi-stage
chiller were selected. A summary of the performances and efficiencies is presented in the
table below.
Concept nr.
overall efficiency
cold output
power output
cold-to-power ratio
heat output
power output
heat-to-power ratio
1
57%
102 kWth
88 kWel
1,2
156 kWth
100 kWel
1,6
2
67%
96 kWth
92 kWel
1,0
156 kWth
100 kWel
1,6
3
105%
1200 kWth
30 kWel
40
1075 kWth
97 kWel
11
4
89%
372 kWth
80 kWel
4,65
361 kWth
98,5 kWel
3,6
5
105%
1200 kWth
30 kWel
40
1075 kWth
97 kWel
11
Table 3-1: Performance data of the installation concepts
42
Installation modeling
The criteria that led to these feasible solutions are individually explained in the following
two paragraphs. Specific properties will be presented in the next chapter where they are
brought to comparison with conventional cold production and energy supply.
3.4.1.1 single-stage hot water-fired
The hot water-fired chiller is a serial product on the market. With the use of a hot water
storage tank the chiller unit can be controlled individually and the T100 CHP-unit can in
this installation concept operate in the normal heating mode. The unit is controlled with
the hot water-temperature that is fed to the storage tank and no modifications have to be
done to the individual controlling system. The efficiency of an exhaust gas-fired chiller is
higher, but as table 3-1 shows, the water-fired type has a higher performance. The singlestage exhaust gas-fired chiller (concept 2) has the same overall efficiency if the heat
exchanger placed behind the chiller (additional invest) is not taken into account. The low
heating performance of the exhaust gas-fired chiller results in the requirement of the heat
exchanger to achieve a high efficiency for heating, and the hot water-fired type is selected
for further investigation.
3.4.1.2 multi-stage exhaust gas-fired
This installation concept can actually fit the chilling load of buildings6 that are suitable
for the power output of the T100 microturbine, since the additional generator enables
different capacities of the chiller to cover peak loads. Part-load operation is controlled by
the amount of hot gases fed to the burner and the special generator. The load of the
microturbine can be individually controlled by the electrical demand of the building and
dependent on the required amount of cold, more or less fuel is added to the high-stage
burner. The heating mode of the chiller results in fouling and wear to chiller components.
Without the use of the special generator however, the high-temperature section can be
isolated from the main shell. A hot water heat exchanger in the high stage then works as
boiler. This saves the power consumption of the working fluid pumps and the lifetime of
the chiller, but means a load reduction of the microturbine or a bad heat recovery since
the high temperature stage only uses a small amount of the total exhaust flow. This chiller
is further investigated as second solution.
6
The energy dynamics of buildings are presented at the beginning of the next chapter.
43
Installation concepts and economics
4 INSTALLATION CONCEPTS AND ECONOMICS
Based on the modeled installation concepts and building energy requirements a review
and validation for commercial use is given in this chapter. Combined heat, cold and
power production with microturbines and the use of absorption chilling was technically
and economically reviewed and compared with conventional electric chilling.
4.1 Applications
The function of a building determines its demand for energy (load: W/m2). Heating,
cooling and lightning are hereby the dominant energy uses. The yearly energy use
(kWh/m2-a) and the amount of operating hours of a CHP system in a year are determined
by the hours of operation of the building. Here, the variations in control of the systems
and components, the concentration of internal loads and the changes in weather make it
hard to determine the actual energy requirement of buildings. Appendix A5 however
shows figures of heat, cold and power requirements for different applications. These
figures present a good reference for the comparison of the demand and load of each
energy form to determine the feasibility of installation concepts in different applications.
4.1.1 Heat, cold and electricity
The space heating or cooling load is defined as the net heat loss or gain resulting from a
set of conditions. Heating and cooling loads result from external climatic factors, internal
occupancy characteristics, and the building design. Ambient temperature (fresh air), solar
radiation, humidity and wind have the major effect on the peak load requirements and
heating, ventilation and air-conditioning systems (HVAC) have to be designed for these
maximum conditions. The type of use of the building however mainly determines the
actual requirement for space conditioning for human comfort. The internal cooling load
i.e. is a function of the heat gain from people, equipment and lightning. A higher intensity
of lighting and a higher density of people therefore result in a higher cooling load.
Estimates for the end-use of the energy consumption of residential and commercial
buildings can be seen in figure 4-1 on the next page. The illustrated breakdown by
category of end-use are based on the total energy consumption in the United States [6].
The left diagram shows that space heating is the predominant end-use in the residential
sector, consuming about 46% of the total residential primary energy in 1988. The trend
44
Installation concepts and economics
during the past decade has however shown a significant shift from fossil fuel use to
electricity use in the residential sector. And as it is not possible to generalize the end use
of primary energy in buildings, the presented diagrams show only a rough insight. A
comparison of both diagrams shows a higher energy use for lighting and space cooling in
the commercial sector. The electricity use in the commercial sector therefore forms a
greater percentage of its total energy use in comparison to the residential sector.
Commonly can also be said that because of larger surface-to-volume ratios, heating and
cooling are more dominant in small offices than in large office buildings. Heating or
cooling would be dominating for space conditioning, depending on the climate.
Residential sector
Commercial sector
Other
13%
Food storage
10%
Other
15%
Food storage
5%
Space heating
46%
Space heating
32%
Water heating
4%
Water heating
15%
Lighting
7% Space cooling
9%
Lighting
28%
Space cooling
16%
NOTE: Includes energy losses associated with electricity generation
SOURCE: U.S. Office of Technology Assessment, 1992
Figure 4-1: Primary energy by end-use
4.1.2 Residential buildings
Residential buildings have low stringent lightning requirements, relatively low equipment
loads, and usually a greater surface-to-volume ratio than commercial buildings do.
Residential buildings are therefore less influenced by internal loads and their energy use
is more subject to weather conditions. The demand for heat and cold is much greater in
comparison to the electricity demand for appliances with only few using hours. Average
design loads for a typical residential building are 65 W/m2 for heating, 50 W/m2 for
cooling and a power demand of 10 W/m2.
45
Installation concepts and economics
4.1.3 Office buildings
The energy use of offices is the most important in the building sector, largely because of
the great building stock number. These commercial buildings have higher electric energy
use and a lower fossil fuel use than residential buildings do. This is a result of inoperable
windows and more stringent humidity control, which generally requires more cooling and
typically more electricity use (ventilation). The dissipated heat by electric lighting and
business machines result in a cooling load of 125 W/m2. Energy is required over longer
periods during the year since office buildings tend to be operated during the day, in 12- to
16-hour periods [7].
4.1.4 Hotels
In the bedrooms of hotels the load is somewhat similar to that in an office. A lower
external load due to smaller windows and balconies that provide external shading is
equaled by the use of filament lamps and a standard color television set. It is however
consequently not easy to generalize about cooling loads in hotels in the way that one can
about office blocks. An important parameter is the ratio of bedroom area to public area.
More public area means a higher cooling load since the public area has a higher specific
refrigeration demand due to a high population density. The mean refrigeration load of
hotels is 129 W/m2 with an energy use of approx. 180 kWh/m2-a.
4.1.5 Super markets
The electric lightning, the population density and open refrigeration cabinets are the main
parameters that determine the heat gains in a super market. Typical high lighting levels of
1400 lux result in an average heat gain of 60 W/m2, depending on the type of installed
lighting. Whereas open refrigerated display cabinets with remote condensers outside the
conditioned space remove approx. 45 W/m2 of heat from the sales area 24 hours per day,
365 days of the year, regardless of the room temperature. This cold credit in form of
electricity use, results in a lower air-conditioning load in summer. It can however also
create a too little heating capacity during the winter period. Overall cooling loads vary
from 90 to 200 W/m2 of the total sales floor area, depending on the illumination level and
the type of refrigerated cabinet used. The average of 150 W/m2 cold load, 65 W/m2 heat
load and 60 W/m2 power load show that the energy use intensities the are highest in food
sales. Its lower stock numbers however limit the market potential of these building types.
46
Installation concepts and economics
4.1.6 Department stores
The total energy use of buildings with sales or service activity is next to the energy use of
office buildings the largest in the building sector, also because of their building stock. A
department store is a complex of different sales areas with different lighting levels and
population density. The energy demand thus varies greatly for each area and cooling
systems tend to be designed with sufficient flexibility to be able to adapt to changes in
layout of the department store. Shops rather have spot lighting than fluorescent lighting
tubes and a larger amount heat is dissipated. Department stores therefore have a low
heating demand of 35 W/m2 and the cooling load is at 180 W/m2 substantially higher than
in any other building types.
4.2 Technical review
To be able to investigate the installation concepts further, an office building located in the
middle of Europe with a floor space of 2500m2 was selected as a reference. This size is
based on the design load7 of this building type and the heating capacity of the T100
microturbine CHP-unit. Three different energy concepts were investigated to cover the
energy demand throughout the year.
DESIGN LOADS OF A 2500 M2 OFFICE BUILDING
Heat
Cold
Power
137,5 kW
312,5 kW
87,5 kW
One concept is designed with the hot water-fired absorption chiller and has an electrical
chiller to cover peak cooling demand. The second concept is build up out of the multistage absorption chiller fired by the exhaust of the microturbine and the third concept
presents a conventional energy concept with hot water boiler, electric chilling and power
supply by the grid. The performance data and specific properties of these different energy
concepts are presented together with the installation components.
7
See appendix A5: Building energy dynamics
47
Installation concepts and economics
4.2.1 Hot water-fired absorption chiller
The concept of this installation is quite similar to that of a conventional CHP-system.
Figure 4-2 shows the possibility to isolate the hot water storage tank and feed the hot
water directly to the heating system to reduce the power consumption by using only one
pumping cycle. The boiler can cover the tap water load and supply heat when the CHPunit operates at part load or is out of operation.
The absorption chiller and electric chiller cover the cooling load. The absorption chiller
can provide approximately one third of peak cooling demand and since air-conditioning
systems tend to be operated between 50 and 75% load, it is recommended to install two
25 kWel air-cooled chillers. These two units would then each provide approx. 100 kWcold
without the requirement for a cooling tower and thus no additional power consumption
and invest. One of both chillers can operate at full-load to cover 65% of design load and
achieve a high efficiency since part-load performance of the electric chiller results in a
bad COP. The dashed line in figure 4-2 shows the possibility to fire the absorption chiller
from the boiler. This option is however not taken into account since a more complex
controlling of the chiller has a negative effect on the operation of the installation concept.
Cogeneration system with hot water-fired chiller
Boiler
P o we r
T100
CHP-unit
P o we r
lo a d
Hot water
storage
Absorption
chiller
Electric
chiller
Spa ce
co o ling
lo a d
Spa ce
he a ting
lo a d
Ta p
wa te r
lo a d
Figure 4-2: Energy concept A with hot water-fired and electric chiller
48
Installation concepts and economics
The operation of the two electric chillers during the summer peak requires 50 kWel of
power from the grid since the CHP-unit is designed to cover the power load of the
building and the power consumption of the absorption chiller. Table 4-1 below shows the
components for this energy concept with the individual consumption and supply of the
different energy forms.
Component
Energy form
Consumption
Supply
T100 CHP-unit
fuel
333 kWpe
-
power
-
100 kWel
heat
-
156 kWth
power
12 kWel
-
cold
-
102 kWth
power
58 kWel
-
cold
-
213 kWth
fuel
145 kWpe
power
-
Absorption chiller
Electric chiller
Grid connection
58 kWel
Table 4-1: Installation components for hot water fired chiller concept
These components present the complete installation for this energy concept. The power
consumption of the cooling water pump and the cooling tower fan are noted under the
absorption chiller since these component are required for the operation of the chiller. The
power supplied by the grid is generated with an electrical efficiency of 40%8
(transmission losses of 2% included) [12] which results in 145 kWpe of used fuel. The
boiler is mainly used for the tap water load in this concept and it is not taken into account
as a component of this investigated energy concept.
8
The 40% present a mix of 50% for a combined cycle and 32% for a coal fired simple cycle
49
Installation concepts and economics
4.2.2 Multi-stage absorption chiller
The installation concept with the multi-stage chiller presents a stand-alone energy
concept without the requirement for a grid connection. This energy concept is based on
concept number 4 of the last chapter. The chilling load of the reference office building is
not as high as the capacity and a somewhat smaller chiller was therefore selected. The
burner and high-temperature stage are sized for a capacity of 315 kWcold to guarantee
chilling without the operation of the T100.
The heating load is normally covered with the T100 and in the case of failure the total
heating load can be covered by be the absorption chiller in heating mode. For the chilling
mode a bypass for the hot water heat exchanger in the T100 leads the hot gases to the
chiller through the exhaust gas piping system. Dependent on the load of the microturbine
and the chiller, hot gases is exhausted or ambient air is taken from the outside intake/
outlet pipe. The fan feeds the required amount of exhaust to the high temperature burner
and the residual amount of hot gases that enter the chiller through the small pipe are
introduced into the special generator9 for free generation of refrigerant.
Cogeneration system with multi-stage chiller
Boiler
T100
CHP-unit
P o we r
lo a d
Ex ha ust
Absorption
chiller
Hot water
storage
Spa ce
co o ling
lo a d
Spa ce
he a ting
lo a d
Ta p
wa te r
lo a d
Figure 4-3: Energy concept B with multi-stage chiller
9
The schematics of a multi-stage chiller can be found in figure 2-13 on page 21
50
Installation concepts and economics
The optional boiler (tap water load) shown in figure 4-3 can be replaced by the hot water
heat exchanger in the high-temperature section of the chiller. Here, heat of the steam
(refrigerant vapor) is transferred to the water flowing through the hot water tubes.
Component
Energy form
Consumption
Supply
T100 CHP-unit
fuel
333 kWpe
-
power
-
100 kWel
heat
-
156 kWth
fuel
203 kWpe
-
power
20 kWel
-
cold
-
315 kWth
Absorption chiller
Table 4-2: Installation components for multi-stage chiller concept
The absorption chiller generates 245 kWcold with a COP of 1,2 in the high-temperature
stage and the 90 kWth of recovered heat from the microturbine exhaust in the special
generator results in approx. 70 kWcold.
4.2.3 Electric chiller
The conventional solution for energy supply is presented as last energy concept.
Conventional energy concept
Grid
connection
P o we r
lo a d
Electric
chiller
Spa ce
co o ling
lo a d
Boiler
Spa ce
he a ting
lo a d
Ta p
wa te r
lo a d
Figure 4-4: Energy concept C with electric chiller and boiler
51
Installation concepts and economics
The boiler is designed to cover the heating demand and the electric chiller covers the
cooling load. The fuel consumption of the boiler and that for the generation of consumed
power from the grid is shown in table 4-3.
Component
Energy form
Consumption
Supply
Fuel
185 kWpe
-
heat
-
156 kWth
power
73 kWel
-
cold
-
315 kWth
fuel
403 kWpe
-
power
-
161 kWel
Boiler
Electric chiller
Grid connection
Table 4-3 : Installation components for conventional energy concept
The power consumption of the chiller is 73 kWel at full load. The bad part load
performance however results in an average COP of approx. 3,0 and a power consumption
of 105 kWhel will therefore be used for every operating hour in the economical review.
4.2.4 Concept review
This last technical paragraph presents the energy data used for the economical review.
The energy concepts are here brought to comparison with their fuel use and thus overall
efficiency. The energy consummations of the three different concepts are presented for
cooling and heating throughout the year. The overall efficiency of the different energy
concepts is calculated in the following way:
Overall efficiency = η tot =
cold or heat + power 315 kWcold + 88 kWel
=
= 79 %
333 + 177 kW pe
fuel use
The example is given for concept A in table 4-4 below. The fuel use is calculated with the
fuel input of the T100 microturbine and the fuel equivalent of the 71 kWel electricity from
the grid (ηel 40% incl. transmission losses) for the electric chiller (COP 3,0). The
recovery of heat at the power plant is hereby not taken into account.
52
Installation concepts and economics
COOLING
Concept
Fuel use
Electricity use
ηtot
A. Hot water-fired and electric chiller
333 kWpe
71 kWel
79 %
B. Multi-stage chiller
536 kWpe
8 kWe
73 %
-
193 kWel
84 %
C. Electric chiller
Table 4-4: Energy consumption for cold and power
The high efficiency of the electric chiller results in a higher overall efficiency than the
energy concepts with microturbine and absorption chiller.
HEATING
During heating operation there is 100 kWel available from the microturbine CHP-unit.
The fuel equivalent for the electricity from the grid is however calculated for the actual
building consumption of 88 kWel and the surplus of electricity (12 kWel) from on-site
generation with the microturbine is sold to the grid.
Concept
Fuel use
Electricity use
ηtot
A. Hot water-fired and electric chiller
333 kWpe
-
77 %
B. Multi-stage chiller
333 kWpe
-
77 %
C. Electric chiller
185 kWpe
88 kWel
60 %
Table 4-5: Energy consumption for heat and power
SIZE AND WEIGHT
The space requirement for the microturbine and the absorption chillers (footprint and
service clearance) are the main dimensions for the size of the machine room. The cooling
towers and the air-cooled compressor chillers with weather-resistant design are both
installed outside. The dimensions and operating weight are presented in appendix A6:
Size and weight of installation components.
53
Installation concepts and economics
4.3 Economical review
Next to the comparison of efficiencies in the technical review, the economy of the two
installation concepts was investigated with the German market conditions. After the
presentation of the basic economical data a cost comparison is made between the
concepts A, B and C (described in the paragraph above). The absorption chiller concept
with the cheapest production cost will then be investigated further. A direct cost
comparison between the selected absorption chiller and the electrically driven compressor
chiller then results in a minimum amount of operating hours for a cost advantage. A static
economical review is then presented to validate the use of absorption chiller in
combination with the microturbine.
4.3.1 Basic economical data
The calculation of the total costs is based on the figures presented in table 4-6 below. The
investment costs for the individual installation components are presented in appendix 7:
Investment costs. The annual capital costs are calculated with an interest rate of 7% and a
depreciation period of 10 years. The subsequent HVAC-system has no relevance for the
comparison and is not included in the calculations. This is also the case for the grid
connection. Only in specific cases costs for the extension of the grid connection can be
avoided by installing on-site generation systems. This is not taken into account in this
review.
Performance
Fuel
Power
Heat
Cold
Consumption cost
Fresh water price €/m3
Waste water price €/m3
Gas price
€/MWh
Electricity price €/MWh
Cold price
€/MWh
333 kW
100 kW
156 kW
102 kW
2
1,5
28
92
53,31
Capital cost
Interest rate
%
depreciation period years
Depreciation factor Maintenance cost[5]
Absorption chiller €/year
Cooling tower
€/year
Compressor chiller €/year
Boiler
€/year
Microturbine
€/h
7
10
0,1424
1%of invest
2%of invest
3%of invest
1%of invest
1,1
Table 4-6: Basic data for economical review
54
Installation concepts and economics
The costs for operation are mainly determined by the consumption of natural gas and
electricity. The gas and electricity prices are therefore the most important factors for a
good economy. The energy prices used for this review present the market prices in
Germany. The net costs in €/MWh are based on 55 DM/MWh for natural gas (HHV) and
180 DM/MWh for electricity.
TAX CREDITS
The use of fuel and electricity in Germany is connected with a taxation of the used
amount in MWh. This taxation is part of the ecological tax law and is paid by the end
users of energy (included in the prices in table 4-6). The fuel used for electricity
production with combined heat and power (ηtot ≥ 70%) is excluded from the taxation.
Also the power itself produced by CHP is excluded. The following tax credits[10] can
therefore be applied for the consumed natural gas and produced electricity by the
microturbine in an office building.
Natural gas tax
3,47 € /MWh
Electricity tax
12,80 € /MWh
HEAT PRICE
The cost credits for heat and cold in the payback calculations later in this chapter are
based on own calculations presented in the tables below. The heat price is determined
with the fixed costs of the investment for a boiler (See appendix A7) and the operation
cost for 3500 heating hours. The heat price in €/MWh can be found in the table below.
Fixed costs
Capital costs €/year
Insurance €/year
Variable costs
Fuel
€/year
Maintenance€/year
Yearly costs
925,45 €
65,00 €
16.758,00 €
130,00 €
17.878,45 €
Production
Heat MWh/year
546MWh
Costs per MWh
32,74 € /MWh
at
3500
Operating hours
Figure 4-5: Cost calculation for heat price
55
Installation concepts and economics
COLD PRICE
The cold price is calculated with the cold production with an electric chiller for 1500
operating hours a year. The electricity price of 92 €/MWh used as a constant price results
in a cold price of 53,31 €/MWh. This price will be used as cost credit for the produced
cold with the absorption chiller in the payback calculations.
Fixed costs
Capital costs €/year
Insurance
€/year
Variable costs
Electricity
€/year
Maintenance €/year
Yearly costs
2.705,17 €
190,00 €
Production
Cold MWh/year
153 MWh
Costs per MWh
53,31 €/MWh
4.692,00 €
570,00 €
8.157,17 €
at
1500
Operating hours
Figure 4-6: Cost calculation for cold price
4.3.2 Concept review
The yearly energy costs for each energy concept are presented and brought to discussion
for three different cases with different amount of operating hours.
Yearly operating hours
Daily operating hours
Months
Case 1
Case 2
Case 3
Chilling Heating Chilling Heating Chilling Heating
500
3000
1500
3000
1500
1500
6
16
12
16
12
12
2,5
6
4
6
4
4
Table 4-7: Operating hours for different user behavior
The variation of the amount of operating hours shown in table 4-6 above present different
user behavior of the reference office building. Other types of building with different
design loads (See page 42) would have a somewhat different energy concept and/ or
different building size. The cost comparison of the investigated energy concept for the
office building however presents a good reference for other building types since the
market conditions are similar.
56
Installation concepts and economics
The cases with different amount of operating hours for chilling and heating were
investigated separately. The individual costs for chilling and heating are presented in
appendixes A9 through A11 and the costs for case 1 are described in detail below. A cost
advantage is achievable in cases 1 and 2 for concept A (water-fired absorption chiller) if a
minimum of 2500 heating hours is achieved. Concept B (multi-stage absorption chiller)
shows a negative result for all operating cases. The review of case 3 in appendix A11
shows that the fuel and electricity costs of concept A and B are significantly lower than
that of concept C. The high capital costs for the investment however, make both concept
A and B unprofitable for this short operating period.
CASE 1
The capital costs of the two absorption chiller concepts A and B present 40 and 42% of
the yearly energy costs. With the electrical chiller this is only a percentage of 18% and
the high investment for the microturbine and absorption chiller add up to high yearly
capital costs. The costs for the electricity consumption of the compressor chiller in the
cooling period present 15% of the yearly costs. The percentage of operating costs
(electricity and fuel) for the absorption chiller concepts do not differ much. The costs for
heating operation (64%) for the conventional energy concept are however significantly
higher than for the two installation concepts. This shows the difference of operating costs
between combined heat and power generation with the T100 and conventional energy
solution. Concept B can not be operated economically with the assumed energy costs.
Based on the acknowledgement that absorption chillers can only be operated with a cost
advantage in combination with electric chillers. The hot water-fired single-stage
absorption chiller (concept A) is further investigated in the rest of this chapter.
57
Installation concepts and economics
4.3.3 Cost comparison calculation
For the selection between two alternatives a comparison can be made between the
production costs of both.. With the cost comparison, the capacity of the two compared
investment alternatives should be the same. The discussion is based on the comparison of
costs where the economy of the investments are not taken into account. The economy as
such is presented later in this chapter with the calculation of liquidation. The critical
amount of MWh (or operating hours) can be determined with the following formula [11]:
Fixed cos ts1 + s lim it * Variable cos ts1 = Fixed cos ts 2 + s lim it * Variable cos ts 2
4.3.3.1 cost comparison chillers
The yearly production costs of a water-fired absorption chiller and a compressor chiller
are brought to comparison in appendix AX based on 1500 operating hours per year at
similar chilling power of 102 kW. The total costs are the sum of yearly fixed costs
(depreciation, insurance, etc.) and variable cost (energy consumption, maintenance) based
on the amount of yearly operating hours. The consumption costs (electricity) for the
compressor chiller are calculated for a part load COP of 3.0 and the inserted heat into the
absorption chiller is not taken into account as consumption costs since the (waste) heat of
the microturbine does not present additional fuel consumption than for microturbine
itself.
The absorption chiller has a higher invest and therefore higher fixed costs. The variable
costs (operation costs) are however lower than the alternative with the compressor chiller.
A limit calculation was made to determine the amount of MWhcold or operating hours
where the alternative with the absorption chiller has a cost advantage to the compressor
chiller. The limit value for the investigated case is then:
s lim it =
Fixed cos ts 2 − Fixed cos ts1
3.845 € − 6.577 €
=
= 123 MWh
Variable cos ts1 − Variable cos ts 2 12,16 € / MWh − 34,39 € / MWh
58
Installation concepts and economics
The critical amount of operating hours where the total yearly costs of both alternatives
are similar is herewith determined at approx 1200 hours (123 MWhcold and 102 kWcold).
The determination of slimit is also be presented graphically in figure 4-5 below.
Cost comparison absorption vs. compressor chiller
Production costs [€/year]
10.000 €
8.000 €
6.577 €
6.000 €
3.845 €
4.000 €
Absorption chiller
Compressor chiller
2.000 €
0
200
400
600
800
1000
Operating hours [h/year]
1200
slimit=
1200
1400
Figure 4-7: Cost comparison chillers
This cost comparison does not give an absolute reference for the validation of the
economy of the investments. It can only be used for the selection between two
alternatives. The economy should the be determined individually.
4.3.3.2 cost comparison microturbine
Looking at the microturbine as alternative to the conventional energy solution there is
also a limit amount of operating hours where the production costs of the microturbine
starts to have a cost advantage to the production of heat with a boiler and electricity
consumption from the grid. The costs (fixed and variable) are presented in appendix AX
based on 3500 hours per year (heating period) at similar 100 kW power and 156 kW heat.
s lim it =
Fixed cos ts 2 − Fixed cos ts1
1.315 € − 16.190 €
=
= 951 MWh
Variable cos ts1 − Variable cos ts 2 37,47 € / MWh − 53,11 € / MWh
59
Installation concepts and economics
The yearly limit of production is 951 MWh of heat and power. The total performance of
the microturbine is 100 kW power and 156 kW heat. The yearly limit of 951 MWh
should therefore be divided by 256 kW of total installed power. This results in a
minimum of 3715 yearly operating hours as shown in figure 4-8 below.
Production costs [€/year]
Production costs microturbine vs. conventional solution
60.000 €
51.822 €
40.000 €
20.000 €
Microturbine
16.190 €
Conventional
1.315 €
-€
0
1000
2000
Operating hours [h/year]
3000
4000
5000
slimit= 3715
Figure 4-8: Comparison of productions costs for microturbine
Both absorption chiller and microturbine are now brought to comparison with the other
cost alternatives. These cost comparisons however do not present the economy of the
investments. It is a selection criteria that shows the minimum production (operating
hours) before a cost benefit is made with the higher investment. For the economy of the
installation concepts the liquidation and the return of invest are presented in the following
paragraphs.
4.3.4 Liquidation and Return of invest
The liquidation and the return of invest (ROI) are both based on the cost comparison
between two alternatives. Aim of the calculation of the liquidation is the determination of
the period in which the investment is paid back by the revenues. The liquidation period is
the ratio between investment and the yearly cash flow.
60
Installation concepts and economics
Liquidation ( years) =
Investment [€]
Cash flow [€ / year ]
The cash flow is the yearly profit together with the yearly capital costs as released capital
of the investment – under the condition that they are covered by the revenues. An
investment is then validated positive when the liquidation period is as short as possible or
shorter than the period set by the investor. A shorter liquidation period presents a smaller
risk for the investment. A liquidation period of 3-4 years should be achieved to keep the
risk of the investment to a minimum. The basic condition is that the liquidation period is
shorter than the using period of the investment object.
The aim of the return of invest is to determine the productivity of the investment as the
ratio of the realized profit to the made investment. The ROI is calculated as follows:
ROI =
Yearly profit [€ / year ]
* 100%
Investment [€]
The ROI can be used to measure the productivity (average interest) of the investment to a
set minimum by the investor. Where expected return of invest > market interest rate[LX].
4.3.4.1 liquidation microturbine
The liquidation of the microturbine is based on the cost comparison made above. This
comparison of production costs resulted in a minimum of 3715 yearly operating hours.
As discussed at the end of the comparison an economy is not guaranteed with this
comparison. The calculations of the liquidation period and the ROI of the investment for
the microturbine are presented in appendix AX. In this review the heat and cold prices
calculated on pages 50 and 51 are used for the calculation of the earnings for the
produced heat and cold. Also the presented tax credits for natural gas and electricity
consumption are taken into account as earnings since the taxes included in the price per
MWh are reimbursed. A summary of the important data is given below.
61
Installation concepts and economics
Cash flow = profit + capital cos ts = 11.285 € + 11.390 € = 22.675 €
ROI =
Yearly profit [€ / year ]
11.285 €
* 100 % =
*100 % = 14,1%
Investment [€]
80.000 €
Liquidatio n ( years ) =
Investment [€]
80.000 €
=
= 3,5 years
Cash flow [€ / year ] 22.675 €
Without the tax credits the liquidation period would be 5,9 years with a ROI of 2,8%.
This is not a good result since the expected return of invest should be higher than the
market interest rate which is not the case.
4.3.4.2 liquidation of microturbine and absorption chiller
The review of the economy of the combined heat, cold and power installation with the
hot water-fired absorption chiller is based on the minimum amount of operating hours
resulting from the cost comparison calculations above. The sum of these limit values is
4915 operating hours for the microturbine whereof 1200 chilling hours. The economical
results are given below where the calculation of the liquidation for this case is presented
in Appendix AX.
Cash flow = 22.585 €
ROI = 5,8%
Liquidatio n = 5,0 years
The additional investment costs for the absorption chiller result in a longer liquidation
period. The cash flow is however nearly the same as with the microturbine alone although
the yearly capital costs are higher. This is the result of lower profits which can be seen
out of the lower return of invest.
62
Installation concepts and economics
4.3.5 Sensitivity analysis for a good economy
To achieve a similar economy with the additional investment of the absorption chiller as
with the microturbine (Liquidation period of 3,5 years and a ROI of 14,1%) the cooling
period should amount 3300 hours. This results in a total of 7015 operating hours for the
microturbine. The economical results are presented in appendix AX.
The produced heat by the microturbine that is used for the cold production is not credited
with the heat price. This affects the economy of the microturbine since “the heat is given
away for free” to the chiller. These “costs” and the operation costs of the chiller for the
production of cold result in the total variable costs per MWh cold. The amount of
operating hours to achieve a cost benefit on the additional invest of the absorption chiller
with the assumed cold price is 3300 hours. The additional yearly profit to attain a similar
liquidation period and ROI as with the microturbine alone should be higher due to the
additional made investment.
Additional required profit = 15.810 € − 11.285 € = 4.525 €
Even more operating hours should actually be made to achieve a higher ROI, which is
normally the aim of additional made investments.
4.3.6 Decision criteria
AMOUNT OF OPERATING HOURS
The main decision criteria for a specific installation concept is a good economy. The
overall efficiency plays an important role here for the conversion of prime energy in the
desired energy forms. The amount of yearly operating hours is however more important
for the application of combined heat, cold and power systems. A critical amount of
running hours determines the profitability of the investment at a fixed set of market
parameters such as natural gas price and electricity price.
63
Installation concepts and economics
ENERGY PRICES
Other than calculating the amount of operating hours as a variable the economy can be
altered by the energy prices as most critical external factors. The two cases 1 and 2 in
table 4-7 present realistic amount of operating hours of commercial buildings. The prices
for electricity and natural gas are changed to realize a similar liquidation as for the
microturbine with the given amount of operating hours for each case. For a good
economy the natural gas price should fall and/ or the electricity price should rise.
Case 1
Case 2
fall
51%
38%
Gas price
14,3
€/MWh
20,3
€/MWh
Rise
56%
31,5
Electricity price
144
€/MWh
121
€/MWh
Figure 4-9: Variation of energy costs
If the gas price drops simultaneously with a rising electricity price, which is normally the
case, the total price drop/ rise should always be 100% of the required price drop/ rise.
This means that if the gas price in case 1 falls with 25,5% (50%) the electricity price
should then rise 28% (50%). The same can be applied to case 2.
WET OR DRY COOLING TOWER
The water price can play an important role when there is a lack of water. Costs for makeup water could therefore make the dry cooling tower a better solution than the wet
cooling tower. The operation of a dry cooling tower (1500 chilling hours) is however
only cheaper in operation than a wet cooling tower (20 €/kW) when the fresh water price
is higher than 3 €/m3 and the waste water price is above 2 €/m3. This is due to the higher
investment costs (60 €/kW) and the four times higher power consumption for the fan of
the dry cooling tower. For a lower amount of operating hours the water price has to be
even higher for a cheaper operation of the dry cooling tower, since the water costs are
lower but the capital costs are constant. A comparison of the costs is given in appendix
12: Wet or dry cooling tower. The costs can be compared with A10: Energy costs case 2
to see an influence of higher water costs to the operating costs in total.
64
Conclusions and prospects
5 CONCLUSIONS AND PROSPECTS
The results show that the application of absorption chillers with microturbines can only
be realized economically in combination with electric chilling. The high efficiency of the
compressor chillers and the current gas and electricity prices make the production of cold
with fuel as only energy source unprofitable.
The investigation present the single-stage hot water-fired absorption chillers as most
suitable for chilling with microturbines. This is the same chiller type as used for
reciprocating engines in the power range ≥ 500 kWcold with the use of district chilling
systems. The expected advantage of the exhaust as only heat source with sufficient
residual oxygen for supplementary firing did not result in an economic operation of
double-stage absorption chillers in this power range since the heat of the microturbine
exhaust cannot be recovered with an as high efficiency as in the hot water heat
exchanger.
Combined heat, cold and power systems are not widely spread in small office buildings
(≤ 2500 m2), that represent a large stock number, because the investment costs are more
critical when the installed power is smaller. The cold-to-power ratio of the building is 3,6
and the single-stage absorption chiller provides one third of the design chilling load,
where the electric chiller covers the peak load. Investigations show that the application of
microturbine and absorption technology for this small power range can operate
economically if the installation achieves over 7000 running hours (3300 chilling and
3700 heating).
The single-stage exhaust gas-fired absorption chiller should be optimized in its heating
capacity and efficiency. Also the costs resulting from wear (shorter lifetime) and fouling
during heating operation of the chiller should be compared with the cost savings for the
hot water heat exchanger in the T100 microturbine CHP-unit. This would support a
choice between heating with the chiller unit or the T100.
65
Literature
6 LITERATURE
[1]
[2]
Erwan Cotard of Cogen Europe
European emissions standards for cogeneration (gas turbines and
reciprocating engines - 2000)
Cogen Europe Briefing Paper, Briefing No. 11, May 2000.
http://www.gascooling.org
Gas Cooling Technical Library
Presentation 2. Engine Basics of Presentations Online
[3]
B. Utesch, G. Mertz
Asorptionskälteanlagen, Grundlagen und Referenzen
ASUE, Hamburg, Oktober 1995
[4]
B. Utesch, G. Mertz
Asorptionskälteerzeugung im Überblick, Kühlen mit Erdgas
ASUE, Hamburg, Juni 1997
[5]
C. Müller
Wärme macht Kälte, Absorptionskälteerzeugung in der Praxis
ASUE, Fachtagung 6. Dezember 2000, Göppingen
[6]
Bruce D. Hunn
Fundamentals of Building Energy Dynamics
MIT, Boston, 1996
[7]
W.P. Jones
Air conditioning, Applications and Design
Second Edition 1997, ISBN 0-34064554
[8]
John Cerisano
Gas Cooling vs. Electric Cooling, Comparing the Costs
Four Seasons Controlled Climates Ltd, Woodbrigde Ontario, Canada,
February 26, 2001
[9]
Lambert Kuijpers, Ozone Operations Resource Group(OORG)
Trends in refrigeration technology in developed countries and in
developing country projects.
The World Bank 17th OORG meeting, 3 November 2000
http://www-esd.worldbank.org/mp/
[10]
http://www.stromsteuer.de
[11]
Warnecke, Bullinger, Hichert, Voegele
Wirtschaftlichkeitsrechnung für Ingenieure
3., überarbeitete Auflage, Carl Hanser Verlag München, Wien, 1996
66
Literature
[12]
Karlheinz Albert (†), Ottomar Apelt, Gregor Bär, Hans-Jürgen Koglin
Elektrischer Eigenbedarf, Energietechnik in Kraftwerken und Industrie
VDE-Verlag GmbH, Berlin, Offenbach, 1996, ISBN 3-8007-2179-1
67
Literature
7 APPENDIX
APPENDIX 1: ADSORPTION CHILLER ..............................................................................................A1
APPENDIX 2: ENVIRONMENTAL CONCERNS OF REFRIGERANTS..........................................A2
APPENDIX 3: PIPING SYSTEM ABSORPTION CHILLER ..............................................................A3
APPENDIX 4: ARI STANDARD 550/590-1998 ......................................................................................A4
APPENDIX 5: BUILDING ENERGY DYNAMICS ...............................................................................A5
APPENDIX 6: SIZE AND WEIGHT OF INSTALLATION COMPONENTS ....................................A6
APPENDIX 7: INVESTMENT COSTS ...................................................................................................A7
APPENDIX 8: ENERGY CONSUMPTION DATA................................................................................A8
APPENDIX 9: ENERGY COSTS CASE 1...............................................................................................A9
APPENDIX 10: ENERGY COSTS CASE 2...........................................................................................A10
APPENDIX 11: ENERGY COSTS CASE 3...........................................................................................A11
APPENDIX 12: WET OR DRY COOLING TOWER..........................................................................A12
APPENDIX 13: COST COMPARISON CHILLERS ...........................................................................A13
APPENDIX 14: COST COMPARISON MICROTURBINE................................................................A14
APPENDIX 15: LIQUIDATION OF MICROTURBINE.....................................................................A15
APPENDIX 16: LIQUIDATION OF
APPENDIX 17: ECONOMY OF
MICROTURBINE AND ABSORPTION CHILLER ........A16
MICROTURBINE AND ABSORPTION CHILLER ..............A17
68
Appendix
APPENDIX 1: ADSORPTION CHILLER
The adsorption chiller works with water as refrigerant and the cooling cycle is realized by
the principle of the adsorption of the refrigerant vapor by silica gel. Two heat exchangers
that are filled with silica gel granulate alternate in function as generator and adsorber. The
generator is the section with a certain amount of stored water (refrigerant) that is
regenerated with hot water as heat source at vacuum pressure. The produced refrigerant
vapor is introduced into the condenser and condenses by rejecting its heat to the cooling
water. This liquid refrigerant is then provided to the lower pressure evaporator where
flash boiling occurs, which cools the chilled water down to the saturation temperature.
The dry section works as adsorber where the refrigerant coming from the evaporator is
adsorbed by the dry silica gel and rejects its acquired heat to the cooling water that is
pumped through the adsorber tubes. The control valves switch the two sections in a 10minute cycle.
Hot water cycle
4
6
9. Evaporator
10. Adsorber
11. Generator
12. Condenser
13. Refrigerant pump
14. Control valves
2
3
6
Cold water in
1
Cold water out
5
Figure A10: Schematics of an adsorption chiller
A 1
Appendix
APPENDIX 2: ENVIRONMENTAL CONCERNS OF REFRIGERANTS
Compressor chillers work with refrigerants that have thermophysical properties that
enable a high coefficient of performance for the refrigeration system. Commonly known
refrigerants R11 and R12 are chlorofluorocarbons (CFCs). They are stable, non-toxic and
inflammable compounds. These qualities made them very attractive in history and they
therefore replaced the toxic and flammable ammonia in spite of its higher efficiency.
These chlorine containing chemicals were, however found out to be harmful to the ozone
layer and have therefore been totally phased-out. The R22 hydrochlorofluorocarbons
(HCFCs) were widely accepted as suitable replacements for the chlorofluorocarbons
since they have a much lower ozone depleting potential (ODP). These refrigerants are
however also added to the list of compounds to be phased out. They can not be used for
new equipment anymore and servicing/ recharging is not allowed with virgin HCFCs
after 1.1.2010.
Non ozone depleting options have therefore taken over. This includes hydrofluorocarbons
as HFC-134a and HFC-410A . These refrigerants do not deplete the ozone layer (nonODP) because they do not contain chlorine. They however have a significant high global
warming potential (GWP) and therefore face growing opposition. The future of
environmentally safer refrigeration lies with natural substances, such as hydrocarbons
(R290, R1270), carbon dioxide (R744), ammonia (R717), water (R718) and air. A
comparison between refrigerants with critical properties is given in table A-1 below.
Refrigerant
Type
Formula
ODP10 GWP
Remark
R11
R12
R22
CFC
CFC
HCFC
CFCL3
CF2CL2
CHF2CL
1,0
0,9
0,05
1,0
3,0
0,37
totally prohibited
totally prohibited
prohibited for
new installations
R134a
R290
R1270
R717
HFC
Butane
Propene
Ammonia
C2H2F4
C3H8
C3H6
NH3
0
0
0
0
0,26
0
0
0
11
flammable
flammable
toxic, flammable
and corrosive
Table A2
10
11
ODP: Ozone depleting potential; reference value R11
GWP: Global warming potential; reference value R11
A 2
Appendix
APPENDIX 3: PIPING SYSTEM ABSORPTION CHILLER
Chilling/ heating water system
hot water system
Cooling water system
auto water makeup
fan coil
expansion
water tank
hot water tank
cooling tower
by-pass
valve
distribution tank
hot water pump
cooling water pump
Exhaust system
gas supply
oil supply
from the storage oil tank
exhaust stack
Fuel system
2
O
O3
4
O
O1
1.
2.
3.
4.
Generator
Condenser
Evaporator
Absorber
back to the storage oil tank
chilled/heating
water pump
vent valve
water tank
water makeup
daily oil tank
fouling collector
valve (open)
ball valve
flow meter
temperature sensor
filter
valve (closed)
solenoid valve
pressure gauge
flow control
soft connector
check valve
safety valve
thermometer
oil level probe
A 3
Appendix
APPENDIX 4: ARI STANDARD 550/590-1998
Table 1. Standard Rating Conditions
Water-Cooled
Evaporatively-Cooled
Condenser Water
Entering
85°F [29.4°C]
Flow Rate
3.0 gpm/ton [0.054 L/s per kW]
Condenser Fouling Factor Allowance
Water-Side
0.00025 h @ ft · °F/Btu
[0.000044 m @ °C/W]
Air-Side
0.000 h @ ft · °F/Btu
[0.000 m @ °C/W]
Air-Cooled
0.000 h @ ft · °F/Btu
[0.000 m @ °C/W]
Entering Air
Dry Bulb
95°F [35.0°C]
Wet Bulb
75°F [23.9°C]
Evaporator Water
Leaving
44°F [6.7°C]
Flow Rate
2.4 gpm/ton [0.043 L/s per kW]
Evaporator Fouling Factor Allowance
Water-Side
0.0001 h @ ft °F/Btu [ 0.000018 m @ °C/W]
7.1.1.1.1 Condenserless
Water or
Air Cooled
Evaporatively Cooled
Saturated Discharge
105°F [40.6°C]
125°F [51.7°C]
Liquid Refrigerant
98°F [36.6°C]
105°F [40.6°C]
Barometric Pressure - 29.92 in. of Hg [101 kPa]
A 4
Appendix
APPENDIX 5: BUILDING ENERGY DYNAMICS
The energy requirements of different types of buildings are shown in the table below. The given range takes the variation of weather conditions
and building construction into account. The data is based on the book: Air conditioning, Applications and design [7].
2
Residential building Design load [W/m ]
range
Energy use [kWh/m2-a]
range
Office building
Design load [W/m2]
range
Energy use [kWh/m2-a]
range
Hotel
Design load [W/m2]
range
Energy use [kWh/m2-a]
range
Super market
Design load [W/m2]
range
Energy use [kWh/m2-a]
range
Department store
Design load [W/m2]
range
Energy use [kWh/m2-a]
range
Heat
65
50-70
135
100-200
55
45-60
135
Cold
50
40-70
45
35-70
125
85-200
180
Power
10
5-15
25
15-55
35
20-50
100
60
50-70
150
129
85-180
180
Heat-to-power ratio Cold-to-power ratio
6,5
5,0
5,4
1,8
1,6
3,6
1,4
1,8
25
30-40
100
2,4
5,2
1,5
1,8
65
150
60
45-80
90-200
35-85
135
450
300
100-300 300-650 250-400
35
180
45
20-45 140-210 35-60
120
350
150
90-300 300-450 100-180
1,1
2,5
0,5
1,5
0,8
4,0
0,8
2,3
Table A5: Energy dynamics of buildings
A5
Appendix
APPENDIX 6: SIZE AND WEIGHT OF INSTALLATION COMPONENTS
The different sizes of the used installation components are shown in the table below. The actual space requirement is based on the size of the
component and the service clearance. Absorption chillers require a clearance for the removal of tubes that approximately have the length of the
chiller. The microturbine and the absorption chiller are designed for indoor installation. The cooling towers and the electric chillers with outdoor
casing and build-in condenser fans are naturally installed outside and indoor space requirement is therefore only required for the microturbine
and absorption chiller.
7.1.1.1.1.1 Size and
weight
Performance
Length
Width
Height
Footprint
Space requirement
Operating weight
Microturbine Water-fired Cooling Electric Gas-fired Cooling Electric
chiller
chiller absorption tower
absorption tower
chiller
chiller
mm
mm
mm
m2
m2
kg
100 kWel
2920
870
1900
2,5
9,3
2000
102 kWth 250 kWth 100 kWth 315 kWth 715 kWth 315 kWth
2475
1226
4100
6400
2220
4800
1040
1226
1215
1550
2220
2435
2000
3020
2325
2140
3480
2435
2,6
1,5
5
10
5
12
12,6
11,5
21,4
19,4
2600
660
1550
6300
2800
3840
Table A6: Size and weight of installation components
A6
Appendix
APPENDIX 7: INVESTMENT COSTS
The investment costs for the three different installation concept are presented with their
individual components. The prices are based on inquiries from equipment suppliers in
Europe. The price for the electric chillers is based on an air-cooled compressor chiller
(190 €/kWcold) with hydrocarbon as refrigerant. The for economical comparison often
used liquid-cooled compressor chiller (475 €/kWcold), has a higher COP but is rather used
for freezing applications.
Investment data
Water-fired Multi-stage Compressor
absorption + absorption
chiller
electric chiller chiller
7.1.1.1.1.2 Invest
Microturbine
Absorption chiller
Compressor chiller
Cooling tower
Boiler
Savings hot water boiler
Installation cost
€
Total
Annual capital cost
80 000
27 500
38 000
5 000
10%
80 000
70 000
-
15 000
-2 000
10%
60 000
6 500
10%
€
165 550
179 300
73 150
€/year
23 571
25 528
10 415
Table A7: Investments for installation components
A 7
Appendix
APPENDIX 8: ENERGY CONSUMPTION DATA
The table below presents the energy consumption of the installation components for the
different energy concepts. The use of electricity and fuel is shown separately next to the
water consumption for the evaporative water loss of the cooling tower. A negative value
presents the generation of power by the microturbine and the surplus of power in the
heating column is sold to the grid.
7.1.2 Consumption data
Water-fired Multi-stage Compressor
absorption + absorption
chiller
electric chiller chiller
7.1.3 Chilling
7.1.4 Power consumption kWel
Building consumption
Microturbine generation
Chiller
Hot water pumping
Cooling tower fan
Cooling water pumping
Total from grid (cooling) kWel
88
-100
71
5
2
3,5
69,5
88
-100
2
5
13
8
333
333
333
203
536
0,42
0,1
1,2
0,3
0,55
0,2
Power consumption
kWel
Building consumption (grid)
Microturbine generation
Total from grid (heating) kWel
88
-100
-12
88
-100
-12
88
88
Fuel consumption
Microturbine
Boiler
333
-
333
-
185
Fuel consumption
Microturbine
Chiller
Total fuel consumption
Water consumption
Fresh water usage
Waste water
kWpe
kWpe
88
105
5
2,5
200,5
m3/h
Heating
kWpe
Table A8: Energy consumption of installation components
A 8
Appendix
APPENDIX 9: ENERGY COSTS CASE 1
Operating hours
chilling
Economical data
500
3000
Water-fired Multi-stage Compressor
absorption + absorption
chiller
chiller
electric chiller
Capital cost
€/year
Cooling cost
Electricity use
Fuel
Fresh water
Waste water
€/year
€/year
Heating cost
Fuel
Electricity use
heating
23 571
25 528
10 415
3 197
4 246
420
75
7 938
368
6 834
1 200
225
8 627
9 223
0
0
0
9 223
25 475
-3 312
22 163
25 475
-3 312
22 163
14 153
24 288
38 441
275
100
1 140
700
300
€/year
€/year
Maintenance cost €/year
Absorption chiller
Cooling tower
Electric chiller
Boiler
Microturbine
€/year
-
-
1 800
65
3 850
5 365
3 850
4 850
-
59 943
Total
€/year
59 036
61 168
Profit
€/year
908
-1 224
40%
13%
38%
9%
100%
42%
14%
36%
8%
100%
Capital cost
Cooling cost
Heating cost
Maintenance cost
-
1 865
18%
15%
64%
3%
100%
Table A9: Energy costs case 1
A 9
Appendix
APPENDIX 10: ENERGY COSTS CASE 2
Operating hours
chilling
Economical data
7.1.4.1 Capital
cost
Cooling cost
Electricity use
Fuel
Fresh water
Waste water
1500
3000
Water-fired Multi-stage Compressor
absorption + absorption
chiller
chiller
electric chiller
€/year
23 571
25 528
10 415
9 591
12 737
1 260
225
23 813
1 104
20 502
3 600
675
25 881
27 669
0
0
0
27 669
25 475
-3 312
22 163
25 475
-3 312
22 163
14 153
24 288
38 441
275
100
1 140
700
300
€/year
€/year
Heating cost
Fuel
Electricity use
heating
€/year
€/year
Maintenance cost €/year
Absorption chiller
Cooling tower
Electric chiller
Boiler
Microturbine
€/year
-
-
1 800
65
4 950
6 465
4 950
5 950
-
78 389
Total
€/year
76 011
79 522
Profit
€/year
2 378
-1 132
31%
31%
29%
9%
100%
32%
33%
28%
7%
100%
Capital cost
Cooling cost
Heating cost
Maintenance cost
-
1 865
14%
35%
49%
2%
100%
Table A10: Energy costs case 2
A 10
Appendix
APPENDIX 11: ENERGY COSTS CASE 3
Operating hours
chilling
Economical data
7.1.4.2 Capital
cost
Cooling cost
Electricity use
Fuel
Fresh water
Waste water
1500
€/year
25 528
10 415
9 591
12 737
1 260
225
23 813
1 104
20 502
3 600
675
25 881
27 669
0
0
0
27 669
12 737
-1 656
11 081
12 737
-1 656
11 081
7 076
12 144
19 220
275
100
1 140
700
300
€/year
Maintenance cost €/year
Absorption chiller
Cooling tower
Electric chiller
Boiler
Microturbine
€/year
Profit
23 571
€/year
€/year
7.1.4.3 Total
1500
Water-fired Multi-stage Compressor
absorption + absorption
chiller
chiller
electric chiller
€/year
Heating cost
Fuel
Electricity use
heating
-
-
-
1 800
65
3 300
4 815
3 300
4 300
-
€/year
63 280
66 791
59 169
€/year
-4 111
-7 621
37%
38%
18%
8%
100%
38%
39%
17%
6%
100%
Capital cost
Cooling cost
Heating cost
Maintenance cost
1 865
18%
47%
32%
3%
100%
Table A11: Energy costs case 3
A 11
Appendix
APPENDIX 12: WET OR DRY COOLING TOWER
The operation of a dry cooling tower is only cheaper when the fresh water price is higher
than 3 €/m3 and a waste water price of 2 €/m3.
chilling
1500
Operating hours
Economical data
Capital cost
Cooling cost
Electricity use
Fuel
Fresh water
Waste water
heating
3000
Water-fired Water-fired
absorption
absorption
chiller + wet chiller + dry
cooling tower cooling tower
€/year
23 571
25 137
€/year
9 591
12 737
1 890
300
24 518
10 378
12 737
0
0
23 115
€/year
275
100
1 140
4 950
6 465
275
100
1 140
4 950
6 465
Total
€/year
76 717
76 880
Power consumption
Cooling tower fan
Other consumption
Microturbine generation
Total from grid (cooling)
kWel
2
167,5
-100
69,5
7,7
167,5
-100
75,2
€/year
Maintenance cost
Absorption chiller
Cooling tower
Electric chiller
Microturbine
€/year
kWel
Water consumption
Fresh water usage
Waste water
m3/h
Invest
Cooling tower
Other installation
components
Total
€
Annual capital cost
0,42
0,1
5 000
145 500
15 000
145 500
€
165 550
176 550
€/year
23 571
25 137
Table A12: Yearly costs for a wet and dry cooling tower
A 12
Appendix
APPENDIX 13: COST COMPARISON CHILLERS
Alternative 1 Alternative 2
Cost comparison chillers
Invest
Absorption chiller
Compressor chiller
Cooling tower
Total Invest
Performance
Absorption chiller
Electric chiller
Consumption per kWh cold
Heat
COP 0,7
Power
COP 3,0
Water-fired
absorption
chiller
Compressor
chiller
27.500 €
-€
5.000 €
32.500 €
-€
19.000 €
-€
19.000 €
102 kW
102 kW
MWh/year
MWh/year
Capital costs
Calculative interest12
Insurance
Fixed costs
€/year
€/year
€/year
Maintenance costs
Electricity costs
Water costs
Variable costs
Sum of variable costs
Total yearly costs
€/year
€/year
€/year
€/MWh
€/year
219 MWh
51 MWh
4.627 €
1.625 €
325 €
6.577 €
42,99 €
375 €
-€
1.485 €
1.860 €
12,16 €
8.437 €
Production
Operating hours
2.705 €
950 €
190 €
3.845 €
25,13 €
570 €
4.692 €
-€
5.262 €
34,39 €
9.107 €
123 MWh
1205 hours
Table A13: Cost comparison absorption vs. compressor chiller
12
The calculative interest presents the minimal desired profit where the investment is profitable for a
company (expected return of invest > market interest rate) Here calculated with 10%of 1/2 Invest
A 13
Appendix
APPENDIX 14: COST COMPARISON MICROTURBINE
Cost comparison microturbine
Microturbine
Invest
Microturbine
Boiler
80.000 €
-€
-€
80.000 €
Total invest
Performance
Microturbine
Boiler
Power
Heat
Heat
Grid
+
Boiler
-€
6.500 €
-€
6.500 €
100 kW
156 kW
156 kW
Consumption
Fuel
Power
1.232 MWh
205 MWh
370 MWh
Capital costs
Calculative interest13
Insurance
Fix costs
€/year
11.390 €
4.000 €
800 €
16.190 €
925 €
325 €
65 €
1.315 €
Maintenance costs
Electricity costs
Fuel costs
Variable costs
Sum of variable costs
Total yearly costs
€/year
€/year
€/year
4.070 €
-€
31.419 €
35.489 €
37,47 €
51.679 €
130 €
34.040 €
16.134 €
50.304 €
53,11 €
51.619 €
Production
Operating hours
€/MWh
951 MWh
3715 hours
Table A14: Cost comparison microturbine vs. conventional energy solution
13
The calculative interest presents the minimal desired profit where the investment is profitable for a
company (expected return of invest > market interest rate) Here calculated with 10%of 1/2 Invest
A 14
Appendix
APPENDIX 15: LIQUIDATION OF MICROTURBINE
Operating hours 3715 hours
Liquidation
Costs
Capital costs microturbine
Capital costs chillers
Insurance
Total fix costs
Maintenance microturbine
Maintenance chiller equipment
Water costs
Fuel costs
Variable costs
Total yearly costs
Earnings
Power
Heat
Cold
Power tax credit
Fuel tax credit
Total yearly earnings
€/year
€/year
€/year
€/year
11.390 €
-€
800 €
12.190 €
4.087 €
-€
-€
34.639 €
38.725 €
50.915 €
(372 MWh)
(580 MWh)
( MWh)
(372 MWh)
(1237 MWh)
€/year
34.178 €
18.974 €
-€
4.755 €
4.293 €
62.200 €
€/year
€/year
€/year
€/year
Yearly profit (costs-earnings)
Cash flow
11.285 €
22.675 €
Return of Invest (ROI)
Liquidation
14,1%
3,5 years
Table A15: Liquidation of microturbine
A 15
Appendix
APPENDIX 16: LIQUIDATION OF
MICROTURBINE AND ABSORPTION CHILLER
Operating hours
Liquidation
Costs
Capital costs microturbine
Capital costs chillers
Insurance
Total fix costs
Maintenance microturbine
Maintenance chiller equipment
Water costs
Fuel costs
Variable costs
Total yearly costs
Earnings
Power
Heat
Cold
Power tax credit
Fuel tax credit
Total yearly earnings
4915 hours (1200 hours chilling)
€/year
€/year
€/year
€/year
€/year
€/year
(433 MWh)14
(580 MWh)
(123 MWh)
(433 + 41 MWh)15
(1638 MWh)
11.390 €
4.627 €
1.125 €
17.142 €
5.407 €
375 €
1.188 €
45.827 €
52.797 €
69.939 €
39.792 €
18.974 €
6.525 €
6.058 €
5.679 €
77.029 €
Yearly profit (costs-earnings)
Cash flow
7.090 €
23.107 €
Return of Invest (ROI)
Liquidation
6,3%
4,9 years
Table A16: Liquidation of microturbine and absorption chiller
14
The 12 kWel of power consumption by the chiller equipment is taken into account
The 41 MWhel presents the amount of electricity that would normally be used by the compressor chiller.
This gives a tax credit for the absorption chiller next to the tax credit for the microturbine
15
A 16
Appendix
APPENDIX 17: ECONOMY OF
MICROTURBINE AND ABSORPTION CHILLER
Operating hours
Liquidation
Costs
Capital costs microturbine
Capital costs chillers
Insurance
Total fix costs
Maintenance microturbine
Maintenance chiller equipment
Water costs
Fuel costs
Variable costs
Total yearly costs
Earnings
Power
Heat
Cold
Power tax credit
Fuel tax credit
Total yearly earnings
6715 hours (3000 hours chilling)
€/year
€/year
€/year
€/year
€/year
€/year
(591 MWh)16
(580 MWh)
(306 MWh)
(591 + 102 MWh)17
(2236 MWh)
€/year
11.390 €
4.627 €
1.125 €
17.142 €
7.387 €
375 €
2.970 €
62.611 €
73.342 €
90.485 €
54.365 €
18.974 €
16.313 €
8.869 €
7.759 €
106.280 €
Yearly profit (costs-earnings)
Cash flow
15.796 €
31.813 €
Return of Invest (ROI)
Liquidation
14,0%
3,5 years
Table A17: Economy of microturbine and absorption chiller
16
The 12 kWel of power consumption by the chiller equipment is taken into account
The 102 MWhel presents the amount of electricity that would normally be used by the compressor chiller.
This gives a tax credit for the absorption chiller next to the tax credit for the microturbine
17
A 17