micro-CHP, fuel cells, microgeneration
Krzysztof KLOBUT*, Jari IHONEN, Jussi IKÄHEIMO
Overview of micro-scale CHP technologies for distributed generation in the residential sector
Cogeneration (combined heat and power, CHP) systems have the ability to produce both electricity and useful thermal energy from a single energy carrier, for example natural gas. Several manufacturers have developed, or are developing, small-scale CHP products aiming for residential applications. Such CHP systems can be based on: fuel cells, internal combustion engines, external
combustion Stirling engines and micro-turbines. The commercial viability of cogeneration plants for
residential scale buildings has yet to be gained. Also the true potential for residential cogeneration energy and emissions savings is yet to be firmly established. This paper provides a review of various
cogeneration technologies suitable for residential applications and discusses their features.
INTRODUCTION
Cogeneration systems in the residential sector have the ability to produce both useful thermal energy and electricity from a single source of fuel such as natural gas.
They can have several benefits compared to traditional solutions, such as increased
reliability of power supply. As residential scale cogeneration technologies are just
entering the market with heavy subsides, the potential for residential cogeneration
energy and emissions savings is yet to be firmly established. Technologies available
and under development for residential applications can be grouped according to the
size into single-family (<10 kWel) and multifamily (10–30 kWel) applications, commercial (5–100 kWel) and institutional cogeneration (20–100 kWel) systems. Such
CHP systems can be based on: fuel cells, internal combustion engines, external combustion Stirling engines and micro-turbines (Figure 1).
The use of cogeneration plant for residential scale buildings has yet to become
commercially viable though several manufacturers have developed products or are
developing products suitable for residential scale use. Thermal storage provides an
__________
* VTT Technical Research Centre of Finland.
opportunity to store the heat from micro-CHP units, thus decoupling the unit dispatch
from heat demand and providing economical benefits [8]. However, the applicability
of thermal storage (hot water tank) is limited to applications in which there is sufficient space available.
80
SOFC-/MCFC - gas turbine
60
efficiencyel
combined process
SOFC / MCFC
steam inject.
gas turbine
diesel-engine
PAFC
40
PEFC
gas engine
gas turbine
SOFC
20
micro gas turbine
0
1
10
100
1.000
10.000
100.000
capacity [kW]
sources: Bohn, Siemens-Westinghouse, Thyssengas, Ruhrgas, EnRef. Frankfurt, Sydkraft, SulzerHexis
Figure 1. Electrical efficiency performance of different technologies as a function of capacity [1].
FUEL CELL (FC) BASED SYSTEMS
A fuel cell converts the chemical energy of a fuel and oxygen continuously into
electrical energy. Typically, the fuel is hydrogen. Thus, the energy incorporated in the
reaction of hydrogen and oxygen to water will be transformed into electrical energy.
Fuel cells can be categorised according to the electrolyte material and, correspondingly, the required operating temperatures into low, medium and high temperature applications. Table 1 lists the main characteristics of the main fuel cell types.
The performance of fuel cell systems is a function of the type of fuel cell and its
capacity. The optimization of electrical efficiency and performance characteristics of
fuel cell systems poses an engineering challenge because fuel cell systems are a combination of chemical, electrochemical, and electronic subsystems. Selected performance data for fuel cell systems are presented in the Table 2.
The cost distribution between different subsystems varies significantly depending
on the source. Based on the study by Directed Technologies, Inc the system level
components, excluding the stack and heat exchangers, are not strongly dependent on
the system size [3]. This indicates that system cost per kW may be significantly larger
in 1 kW systems compared to 10 kW systems.
Table 1. Types of fuel cells and main characteristics (updated after [6]).
Electrolyte
AFC
KOH
Temperature
Ion
60-90 oC
OH-
 system, el
PEFC
Proton
conducting
membrane
60-90 oC
H+
30-42 %
DMFC
Proton
conducting
membrane
55-65oC
H+
PAFC
Phosphoric
acid
MCFC
Carbonate
melt
SOFC
Y stabilised
ZrO2
200 oC
H+
38-42 %
800 – 1000 oC
O230-55 %
(w/ GT> 60%)
47-50 %
650 oC
CO3250-55 %
(w/ ST >
55%)
n.a.
CHP
CHP, CC
CHP, CC
(natural gas)
 system, el
38-50 %
(hydrogen)
Favoured
application
Space,
military,
portable
Mobile,
portable,
CHP
2 – 200
Mobile,
portable
n.a.
200 –
2 – 100 000
100 000
First
Subsidised
Commercial
Small series Subsidised
Subsidised
commercial
commercial
production
commercial
commercial
production
production
(200 kWel)
production
production
ST: Steam turbine; GT: Gas turbine; CHP: Combined Heat and Power Production; AFC: Alkaline Electrolyte
Fuel Cell; PEFC: Polymer Electrolyte Membrane Fuel Cell; DMFC: Direct Methanol Fuel Cell; PAFC:
Phosphoric Acid Fuel Cell; MCFC: Molten Carbonate Fuel Cell; SOFC: Solid Oxide Fuel Cell; n.a. not
available
Power range
[kWel]
Status
50 – 10 000
Table 2. Performance characteristics of fuel cell based cogeneration systems [5].
Performance Characteristics
System 1
System 2
System 3
System 4
System 5
Fuel Cell Type
PEMFC
PEMFC
PAFC
SOFC
MCFC
10
200
200
100
250
30 %
35 %
36 %
45 %
43 %
Fuel Input kW
29
586
557
234
586
Operating Temperature [oC]
70
70
200
950
650
Nominal Electricity Capacity [kW]
Electrical Efficiency [%], HHV
Cogeneration Characteristics
Heat Output [kW]
12
211
217
56
128
Total Overall Efficiency [%], HHV
68 %
72 %
75 %
70 %
65 %
Power / Heat Ratio
0.77
0.95
0.92
1.79
1.95
53.6 %
65.0 %
70.3 %
65.6 %
59.5 %
Effective Electrical Efficiency [%], HHV
HHV = Higher Heating Value
Actual cost data from Japan also indicates that cost targets for 1 kW systems may
be difficult to reach if fuel cell systems cannot be simplified [8].
Table 3. Known sale prices for fuel cell micro-CHP systems [8].
INTERNAL COMBUSTION ENGINE (ICE) BASED COGENERATION SYSTEMS
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 turbo-charger and intercooler. 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. Cooling systems for
diesel engines are more complex in comparison to the cooling systems of spark ignition engines and temperature are often lower, usually 85oC maximum, thus limiting
the heat recovery potential.
Efficiency: Reciprocating internal combustion engines have mechanical efficiencies that range from 25–30%. The overall efficiency in cogeneration mode reaches
approximately 80–85 (% HVV).
Part-load performance: The percentage of fuel energy input used in producing mechanical work, which results in electrical generation, remains fairly constant until 75%
of full load, and thereafter starts decreasing. The amount of useful heat derived from a
cogeneration system increases as the efficiency of electric power delivered decreases.
Costs: Generally, reciprocating internal combustion based cogeneration systems
less than 500 kW in size cost between 800 and 3,020 $/kW (630 and 2,370 €/kW),
with higher cost for smaller cogeneration systems. Estimated capital costs indicate that
the cost per unit capacity decreases with increasing engine size. [4]
STIRLING ENGINE BASED COGENERATION SYSTEMS
Stirling engine differs from internal combustion engine by the fact that cylinder is
closed and combustion process takes place outside of it. Piston is moved by pressure
changes due to heating and cooling of working gas. Stirling engines operate smoothly,
resulting in lower vibration, noise level and emissions than reciprocating internal
combustion engines. Also, the external combustion process allows the use of a large
variety of fuels and longer fuel retention times in the combustion chamber compared
to internal combustion engines. As a result, the control, and hence the efficiency of
combustion is higher.
Efficiency and part-load performance: It is expected that while the full load efficiency can be 35–50%, the efficiency at 50% load can be expected to be in the 34–
39% range. Since the technology is still in the development phase, there is no statistical data for the reliability and availability of Stirling engines. However, it is expected
that the reliability of Stirling engines will be comparable to that of diesel engines.
Costs: Presently, the investment cost for the unit is still about twice as high as an
internal combustion engine driven cogeneration unit of the same capacity, although it
is more economical when considering the maintenance costs of Stirling engines, i.e.
0.013 $/kWh (0.010 €/kWh) as compared with $0.018 $/kWh (0.014€/kWh) of internal combustion engine driven cogeneration systems. [4]
SUMMARY
Properties of different small CHP generation systems are compared by means of
spider graphs in the following figures. Different features of the systems are valued
using a scale from 0 (worst) to 5 (best).
Costs
Kustannukset
Stirling-moottorit
Polttokennot
Hyötysuhde
Efficiency
5
Hyötysuhde
Efficiency
5
4
Lifetime
Elinikä
Costs
Kustannukset
3
2
2
1
Noise
Melu
4
3
0
1
Size
Koko
Emissions
Päästöt
Fuel
Polttoaine
Lifetime
Elinikä
Melu
Noise
0
Polttoaine
Fuel
Power
Control
Tehonsäätö
Figure 2. Features of Stirling engines (left) and fuel cells (right). [7]
Size
Koko
Päästöt
Emissions
Tehonsäätö
Power
Control
In Table 4 estimation is given regarding how well different technologies fit different types of buildings in terms of application as distributed energy generation systems.
Table 4. Estimated technical applicability of different technologies to different buildings [7].
Building type
Gas and
diesel engines
Gas
turbines
Stirling
engines
Fuel cells
Single-family house
-
--
++
++
Attached row house
+
-
++
++
Apartment house
+
-
+
+
Office building
+
-
-
+
Hotel, spa, etc.
++
+
-
+
Greenhouse
++
+
--
+
Small industry: workshop, sawmill etc.
++
++
--
+
District heating
++
+
--
+
Fitting category: -- very poor - poor + good ++ excellent
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and power generation. Energy and Environmental Science, 2009, No. 2, p. 729-744.
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Electrolyte Membrane (PEM) Fuel Cell Systems. 1999, Directed Technologies Inc. URL:
http://www.directedtechnologies.com/publications/fuel_cell/FinalStationPEMFCS.pdf
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ISBN 951-38-6731-5; 951-38-6732-3 http://www.vtt.fi/inf/pdf/tiedotteet/2005/T2305.pdf
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of
Birmingham.
September
2009.
Available
at:
http://etheses.bham.ac.uk/641/