2012/2013
2012 / 2013
FINAL YEAR PROJECT
Submitted in fulfillment of the requirements for the
ENGINEERING DEGREE FROM THE LEBANESE UNIVERSITY
Faculty of Engineering - Branch III
Major : Mechanical Engineering
By :
Mohammad Mahdi
________________________________________________
Cogeneration and Tri-generation of Diesel Generators
Supervised by:
Dr. Farouk Fardoun
Defended on Monday 8, July 2013 in front of the jury:
Dr. Rafic Younis
Dr. Farouk Fardoun
Dr. Hassan Lakkis
Dr. Ali Sayegh
President
Member
Member
Member
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ACKNOWLEDGEMENT
First of all, thanks to God, for giving me strength and courage to complete this research.
I would like to express my gratitude to my supervisor Dr. Farouk Fardoun, whose invaluable
guidance and support was very helpful throughout my research.
A similar level of gratitude is due to the doctors. It is unlikely that I would have reached
completion without their comments and remarks.
Much appreciation expressed to Dr. Rafic Younes, Dr.Mohamad Hamdan and Dr. Mohammad
Hammoud for providing all the necessary facilities throughout the five years.
My gratitude also goes to Dr.Ousama Ibrahim and Ghaddar Company for providing me with the
customer data and other helpful information for the project.
I express my appreciation to everyone who has involved directly and indirectly to the success of
this research. To my lovely friends who have supported me throughout the process.
Last but not least, I would have not finished this project without the support of my family who
has always been there for me whenever I need them, the encouragement they give to keep me
going and their love to empower me that never fails all the time.
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Abstract
There is a growing potential for the use of cogeneration and tri-generation systems in the
residential sector, because they have the ability to produce both useful thermal energy and
electricity from a single source of fuel such as diesel, which leads to increase the engine
efficiency and decrease the gas emission.
In our study a cogeneration and tri-generation systems on diesel power generators are design
based on the parameters of the exhaust gasses of the engine. The cogeneration system consists of
heat exchangers, and two hot storage tanks, which are responsible for producing space heating
load and DHW for the residential sector in winter. While tri-generation system consists of heat
exchangers, absorption chiller, hot storage tank, and cold storage tank, and it is responsible for
producing space cooling load and DHW for the residential sector in summer.
These systems are simulated in a TRNSYS (transient system simulation software) to study the
performance of operation, and to calculate the output parameters of the systems including the
storage tank temperatures and the energy rate absorbed from the engine with respect to time.
The purposes of this research work are to calculate the space heating, space cooling, and DHW
loads provided by the systems with respect to the generator power (KVA), to determine the
recovered percentage of this loads, and to estimate the economy of energy and the quantity of
pollution reduction with respect to the generator power (KVA).
An optimization study was carried out to find the optimum tank size for heating and cooling
storage tank with respect to (KVA) of power generator.
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Table of contents
ACKNOWLEDGEMENT ............................................................................................................................ 0
Abstract ......................................................................................................................................................... 2
Table of contents ........................................................................................................................................... 3
Table of Figures ............................................................................................................................................ 4
Table of Tables ............................................................................................................................................. 7
Chapter 1
Cogeneration and Tri-Generation Description ...................................................................... 8
1-1 Introduction: ....................................................................................................................................... 8
1-2 Definition:........................................................................................................................................... 9
1-3 Diesel Engine Based Cogeneration System ...................................................................................... 10
1-4 Concepts ........................................................................................................................................... 11
1-5 Heat recovery:................................................................................................................................... 13
Chapter 2
Heating Systems .................................................................................................................. 15
2-1 Cogeneration System Modeling ....................................................................................................... 15
2-2 Exhausts heat exchangers: ................................................................................................................ 16
2-2-1 Case Study ................................................................................................................................. 22
2-2-2 General Study ............................................................................................................................ 23
2-3 Heating Energy Demands ................................................................................................................. 27
2-3-1 Case Study ................................................................................................................................. 27
2-3-2 General Study ............................................................................................................................ 35
Chapter 3
Cooling Systems ................................................................................................................. 38
3-1 Description of Tri-Generation System: ............................................................................................ 38
3-2 Absorption chillers ........................................................................................................................... 39
3-2-1 Absorption chillers design: ........................................................................................................ 41
3-2-2 Case Study ................................................................................................................................. 42
3-2-3 General Study ............................................................................................................................ 43
3-3 Cooling Energy Demands Calculation: ............................................................................................ 45
3-3-1 Case study: ................................................................................................................................ 45
3-3-3 general study ............................................................................................................................. 48
Chapter 4
Energy Economy and Pollution Reduction ......................................................................... 49
4-1 Energy Conservation ........................................................................................................................ 49
4-1-1 Case Study ................................................................................................................................. 49
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4-1-2 General Study ............................................................................................................................ 52
4-2 Pollution reduction ........................................................................................................................... 54
Conclusion .................................................................................................................................................. 55
References ................................................................................................................................................... 56
Table of Figures
Figure ‎1.1 Forecasting of the power shortage. ............................................................................... 8
Figure ‎1.2 Diesel Engine Based Cogeneration systems ............................................................... 11
Figure ‎1.3 Percent energy distribution in a 30 KVA power generator at full load conditions .... 12
Figure ‎1.4 Calorific (input), gross engine power, exhaust gas, coolant/lubricating-oil energy and
radiation to surroundings flow rates in function of power generator KVA at full load conditions.
....................................................................................................................................................... 12
Figure ‎1.5 Conceptual Schematic Flow Diagram of a CCHP System. ........................................ 13
Figure ‎1.6 Schematic Flow Diagram for Heat Recovery System ............................................... 14
Figure ‎2.1 Schematic Flow Diagram for distribution of engine based cogeneration output energy
....................................................................................................................................................... 15
Figure ‎2.2 Shell and tube heat exchanger-one path tube side one path shell side. ...................... 16
Figure ‎2.3 The variation of Overall heat transfer coefficient of heat exchanger with respect to
generator power ............................................................................................................................ 24
Figure ‎2.4 The variation of area of heat exchanger with respect to generator power .................. 24
Figure ‎2.5 The variation of heat absorbed by water with respect to generator power ................. 25
Figure ‎2.6 The variation of convective heat transfer coefficient on shell side with respect to
generator power ............................................................................................................................ 25
Figure ‎2.7 The variation of convective heat transfer coefficient on tube side with respect to
generator power ............................................................................................................................ 26
Figure ‎2.8 The variation of effectiveness of heat exchanger with respect to generator power ... 26
Figure ‎2.9 The variation of efficiency of heat exchanger with respect to generator power ........ 27
Figure ‎2.10 TRNSYS cycle of 20KVA generators for providing heating load. .......................... 28
Figure ‎2.11 the probable generator functioning hours in almost all regions in Lebanon. ........... 29
Figure ‎2.12 TRNSYS results of the cycle for 20KVA generators for providing heating load. ... 29
Figure ‎2.13 The variation of coverable of heating load with respect to tank volume ................. 31
Figure ‎2.14 TRNSYS cycle of 20KVA generators for providing heating loads and DHW. ....... 32
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Figure ‎2.15 TRNSYS results for DHW of the cycle for 20KVA generators. .............................. 33
Figure ‎2.16 TRNSYS results for heating load of the cycle for 20KVA generators. ................... 34
Figure ‎2.17 The variation of heating load with respect to generator power ................................ 36
Figure ‎2.18 The variation of heating load with respect to generator power ................................ 37
Figure ‎2.19 The variation of hot water consumption with respect to generator power ............... 37
Figure ‎3.1 Schematic Flow Diagram for distribution of engine based tri-generation output
energy. ........................................................................................................................................... 38
Figure ‎3.2 A single-effect absorption chillers cycle. ................................................................... 39
Figure ‎3.3 The variation of capacity of absorption chillers used with respect to generator power
....................................................................................................................................................... 44
Figure ‎3.4 The variation of exhaust energy rate supplied to chillers generator with respect to
generator power ............................................................................................................................ 44
Figure ‎3.5 The variation of exhaust energy rate supplied to chillers generator with respect to
generator power ............................................................................................................................ 45
Figure ‎3.6 TRNSYS cycle of 20KVA generators for providing cooling load. ............................ 46
Figure ‎3.7 The TRNSYS results for the cooling cycle. ............................................................... 46
Figure ‎3.8 The variation of coverable of cooling load with respect to tank volume ................... 47
Figure ‎3.9 The variation of cooling load with respect to generator power .................................. 48
Figure ‎4.1 The variation of energy rate taken from heat source for TRNSYS cycle of 20KVA
generators for providing heating load only ................................................................................... 49
Figure ‎4.2 The variation of energy rate taken from heat source for TRNSYS cycle of 20KVA
generators for providing heating load and DHW .......................................................................... 50
Figure ‎4.3 The variation of energy rate taken from heat source for TRNSYS cycle of 20KVA
generators for providing cooling load ........................................................................................... 51
Figure ‎4.4 The variation of the economy of system producing heating load only with respect to
generator power ............................................................................................................................ 52
Figure ‎4.5 The variation of the economy of system producing heating load and DHW with
respect to generator power ............................................................................................................ 53
Figure ‎4.6 The variation of the economy of system producing cooling load with respect to
generator power ............................................................................................................................ 53
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Table of Tables
Table 1: Shell & Tube heat exchanger tube outer diameters (do ) and tube thicknesses (t). ....... 19
Table 2: Thermal conductivity of some commonly used tube materials. .................................... 20
Table 3: The gas flow rate and exhaust gas temperature of some generator power .................... 23
Table 4: The outlet parameters for heat exchanger of some generator power ............................. 23
Table 5: The time coverable and energy coverable of heating load with respect to tank volume.
....................................................................................................................................................... 31
Table 6: The heating load provided with respect to generator power .......................................... 35
Table 7: The heating load providing and hot water consumption with respect to generator power
....................................................................................................................................................... 36
Table 8: The exhaust gas flow rate, temperature, and chilled water flow rate of some generator
power............................................................................................................................................. 43
Table 9: The outlet parameters for absorption chillers of some generator power ....................... 43
Table 10: The time coverable and energy coverable of cooling load with respect to tank volume.
....................................................................................................................................................... 47
Table 11: The cooling load provided with respect to generator power ....................................... 48
Table 12: The energy economy of the three systems with respect to generator power ............... 52
Table 13 Quantity of reduction of CO2 for the three systems with respect to generator power .. 54
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Chapter 1 Cogeneration and Tri-Generation
Description
1-1 Introduction:
Electricity of Lebanon (EDL-Electricite´ du Liban) is public institution with an industrial and
commercial vocation under the control of the Ministry of Energy and Water (MEW). Electricity,
is principally generated through thermal power plants, in addition to a small amount (5–12%)
produced from renewable energy resources through the several, long-ago established
hydropower plants.
EDL is facing huge technical, administrative and financial problems. Technically, there is a
serious deficit in the generation capacity which is unable to meet the demand (Fig.1.1).
Figure ‎1.1 Forecasting of the power shortage.
Data source: Fardoun et al., 2012.
The electrical capacity shortage has created an informal back-up self-generation system which is
estimated to represent up to 30% of all electricity generated (World Bank, 2009b) and plays a
complementary role that is participating in assuring nearly 100% electrification together with
EDL. Thus, the Lebanese are paying a double electrical bill, one for EDL and the other for the
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self-generation which is almost twice the EDL bill (World Bank, 2009a). Considering this fact,
the Lebanese are compelled to pay a high monthly electricity bill as compared with other
countries in the region (World Bank, 2008), while suffering from a low quality service [1].
The main source of the electrical capacity of this self-generation systems are the diesel
generators. Cogeneration and tri-generation systems make an energy audit and pollution
reduction for this type of engine. Thus, Cogeneration and tri-generation systems provide an
alternative for the world to meet and solve energy-related problems, such as energy shortages,
energy supply security, emission control, the economy and conservation of energy, etc.
1-2 Definition:
Cogeneration systems (also known as combined heat and power, CHP) and tri-generation
systems (also known as Combined cooling, heating and power, CCHP) are proven and reliable
technologies with a history of more than 100 years, which was utilized mainly in large-scale
centralized power plants and industrial applications [2]. Knight and Ugursal state that its first
appearance was in industrial plants in the 1880s when steam was the primary source of energy in
industry and electricity was just surfacing as a product for both power and lighting.
With respect to single-family dwellings, there are several systems that could be applicable
including reciprocating internal combustion engine (ICE) (spark ignition – gasoline, natural gas,
propane, or compression ignition – diesel), micro gas turbine based systems, fuel cell based
systems and Stirling engine based systems. Onovwiona and Ugursal showed that any of these
systems could be used in place of a boiler or furnace and used to produce the required thermal
and electrical energy while surplus energy could be sold to the local utility grid or stored in an
energy storage device .Watson et al. found that the payback time for micro-CHP is considerably
shorter than other renewable micro generation systems namely PV and wind.
Typically, CHP is the simultaneous production of electrical or mechanical energy (power) and
useful thermal energy from a single energy stream. In electricity generation, the waste heat can
be recovered from the coolant/lubricating oil and exhaust gases to be used in heating purposes
such as space heating and domestic water heating. A slight difference between CCHP and CHP
is that thermal or electrical/mechanical energy is further utilized to provide space or process
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cooling capacity in a CCHP application. In winter, many CCHP systems can be seen as CHP
units, when there is no cooling demand of building air-conditioning [2].
In other words, CHP system is CCHP without any thermally activated equipment for generating
cooling power, though this difference will change the structure of systems to some extent.
Cogeneration, or the simultaneous production of electricity and useful heat using single fuel
stream, offers a way of increasing fuel efficiency while decreasing emissions when compared to
conventional electrical and thermal energy generation. Residential cogeneration is an attractive
option for increasing energy efficiency and decreasing greenhouse gas (GHG) emissions as
residential cogeneration systems can achieve energy conversion efficiencies up to 80% (based
on HHV) as compared to 30–35% (HHV) efficiency obtained through conventional fossil fuel
based electricity production and up to 55% (HHV) for combined cycle plants [4].
Higher efficiency translates into reduced GHG emissions and reduced fuel costs. Moreover,
there are several technologies suitable for residential cogeneration currently available or under
development including reciprocating internal combustion engine, micro-turbine, fuel cell, and
reciprocating external Sterling engine based cogeneration systems [3].
1-3 Diesel Engine Based Cogeneration System
Diesel engines are primarily used for large-scale cogeneration, although they can also be used
for small-scale cogeneration. These engines are mainly four-stroke direct injection engines fitted
with a turbocharger (greater than 30 KVA) and air intercooler (greater than 100 KVA). Diesel
engines run on diesel fuel or heavy oil, or they can be set up to operate on a dual fuel mode that
burns primarily natural gas with a small amount of diesel pilot fuel. Stationary diesel engines run
at speeds between 500 and 1500 rpm. In electricity generation, an electric generator is a device
that converts mechanical energy to electrical energy, A generator forces electric current to flow
through an external circuit.
Today, highly efficient packaged cogeneration units, as small as 1 kW electric and 3 kW thermal,
are available that can be used for variety of residential, commercial and institutional applications
[3]. These robust and high-efficiency cogeneration units are currently being used for meeting the
base load requirement of a building or facility, as well as for backup or peak shaving
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applications. The advantages packaged reciprocating diesel engines based cogeneration
technology has over other cogeneration technologies are low capital cost, reliable onsite energy,
low operating cost, ease of maintenance, and wide service infrastructure.
The basic elements of a reciprocating diesel engines based cogeneration system are the engine,
generator, heat recovery system, exhaust system, controls and acoustic enclosure. The generator
is driven by the engine, and the useful heat is recovered from the engine exhaust and cooling
systems. The architecture of a typical packaged internal Diesel Engine Based Cogeneration
system is shown in Fig 1.2[5].
Figure ‎1.2 Diesel Engine Based Cogeneration systems
1-4 Concepts
The input fuel energy is distributed into thermal energy and electrical energy. The thermal
energy is dispersed for coolant, lubricating oil, exhaust gas, and radiation to surroundings as seen
in Fig 1.3. This energy balance differs from engine to another which depends on the power
generator.as seen in Fig 1.4.
In single generation diesel engine, the useful energy is only the electrical energy, while the
thermal energy is a waste heat.
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radiation to
surroundings
7%
coolant and
lubricating oil
22%
gross engine
power
40%
exhaust gas
31%
Figure ‎1.3 Percent energy distribution in a 30 KVA power generator at full load conditions
Energy flow rate (kW)
350
300
Calorific (input-fuel) (heat
of combustion)
250
gross engine power
200
150
exhaust gas
100
coolant and lubricating oil
50
0
0
50
100
150
200
radiation to surroundings
Power generator KVA
Figure ‎1.4 Calorific (input), gross engine power, exhaust gas, coolant/lubricating-oil
energy and radiation to surroundings flow rates in function of power generator KVA at full
load conditions.
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The main concept of cogeneration and tri-generation is to recover the waste heat from the
cooling systems(Jacket water and lube oil) and combustion gases to be used in heating purposes
such as space heating, domestic water heating (cogeneration) and to drive absorption chillers for
cooling applications (tri-generation). Cogeneration technologies for residential, commercial and
institutional applications can be classified according to their prime mover and from where their
energy source is derived.
Figure ‎1.5 Conceptual Schematic Flow Diagram of a CCHP System
1-5 Heat recovery:
Not all of the heat produced in a Diesel Engine Based Cogeneration system can be captured in
on-site electric generation, because some of the heat energy is lost as low temperature heat
within the exhaust gases and as radiation and convection losses from the engine and generator.
There are four sources, where usable waste heat can be derived from a reciprocating diesel
engine based cogeneration system: exhaust gas, engine jacket cooling water, lube oil cooling
water and turbocharger cooling. Heat from the engine jacket cooling water accounts for up to
30% of the energy input while the heat recovered from the engine exhaust represents 30–50%.
Thus, by recovering heat from the cooling systems and exhaust, approximately 70–80% of the
energy derived from the fuel is utilized to produce both electricity and useful heat.
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The heat recovered from the engine jacket can yield hot water with temperatures between 85 and
90 oC, while the heat recovered from the engine exhaust gases as hot water or low-pressure
steam is from 100 to120 oC. The recovered heat can therefore be used to generate hot water or
low-pressure steam for space heating, domestic hot water heating, or absorption cooling.
Heat recoveries from reciprocating diesel engine based cogeneration systems cannot be made
directly to a building’s heating medium because of problems associated with pressure, corrosion,
and thermal shock. Therefore, shell and tube heat exchangers or plate heat exchangers are used
to transfer heat from the engine cooling medium to the building’s heating medium.
Figure ‎1.6 Schematic Flow Diagram for Heat Recovery System
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Chapter 2 Heating Systems
2-1 Cogeneration System Modeling
In this work, the cogeneration system is assumed to address the space and domestic hot water
heating and electrical loads of the residence; cooling loads are not considered. Two kinds of
energy are provided from the engine. The electrical energy derived from the generator provides
the electrical demand of the building, while, the thermal energy derived from the exhaust gas is
transferred to hot water storage tanks, which is used to meet the space and DHW heating energy
demands. The space heating requirements were met by the heating coil, which was fed by the hot
water storage tank.
The system will consist of engine, generator, exhaust heat exchanger, two hot water storage
tanks, and exhaust controller system as we see in Fig.2.1
Exhaust
heat
exchanger
Engine
Power
Generator
Storage
Tank
Space
heating
Storage
Tank
Domestic
Hot water
Electrical
Demands
Losses
Figure 2.1 Schematic Flow Diagram for distribution of engine based cogeneration output energy
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2-2 Exhausts heat exchangers:
The exhaust heat exchangers used is a shell and tube heat exchangers. A shell and tube heat
exchanger is a class of heat exchanger designs. As its name implies, this type of heat exchanger
consists of a shell (a large pressure vessel) with a bundle of tubes inside it. One fluid runs
through the tubes, and another fluid flows over the tubes (through the shell) to transfer heat
between the two fluids. The set of tubes is called a tube bundle, and may be composed by several
types of tubes: plain, longitudinally finned, etc.
Two fluids, of different starting temperatures, flow through the heat exchanger. One flows
through the tubes (the tube side) and the other flows outside the tubes but inside the shell (the
shell side). Heat is transferred from one fluid to the other through the tube walls, either from tube
side to shell side or vice versa. The fluids can be either liquids or gases on either the shell or the
tube side as seen in Fig.2.2. In order to transfer heat efficiently, a large heat transfer area should
be used, leading to the use of many tubes. In this way, waste heat can be put to use. This is an
efficient way to conserve energy.
Figure ‎2.2 Shell and tube heat exchanger-one path tube side one path shell side.
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The exhaust gasses pass through the shell side while the water passes through the tube side.
For the design of exhaust heat exchanger three types of inputs are required, they are exhaust
specifications, water specifications, and shell and tube heat exchanger specifications.
In the exhaust specifications for design we consider the mass flow rate of exhaust and exhaust
gas temperature inlet to heat exchanger (i.e. stack gas) at 100% engine load to maximize the
potential of exhaust heat energy usage. The exhaust gas temperature at outlet to heat exchanger is
generally taken to be 177oC (450K), which is the dew point of sulfurous acid to avoid corrosion
of the exhaust system.
The water specifications required are physical properties of coolant (which include density,
specific heat, dynamic viscosity, and thermal conductivity), and water inlet and outlet
temperature.
Detailed shell and tube heat exchanger specifications required are outer diameter of tube,
thickness of tube, thermal conductivity of tube material, convective heat transfer coefficients of
shell and tube side fluids and fouling resistances of shell and tube side fluids.
Then the input parameters required for the design of exhaust heat exchanger are:
(1) Exhaust side of heat exchanger:
i. Mass flow rate of exhaust gasses (
e)
in kg/s
ii. Temperature of exhaust at inlet to the heat exchanger (Tein) in Kelvin (K)
iii. Temperature of exhaust at outlet to the heat exchanger (Teout) in Kelvin (K)
iv. Specific heat of exhaust gasses (Cpe) in J/Kg-K
(2) Coolant side of heat exchanger (the coolant here is water)
i. Specific heat of coolant (Cpc) in J/Kg-K
ii. Density of coolant (ρc) in kg/m3
iii. Thermal conductivity of coolant (khtc) in W/m-K
iv. Dynamic viscosity of coolant (μc) in Pa-s or Kg/m-s
v. Temperature of coolant outlet to the heat exchanger (Tc2) in Kelvin
vi. Temperature of coolant inlet to the heat exchanger (Tc1) in Kelvin
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(3) Exhaust Heat exchanger properties:
i. Tube outer diameter (do) in inches
ii. Thickness of the tube (t) in inches
iii. Thermal conductivity of tube wall material (kw) in W/m-K
iv. Convective heat transfer coefficient on tube side (hi) in W/m2-K
v. Convective heat transfer coefficient on shell side (ho) in W/m2-K
vi. Fouling resistance on tube side fluid (Rfi) in m2-K/W
vii. Fouling resistance on shell side fluid (Rfo) in m2-K/W
The output parameters for the design of exhaust heat exchanger are:
i. Overall heat transfer coefficient of heat exchanger (UEHX) in W/m2-K
ii. Area required for shell and tube heat exchanger (AEHX) in m2
iii. Mass flow rate of coolant (
iv. Heat absorbed by coolant (
c)
c)
in kg/s.
in W
v. Effectiveness of exhaust heat exchanger (ξEHX)
vi. Efficiency of exhaust heat recovery (ȠEHX)
The mass flow rate of coolant is calculated by the following equation
m·e C pe (Tein - Teout ) = m·c C pc (Tc2 - Tc1 )
m·e C pe (Tein Teout ) U EHX AEHX (Tlm,e )
Where
lme is the logarithmic mean temperature difference and is given by the relation,
lme
((
ln(( ein
) (
c ) ( eout
))
c
Other temperature relations which will be used in the design of exhaust heat exchanger
calculations are average exhaust temperature (Teavg), average coolant temperature (Tcavg) and film
temperature (Tefilm).
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The convective heat transfer coefficient on shell side (ho) and convective heat transfer
coefficient of tube side (hi) are calculated by the following equation:
The exhaust heat exchanger tube outer diameter (do) and thickness of the tube (t) are
standardized by Tubular Exchanger Manufacturers Association [TEMA] and are tabulated in
Table1.
Table 1: Shell & Tube heat exchanger tube outer diameters (do ) and tube thicknesses (t).
Page | 19
The Thermal conductivity depends on the type of tube materials as seen at table 2.
Tube Material
Carbon Steel
High Alloy Steel
Low Alloy Steel
Nickel
Nickel-Chromium-Iron
Aluminum
Aluminum Alloys
Copper
Titanium Alloys
Aluminum brass 76/22/2
Cupronickel (70/30)
Cupronickel (90/10)
Thermal Conductivity (W/m-K)
52
16
35
52
17.3
204
156
390
21
100.4
29
45
Table 2: Thermal conductivity of some commonly used tube materials.
In determination ho of by above equation we use the properties of air for calculating Reynolds
number (REDE) and Prandtl number (PRe) and thermal conductivity (ke) at film temperature
(Tefilm). Note that Prandtl number for air is constant and is equal to 0.7 (Pre = 0.7) over a wide
range of temperature from 0 oC to over 1000 oC.
Where is
the velocity of exhaust,
is the kinematic viscosity of air which is function of
temperature, and is calculated as follow
And ke is the thermal conductivity of air and is calculated as follow
In determination hi for by above equation we use the properties of coolant for calculating
Reynolds number (REDC) and Prandtl number (PRe) which are calculated by equations,
Where di is the tube inner diameter.
Page | 20
And Vc is velocity of coolant in the tube and is given by relation,
Where At is the cross-sectional area of heat exchanger tube and is calculated by relation,
Now we that calculate UEHX by using the following equation
(
Heat absorbed by coolant (
⁄ )
) is straight forward and is calculated by relation,
(
)
Effectiveness of exhaust heat exchanger (ξEHX) is calculated by relation
(
Where min is the minimum of
e and
c
)
which are the heat capacity rates of exhaust and
coolant respectively and can be calculated by the relation,
Efficiency of exhaust heat recovery (ȠEHX) is defined by Eq. 3.21 and calculated by reducing the
exhaust outlet temperature (Teout) to ambient temperature of 300K
(
)
Page | 21
2-2-1 Case Study
The heat exchangers selected for 20KVA diesel generators to recover heat from the engine
exhaust gas according to the maximum heat output from the exhaust gas has the following
parameters:
(1) Exhaust side of heat exchanger:
m·e =0.036kg/s, Cpe =1120 j/kg.K, Tein =713 K, Teout =450 K.
(2) Coolant side of heat exchanger :( the coolant here is water)
Cpc = 4180 j/kg.K, ρc =1000kg/m^3, kc = 0.6kg/m.K, μc= 0.000355pa.s, Tc2=343 K, Tc1=293 K.
(3) Exhaust Heat exchanger properties:
do=1 inch, t =0.065 inch kw=35w/m.K(the tube material is Low Alloy Steel ),hi=1203 w/m^2.K,
ho= 19.25w/m^2.K, Rfi=0 m^2.K/w, Rfo= 0.00176 m^2.K/w.
Then the output parameters for the design of exhaust heat exchanger are:
UEHX = 18.227 W/m^2.K
AEHX = 2.335 m^2
m·c =0.05kg/s
c=10.45
KW
ξEHX =0.6171
ȠEHX= 62.75%
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2-2-2 General Study
By making the same study for all types of generators according to the following table:
Generator power
20
30
45
60
80
100
713
773
765
830
787
816
0.036
0.043
0.055
0.0707
0.0875
0.1062
(KVA)
Exhaust temperature
(oK)
Gas flow (kg/sec)
Table 3: The gas flow rate and exhaust gas temperature of some generator power
And by taking the same tube outer diameter (do) in inches, thickness of the tube (t) in inches, and
the tube material (Low Alloy Steel).
We indicate the following table:
Generator power
20
30
45
60
80
100
UEHX (W/m^2.K)
18.227
20.267
22.81
26.234
28.807
31.895
AEHX (m^2)
2.335
2.833
3.173
3.935
4.153
4.763
ho(w/m^2.K)
19.25
21.367
24.141
27.8
30.744
34.2
hi(w/m^2.K)
1203
1574.6
1925
2741.5
3050.6
3792
Heat recovered (KW)
10.45
14.63
18.81
29.26
33.44
43.89
Effectiveness ξEHX
0.6171
0.6329
0.6469
0.6881
0.6907
0.7055
Efficiency ȠEHX(%)
62.75
64.22
65.67
69.72
70.07
71.51
(KVA)
Table 4: The outlet parameters for heat exchanger of some generator power
Page | 23
And the following graphs:
UEHX (W/m^2.K)
35
30
25
20
15
10
5
0
0
20
40
60
80
100
Generator power (KVA)
Figure ‎2.3 The variation of Overall heat transfer coefficient of heat exchanger with respect
to generator power
AEHX (m^2)
6
5
4
3
2
1
0
0
10
20
30
40
50
60
Generator power (KVA)
70
80
90
100
Figure ‎2.4 The variation of area of heat exchanger with respect to generator power
Page | 24
Heat absorbed (KW)
50
45
40
35
30
25
20
15
10
5
0
0
20
40
60
80
100
Generator power (KVA)
Figure ‎2.5 The variation of heat absorbed by water with respect to generator power
ho(w/m^2.K)
40
35
30
25
20
15
10
5
0
0
20
40
60
80
100
Generator power (KVA)
Figure ‎2.6 The variation of convective heat transfer coefficient on shell side with respect to
generator power
Page | 25
hi(w/m^2.K)
4000
3500
3000
2500
2000
1500
1000
500
0
0
20
40
60
80
100
120
Generator power (KVA)
Figure ‎2.7 The variation of convective heat transfer coefficient on tube side with respect to
generator power
Effectiveness ξEHX
0.71
0.7
0.69
0.68
0.67
0.66
0.65
0.64
0.63
0.62
0.61
0
20
40
60
80
100
Generator power (KVA)
Figure ‎2.8 The variation of effectiveness of heat exchanger with respect to generator
power
Page | 26
Efficiency ȠEHX(%)
72
71
70
69
68
67
66
65
64
63
62
0
20
40
60
80
100
120
Generator power (KVA)
Figure ‎2.9 The variation of efficiency of heat exchanger with respect to generator power
2-3 Heating Energy Demands
Each type of engines provides a thermal energy which is used to meet the space and DHW
heating energy demands. For calculation the energy demands and the heating load provided by
the generator and for verification the percentage of coverage that this heating load can cover, we
use TRNSYS for dynamic simulation in function of time.
2-3-1 Case Study
Two TRNSYS cycles has done for 20KVA diesel generators:
1) Space heating: In the first cycle the thermal energy derived from the exhaust gas is
transferred to one storage tank, which is used to meet the space heating profile only. This cycle
calculates the heating load that can be provided by 20KVA generator, and the energy taken from
the engine.
The cycle as seen in Fig.2.10 consists of:
1- One shell and tube heat exchanger with overall heat transfer coefficient = 154 KJ/hr.K.
2- One single speed pump with rated flow rate=180 Kg/hr.
Page | 27
3- One storage tank with tank volume= 350 l, a high aspect ratio (height/diameter), and
insulation thickness from 4 to 6 cm which yield to tank loss coefficient= 0.833 w/m^2.K.
4- A generator function profile as seen in Fig 2.11.
5-Thermostat that control the temperature of water in storage tank (The maximum temperature of
water is 90 oC)
6-Two plotter to calculate the outer parameters needed.
7-Control systems to control the return temperature from load to the tank.
8- Heating load profile taken from storage tank.
According to typical days, the daily heating load that can be used consist three types of:
a) Worst case in the year.
b) Average day in the season.
c) Most frequent day in the season.
In this cycle the heating load is the most frequent day in the season as seen in Fig.18
Figure ‎2.10 TRNSYS cycle of 20KVA generators for providing heating load.
Page | 28
Generator function
Figure 2.11 the probable generator functioning hours in almost all regions in Lebanon.
The results of the cycle are shown in the following graphs
Heating load (W
ank emperature (oC
Figure ‎2.12 TRNSYS results of the cycle for 20KVA generators for providing heating load.
Page | 29
In a fan coil unit heating system, the temperature of water enters the coil should be greater than
60 oC, to give the heating load. In other word, when the tank temperature is greater than 60 oC,
then the heating load is provided.
The results show that a 20KVA generator provides 7.5 KW heating load with:
A) The overall percentage of coverage of time for 7.5 KW heating load is 71% distributed over:
1) 88% percentage of coverage of time, when the generator is functioning.
2) 62.5% percentage of coverage of time, when the generator is stopping.
B) The overall percentage of coverage of energy for 7.5 KW heating load is 64% distributed
over:
1) 87.2% percentage of coverage of energy, when the generator is functioning.
2) 40% percentage of coverage of energy, when the generator is stopping.
Now at this point, there is a discussion about the tank volume and its calculation. The mass flow
rate of water required for heating cycle is calculated by using the following equation:
Where
is the difference in temperatures between the inlet and the outlet temperature of
FCU. In heating load generally
.
From this equation we deduce that the mass flow rate of water required for heating is huge.
Therefore the tank needed should have a high volume to provide the load when the generator is
off. But high volume tank need more times to increase its temperature which yield to a problem.
Then the volume of tank will depend on the time needed to rise its temperature to the needed
temperature. Then the favorable tank is the tank that offers the greatest percentage of coverable
of time and energy. By making the same study above for different volume of tanks we indicate
the following table:
Page | 30
Tank volume (L)
150
250
300
350
450
550
650
Time coverable (%)
67
70.3
73
75
77
79
75
Energy coverable(%)
54.3
60.5
61.5
64
69
70
67
Table 5: The time coverable and energy coverable of heating load with respect to tank
volume.
90
80
70
Time coverable (%)
60
Energy coverable(%)
50
40
0
100
200
300
400
500
600
700
Tank volume (L)
Figure ‎2.13 The variation of coverable of heating load with respect to tank volume
From the results shown above we deduce that the 550L volume of tank is the favorable volume
used for 20 KVA generators.
2) Space heating and DHW: In the second cycle, the thermal energy derived from the
exhaust gas is transferred to two storage tanks, which are used to meet the space and DHW
heating profile. In this cycle, the priority is to provide a DHW for 1 family (6 persons) from the
thermal energy, and the rest of energy is used to meet the space heating load.
The cycle as seen in Fig.2.14 is consisting of:
1- Two shell and tube heat exchangers connected in series: one for DHW with overall heat
transfer coefficient = 30 KJ/hr.K, and the other for heating load with overall heat transfer
coefficient = 130 KJ/hr.K.
2-Two single speed pumps with rated flow rate=180 Kg/hr.
Page | 31
Figure ‎2.14 TRNSYS cycle of 20KVA generators for providing heating loads and DHW.
Page | 32
3-Two storage tanks: one for DHW with tank volume= 250 l, and the other for heating load tank
volume= 350 l, and have the same characteristic as the first cycle.
4-A generator function profile is seen in Fig 2.11.
5- Two thermostats.
6-Heating load profile is taken from its storage tank.
7-Hot water consumption profile (from LCECP Lebanon) taken from its tank.
8-Four plotter to calculate the outer parameters needed.
9-Control systems to control the return temperature from load to the tank.
The results of this cycle are shown in the following graphs
Hot water consumption (Kg hr
ank emperature (oC
Figure ‎2.15 TRNSYS results for DHW of the cycle for 20KVA generators.
Page | 33
Heating load (W
ank emperature (oC
Figure ‎2.16 TRNSYS results for heating load of the cycle for 20KVA generators.
The results show that a 20KVA generator provides:
1) Domestic Hot Water for 1 family that consist of 6 persons (261 l/day)
2) 5 KW heating load with:
A) The overall percentage of coverage of time for 5 KW heating load is 83.5% distributed over:
1) 87.5% percentage of coverage of time, when the generator is functioning.
2) 79 % percentage of coverage of time, when the generator is stopping.
B) The overall percentage of coverage of energy for 5 KW heating load is 77% distributed over:
1) 86% percentage of coverage of energy, when the generator is functioning.
2) 68% percentage of coverage of time, when the generator is stopping.
Page | 34
If we want to provide the same heating load as cycle one, then the results are:
1) Domestic Hot Water for 1 family that consist of 6 persons (261 l/day)
2) 7.5 KW heating load with:
A) The overall percentage of coverage of time for 5 KW heating load is 58% distributed over:
1) 67% percentage of coverage of time, when the generator is functioning.
2) 50 % percentage of coverage of time, when the generator is stopping.
B) The overall percentage of coverage of energy for 5 KW heating load is 55% distributed over:
1) 71% percentage of coverage of energy, when the generator is functioning.
2) 40% percentage of coverage of time, when the generator is stopping.
2-3-2 General Study
By making the same study for all types of generators we obtain the following results:
i) If all the thermal energy derived from the exhaust gas is transferred to one storage tank, which
is used is to meet the space heating profile only:
Generator power
20
30
45
60
80
100
7.5
14
18
24
34
37.5
Time coverable (%)
79
77
78
76.5
79
78
Energy coverable(%)
70
68.5
67
67
66
65
(KVA)
Maximum Heating
Load (kW)
Table 6: The heating load provided with respect to generator power
Page | 35
heating load (kw)
40
35
30
25
20
15
10
5
0
0
20
40
60
80
100
Generator Power
Figure ‎2.17 The variation of heating load with respect to generator power
ii) If the thermal energy derived from the exhaust gas is transferred to two storage tanks, which
are
used to meet the space heating profile and DHW heating with 100% coverable profile.
Generator power
20
30
45
60
80
100
DHW used (l/day)
264
528
792
1056
1320
1584
Heating load (kW)
5
7.5
10
15
20
25
Time coverable (%)
83.5
82
80
82
80
84
Energy coverable(%)
77
76
76
74
75
77
(KVA)
Table 7: The heating load providing and hot water consumption with respect to generator
power
Page | 36
heating load (kw)
30
25
20
15
10
5
0
0
20
40
60
80
100
Generator power
Figure ‎2.18 The variation of heating load with respect to generator power
hot water consumption(l/day)
1800
1600
1400
1200
1000
800
600
400
200
0
0
20
40
60
80
100
Generator Power KVA
Figure ‎2.19 The variation of hot water consumption with respect to generator power
Page | 37
Chapter 3 Cooling Systems
3-1 Description of Tri-Generation System:
In this work, the system is assumed to address the domestic hot water heating, cooling loads and
electrical loads of the residence; heating loads are not considered. Here, the thermal energy
derived from the exhaust gas is transferred to two tanks that comprised of one cold water storage
tank which is used to meet the space cooling energy demands, and one hot water storage tank for
domestic hot water. The cooling power is generated by absorption chillers that utilized the
exhaust gas of the engine. The space cooling requirements were met by the cooling coil, which
was fed by the cold water storage tank.
The system will consist of engine, generator, exhaust heat exchanger, one hot water storage
tank, absorption chillers, and exhaust controller system as we see in Fig.3.1.
Losses
l
Engine
Exhaust
heat
exchangers
Absorption
chillers
Power
Generator
Storage
Tank
Domestic
Hot water
Space
Cooling
Electrical
Demands
Figure 3.1 Schematic Flow Diagram for distribution of engine based tri-generation output
energy.
Page | 38
3-2 Absorption chillers
Absorption chillers are one of the commercialized thermally activated technologies widely
applied in existing CCHP systems; they are similar to vapor compression chillers, with a few key
differences. The basic difference is that a vapor compression chiller uses a rotating device
(electric motor, engine, combustion turbine or steam turbine); to raise the pressure of refrigerant
vapors, while an absorption chiller uses heat to compress the refrigerant vapors to a high
pressure. Therefore, this thermal compressor has no moving parts.
Basic absorption cycle is illustrated in Fig.3.2. After the evaporator of absorption chiller
generates cooling power, vapor generated in the evaporator is absorbed into a liquid absorbent in
the absorber. The absorbent that has taken up refrigerant with spent or weak absorbent is pumped
to the generator. The refrigerant is released again as a vapor by waste heat from steam, hot water
or hot exhaust gas, and vapor is to be condensed in the condenser. The regenerated or strong
absorbent is then led back to the absorber to pick up refrigerant vapor anew. Heat is supplied to
the generator at a comparatively high temperature and rejected from the absorber at a
comparatively low level, analogously to a heat engine.
Figure ‎3.2 A single-effect absorption chillers cycle.
Page | 39
The most common working fluids for absorption chillers are water/NH3 and LiBr/water,
although there are 40 refrigerant compounds and 200 absorbent compounds available in the
literature.
In lithium bromide/water (LiBr-water) systems, water is the refrigerant and these systems are
typically used for cooling fluids to as low as 40 oF and thus, cannot be used for freezing
applications (refrigeration). While in water/ammonia systems (aqua-ammonia refrigeration,
AAR), ammonia is the refrigerant, and these systems are typically used for refrigeration (< 32 oF)
applications, down to –60 oF. . Fig. 2 shows a single-effect system using non volatility absorbent
such as LiBr/water. When volatility absorbent such as water/NH3 is used, the system requires an
extra component called a rectifier, which will purify the refrigerant before entering the
condenser.
Although water/ammonia systems can be used for cooling (non-freezing) applications, they are
significantly more expensive (higher installed cost) than the lithium bromide/water chillers and
therefore, are not generally used for cooling applications.
Depending on how many times the heat supply is utilized within the chiller; absorption chillers
can be divided into single-effect, double-effect and triple effect. A single-effect absorption
refrigeration system is the simplest and most commonly used design because the heat utilized for
these chillers is recycled thermal energy and the lower operating temperatures of these chillers
allow more heat to be recovered from the generator. With engine generators, a single-effect
absorption chiller will generate more cooling per kW of engine generator than a double-effect
chiller and cost significantly less to purchase.
Absorption chillers can also be used in chilled water storage systems to produce chilled water
during off-peak electric load periods when the cost of electricity is low and the demand for
cooling is low. The stored chilled water is then drawn upon during the peak cooling periods
when electricity costs are high, to supplement the chiller operation. The storage system helps to
reduce the chiller capacity requirement and total installed cost of chillers.
Page | 40
3-2-1 Absorption chillers design:
For the design of absorption chiller three type of inputs are required, they are exhaust
specification, water specification, and absorption chillers specification.
In my design, the type of absorption chillers is a single-effect absorption refrigeration system,
with lithium bromide/water (LiBr-water) are the working fluids.
The input parameters required for the design of absorption chillers are:
(1)Exhaust gas specification:
i. Mass flow rate of exhaust gasses (me) in kg/s.
ii. Temperature of exhaust at inlet to the heat exchanger (Tein) in Kelvin (K).
iii. Temperature of exhaust at outlet to the heat exchanger (Teout) in Kelvin (K).
iv. Specific heat of exhaust gasses (Cpe) in J/Kg-K.
(2) Chilled water (evaporator) specification:
i. Specific heat of water (Cpch) in J/Kg-K.
ii. Temperature of coolant inlet to the absorption chillers (Tch1) in Kelvin.
iii. Mass flow rate of chilled water (mch) in kg/s.
(3) Cooling water (condenser and absorber) specification:
i. Specific heat of water (Cpc) in J/Kg-K.
ii. Temperature of water outlet from absorption chillers (Tc2) in Kelvin.
iii. Temperature of coolant inlet to the absorption chillers (Tc1) in Kelvin.
(4) Absorption chillers specification:
i- Chilled water stream set point (Tch,set) in Kelvin.
ii- Machine coefficient of performance(COP).
iii-The power of the pump (
) in W.
The output parameters for the design of absorption chillers are:
i- Energy rate that must be supplied to the generator of absorption chillers (
) in W
ii- Temperature of chilled water outlet from absorption chillers (Tch2) in Kelvin
iii- mass flow rate of cooling water (mc) in kg/s
Page | 41
iv- The cooling capacity of the chillers (capacity) in W
v- The amount of energy rate that must be removed from the chilled water stream in order to
reach the set point temperature (
) in W.
vi- Energy rate added to cooling water stream(
) in W.
The energy rate supplied to the generated is calculated by the following equation:
(
)
The capacity of the chillers is calculated by the following equation:
The removed energy is calculated by the following equation:
(
)
The outlet chilled water is calculated by the following equations:
(
(
)
(
)
)
The energy rate added to cooling water stream is calculated by the following equation:
The mass flow rate of cooling water is calculated by the following equation:
(
)
3-2-2 Case Study
The absorption chillers selected for 20KVA diesel generators to recover heat from the engine
exhaust gas according to the maximum heat output from the exhaust gas has the following
parameters:
(1)Exhaust gas specification:
m·e =0.036kg/s, Cpe =1120 J/kg.K, Tein =713 K, Teout =450 K.
(2) Chilled water specification:
Cpch=4180 J/kg.K, Tch1=288 K, mch = 0.24 kg/s.
(3) Cooling water specification:
Cpc=4180 J/kg.K, Tc1=293 K, Tc2=318 K.
Page | 42
(4) Absorption chillers specification:
Tch,set = 280 K, COP = 0.7,
=100 W.
Then the output parameters for the design of absorption chillers for 20 KVA generators are:
3-2-3 General Study
By making the same calculation for all type of generators according to the following table:
Generator power
20
30
45
60
80
100
713
773
765
830
787
816
gas flow rate (kg/sec)
0.036
0.043
0.055
0.0707
0.0875
0.1062
Chilled water flow
0.24
0.35
0.42
0.65
0.8
0.93
(KVA)
Exhaust temperature
(Co)
rate (kg/sec)
Table 8: The exhaust gas flow rate, temperature, and chilled water flow rate of some
generator power
And by taking the same chilled water inlet temperature, same cooling water specification, and
same absorption chillers specification we indicate the following results:
generator power
(KVA)
Cooling capacity (W)
(W)
20
30
45
60
80
100
7423
11000
13500
21000
26000
30500
10604.3
15714.3
19285.7
30000
37142.9
43571.4
(W)
18127.3 26814.3
32885.7
51100
63242.9
74171.4
Table 9: The outlet parameters for absorption chillers of some generator power
Page | 43
Cooling Capacity (w)
35000
30000
25000
20000
15000
10000
5000
0
0
20
40
60
80
100
generator power (KVA)
Figure ‎3.3 The variation of capacity of absorption chillers used with respect to generator
power
Exhaust Energy Rate (W)
50000
45000
40000
35000
30000
25000
20000
15000
10000
5000
0
0
20
40
60
80
100
generator power (KVA)
Figure ‎3.4 The variation of exhaust energy rate supplied to chillers generator with respect
to generator power
Page | 44
Condenser Energy Rate (W)
80000
70000
60000
50000
40000
30000
20000
10000
0
0
20
40
60
80
100
generator power(KVA)
Figure ‎3.5 The variation of exhaust energy rate supplied to chillers generator with respect
to generator power
3-3 Cooling Energy Demands Calculation:
The same work done for heating demands is prepared for cooling demands. The energy
demands, the heating load provided by the generator, and the percentage of coverage are
calculated by using TRNSYS for dynamic simulation in function of time.
3-3-1 Case study:
Fig.3.6 show the TRNSYS cycle used for calculation cooling loads for 20 KVA diesel
generators that consist of:
1-One absorption chiller having the following parameters (capacity=7423 W, COP=0.7, Pump
electrical power = 100 W, set temperature =7
).
2- One cold storage tank with tank volume= 500l and tank loss coefficient= 0.833 w/m^2.K.
3-A power generator function profile is seen in Fig 2.11.
4-cooling load profile is taken from storage tank (for a typical small home in Beirut from design
builder).
5-Two plotters to calculate the outer parameters needed.
6- Control systems to control the return temperature from load to the tank.
Page | 45
Figure ‎3.6 TRNSYS cycle of 20KVA generators for providing cooling load.
The results of this cycle are shown in the following graphs:
Figure ‎3.7 The TRNSYS results for the cooling cycle.
Page | 46
In a fan coil unit cooling system, the temperature of water enters the coil should be less than
10oC, to give the cooling load. In other word, when the tank temperature is less than 10 oC, then
the cooling load is provided.
The results show that a 20KVA generator provides 5 KW cooling load with:
A) The overall percentage of coverage of time for 5 KW cooling load is 71% distributed over:
1) 92% percentage of coverage of time, when the generator is functioning.
2) 62.5% percentage of coverage of time, when the generator is stopping.
B) The overall percentage of coverage of energy for 5 KW cooling load is 48 % distributed over:
1) 80% percentage of coverage of energy, when the generator is functioning.
2) 50% percentage of coverage of time, when the generator is stopping.
The same calculation is done for different tank volumes to predict the favorable tank.
Tank volume (L)
300
500
600
800
1000
1200
1500
Time coverable (%)
69
71
75
77
81
83
81
Energy coverable (%)
48
48
54
63
72
73
72
Table 10: The time coverable and energy coverable of cooling load with respect to tank
volume.
90
80
70
Time coverable (%)
60
Energy coverable (%)
50
40
0
300
600
900
1200
1500
Tank volume
Figure ‎3.8 The variation of coverable of cooling load with respect to tank volume
Page | 47
From the results shown above we deduce that the 1200L volume of tank is the favorable volume
used for 20 KVA generators.
3-3-3 general study
By making the same study for all types of generators we obtain the following results:
Generator power
(KVA)
Cooling load (kW)
20
30
45
60
80
100
5
8
13.5
20
24
30
Time coverable (%)
83
83
77
75
75
71
Energy coverable (%)
73
72.5
63
54
54
52
Table 11: The cooling load provided with respect to generator power
Cooling load (kW)
35
30
25
20
15
10
5
0
0
20
40
60
80
100
Generator power (KVA)
Figure ‎3.9 The variation of cooling load with respect to generator power
Page | 48
Chapter 4 Energy Economy and Pollution
Reduction
4-1 Energy Conservation
Energy conservation refers to reducing energy through using less of an energy service. Energy
conservation differs from efficient energy use, which refers to using less energy for a constant
service. Energy conservation and efficiency are both energy reduction techniques.
Even though energy conservation reduces energy services, it can result in increased financial
capital, environmental quality, national security, and personal financial security. It is at the top of
the sustainable energy hierarchy.
4-1-1 Case Study
The three TRNSYS cycles which are applied for 20KVA generators calculate the energy rate
taken from the exhaust gas of the engine, which is the heat source for heat exchanger and
absorption chiller. The results for space heating cycle alone, space heating and DHW cycle, and
space cooling are shown in Fig 4.1, Fig 4.2, and Fig 4.3 respectively.
1) Space heating:
Figure ‎4.1 The variation of energy rate taken from heat source for TRNSYS cycle of 20KVA
generators for providing heating load only
Page | 49
The storage tank used for heating loads with volume= 350 l, a high aspect ratio
(height/diameter), and insulation thickness from 4 to 6 cm which yield to tank loss coefficient=
0.833 w/m^2.K. Moreover the set point is taken 90 oC (The maximum temperature of water).
From the above figure we indicate that the energy taken from heat source for 48 hr. is equal
The heating load is used for 6 month per year.
Therefore the energy saved from using cogeneration to provide heating load from 20KVA
generator is equal
2- Space heating and DHW:
Figure ‎4.2 The variation of energy rate taken from heat source for TRNSYS cycle of 20KVA
generators for providing heating load and DHW
The storage tank used for heating loads with volume= 350 l, a high aspect ratio
(height/diameter), and insulation thickness from 4 to 6 cm which yield to tank loss coefficient=
0.833 w/m^2.K.moreover the set point is taken 90 oC (The maximum temperature of water is 90)
While the storage tank used for DHW with volume=250 l, and same characteristic above.
Page | 50
From the above figure we indicate that the energy taken from heat source used for DHW for 48
hr. is equal
From the above figure we indicate that the energy taken from heat source used for space heating
for 48 hr. is equal
The heating load and DHW is used for 6 month par year.
Therefore the energy saved from using cogeneration to provide heating load and DHW from
20KVA generator is equal
3- Space cooling:
Figure ‎4.3 The variation of energy rate taken from heat source for TRNSYS cycle of 20KVA
generators for providing cooling load
The storage tank used for cooling loads with volume= 350 l, a high aspect ratio
(height/diameter), and insulation thickness from 4 to 6 cm which yield to tank loss coefficient=
0.833 w/m^2.K.moreover the set point is taken 90 oC (The maximum temperature of water is
90).
From the above figure we indicate that the energy taken from heat source for 48 hr. is equal
Page | 51
The cooling load is used for 6 month per year.
Therefore the energy saved from using cogeneration to provide heating load from 20KVA
generator is equal
4-1-2 General Study
By making the same calculation for some generator power we indicate the following table:
Generator power
(KVA)
20
30
45
60
80
100
15150
28900
35600
54500
68000
79000
17650
29350
36450
55500
63000
82000
15600
26000
38000
55400
61500
81000
Economy for heating
load only (KWh/year)
Economy for heating
load and DHW
(KWh/year)
Economy for cooling
load only (KWh/year)
Table 12: The energy economy of the three systems with respect to generator power
Economy for heating load only (KWh/year)
90000
80000
70000
60000
50000
40000
30000
20000
10000
0
0
20
40
60
80
100
Generator Power (KVA)
Figure ‎4.4 The variation of the economy of system producing heating load only with
respect to generator power
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Economy for heating load and DHW (KWh/year)
90000
80000
70000
60000
50000
40000
30000
20000
10000
0
0
20
40
60
80
100
Generator power(KVA)
Figure ‎4.5 The variation of the economy of system producing heating load and DHW with
respect to generator power
Economy for cooling load only (KWh/year)
90000
80000
70000
60000
50000
40000
30000
20000
10000
0
0
20
40
60
80
100
Generator Power(KVA)
Figure ‎4.6 The variation of the economy of system producing cooling load with respect to
generator power
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4-2 Pollution reduction
The primary pollutants associated with reciprocating diesel engines are oxides of nitrogen
(NOx), carbon monoxide (CO), carbon dioxide (CO2) , volatile organic compounds (VOCs—
unburned, non-methane hydrocarbons), and oxides of sulfur (SOx).
But the main pollutant gas of the combustion equation is carbon dioxide (one of products of
combustion equation), which depend on the produced energy of the engine, according to the
following equation:
Table 12 shows the energy economy of cogeneration and tri-generation systems. By using the
equation above we indicate the quantity of reduction of emission of CO2, and the results are
tabulated in the following table:
Generator power
20
30
45
60
80
100
(KVA)
Pollution reduction for
4393.5
8381
10324
15805
19720
22910
heating load only (Kg
of CO2)
Pollution reduction for
8511.5
10570.5
16095
18270
23780
heating load and DHW 5118.5
(Kg of CO2)
Pollution reduction for
4524
7540
11020
16066
17835
23490
cooling load only (Kg
of CO2)
Table 13 Quantity of reduction of CO2 for the three systems with respect to generator
power
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Conclusion
Tri-generation systems (combined cooling, heating and power production) make the diesel power
generators more energy-efficient, and reduce greenhouse gas emission.
In our study, waste heat of exhaust gas of diesel power generator is recovered by using shell and
tube heat exchangers and also providing heat to the generator of absorption chiller to cover the
space heating, space cooling, and DHW loads.
It is found that:
1) The heating load provided by the system increases with respect to the generator power (KVA)
and it is approximately according to the following equation:
(
)
(
)
2) The cooling load provided by the system increases with respect to the generator power (KVA)
and it is approximately according to the following equation:
(
)
(
)
3) The DHW consumption load could be fully covered all over the year (winter and summer).
4) Up to 75% of the space heating demands could be covered by using the optimum size of the
hot storage tank.
5) Also, The space cooling demands could be covered up to 75%, when using the optimum size
of the cold storage tank.
In addition, it is noticed that:
1) It is better to integrate an auxiliary heating source to the hot storage tank, in order to fully
recover the space heating load during non-operating hours of the power generator.
2) It is better to integrate an auxiliary heating source connected to the generator of absorption
chiller, or assimilate an electric chiller connected to the cold storage tank, in order to fully
recover the space cooling load during non-operating hours of the power generator.
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3) Improvement of residential isolation measures will reduce the heating and cooling loads and
consequently the recovered energy from the exhaust gas could fully recover these loads.
4) A yearly heating and cooling energy analysis could be carried out to determine the annual
recovering fraction of the tri-generation system.
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