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Study on capacity optimization of PEM fuel cell and hydrogen mixing gas-engine compound generator
International Journal of Hydrogen Energy 32 (2007) 4329 – 4339 www.elsevier.com/locate/ijhydene
Study on capacity optimization of PEM fuel cell and hydrogen mixing gas-engine compound generator Shin’ya Obara a,∗ , Itaru Tanno b a Department of Mechanical Engineering, Ichinoseki National College of Technology, Takanashi, Hagisho, Ichinoseki, Iwate 021-8511, Japan b Department of Mechanical Engineering, Tomakomai National College of Technology, 443 Nishikioka, Tomakomai, Hokkaido 0591275, Japan
Received 4 December 2006; received in revised form 10 May 2007; accepted 10 May 2007 Available online 27 June 2007
Abstract Development of a small-scale power source not dependent on commercial power may result in various effects. For example, it may eliminate the need for long distance power-transmission lines, and mean that the amount of green energy development is not restricted to the dynamic characteristics of a commercial power grid. Moreover, the distribution of the independent energy source can be optimized with regionality in mind. This paper examines the independent power supply system relating to hydrogen energy. Generally speaking, the power demand of a house tends to fluctuate considerably over the course of a day. Therefore, when introducing fuel cell cogeneration into an apartment house, etc., low-efficiency operations in a low-load region occur frequently in accordance with load fluctuation. Consequently, the hybrid cogeneration system (HCGS) that uses a solid polymer membrane-type fuel cell (PEM-FC) and a hydrogen mixture gas engine (NEG) together to improve power generation efficiency during partial load of fuel cell cogeneration is proposed. However, since facility costs increase, if the HCGS energy cost is not low compared with the conventional method, it is disadvantageous. Therefore, in this paper, HCGS is introduced into 10 household apartments in Tokyo, and the power generation efficiency, carbon dioxide emissions and optimal capacity of a boiler and heat storage tank are investigated through analysis. Moreover, the system characteristics change significantly based on the capacity of PEM-FC and NEG that compose HCGS. Therefore, in this study, the capacity of PEM-FC and that of NEG are investigated, as well as the power generation efficiency, carbon dioxide emissions and the optimal capacity of a boiler and heat storage tank. Analysis revealed that the annual average power generation efficiency when the capacity of PEM-FC and NEG is 5 kW was 27.3%. Meanwhile, the annual average power generation efficiency of HCGS is 1.37 times that of the PEM-FC independent system, and 1.28 times that of the NEG independent system, respectively. 䉷 2007 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. Keywords: PEM fuel cell; Hydrogenation gas engine; Compound generator; Cogeneration; Partial load; Operation planning
1. Introduction Cogeneration using a solid polymer membrane-type fuel cell (PEM-FC) has a high maximum power generation efficiency compared with other power plants. However, reduced power generation efficiency during partial-load operation of a system with a reformer remains an issue. In particular, in general houses, partial-load operations with significant fluctuation of power load and low efficiency occur frequently. On the other hand, hydrogen mixing technology is studied, concerning
∗ Corresponding author. Tel./fax: +81 191 24 4835.
E-mail addresses: [email protected], [email protected] (S. Obara), [email protected] (I. Tanno).
exhaust gas cleanup and improved efficiency at the time of partial load of gas-engine cogeneration [1–4]. If the hydrogen rate of fuel is increased when the load is small, studies confirm that exhaust cleanup and brake thermal efficiency improve [1–4]. Although the facility cost of a hydrogen/gas hybrid engine (NEG) cogeneration is low compared with PEM-FC, the maximum power generation efficiency is inferior. However, if the partial-load characteristics of NEG are better compared with PEM-FC, the introduction of PEM-FC to a house may be disadvantageous. Therefore, in this paper, a PEM-FC and NEG hybrid cogeneration system (HCGS) having the maximum power generation efficiency of PEM-FC and the partial-load characteristics of NEG is proposed. In HCGS, PEM-FC is operated corresponding to a base load, and NEG is operated corresponding to a fluctuating load. In this case, since PEM-FC operates
0360-3199/$ - see front matter 䉷 2007 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2007.05.003
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Nomenclature C C E g q R t U
N R S
capacity, kW maximum load, kW power, kW CO2 emission, g/(s kW) primary power input, kW load factor, % sampling time flow rate, m3 /s
Equipment C/O CGS DC/AC FC G/T HCGS I/T NEG PEM-FC Vb
Greek symbols
efficiency, %
Subscripts B F
gas engine generator reformer boiler
burner installed in a reformer fuel cell
carbon monoxide oxidation equipment cogeneration DC–AC converter fuel cell generator hybrid cogeneration inverter gas engine cogenerator solid polymer membrane-type fuel cell valve
Controller
City gas system (1)
Vb1 Vb4
City gas system (2)
Vb2 City gas system (3)
Vb3
Reformer Burner
Compressor
PEM-FC
Vb5
To heat storage tank
G/T
To heat storage tank C/O
Dryer
Gas engine (NEG)
I/T Power output
DC /AC
To heat storage tank Exhaust heat input Boiler
Heat output
Tap water Heat storage tank City gas system (4) Fig. 1. HCGS block diagram.
at a constant load, the capacity of the fuel cell is designed to be optimal and can always be operated at high power generation efficiency. On the other hand, NEG is operated corresponding to fluctuating loads except for the base load. Therefore, a low-load operating range, where PEM-FC is disadvantageous, can correspond to NEG. The facility cost of HCGS increases compared with the case where PEM-FC or NEG is operated independently. Therefore, in order for the introduction of HCGS to city areas to be effective, compared with the individual operation of PEM-FC or NEG, the power cost and carbon dioxide emissions must be advantageous. However, the power generation efficiency and the carbon dioxide emissions of PEM-FC, NEG, and HCGS are dependent on the load pattern of a building. Moreover, since the exhaust heat characteristics differ according to PEM-FC, NEG, or HCGS, the optimal capacity of the heat storage tank and boiler of each system also varies, while the power generation efficiency and exhaust heat characteristics of HCGS change with the capacity of PEM-FC and NEG to be introduced. Therefore, in this paper, the relations among the “capacity of PEM-FC and NEG”, “power generation efficiency”,
“carbon dioxide emissions”, and “the optimal capacity of a boiler and a heat storage tank” are investigated with reference to the introduction of HCGS into 10 household apartment houses in Tokyo. 2. HCGS scheme A block diagram of HCGS proposed in this paper is shown in Fig. 1. City gas includes supply systems to NEG, to a reformer, to a heat source burner, and to a boiler, respectively, where the Japanese conditions apply to the composition of “city gas”. Accordingly, “city gas” consists mainly of methane. The extent of the HCGS city gas consumption is calculated by Utotal,t = UN,t + UR,t + UB,t + US,t .
(1)
The operation model of HCGS is shown in Fig. 2(a). In the operation of HCGS, PEM-FC is used for a base load and NEG is used for other fluctuating loads. Therefore, a high load of PEMFC is always highly efficient, and it can be operated. CN and CF in this figure express the maximum electric load of NEG and
S. Obara, I. Tanno / International Journal of Hydrogen Energy 32 (2007) 4329 – 4339
Fluctuating load (NEG)
Base load (FC) 0
6
12 Time[Hour]
18
24
PEM-FC operation
HCGS operation
Power generation efficiency
Power consumption [kW]
FC maximum power generation efficiency operation
4331
Operation of NEG with hydrogenation
0
20
40 60 80 Load factor R [%]
100
120
Fig. 2. HCGS operation method: (a) base load and fluctuating load operation, (b) efficiency characteristic of HCGS.
16
6
CO2 emission [g/s]
City-gas
3 2
12 0.6 8
Total
4
0.2
Hydrogen
1
0.4
Engine emission
Burner
Reforming 0.0
0 0
0 0
0
10 15 5 Production of electricity [kW] 20
40 60 Load factor R [%]
80
0
5 10 15 Production of electricity [kW] 20
40 60 Load factor R [%]
80
100
(R ≥ 60 %) g'N = 7×10−7 R3 − 0.0004R2 + 0.0601R − 2.11 (R < 60 %) g'N = 3×10−6 R2 + 0.0027 R + 0.00346
100
90 80 70 60
NEG capacity= 10kW 7kW 5kW
20
Total efficiency 3kW
40 Output [kW]
Power generation efficiency [%]
25
15 10
30 Heat output (Cooling hotwater and exhaust)
20
Overall efficiency [%]
Fuel consumption [g/s]
4
CO2 emission [g/(skW)]
0.8 5
Power output (Generating end)
10 5 0 0
20
40 60 80 Load factor R [%]
100
0
10
20 30 40 Fuel supply [kW]
50
Fig. 3. Output characteristics of the hydrogen mixing gas engine cogenerator [1]: (a) amount of CH4 and hydrogen mixing when getting the maximum thermal efficiency, (b) CO2 emission characteristics of NEG, (c) relation between a production of electricity and gross power generation efficiency, (d) output characteristics of 10-kW NEG.
PEM-FC, respectively. It is necessary to determine the capacity of NEG and PEM-FC (CN , CF ) introduced into HCGS with a value exceeding CN and CF of the figure. Since the load factor
(load/capacity) changes according to the capacity setup of CN and CF , the fuel consumption and carbon dioxide emissions of HCGS also change. So, in this paper, the fuel consumption and
85 Total efficiency
75
Output [kW]
12
65 Heat output (Reformer and cell stack)
8
4 Power output (Generating end)
F Efficiency
30
2.0
20 gF CO2 total '
1.0
10 Reforming CO2 Burner CO2
0.0
0 0
40 60 80 100 120 Load factor R [%] F = −0.0028 R2 + 0.598 R + 0.150 g'F = −3×10−6 R3 + 0.0009R2 − 0.0914R + 4.174
0 10
12
14 16 Fuel supply [kW]
60
80 Load factor R [%]
3.0
CO2 emission g'F [g/(skW)]
16
Power generation efficiency F [%]
S. Obara, I. Tanno / International Journal of Hydrogen Energy 32 (2007) 4329 – 4339
Overall efficiency [%]
4332
18
20
100
Fig. 4. Output characteristics of the PEM-FC [8,9]: (a) output characteristics of 5-kW PEM-FC, (b) the characteristics model of the load factor of a PEM-FC with reformer, and power generation efficiency. The area of the electrode including the anode and cathode of the fuel cell stack is 1 m2 , respectively, and the reformer efficiency is 73%.
carbon dioxide emissions when changing the value (relatively) of CN and CF are investigated. Fig. 2(b) shows a model showing the relation of the load factor and power generation efficiency of NEG and PEM-FC. In the low-load operating range of NEG, an improvement in brake thermal efficiency is expected by increasing the hydrogen concentration of fuel [1]. However, there is no effect of hydrogen mixing in the high-load operating range of NEG. Moreover, as shown in Fig. 2(b), compared with PEM-FC, the maximum efficiency point of NEG is low. Therefore, operation in a low-load zone of NEG is advantageous, and the same applies to operation in a high-load zone of PEM-FC. Since HCGS has the characteristics of the power generation efficiency of PEM-FC and NEG, it is expected to be comparable to the individual operation of PEM-FC or NEG, in which power generation efficiency is advantageous.
meanwhile, shows the carbon dioxide emission characteristics of NEG, calculated based on the reference examination result [1]. In this figure, the approximate expression showing the relation between load factor R and CO2 emissions is shown. These approximate expressions differ bordering on 60% of the load factor, because the fuel supplied to NEG has a high hydrogen rate in a low-load range and the rate of city gas is high within a high-load range. In a low-load range, the carbon dioxide ratio discharged by city gas reforming for hydrogen manufacture and the burner for reformers is high. On the other hand, in a high-load range, the rate of carbon dioxide generated in engine city gas burning is high. Fig. 3(c) shows the relationship between the load factor and the generation efficiency of NEG. Although reformed gas is supplied to NEG, reformer efficiency is included in the power generation efficiency shown in this figure. Eq. (2) defines reformer efficiency.
3. Equipment characteristics
R = (qH2 /qCH4 ) × 100(%),
3.1. Output characteristics of NEG
where qH2 is the calorific power of hydrogen in reformed gas, and qCH4 expresses the calorific power of the city gas supplied to a reformer, while the calorific power of the city gas for reforming and the city gas for heat-source burners is included in qCH4 . In this paper, R was set at 73% [5–7]. Generally, engine thermal efficiency increases, so that the capacity expands. Consequently, as shown in Fig. 3(c), the NEG power generation efficiency characteristics are separated into each capacity of NEG, which are then introduced into the analysis. Fig. 3(d) shows the power and heat output of 10-kW NEG, and a model of overall efficiency. The heat outputs are engine exhaust, cooling water, and reformer exhaust heat. However, the overall efficiency in this figure assumes a case where all the power and heat outputted by NEG are consumed. Introduced in
Past examination results are used for the output characteristics of a city gas engine with hydrogen mixing [1]. The examined hydrogen mixing engine is 857 cc in a single cylinder, and lean burn is enabled by injecting reformed gas into an inlet pipe. Fig. 3(a) shows the model of the city gas consumption (CH4 ) of NEG, the amount of hydrogen mixing, and the production of electricity. These characteristics were calculated from the hydrogen mixing rate, based on the reference examination result [1]. In Fig. 3(a), when the electricity production exceeds 14 kW, the amount of hydrogen mixing is zero, because high thermal efficiency can be obtained, even if there is no hydrogen mixing within the large operating range of engine power. Fig. 3(b),
(2)
S. Obara, I. Tanno / International Journal of Hydrogen Energy 32 (2007) 4329 – 4339
the Case Study are 3- to 10-kW NEG. The difference between the generation efficiency and the carbon dioxide emissions of 3-kW NEG and 10-kW NEG is about 6%. Fig. 5(b) described in Section 3.3 explains the exhaust heat of the NEG.
4333
60
76
80
Production of heat [kW]
72
3.2. Output characteristics of PEM-FC Fig. 4(a) shows a model of the output characteristics and overall efficiency of 5-kW PEM-FC with a city gas reformer [8,9]. The heat output includes the exhaust heat of the reformer and cell stack. Moreover, the power output is the value of the inverter outlet. Overall efficiency assumes the case where all of the power and heat to output are consumed. Fig. 4 (b) shows the model of the carbon dioxide emissions and generation efficiency of PEM-FC of Fig. 4(a). The carbon dioxide discharged by the operation of PEM-FC is based on a reforming reaction (Eqs. (3) and (4)) and city gas burning of a reformer burner (Eq. (5)). The power generation efficiency F of Fig. 4(b) was calculated from Eq. (6). EF,t , on the right-hand side of Eq. (6), shows the power of the inverter outlet, qR,CH4 ,t shows the calorific power of CH4 supplied to the reformer, and qB,CH4 ,t shows the calorific power of CH4 supplied to the heat-source burner of the reformer. The maximum generation efficiency of the PEM-FC model shown in Fig. 4(b) is 32%.
68
50
Primary po
64
wer input [k
60
W]
56
40
52 48 44
30
40 36
Operation area B
32
20
24 20 16
10
12
28
ar Heat out put ch
act eri sti c in Ar
ea A
Operation area A
0 2
0
6 4 8 Production of electricity [kW]
10
60
85
Production of heat [kW]
75
50
Primary
80
70 power
65
input [k
W]
40
60 55 50
Operation area B
30
45 40
35 30
20
25 20 15
10
Heat
10
ut c ou t p
c te ha r a
r ist i
c in
A re
aA
Operation area A
5
CO + H2 O → CO2 + H2 + 41
kJ/mol,
0 2
0
kJ/mol,
CH4 + 2O2 → CO2 + 2H2 O + 802
(3)
kJ/mol,
F,t = {EF,t /(qR,CH4 + qB,CH4 ,t )} × 100.
(4) (5)
In this paper, the chart showing the city gas calorific power (consumption) supplied to HCGS and the relationship between the production of electricity and the heat output of a system is defined as an “operation map”. The fuel calorific power in Fig. 5 has a lower calorific value. Fig. 5(a) shows the operation map of a 10-kW PEM-FC, and Fig. 5(b) shows the operation map of the 10-kW NEG, respectively. Overall efficiency is calculable because the calorific power of the supply fuel is shown in Fig. 5. The Operation Area A of each figure is the production range of electricity and exhaust heat output when operating PEM-FC or NEG independently. In Area A, when the production of electricity is decided, heat output as shown in the figure occurs. Operation Area B, meanwhile, indicates the range of production of electricity and the heat output (the amount of exhaust heat, and boiler output) when operating a boiler in addition to PEM-FC or NEG. However, the boiler efficiency of Operation Area B was set at 0.9. The operation map describes primary input power (based on city gas LHV, described as city gas consumption below) consumed by a system. When the operation maps of PEM-FC (Fig. 5(a)) and NEG (Fig. 5(b)) are compared, the NEG of Operation Area A is wider. Furthermore, in the Operation Area A of NEG, if the electricity production increases, exhaust heat output will also increase rapidly, but
10
12
NEG operation
38
50 36 48 P rim 34 a ry 46 p ow 32 44 er in put 30 [kW ] 28 Operation area D 26 42 40 24 22 38 36 20 34 t ic 32 18 e r is t c 30 Operation area B ara 16 28 t ch n d C 14 26 tp u a A a 24 u o e 12 t 22 r He a i n A 20 10
(6)
3.3. System operation map
6 4 8 Production of electricity [kW]
PEM-FC operation 30 Production of heat [kW]
CH4 + H2 O → CO + 3H2 − 206
25 20 15 10 5
6
8
Operation area A
Operation area C
0 0
1
2
3 4 7 5 6 Production of electricity [kW]
8
9
10
Fig. 5. Operation map; (a) 10-kW PEM-FC, (b) 10-kW NEG, (c) 10-kW HCGS.
in PEM-FC, even if the production of electricity increases, the increase in exhaust heat output is lost. Fig. 5(c) shows the operation map when introducing 5-kW PEM-FC and 5-kW NEG into HCGS. In HCGS, PEM-FC corresponds to the base load and NEG is operated corresponding to a fluctuating load. The maximum power generation efficiency point of PEM-FC approaches the maximum load rate, as shown in Fig. 4(b). On the other hand, as shown in Fig. 3, NEG shows positive power generation efficiency and exhaust gas characteristics at low load. As for the installation of low-load zone operation of NEG,
10
100
8
80
6
60
4
40
2
20
0
Load factor [%]
S. Obara, I. Tanno / International Journal of Hydrogen Energy 32 (2007) 4329 – 4339
Power [kW]
4334
0 January
March
February
May April
July June
September November August
October
December
Heat [kW]
30 20 10 0 January
March
February
May April
July June
September November August
October
December
Efficiency [%]
Fig. 6. Power (a) and heat (b) demand for 10 houses apartment in Tokyo.
30
27.3%
26.7%
24.7%
21.4%
20.0%
20 10 0
Fuelcalorific power [GJ/ Year]
PEM-FC 0kW PEM-FC 3kW PEM-FC 5kW PEM-FC 7kW PEM-FC10kW NEG 10kW NEG 7kW NEG 5kW NEG 3kW NEG 0kW
800
630GJ/ Year
588GJ/Year
600 400
473GJ/ Year
462GJ/ Year
510GJ/ Year
315GJ/Year
200 0 kW 10 C -F W M 0k PE G NE kW C7 -F W M 3k PE EG N
kW C5 -F W M 5k PE G NE W 3k C W -F k M G7 PE NE
W 0k C W -F k M G0 PE NE l cia ) er m r % m we 40 Co po ncy ie fic
f (E
Fig. 7. Result of annual average power-generation efficiency (a) and fuel consumption (b).
high effectiveness is expected if the generation efficiency and the load factor of PEM-FC and NEG have the relationship as shown in the model in Fig. 2(b). Therefore, the power generation efficiency of HCGS is predicted to improve when compared to circumstances where PEM-FC or NEG is introduced independently.
4. Case study Fig. 6 shows a power and heat demand model for the representative days each month of 10 residence apartment houses in Tokyo and indicates the average load of each sampling time [10]. However, the actual power-demand pattern is a set of
S. Obara, I. Tanno / International Journal of Hydrogen Energy 32 (2007) 4329 – 4339
4335
Heat [kW]
20 15 10 5 January
March
Heat [kW]
February
May April
July June
September August
November
October
December
15 10 5 January
March
Heat [kW]
February
May April
July June
September August
November
October
December
16 14 12 10 8 6 January
March
February
May April
July June
September August
November
October
December
Heat [kW]
14 12 10 8 January
March
February
May April
July June
September August
November
October
December
Heat [kW]
14 13 12 11 10 January
March
February
May April
July June
September August
November
October
December
Fig. 8. Result of the exhaust heat output of an every month representation day: (a) PEM-FC 0 kW, NEG 10 kW; (b) PEM-FC 3 kW, NEG 7 kW (c) PEM-FC 5 kW, NEG 5 kW; (d) PEM-FC 7 kW, NEG 3 kW; (e) PEM-FC 10 kW, NEG 0 kW.
loads that change rapidly within a short time, such as an inrush current. Since a cooling load is included in the electric power demand, there is considerable power demand in the summer season. The annual power demand amount of 10 houses is 57.6 MWh, while the heat demand is 86 GJ and the load
factor of the power every month is shown in Fig. 6. However, when the supply demand balance is calculated by averaging power demand with inrush current, the result will be estimated to be far smaller [11,12]. The amount of city gas consumption is obtained using the operation map of the system shown in
S. Obara, I. Tanno / International Journal of Hydrogen Energy 32 (2007) 4329 – 4339
Fig. 5 for every time the power and heat demand model shown in Fig. 6 is sampled. However, when the exhaust heat output of a system exceeds heat demand, surplus heat is stored in a heat storage tank. On the other hand, when the exhaust heat is less than the heat demand, heat is supplied from a heat storage tank and when the heat still remains insufficient, it is output by a boiler. When exhaust heat exceeds the capacity of the heat storage tank, the excess amount is released using a radiator. In the analysis of this paper, the daily radiation loss of the heat storage tank becomes 5%. Generation efficiency is calculated by dividing the power demand amount by the calorific power of the consumed city gas fuel. Moreover, the value that divides the value adding the production of electricity and the amount of heat output by the calorific power of the city gas fuel supplied to a system is defined as overall efficiency. The carbon dioxide emissions of the system for every sampling time are obtained by giving a load factor to Figs. 3(b) and 4(b), while total carbon dioxide emissions are calculated by adding these values and the amount of carbon dioxide discharged by the boiler. 5. Analysis results and discussions 5.1. Generation efficiency and fuel consumption Fig. 7(a) shows the analysis output of the annual average generation efficiency, and Fig. 7(b) shows the result of the calorific power of the city gas fuel consumed each year. However, the fuel quantity consumed by a boiler is not included in Fig. 7(b). The average generation efficiency in the case of PEM-FC and NEG of 5 kW peaks in this case, as shown in Fig. 7(a). The generation efficiency in this case is 1.37 compared with the PEM-FC individual system and is 1.28 compared with the NEG individual system. This difference is equivalent to the difference in the fuel consumption shown in Fig. 7(b). The conventional method illustrated is the value where the presumed value in a thermal power station is power generation efficiency of 40%, while transmission loss is not included. Since the power generation efficiency of the proposed power system is lower than a thermal power station, there is a considerable fuel amount of heat for the proposed system and it is difficult to improve the power generation efficiency of the independent distributed power supply rather than that of a conventional system. An independent distributed power supply has the economic advantages of low power transmission loss and waste heat loss. Therefore, when exhaust heat can be used effectively, or when the distance between a power plant and demand area is considerable, the merit of an independent source becomes clear. However, it is difficult for the proposed system to be economically feasible. Generation efficiency is higher with the installation of 10-kW NEG rather than that of 10-kW PEMFC. If the energy-demand pattern shown in Fig. 6 is installed into 10-kW PEM-FC or 10-kW NEG, the generation efficiency of PEM-FC will fall compared with NEG. Therefore, a system (NEG) with sufficient generation efficiency at low load has a stronger influence than one (PEM-FC) with high generation efficiency at high load.
0kW PEM-FC, 10kW NEG 3kW PEM-FC, 7kW NEG 5kW PEM-FC, 5kW NEG 7kW PEM-FC, 3kW NEG 10kW PEM-FC, 0kW NEG
25
Boiler capacity [kW]
4336
20 15 10 5 0 0
100
200 300 Heating storage capacity [MJ]
400
Fig. 9. Analysis result of the heating storage capacity and boiler capacity when satisfying a heat balance.
5.2. Heat storage tank and boiler capacity Fig. 8 shows the analysis result of the exhaust heat of a representative day every month. The system with the most exhaust heat is 10-kW NEG (Fig. 8(a)), while 10-kW PEM-FC produces the least, due to the difference in the Operation Area A of the system operation map, as shown in Fig. 5. In other words, although the increase in exhaust heat is lost due to the increase in the production of electricity of PEM-FC, NEG changes rapidly. For this reason, when the power load alters sharply, the exhaust heat characteristics of PEM-FC and NEG differ considerably. Based on the result of Fig. 8, the amount of exhaust heat increases, so that the NEG capacity does the same. Moreover, since there is little heat demand in the summer season, there is considerable surplus exhaust heat. Fig. 9 shows the analysis result of the investigation into the relation between heat storage tank capacity and boiler capacity. Since exhaust heat characteristics also vary according to the capacity of PEM-FC and NEG, the optimum heat storage and boiler capacity differ for each system. As shown in the example of Fig. 5(c), there is little exhaust heat of PEM-FC and NEG that composes HCGS compared with Figs. 5(a) and (b). This is attributable to the low capacity of PEM-FC and NEG. Consequently, periods of time when the exhaust heat of HCGS and the heat supply of a time shift of the heat storage tank do not satisfy heat demand appear. The boiler capacity installed into HCGS is smallest in the order of 5-kW PEM-FC and 5-kW NEG, 3-kW PEM-FC and 7-kW NEG, and 7-kW PEM-FC and 3-kW NEG, respectively. This order is the same as that of the average generation efficiency described in Fig. 7(a). In other words, if generation efficiency is high, the amount of exhaust heat will decrease, and periods of heat shortage emerge. 5.3. Carbon dioxide emissions Fig. 10 shows the analysis result of the carbon dioxide emissions of a representative day every month. The carbon dioxide emissions of the boiler are included in Figs. 10(b), (c), and (d).
Amount of discharge [kg/Hour]
S. Obara, I. Tanno / International Journal of Hydrogen Energy 32 (2007) 4329 – 4339
30 20 10 0 January
March
Amount of discharge [kg/Hour]
February
Amount of discharge [kg /Hour]
July June
September August
November
October
December
40 20 0 March
February
May April
July June
September August
November
October
December
50 30 10 January
March
February Amount of discharge [kg /Hour]
May April
60
January
May April
July June
September August
November
October
December
50 40 30 20 10 January
March
February Amount of discharge [kg/Hour]
4337
May April
July June
September August
November
October
December
32 28 24 20 16 January
March
February
May April
July June
September August
November
October
December
Fig. 10. Result of the carbon dioxide emissions of an every month representation day: (a) PEM-FC 0 kW, NEG 10 kW; (b) PEM-FC 3 kW, NEG 7 kW; (c) PEM-FC 5 kW, NEG 5 kW; (d) PEM-FC 7 kW, NEG 3 kW; (e) PEM-FC 10 kW, NEG 0 kW.
On the other hand, in Figs. 10(a) and (e), heat demand can be fulfilled by storing the exhaust heat of HCGS. Since a large cooling load is added to the system in the summer season, carbon dioxide emissions are considerable when compared with other seasons. Although HCGS (Figs. 10(b), (c) and (d)) is accompanied by the boiler operation, there is a greater amount of
carbon dioxide discharged by power generation than that discharged by the boiler operation. Fig. 11 shows the carbon dioxide emissions of the conventional method (commercial power and boiler). Moreover, Fig. 12 shows the result of the annual carbon dioxide emissions using the conventional method and the proposed system. The amount of carbon dioxide emissions
S. Obara, I. Tanno / International Journal of Hydrogen Energy 32 (2007) 4329 – 4339
Amount of discharge [kg/Hour]
4338
20 16 12 8 4 0 January
March
February
May April
July June
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December
Amount of discharge [ton/Year]
Fig. 11. Result of annual carbon dioxide emissions for conventional method.
300
287 ton/Year 207 ton/Year
200
150 ton/Year
146 ton/Year
167 ton/Year
100 37 ton/Year
0 kW 10 C W -F 0k M G PE NE
W 7k C -F 3kW M PE NEG
W 5k C -F 5kW M PE NEG
W 3k C -F 7kW M PE NEG
W 0k C W -F 0k M 1 PE G NE l na tio en d nv tho Co me
Fig. 12. Result of annual carbon dioxide emissions.
using the conventional method is calculated based on “the investigative commission report of the calculation method of the amount of greenhouse gas discharge (the Ministry of the Environment in Japan, August, 2003)”. There is a peak of 10-kW PEM-FC in the result of Fig. 12. This is because of the considerable load fluctuation of power and the considerable operation frequency of a low-load region in a general house, while the reduced carbon dioxide emissions of a system depend more on a system (NEG) with better operation characteristics under partial load than one (PEM-FC) with a large maximum efficiency point.
(2) The capacity of PEM-FC and NEG that constitute HCGS was changed, and the relationship between the heating storage capacity and boiler capacity was clarified. Since the exhaust heat decreases to enhance the average generation efficiency, a large boiler capacity is required. (3) NEG independent systems produce a small annual amount of carbon dioxide emission. When inputting the energydemand pattern of a general house into HCGS, the carbon dioxide emission characteristics in a partial-load region at low load have a significant influence.
6. Conclusions
Acknowledgment
The PEM-FC and NEG hybrid cogeneration system (HCGS) equipped with a solid polymer membrane-type fuel cell (PEMFC) at base load operation and the hydrogen mixing gas engine (NEG) of a load that follows operation were examined. In this paper, generation efficiency, carbon dioxide emissions, and the optimal capacity of the boiler and heat storage tank were investigated regarding the installation of HCGS into 10 household apartment houses in Tokyo. Consequently, the following conclusions were obtained:
This work was partially supported by a Grant-in-Aid for Scientific Research (C) from JSPS.KAKENHI (17510078).
(1) When the capacity of PEM-FC and NEG that composes HCGS was changed and the annual average generation efficiency was investigated, the case of each remaining at 5 kW was good at 27.3%. This value was 1.37 times that of the PEM-FC individual system, and 1.28 times that of the NEG individual system, respectively.
References [1] Ali M, et al. Development of highly efficient and clean engine system using natural-gas and hydrogen mixture fuel obtained from onboard reforming. NEDO Report ID:03B71006c; 2005 [in Japanese]. [2] Yap D, et al. Effect of hydrogen addition on natural gas HCCI combustion. SAE Technical Paper Series, SAE-2004-01-1972; 2004. [3] Larsen JF, Wallace JS. Comparison of emissions and efficiency of a turbocharged lean-burn natural gas and hythane-fuelled engine. ASME ICE 1995;24:31–40. [4] Swain MR. Usuf MJ. The effects of hydrogen addition on natural gas engine operation. SAE Technical Paper Series, SAE-932775, 1993. [5] Nakamura T, Sei M. Energy related technology. High-efficiency fuel processor for fuel cell system. Technical Report, No. 77, Matsushita Electric Works, Ltd.; 2002, p. 4–9 [in Japanese].
S. Obara, I. Tanno / International Journal of Hydrogen Energy 32 (2007) 4329 – 4339 [6] Oda K. et al. A small-scale reformer for fuel cell application. Sanyo Tech Rev 1999;31(2):99–106 [in Japanese]. [7] Takeda Y. et al. Development of fuel processor for rapid start-up. In: Kimura K, editor. Proceedings 20th energy system economic and environment conference. Tokyo, 2004. p. 343–4 [in Japanese]. [8] Mikkola M. Experimental studies on polymer electrolyte membrane fuel cell stacks. Master’s thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in Technology, Helsinki University of Technology; 2001. p. 58–79. [9] Ibaraki Prefecture Government Office of Education. Modeling of hydrogen energy system. High school active science project research report, Ibaraki, Japan; 2002. [in Japanese].
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[10] Architectural Institute of Japan. The nationwide research study concerning the energy consumption in the house in the 2001 fiscal year, vol. 3; 2002. p. 3–6 [in Japanese]. [11] Andrew W, Steven F. The nature of domestic electricity-loads and effects of time averaging on statistics and on-site generation calculations. Appl Energy 2007;84(4):389–403. [12] Adam H, Matthew L. Impacts of temporal precision in optimization modeling of micro-combined heat and power. Energy 2005;30(10): 1759–79.
Study on capacity optimization of PEM fuel cell and hydrogen mixing gas-engine compound generator Shin’ya Obara a,∗ , Itaru Tanno b a Department of Mechanical Engineering, Ichinoseki National College of Technology, Takanashi, Hagisho, Ichinoseki, Iwate 021-8511, Japan b Department of Mechanical Engineering, Tomakomai National College of Technology, 443 Nishikioka, Tomakomai, Hokkaido 0591275, Japan
Received 4 December 2006; received in revised form 10 May 2007; accepted 10 May 2007 Available online 27 June 2007
Abstract Development of a small-scale power source not dependent on commercial power may result in various effects. For example, it may eliminate the need for long distance power-transmission lines, and mean that the amount of green energy development is not restricted to the dynamic characteristics of a commercial power grid. Moreover, the distribution of the independent energy source can be optimized with regionality in mind. This paper examines the independent power supply system relating to hydrogen energy. Generally speaking, the power demand of a house tends to fluctuate considerably over the course of a day. Therefore, when introducing fuel cell cogeneration into an apartment house, etc., low-efficiency operations in a low-load region occur frequently in accordance with load fluctuation. Consequently, the hybrid cogeneration system (HCGS) that uses a solid polymer membrane-type fuel cell (PEM-FC) and a hydrogen mixture gas engine (NEG) together to improve power generation efficiency during partial load of fuel cell cogeneration is proposed. However, since facility costs increase, if the HCGS energy cost is not low compared with the conventional method, it is disadvantageous. Therefore, in this paper, HCGS is introduced into 10 household apartments in Tokyo, and the power generation efficiency, carbon dioxide emissions and optimal capacity of a boiler and heat storage tank are investigated through analysis. Moreover, the system characteristics change significantly based on the capacity of PEM-FC and NEG that compose HCGS. Therefore, in this study, the capacity of PEM-FC and that of NEG are investigated, as well as the power generation efficiency, carbon dioxide emissions and the optimal capacity of a boiler and heat storage tank. Analysis revealed that the annual average power generation efficiency when the capacity of PEM-FC and NEG is 5 kW was 27.3%. Meanwhile, the annual average power generation efficiency of HCGS is 1.37 times that of the PEM-FC independent system, and 1.28 times that of the NEG independent system, respectively. 䉷 2007 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. Keywords: PEM fuel cell; Hydrogenation gas engine; Compound generator; Cogeneration; Partial load; Operation planning
1. Introduction Cogeneration using a solid polymer membrane-type fuel cell (PEM-FC) has a high maximum power generation efficiency compared with other power plants. However, reduced power generation efficiency during partial-load operation of a system with a reformer remains an issue. In particular, in general houses, partial-load operations with significant fluctuation of power load and low efficiency occur frequently. On the other hand, hydrogen mixing technology is studied, concerning
∗ Corresponding author. Tel./fax: +81 191 24 4835.
E-mail addresses: [email protected], [email protected] (S. Obara), [email protected] (I. Tanno).
exhaust gas cleanup and improved efficiency at the time of partial load of gas-engine cogeneration [1–4]. If the hydrogen rate of fuel is increased when the load is small, studies confirm that exhaust cleanup and brake thermal efficiency improve [1–4]. Although the facility cost of a hydrogen/gas hybrid engine (NEG) cogeneration is low compared with PEM-FC, the maximum power generation efficiency is inferior. However, if the partial-load characteristics of NEG are better compared with PEM-FC, the introduction of PEM-FC to a house may be disadvantageous. Therefore, in this paper, a PEM-FC and NEG hybrid cogeneration system (HCGS) having the maximum power generation efficiency of PEM-FC and the partial-load characteristics of NEG is proposed. In HCGS, PEM-FC is operated corresponding to a base load, and NEG is operated corresponding to a fluctuating load. In this case, since PEM-FC operates
0360-3199/$ - see front matter 䉷 2007 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2007.05.003
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Nomenclature C C E g q R t U
N R S
capacity, kW maximum load, kW power, kW CO2 emission, g/(s kW) primary power input, kW load factor, % sampling time flow rate, m3 /s
Equipment C/O CGS DC/AC FC G/T HCGS I/T NEG PEM-FC Vb
Greek symbols
efficiency, %
Subscripts B F
gas engine generator reformer boiler
burner installed in a reformer fuel cell
carbon monoxide oxidation equipment cogeneration DC–AC converter fuel cell generator hybrid cogeneration inverter gas engine cogenerator solid polymer membrane-type fuel cell valve
Controller
City gas system (1)
Vb1 Vb4
City gas system (2)
Vb2 City gas system (3)
Vb3
Reformer Burner
Compressor
PEM-FC
Vb5
To heat storage tank
G/T
To heat storage tank C/O
Dryer
Gas engine (NEG)
I/T Power output
DC /AC
To heat storage tank Exhaust heat input Boiler
Heat output
Tap water Heat storage tank City gas system (4) Fig. 1. HCGS block diagram.
at a constant load, the capacity of the fuel cell is designed to be optimal and can always be operated at high power generation efficiency. On the other hand, NEG is operated corresponding to fluctuating loads except for the base load. Therefore, a low-load operating range, where PEM-FC is disadvantageous, can correspond to NEG. The facility cost of HCGS increases compared with the case where PEM-FC or NEG is operated independently. Therefore, in order for the introduction of HCGS to city areas to be effective, compared with the individual operation of PEM-FC or NEG, the power cost and carbon dioxide emissions must be advantageous. However, the power generation efficiency and the carbon dioxide emissions of PEM-FC, NEG, and HCGS are dependent on the load pattern of a building. Moreover, since the exhaust heat characteristics differ according to PEM-FC, NEG, or HCGS, the optimal capacity of the heat storage tank and boiler of each system also varies, while the power generation efficiency and exhaust heat characteristics of HCGS change with the capacity of PEM-FC and NEG to be introduced. Therefore, in this paper, the relations among the “capacity of PEM-FC and NEG”, “power generation efficiency”,
“carbon dioxide emissions”, and “the optimal capacity of a boiler and a heat storage tank” are investigated with reference to the introduction of HCGS into 10 household apartment houses in Tokyo. 2. HCGS scheme A block diagram of HCGS proposed in this paper is shown in Fig. 1. City gas includes supply systems to NEG, to a reformer, to a heat source burner, and to a boiler, respectively, where the Japanese conditions apply to the composition of “city gas”. Accordingly, “city gas” consists mainly of methane. The extent of the HCGS city gas consumption is calculated by Utotal,t = UN,t + UR,t + UB,t + US,t .
(1)
The operation model of HCGS is shown in Fig. 2(a). In the operation of HCGS, PEM-FC is used for a base load and NEG is used for other fluctuating loads. Therefore, a high load of PEMFC is always highly efficient, and it can be operated. CN and CF in this figure express the maximum electric load of NEG and
S. Obara, I. Tanno / International Journal of Hydrogen Energy 32 (2007) 4329 – 4339
Fluctuating load (NEG)
Base load (FC) 0
6
12 Time[Hour]
18
24
PEM-FC operation
HCGS operation
Power generation efficiency
Power consumption [kW]
FC maximum power generation efficiency operation
4331
Operation of NEG with hydrogenation
0
20
40 60 80 Load factor R [%]
100
120
Fig. 2. HCGS operation method: (a) base load and fluctuating load operation, (b) efficiency characteristic of HCGS.
16
6
CO2 emission [g/s]
City-gas
3 2
12 0.6 8
Total
4
0.2
Hydrogen
1
0.4
Engine emission
Burner
Reforming 0.0
0 0
0 0
0
10 15 5 Production of electricity [kW] 20
40 60 Load factor R [%]
80
0
5 10 15 Production of electricity [kW] 20
40 60 Load factor R [%]
80
100
(R ≥ 60 %) g'N = 7×10−7 R3 − 0.0004R2 + 0.0601R − 2.11 (R < 60 %) g'N = 3×10−6 R2 + 0.0027 R + 0.00346
100
90 80 70 60
NEG capacity= 10kW 7kW 5kW
20
Total efficiency 3kW
40 Output [kW]
Power generation efficiency [%]
25
15 10
30 Heat output (Cooling hotwater and exhaust)
20
Overall efficiency [%]
Fuel consumption [g/s]
4
CO2 emission [g/(skW)]
0.8 5
Power output (Generating end)
10 5 0 0
20
40 60 80 Load factor R [%]
100
0
10
20 30 40 Fuel supply [kW]
50
Fig. 3. Output characteristics of the hydrogen mixing gas engine cogenerator [1]: (a) amount of CH4 and hydrogen mixing when getting the maximum thermal efficiency, (b) CO2 emission characteristics of NEG, (c) relation between a production of electricity and gross power generation efficiency, (d) output characteristics of 10-kW NEG.
PEM-FC, respectively. It is necessary to determine the capacity of NEG and PEM-FC (CN , CF ) introduced into HCGS with a value exceeding CN and CF of the figure. Since the load factor
(load/capacity) changes according to the capacity setup of CN and CF , the fuel consumption and carbon dioxide emissions of HCGS also change. So, in this paper, the fuel consumption and
85 Total efficiency
75
Output [kW]
12
65 Heat output (Reformer and cell stack)
8
4 Power output (Generating end)
F Efficiency
30
2.0
20 gF CO2 total '
1.0
10 Reforming CO2 Burner CO2
0.0
0 0
40 60 80 100 120 Load factor R [%] F = −0.0028 R2 + 0.598 R + 0.150 g'F = −3×10−6 R3 + 0.0009R2 − 0.0914R + 4.174
0 10
12
14 16 Fuel supply [kW]
60
80 Load factor R [%]
3.0
CO2 emission g'F [g/(skW)]
16
Power generation efficiency F [%]
S. Obara, I. Tanno / International Journal of Hydrogen Energy 32 (2007) 4329 – 4339
Overall efficiency [%]
4332
18
20
100
Fig. 4. Output characteristics of the PEM-FC [8,9]: (a) output characteristics of 5-kW PEM-FC, (b) the characteristics model of the load factor of a PEM-FC with reformer, and power generation efficiency. The area of the electrode including the anode and cathode of the fuel cell stack is 1 m2 , respectively, and the reformer efficiency is 73%.
carbon dioxide emissions when changing the value (relatively) of CN and CF are investigated. Fig. 2(b) shows a model showing the relation of the load factor and power generation efficiency of NEG and PEM-FC. In the low-load operating range of NEG, an improvement in brake thermal efficiency is expected by increasing the hydrogen concentration of fuel [1]. However, there is no effect of hydrogen mixing in the high-load operating range of NEG. Moreover, as shown in Fig. 2(b), compared with PEM-FC, the maximum efficiency point of NEG is low. Therefore, operation in a low-load zone of NEG is advantageous, and the same applies to operation in a high-load zone of PEM-FC. Since HCGS has the characteristics of the power generation efficiency of PEM-FC and NEG, it is expected to be comparable to the individual operation of PEM-FC or NEG, in which power generation efficiency is advantageous.
meanwhile, shows the carbon dioxide emission characteristics of NEG, calculated based on the reference examination result [1]. In this figure, the approximate expression showing the relation between load factor R and CO2 emissions is shown. These approximate expressions differ bordering on 60% of the load factor, because the fuel supplied to NEG has a high hydrogen rate in a low-load range and the rate of city gas is high within a high-load range. In a low-load range, the carbon dioxide ratio discharged by city gas reforming for hydrogen manufacture and the burner for reformers is high. On the other hand, in a high-load range, the rate of carbon dioxide generated in engine city gas burning is high. Fig. 3(c) shows the relationship between the load factor and the generation efficiency of NEG. Although reformed gas is supplied to NEG, reformer efficiency is included in the power generation efficiency shown in this figure. Eq. (2) defines reformer efficiency.
3. Equipment characteristics
R = (qH2 /qCH4 ) × 100(%),
3.1. Output characteristics of NEG
where qH2 is the calorific power of hydrogen in reformed gas, and qCH4 expresses the calorific power of the city gas supplied to a reformer, while the calorific power of the city gas for reforming and the city gas for heat-source burners is included in qCH4 . In this paper, R was set at 73% [5–7]. Generally, engine thermal efficiency increases, so that the capacity expands. Consequently, as shown in Fig. 3(c), the NEG power generation efficiency characteristics are separated into each capacity of NEG, which are then introduced into the analysis. Fig. 3(d) shows the power and heat output of 10-kW NEG, and a model of overall efficiency. The heat outputs are engine exhaust, cooling water, and reformer exhaust heat. However, the overall efficiency in this figure assumes a case where all the power and heat outputted by NEG are consumed. Introduced in
Past examination results are used for the output characteristics of a city gas engine with hydrogen mixing [1]. The examined hydrogen mixing engine is 857 cc in a single cylinder, and lean burn is enabled by injecting reformed gas into an inlet pipe. Fig. 3(a) shows the model of the city gas consumption (CH4 ) of NEG, the amount of hydrogen mixing, and the production of electricity. These characteristics were calculated from the hydrogen mixing rate, based on the reference examination result [1]. In Fig. 3(a), when the electricity production exceeds 14 kW, the amount of hydrogen mixing is zero, because high thermal efficiency can be obtained, even if there is no hydrogen mixing within the large operating range of engine power. Fig. 3(b),
(2)
S. Obara, I. Tanno / International Journal of Hydrogen Energy 32 (2007) 4329 – 4339
the Case Study are 3- to 10-kW NEG. The difference between the generation efficiency and the carbon dioxide emissions of 3-kW NEG and 10-kW NEG is about 6%. Fig. 5(b) described in Section 3.3 explains the exhaust heat of the NEG.
4333
60
76
80
Production of heat [kW]
72
3.2. Output characteristics of PEM-FC Fig. 4(a) shows a model of the output characteristics and overall efficiency of 5-kW PEM-FC with a city gas reformer [8,9]. The heat output includes the exhaust heat of the reformer and cell stack. Moreover, the power output is the value of the inverter outlet. Overall efficiency assumes the case where all of the power and heat to output are consumed. Fig. 4 (b) shows the model of the carbon dioxide emissions and generation efficiency of PEM-FC of Fig. 4(a). The carbon dioxide discharged by the operation of PEM-FC is based on a reforming reaction (Eqs. (3) and (4)) and city gas burning of a reformer burner (Eq. (5)). The power generation efficiency F of Fig. 4(b) was calculated from Eq. (6). EF,t , on the right-hand side of Eq. (6), shows the power of the inverter outlet, qR,CH4 ,t shows the calorific power of CH4 supplied to the reformer, and qB,CH4 ,t shows the calorific power of CH4 supplied to the heat-source burner of the reformer. The maximum generation efficiency of the PEM-FC model shown in Fig. 4(b) is 32%.
68
50
Primary po
64
wer input [k
60
W]
56
40
52 48 44
30
40 36
Operation area B
32
20
24 20 16
10
12
28
ar Heat out put ch
act eri sti c in Ar
ea A
Operation area A
0 2
0
6 4 8 Production of electricity [kW]
10
60
85
Production of heat [kW]
75
50
Primary
80
70 power
65
input [k
W]
40
60 55 50
Operation area B
30
45 40
35 30
20
25 20 15
10
Heat
10
ut c ou t p
c te ha r a
r ist i
c in
A re
aA
Operation area A
5
CO + H2 O → CO2 + H2 + 41
kJ/mol,
0 2
0
kJ/mol,
CH4 + 2O2 → CO2 + 2H2 O + 802
(3)
kJ/mol,
F,t = {EF,t /(qR,CH4 + qB,CH4 ,t )} × 100.
(4) (5)
In this paper, the chart showing the city gas calorific power (consumption) supplied to HCGS and the relationship between the production of electricity and the heat output of a system is defined as an “operation map”. The fuel calorific power in Fig. 5 has a lower calorific value. Fig. 5(a) shows the operation map of a 10-kW PEM-FC, and Fig. 5(b) shows the operation map of the 10-kW NEG, respectively. Overall efficiency is calculable because the calorific power of the supply fuel is shown in Fig. 5. The Operation Area A of each figure is the production range of electricity and exhaust heat output when operating PEM-FC or NEG independently. In Area A, when the production of electricity is decided, heat output as shown in the figure occurs. Operation Area B, meanwhile, indicates the range of production of electricity and the heat output (the amount of exhaust heat, and boiler output) when operating a boiler in addition to PEM-FC or NEG. However, the boiler efficiency of Operation Area B was set at 0.9. The operation map describes primary input power (based on city gas LHV, described as city gas consumption below) consumed by a system. When the operation maps of PEM-FC (Fig. 5(a)) and NEG (Fig. 5(b)) are compared, the NEG of Operation Area A is wider. Furthermore, in the Operation Area A of NEG, if the electricity production increases, exhaust heat output will also increase rapidly, but
10
12
NEG operation
38
50 36 48 P rim 34 a ry 46 p ow 32 44 er in put 30 [kW ] 28 Operation area D 26 42 40 24 22 38 36 20 34 t ic 32 18 e r is t c 30 Operation area B ara 16 28 t ch n d C 14 26 tp u a A a 24 u o e 12 t 22 r He a i n A 20 10
(6)
3.3. System operation map
6 4 8 Production of electricity [kW]
PEM-FC operation 30 Production of heat [kW]
CH4 + H2 O → CO + 3H2 − 206
25 20 15 10 5
6
8
Operation area A
Operation area C
0 0
1
2
3 4 7 5 6 Production of electricity [kW]
8
9
10
Fig. 5. Operation map; (a) 10-kW PEM-FC, (b) 10-kW NEG, (c) 10-kW HCGS.
in PEM-FC, even if the production of electricity increases, the increase in exhaust heat output is lost. Fig. 5(c) shows the operation map when introducing 5-kW PEM-FC and 5-kW NEG into HCGS. In HCGS, PEM-FC corresponds to the base load and NEG is operated corresponding to a fluctuating load. The maximum power generation efficiency point of PEM-FC approaches the maximum load rate, as shown in Fig. 4(b). On the other hand, as shown in Fig. 3, NEG shows positive power generation efficiency and exhaust gas characteristics at low load. As for the installation of low-load zone operation of NEG,
10
100
8
80
6
60
4
40
2
20
0
Load factor [%]
S. Obara, I. Tanno / International Journal of Hydrogen Energy 32 (2007) 4329 – 4339
Power [kW]
4334
0 January
March
February
May April
July June
September November August
October
December
Heat [kW]
30 20 10 0 January
March
February
May April
July June
September November August
October
December
Efficiency [%]
Fig. 6. Power (a) and heat (b) demand for 10 houses apartment in Tokyo.
30
27.3%
26.7%
24.7%
21.4%
20.0%
20 10 0
Fuelcalorific power [GJ/ Year]
PEM-FC 0kW PEM-FC 3kW PEM-FC 5kW PEM-FC 7kW PEM-FC10kW NEG 10kW NEG 7kW NEG 5kW NEG 3kW NEG 0kW
800
630GJ/ Year
588GJ/Year
600 400
473GJ/ Year
462GJ/ Year
510GJ/ Year
315GJ/Year
200 0 kW 10 C -F W M 0k PE G NE kW C7 -F W M 3k PE EG N
kW C5 -F W M 5k PE G NE W 3k C W -F k M G7 PE NE
W 0k C W -F k M G0 PE NE l cia ) er m r % m we 40 Co po ncy ie fic
f (E
Fig. 7. Result of annual average power-generation efficiency (a) and fuel consumption (b).
high effectiveness is expected if the generation efficiency and the load factor of PEM-FC and NEG have the relationship as shown in the model in Fig. 2(b). Therefore, the power generation efficiency of HCGS is predicted to improve when compared to circumstances where PEM-FC or NEG is introduced independently.
4. Case study Fig. 6 shows a power and heat demand model for the representative days each month of 10 residence apartment houses in Tokyo and indicates the average load of each sampling time [10]. However, the actual power-demand pattern is a set of
S. Obara, I. Tanno / International Journal of Hydrogen Energy 32 (2007) 4329 – 4339
4335
Heat [kW]
20 15 10 5 January
March
Heat [kW]
February
May April
July June
September August
November
October
December
15 10 5 January
March
Heat [kW]
February
May April
July June
September August
November
October
December
16 14 12 10 8 6 January
March
February
May April
July June
September August
November
October
December
Heat [kW]
14 12 10 8 January
March
February
May April
July June
September August
November
October
December
Heat [kW]
14 13 12 11 10 January
March
February
May April
July June
September August
November
October
December
Fig. 8. Result of the exhaust heat output of an every month representation day: (a) PEM-FC 0 kW, NEG 10 kW; (b) PEM-FC 3 kW, NEG 7 kW (c) PEM-FC 5 kW, NEG 5 kW; (d) PEM-FC 7 kW, NEG 3 kW; (e) PEM-FC 10 kW, NEG 0 kW.
loads that change rapidly within a short time, such as an inrush current. Since a cooling load is included in the electric power demand, there is considerable power demand in the summer season. The annual power demand amount of 10 houses is 57.6 MWh, while the heat demand is 86 GJ and the load
factor of the power every month is shown in Fig. 6. However, when the supply demand balance is calculated by averaging power demand with inrush current, the result will be estimated to be far smaller [11,12]. The amount of city gas consumption is obtained using the operation map of the system shown in
S. Obara, I. Tanno / International Journal of Hydrogen Energy 32 (2007) 4329 – 4339
Fig. 5 for every time the power and heat demand model shown in Fig. 6 is sampled. However, when the exhaust heat output of a system exceeds heat demand, surplus heat is stored in a heat storage tank. On the other hand, when the exhaust heat is less than the heat demand, heat is supplied from a heat storage tank and when the heat still remains insufficient, it is output by a boiler. When exhaust heat exceeds the capacity of the heat storage tank, the excess amount is released using a radiator. In the analysis of this paper, the daily radiation loss of the heat storage tank becomes 5%. Generation efficiency is calculated by dividing the power demand amount by the calorific power of the consumed city gas fuel. Moreover, the value that divides the value adding the production of electricity and the amount of heat output by the calorific power of the city gas fuel supplied to a system is defined as overall efficiency. The carbon dioxide emissions of the system for every sampling time are obtained by giving a load factor to Figs. 3(b) and 4(b), while total carbon dioxide emissions are calculated by adding these values and the amount of carbon dioxide discharged by the boiler. 5. Analysis results and discussions 5.1. Generation efficiency and fuel consumption Fig. 7(a) shows the analysis output of the annual average generation efficiency, and Fig. 7(b) shows the result of the calorific power of the city gas fuel consumed each year. However, the fuel quantity consumed by a boiler is not included in Fig. 7(b). The average generation efficiency in the case of PEM-FC and NEG of 5 kW peaks in this case, as shown in Fig. 7(a). The generation efficiency in this case is 1.37 compared with the PEM-FC individual system and is 1.28 compared with the NEG individual system. This difference is equivalent to the difference in the fuel consumption shown in Fig. 7(b). The conventional method illustrated is the value where the presumed value in a thermal power station is power generation efficiency of 40%, while transmission loss is not included. Since the power generation efficiency of the proposed power system is lower than a thermal power station, there is a considerable fuel amount of heat for the proposed system and it is difficult to improve the power generation efficiency of the independent distributed power supply rather than that of a conventional system. An independent distributed power supply has the economic advantages of low power transmission loss and waste heat loss. Therefore, when exhaust heat can be used effectively, or when the distance between a power plant and demand area is considerable, the merit of an independent source becomes clear. However, it is difficult for the proposed system to be economically feasible. Generation efficiency is higher with the installation of 10-kW NEG rather than that of 10-kW PEMFC. If the energy-demand pattern shown in Fig. 6 is installed into 10-kW PEM-FC or 10-kW NEG, the generation efficiency of PEM-FC will fall compared with NEG. Therefore, a system (NEG) with sufficient generation efficiency at low load has a stronger influence than one (PEM-FC) with high generation efficiency at high load.
0kW PEM-FC, 10kW NEG 3kW PEM-FC, 7kW NEG 5kW PEM-FC, 5kW NEG 7kW PEM-FC, 3kW NEG 10kW PEM-FC, 0kW NEG
25
Boiler capacity [kW]
4336
20 15 10 5 0 0
100
200 300 Heating storage capacity [MJ]
400
Fig. 9. Analysis result of the heating storage capacity and boiler capacity when satisfying a heat balance.
5.2. Heat storage tank and boiler capacity Fig. 8 shows the analysis result of the exhaust heat of a representative day every month. The system with the most exhaust heat is 10-kW NEG (Fig. 8(a)), while 10-kW PEM-FC produces the least, due to the difference in the Operation Area A of the system operation map, as shown in Fig. 5. In other words, although the increase in exhaust heat is lost due to the increase in the production of electricity of PEM-FC, NEG changes rapidly. For this reason, when the power load alters sharply, the exhaust heat characteristics of PEM-FC and NEG differ considerably. Based on the result of Fig. 8, the amount of exhaust heat increases, so that the NEG capacity does the same. Moreover, since there is little heat demand in the summer season, there is considerable surplus exhaust heat. Fig. 9 shows the analysis result of the investigation into the relation between heat storage tank capacity and boiler capacity. Since exhaust heat characteristics also vary according to the capacity of PEM-FC and NEG, the optimum heat storage and boiler capacity differ for each system. As shown in the example of Fig. 5(c), there is little exhaust heat of PEM-FC and NEG that composes HCGS compared with Figs. 5(a) and (b). This is attributable to the low capacity of PEM-FC and NEG. Consequently, periods of time when the exhaust heat of HCGS and the heat supply of a time shift of the heat storage tank do not satisfy heat demand appear. The boiler capacity installed into HCGS is smallest in the order of 5-kW PEM-FC and 5-kW NEG, 3-kW PEM-FC and 7-kW NEG, and 7-kW PEM-FC and 3-kW NEG, respectively. This order is the same as that of the average generation efficiency described in Fig. 7(a). In other words, if generation efficiency is high, the amount of exhaust heat will decrease, and periods of heat shortage emerge. 5.3. Carbon dioxide emissions Fig. 10 shows the analysis result of the carbon dioxide emissions of a representative day every month. The carbon dioxide emissions of the boiler are included in Figs. 10(b), (c), and (d).
Amount of discharge [kg/Hour]
S. Obara, I. Tanno / International Journal of Hydrogen Energy 32 (2007) 4329 – 4339
30 20 10 0 January
March
Amount of discharge [kg/Hour]
February
Amount of discharge [kg /Hour]
July June
September August
November
October
December
40 20 0 March
February
May April
July June
September August
November
October
December
50 30 10 January
March
February Amount of discharge [kg /Hour]
May April
60
January
May April
July June
September August
November
October
December
50 40 30 20 10 January
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February Amount of discharge [kg/Hour]
4337
May April
July June
September August
November
October
December
32 28 24 20 16 January
March
February
May April
July June
September August
November
October
December
Fig. 10. Result of the carbon dioxide emissions of an every month representation day: (a) PEM-FC 0 kW, NEG 10 kW; (b) PEM-FC 3 kW, NEG 7 kW; (c) PEM-FC 5 kW, NEG 5 kW; (d) PEM-FC 7 kW, NEG 3 kW; (e) PEM-FC 10 kW, NEG 0 kW.
On the other hand, in Figs. 10(a) and (e), heat demand can be fulfilled by storing the exhaust heat of HCGS. Since a large cooling load is added to the system in the summer season, carbon dioxide emissions are considerable when compared with other seasons. Although HCGS (Figs. 10(b), (c) and (d)) is accompanied by the boiler operation, there is a greater amount of
carbon dioxide discharged by power generation than that discharged by the boiler operation. Fig. 11 shows the carbon dioxide emissions of the conventional method (commercial power and boiler). Moreover, Fig. 12 shows the result of the annual carbon dioxide emissions using the conventional method and the proposed system. The amount of carbon dioxide emissions
S. Obara, I. Tanno / International Journal of Hydrogen Energy 32 (2007) 4329 – 4339
Amount of discharge [kg/Hour]
4338
20 16 12 8 4 0 January
March
February
May April
July June
September August
November
October
December
Amount of discharge [ton/Year]
Fig. 11. Result of annual carbon dioxide emissions for conventional method.
300
287 ton/Year 207 ton/Year
200
150 ton/Year
146 ton/Year
167 ton/Year
100 37 ton/Year
0 kW 10 C W -F 0k M G PE NE
W 7k C -F 3kW M PE NEG
W 5k C -F 5kW M PE NEG
W 3k C -F 7kW M PE NEG
W 0k C W -F 0k M 1 PE G NE l na tio en d nv tho Co me
Fig. 12. Result of annual carbon dioxide emissions.
using the conventional method is calculated based on “the investigative commission report of the calculation method of the amount of greenhouse gas discharge (the Ministry of the Environment in Japan, August, 2003)”. There is a peak of 10-kW PEM-FC in the result of Fig. 12. This is because of the considerable load fluctuation of power and the considerable operation frequency of a low-load region in a general house, while the reduced carbon dioxide emissions of a system depend more on a system (NEG) with better operation characteristics under partial load than one (PEM-FC) with a large maximum efficiency point.
(2) The capacity of PEM-FC and NEG that constitute HCGS was changed, and the relationship between the heating storage capacity and boiler capacity was clarified. Since the exhaust heat decreases to enhance the average generation efficiency, a large boiler capacity is required. (3) NEG independent systems produce a small annual amount of carbon dioxide emission. When inputting the energydemand pattern of a general house into HCGS, the carbon dioxide emission characteristics in a partial-load region at low load have a significant influence.
6. Conclusions
Acknowledgment
The PEM-FC and NEG hybrid cogeneration system (HCGS) equipped with a solid polymer membrane-type fuel cell (PEMFC) at base load operation and the hydrogen mixing gas engine (NEG) of a load that follows operation were examined. In this paper, generation efficiency, carbon dioxide emissions, and the optimal capacity of the boiler and heat storage tank were investigated regarding the installation of HCGS into 10 household apartment houses in Tokyo. Consequently, the following conclusions were obtained:
This work was partially supported by a Grant-in-Aid for Scientific Research (C) from JSPS.KAKENHI (17510078).
(1) When the capacity of PEM-FC and NEG that composes HCGS was changed and the annual average generation efficiency was investigated, the case of each remaining at 5 kW was good at 27.3%. This value was 1.37 times that of the PEM-FC individual system, and 1.28 times that of the NEG individual system, respectively.
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