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Fuel cells and energy networks of electricity, heat, and hydrogen: A demonstration in hydrogen-fueled apartments
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Fuel cells and energy networks of electricity, heat, and hydrogen: A demonstration in hydrogen-fueled apartments Hirohisa Aki a,*, Yukinobu Taniguchi a, Itaru Tamura b, Akeshi Kegasa b, Hideki Hayakawa b, Yoshiro Ishikawa c, Shigeo Yamamoto c, Ichiro Sugimoto d a
National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japan Osaka Gas Co., Ltd., 6-19-9, Torishima, Konohana-ku, Osaka City 554-0051, Japan c KRI Inc., 6-19-9, Torishima, Konohana-ku, Osaka City 554-0051, Japan d Advanced Cogeneration and Energy Utilization Center Japan, 1-16-4, Toranomon, Minato-ku, Tokyo 105-0001, Japan b
article info
abstract
Article history:
A demonstration was performed to evaluate our proposal of a residential energy system
Received 13 August 2011
based on fuel cells and energy networks of electricity, hot water, and hydrogen. The
Received in revised form
demonstration was conducted from April 2007 to March 2009 in a small apartment building
7 October 2011
constructed for experimental purposes in Osaka City. Three small proton exchange
Accepted 8 October 2011
membrane fuel cells were installed, and the electricity and hot water from the fuel cells
Available online 5 November 2011
were shared among 6 units via an internal electricity grid and hot water pipe. A hydrogen production facility, a small storage device, and a hydrogen pipe were installed to supply
Keywords:
hydrogen to the fuel cells. Six families went about their normal daily lives using this
Hydrogen system
system. The energy flow from hydrogen production to consumption was demonstrated.
Energy network
The results of fuel cell operation, energy supply, and energy demand, as well as an analysis
Fuel cell
of primary energy saving and CO2 emission mitigation are presented. Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
Distributed generation Residential
combined
heat
and
reserved.
power system Demonstration
1.
Introduction
Combined heat and power (CHP) systems based on fuel cells are promising applications of fuel cell technology [1,2]. Residential fuel cells have been commercially available in Japan since 2009. As of 2010, more than 10,000 units have been installed in general homes, in addition to another 3307 installed through a government demonstration project in 2005e2008 [3]. Current fuel cell systems involve proton exchange membrane fuel cells (PEMFCs) or polymer electrolyte fuel cells (PEFCs). Systems based on solid-oxide fuel cells
(SOFCs), which are more attractive because they have a higher efficiency of electricity generation [4,5], are likely to be commercialized in the near future [6]. Commercialized systems have built-in fuel processors that generate hydrogen from natural gas/kerosene so that the systems can be used without a hydrogen supply. These systems face challenges against broad commercial penetration in terms of the performance or energy demand characteristics of homes. Hence, despite the many studies already conducted on the performance improvement and cost reduction of fuel cells
* Corresponding author. Tel.: þ81 29 861 4194; fax: þ81 29 861 5754. E-mail address: [email protected] (H. Aki). 0360-3199/$ e see front matter Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2011.10.021
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[7e9], providing solutions and knowledge on the operational strategies and optimum system integration that can make full use of the advantages of fuel cells is needed to accelerate penetration [10,11]. More generally, the penetration of fuel cells is an important step toward a hydrogen economy [12]. However, there are no concrete scenarios in which hydrogen networks can be arranged to reach homes or offices. The concept of interconnecting a group of residential homes by energy networks involving electricity, heat (hot water), and hydrogen, with some of the homes possessing fuel cells, has previously been proposed as a solution to these problems [13,14] Such networks that interconnect fuel cell stacks, fuel processors, and other equipment are expected to contribute to the mitigation of associated environmental impacts through flexible and efficient utilization of equipment devices. Starting with small-scale systems, a gradual expansion to larger systems can be achieved by consistently using conventional energy systems. This can also help to avoid large initial investments. An energy interchange system has also been proposed for apartments. The combination of two different types of distributed generation (DG)dPEMFC and gas engines/ SOFCdallows variations in the ratio of heat to electricity supply and enables the load matching necessary for homes. Mathematical models, a PC-based simulator, and an experimental system have been developed to evaluate the proposed system. The effects of the system have been analyzed and verified by the simulator and experimental systems, respectively. As the next step towards practical application, a demonstration has been conducted in existing residential homes to evaluate the proposal. The demonstration was performed between April 2007 and March 2009, in a small apartment building with 18 units constructed for experimental purposes in Osaka City. Two homes on the 3rd floor and four homes on the 4th floor were involved in the demonstration. The six families went about their daily lives as usual using the system. Three small PEM fuel cells with hot water tanks were installed, and the electricity and hot water from the fuel cells were shared via an internal electricity grid and a hot water pipe. A hydrogen production facility, a small storage system, and a hydrogen pipe were installed as infrastructure to supply hydrogen to the fuel cells. The energy flow among the homes was demonstrated, from hydrogen production to consumption. Two cases were examined: a system for detached houses, and a system for apartment buildings.
2.
Fuel cells and energy networks
The first challenge for commercialized residential fuel cells relates to the demand characteristics of residential homes [15]. The installation of fuel cells with capacities that equal or exceed the peak demand is economically unjustified. Therefore, the capacity of commercialized residential fuel cells is set to 700e1000 W. The fuel cells are operated in a griddependent mode and not intended to supply the peak demand. The energy demand in residential homes generally varies. The electricity base load of Japanese homes is usually
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250e500 W with a peak load of 3e5 kW [16]. Their electricity demands have “demand spikes”, which are large loads in a short period such as those caused by the use of microwaves. The load factors of the electricity demands are usually low because of the demand spikes. The maximum value of the combined load of all homes is considerably smaller than the sum of the peak load values of each home by smoothing effect. For a group of homes, combining the electricity load of all homes and considering the group as a single consumer reduces the required total capacity of the equipment including the fuel cells [16]. The implementation of energy networks for a group enables the equipment and energy demand of the homes to be interconnected. Thus, coordinated and flexible operation of the equipment can be achieved [17]. Fuel processors that produce hydrogen from natural gas (methane) or kerosene are installed in the fuel cell systems so that no hydrogen infrastructure is necessary. However, the shortcoming of this scheme is that the response to demand is slow [15]. Fuel processors, which are based on chemical processes, have four different catalytic reactions that must be kept at different temperatures. Thus, fuel processors cannot respond rapidly to changes in load, whereas fuel cell stacks can respond quickly and immediately to spikes in electricity demand. In addition to their slow response, fuel processors are less efficient at partial load operation and require 1 h or more of preheating for the catalytic reactions at start-up. These shortcomings cancel out the advantages of fuel cell stacks. Residential fuel cells are used as CHP systems that provide high total efficiencies by heat recovery. To make full use of this advantage, it is critical to match the supply and demand for both electricity and heat [18,19]. The ratio between electricity generation and hot water generation is fixed at approximately 1.25e1.41, depending on the characteristics of the system [20]. On the other hand, the ratio between electricity demand and hot water demand always varies. This is because hot water consumption varies by season; for example, the hot water consumption in winter is five times that in summer. Based on the considerations above, the authors have designed and proposed typical system configurations for detached houses and apartment buildings. An example of the system configurations for a group of eight detached houses is shown in Fig. 1. The electricity generated from the fuel cells installed in residential homes can be shared because the fuel cells are interconnected with a grid. Therefore, the homes share virtually all of the fuel cells. CHP systems should be installed near points of hot water consumption to avoid heat loss. Fuel cells, especially PEMFCs, can be scaled flexibly because of their structure [21]. Therefore, it is preferable to install small fuel cells in each home than to install a single large fuel cell in one place, especially for detached houses because high heat isolation of hot water pipe is impossible. Hot water from the heat exchangers of fuel cells should primarily be used in homes where fuel cells are installed, after which the rest should be transported to other homes. Fig. 2 shows an example of an apartment building with 100 apartment units (10 10). In the case of an apartment building, it is possible to install larger systems in public rooms
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Fig. 1 e Example of the proposed system for eight detached houses.
such as the electric room. A hybrid generation system, which combines two or more types of generation systems (e.g., PEMFCs and SOFCs), enables variations in the ratio of heat to electricity supply depending on the demand. The authors propose the installation of PEMFCs into some of the homes along with gas engines or SOFCs. A centralized hydrogen production system with purifiers [pressure swing adsorption (PSA) units] is chosen for the apartment buildings. It can provide pure hydrogen and is better suited for the operation of fuel cell stacks; however, it is unavailable for detached houses owing to technical constraints in PSA production.
In these apartment buildings, the hot water is circulated horizontally and shared between all units on the same floor. Sharing hot water between all units is another option; this depends on the trade-off between the electricity consumption involved in pumping up the water and its utilization.
3.
Demonstration project
A demonstration project was initiated to evaluate a previous proposal. Its aims were as follows: 1) to evaluate the technological feasibility of a hydrogen-based system, 2) to improve
Fig. 2 e Example of the proposed system for an apartment building (10 3 10 [ 100).
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the operational algorithms to adapt the system to more practical applications, and 3) to verify the effects of the proposed system on reducing primary energy consumption and CO2 emission. The demonstration was conducted at the Experimental Complex NEXT21 (Fig. 3) in Osaka, Japan. NEXT21 is an apartment building in which various experiments and demonstrations can be conducted [22]. The seven-story building includes a basement with 18 families living on the 3rd to 6th floors. The allocated rooms are completely separated from one another to simulate detached houses. In addition, the plans of all rooms are completely different. Each floor has a large space to install pipes for hot water and gas as well as electric cables beneath the floor. The large space enables complete modifications of these pipes if necessary.
3.1.
System configuration
Two homes on 3rd floor and four homes on 4th floors were involved in the demonstration project. A hydrogen production facility, which consists of three sets of a fuel processor (steam reforming of methane, 1.5 Nm3/h each) and a PSA, and a small hydrogen tank (200 L) were installed on the roof of the building. Hydrogen pipes were installed for the entire area involved in the demonstration; in addition to electricity and water, hydrogen was supplied as part of the basic infrastructure. In this demonstration, the operation of the hydrogen production facility was beyond the scope of evaluation. One and two PEMFCs were installed on 3rd and 4th floors, respectively. Hot water tanks were installed next to the fuel cell stacks to use the recovered heat to produce hot water, which was transported through a pipe. Auxiliary boilers were provided to all six homes as a backup. Each PEMFC consisted of a fuel cell stack, a heat recovery system, and an inverter for grid connection. The most significant departure from commercialized residential fuel cell
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systems was that the PEMFC was not equipped with a fuel processor to make hydrogen from hydrocarbon fuels. The design specifications of the installed PEMFCs are shown in Table 1. Note that the electricity generation efficiency is the system output efficiency, which depends not only on the characteristics of the fuel cell stack but also on inverter performance. The actual heat recovery efficiency was approximately 40% according to our measurements. Two types of demonstrations were performed. Fig. 4 represents the implemented energy systems. Detached house case: A simulation of the system for a group of detached homes. Four homes on 4th floor were involved. Two of the four homes had PEMFCs. Electricity and hot water were interchanged or shared. The interchange of hot water was possible between two tanks. Apartment building case: A simulation of the system for an apartment building. The PEMFCs and hot water tanks were shared. Two hot water tanks on the 4th floor were connected sequentially and used as a single tank. Electricity was shared among all six homes, and hot water was circulated and shared among two/four homes on each floor.
3.2.
Instrumental and control system
Monitoring and control servers were installed underground, and an Ethernet-based private network was installed in the entire area used for the demonstration. The signals from instruments such as flow meters and thermocouples were converted to TCP/IP by signal converters and collected by the monitoring server. The fuel cells were equipped with TCP/IP communication function. Each signal converter or fuel cell was assigned a unique IP address. The energy demands of homes (e.g., electricity consumption for air conditioning and lighting, hot water consumption
Fig. 3 e Demonstration site.
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Table 1 e Design specifications of fuel cells and hot water tanks. Item Type Fuel Electric output
Heat recovery Dimensions
Design specifications Polymer Electrolyte Membrane fuel cell (PEM-FC) 99.99% hydrogen Rated output: 700 W Efficiency at rated operation: 40% (HHV) Output: AC 200 V Efficiency at rated operation: 30% (HHV)a Temperature: 60e70 C Generation units: 300 W 1250 H 440 D (m) Hot water tanks: 370 L 1 (3rd floor), 200 L 2 (4th floor)
a According to the authors’ measurements, the actual efficiency is 40%.
flow. The flow always varied and often caused errors, and small errors, e.g., 2e3 C in the measured temperature were critical for evaluations of the hot water system efficiency. Although the complex state-of-the-art boiler (Fig. 5) provided more comfort to inhabitants, it also posed difficulties for investigators. Fig. 5 shows the hot water system of each apartment. The mixers always regulated the flows of the tap water and the hot water from the tanks to maintain the temperature of the hot water supplied to the apartment units. Each unit was equipped with an auxiliary boiler as a backup, which was automatically turned on when the hot water temperature was low. When the valves were opened, the cold water in hot water pipe flowed into the mixer. The cold water was heated by the auxiliary boiler.
3.3. for bathing, etc.), as well as the states of the equipment and energy flow, were measured every 2 s and stored on a server via Ethernet. The major measurement points are summarized in Table 2. Accurate measurement of the hot water system was challenging because heat values could not be measured directly and were calculated from the temperature and water
System operation
The fuel cells were operated to match the electricity load unless the hot water tanks became full. Details of the operational strategies are given below. Detached house case: The two fuel cells shared the sum of electricity load of four units equally when the sum of electricity loads was smaller than the rated output of the fuel cells.
Fig. 4 e Energy systems of the demonstration. (a) Detached house case (boilers for emergency backup only). (b) Apartment house case (boilers for emergency backup only).
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Fuel cells, hot water tanks, others Hydrogen consumption Electricity generation (AC)
Apartments Electricity consumption Electricity consumption of air conditioner Natural gas consumption
Recovered heat (hot water) temperature Hot water tank level
Natural gas consumption of aux. boiler Cold/hot water flow & temperature to/from aux. boiler Hot water flow & temperature from hot water tank
Hot water flow and temperature
The rest of the time, the fuel cells were operated at the rated output (Fig. 6). The hot water was stored in the hot water tanks. A fuel cell was stopped when the adjacent hot water tank became full. Hot water was supplied to two units from a hot water tank. The two hot water tanks were interconnected by an interchange pipe. When one hot water tank became empty while the other had sufficient hot water, the valve on the interchange pipe was opened to supply hot water to the empty tank. Apartment building case: The three fuel cells shared the sum of the electricity load of six units equally. The hot water tanks of the 4th floor were sequentially connected to single tank. The hot water was circulated on each floor. The circulation pumps were not always operated but turned on every hour only when the temperature of the circulation pipe was low. The hydrogen production facility was turned on or off depending on the pressure of the hydrogen tank. It was independent of the other components of the demonstration system. The demonstrations were conducted for three weeks each season (spring, summer, fall, winter) in the following order: 1) energy demand measurements without fuel cells to establish a baseline, 2) the detached house case, and 3) the apartment building case.
4.
Results
The entire system functioned well; no major faults or troubles occurred during the demonstration. This indicates that that
Aux. boiler Kitchen, others
Load, ooutput (kW W)
Table 2 e Measurement points.
Bath tab
(Gas boiler)
Shower kitchen
Mixer
Measurement point
Natural gas Tap water
Hot water (from tank) Fig. 5 e Details of hot water system for each apartment and measurement system.
Sum of electricity demand
Rated output of fuel cells FC#2
FC#1 Time
Fig. 6 e Fuel cell operation (Detached house case).
the hydrogen-based system is technologically available to residential homes at the present time.
4.1.
Operational results
The operational results in fiscal years (FY) 2007 and 2008 are summarized in Table 3. The average of five days (three weekdays and two days of the weekend) and the average of three weekdays are shown for FY2007 and FY2008, respectively. Note that the number of units is four in the Detached house case and six in Apartment building case. The conditions of the columns are different, e.g., the behavior and energy demands of residents vary day by day. “Heat loss” was calculated as “Fuel cells” plus “Aux. boilers” minus “Demand,” because it cannot be measured directly. The contribution of the fuel cells to electricity demand was small in the summer. Fuel cells occasionally stopped because hot water demand was small and the hot water tanks became full. Although the installed system reduced such problems by sharing the hot water, they could not be reduced completely. The majority of hot water demand was supplied by fuel cells except in the winter. The demand was too large to be supplied only by fuel cells, and required compensation by the auxiliary boilers in the winter. The installed systemda CHP system with auxiliary boilersdis suitable for residential dwellings with large variations in hot water demand. The demand in the winter was 3e5 times that in the summer. In the fall of 2007, the hot water demand of the Apartment building case was significantly larger than that of the Detached house case, because the ambient temperature during the Apartment building case was low. The heat loss in the tanks, circulation, and branch pipes between the main circulation pipe and each apartment was substantial. Contrary to expectations, the heat loss was large in the summer and exceeded the demand. Little hot water was consumed. The hot water often stayed in the pipe and became cold because of the low demand. The majority of the external energy supply was hydrogen. That is, the units depended on hydrogen for most of their enduse energy.
4.2.
System operations
The fuel cell operation, energy supply, and energy demand are shown in Fig. 7.
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Table 3 e Operational summary of average energy supply and demand. Spring
(a) Detached house case Energy balance Electricity (kWh) Demand Fuel cells Grid Fuel cells contributed Hot water (MJ) Demand Fuel cells Loss Aux. boilers Fuel cells contributed External energy supply (MJ) Electricity Natural gas Hydrogen Covered by hydrogen Misc. H.W./electricity demands Avg. temp. of tap water (b) Apartment building case Energy balance Electricity (kWh) Demand Fuel cells Grid Fuel cells contributed Hot water (MJ) Demand Fuel cells Loss Aux. boilers Fuel cells contributed External energy supply (MJ) Electricity Natural gas Hydrogen Covered by hydrogen Misc. H.W./electricity demands Avg. temp. of tap water
Summer
Autumn
2007
2008
2007
2008
2007
41.6 27.9 13.7 67.0%
50.7 31.8 18.9 62.7%
53.1 23.3 29.8 43.9%
63.6 24.1 39.5 37.9%
95.9 98.5 42.8 40.3 58.0%
84.7 115.9 48.1 16.9 80.0%
47.3 81.5 51.2 17.1 64.0%
49.5 41.7 242.8 72.7%
68.0 24.8 295.6 76.1%
0.64 20.8
2007
2008
37.4 29.7 7.7 79.5%
47.0 28.5 18.5 60.6%
48.8 30.2 18.6 61.8%
42.4 93.6 64.1 12.9 69.6%
67.2 106.7 52.5 13.1 80.5%
200.5 89.4 30.1 141.2 29.6%
200.2 118.7 52.2 133.7 33.2%
107.3 15.8 200.6 62.0%
142.1 11.4 215.0 58.4%
27.6 21.3 267.6 84.5%
66.7 199.8 254.2 48.8%
67.0 199.6 287.8 51.9%
0.46 22.6
0.25 26.4
0.19 30.3
0.50 20.9
1.18 9.7
1.14 12.0
66.1 43.2 23.2 65.0%
74.8 49.1 25.7 65.6%
81.3 42.2 39.1 51.9%
86.3 37.9 48.5 43.9%
70.4 45.3 25.2 64.2%
88.3
75.6 48.7 26.8 64.5%
111.4 151.6 85.8 45.5 59.1%
155.0 159.1 70.6 66.6 57.0%
63.9 144.5 102.9 22.4 65.0%
59.9 139.0 104.4 25.4 57.6%
185.4 152.6 72.5 105.3 43.2%
309.8
272.2 184.1 81.5 169.6 37.7%
83.4 52.7 372.9 73.3%
92.5 100.5 448.1 69.9%
140.8 27.0 362.6 68.4%
174.4 34.1 336.3 61.7%
90.8 163.2 401.5 61.3%
47% 23.4
58% 22.0
22% 28.4
19% 29.8
73% 18.3
Fig. 7(a) shows an example of the supply and demand of electricity, including fuel cell operation in the Apartment building case of a weekday in February 2009. The figure shows the total electricity demand, electricity from a grid of six units, and total electricity generation of the fuel cells. The majority of electricity demand was supplied by the fuel cells. The fuel cells followed the demand variations during lowdemand hours (2:00e4:00, 4:30e6:00, 10:00e14:00), and kept the rated output during other hours. This shows that PEMFCs without fuel processor have enough load following capability. The hot water supply and demand varies by season. Fig. 7(b) shows the hot water supply and demand on each floor of the Apartment building case of a weekday in February 2009. The figure shows the hot water tank level (estimated by thermocouples), total hot water demand, operations of
2008
Winter
96.6 257.0 455.0 56.3% 97% 9.2
100% 11.3
auxiliary boilers of units, and hot water from fuel cell(s). The hot water tank level varied as hot water was consumed. The variation in the tank level on the 3rd floor is simpler than that of the 4th floor because there are only two units on the 3rd floor. The auxiliary boilers often used to heat up the water in the branch pipe, because this water was not circulated and its temperature decreased rapidly in the winter. In peak hours, the hot water tank became empty on the 4th floor. The fuel cells could not cover all of the demand, without assistance from the auxiliary boilers. The supply and demand for electricity and hot water in the Detached house case of a weekday in July 2008 is shown in Fig. 7(c). The hot water demand was low, and the hot water tank level reached full capacity at around 17:30. The fuel cell then stopped for 1.5 h until the tank level decreased. Although the heat of the reaction in the fuel cell stack must be removed
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Fig. 7 e Operations of the system. (a) Apartment building case in winter. (b) Hot water supply and demand. (c) Detached house case in summer.
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by heat recovery system, it cannot be removed if the tank becomes full. Therefore, the fuel cell must be stopped. The load duration curves of electricity demand and electricity generation by fuel cells are shown in Fig. 8. Most of the base demand was supplied by the fuel cells. The load factors of the fuel cells were high. If fuel cells are installed in all homes at a high initial cost, more demand would be covered, but load factor will become low. The average capacity of fuel cells per home is only 350 W. However, most of demand was covered. This is an effect of networking.
4.3.
Analysis
The effects of the proposed system on primary energy conservation and CO2 mitigation were analyzed. The length of the hot water pipe was too long, since the layout of equipment could not be optimized. As mentioned above, this caused extensive heat loss. This heat loss should be considered when evaluating the effects for practical applications. The amount of heat loss generally depends on the length of the pipe from the tank. The longest distance from the tank to the units is 67.6 m on the 4th floor in the Apartment building case. This could be halved if the system is optimized and installed in a general apartment building. The heat loss could be halved as well. The operation of auxiliary boilers will decrease if the heat loss is reduced. The external energy supply in Table 3 was recalculated based on these assumptions. Fig. 9 plots the CO2 Fig. 9 e Evaluation of primary energy saving and CO2 mitigation. (a) Detached house case. (b) Apartment building case.
emission and primary energy consumption calculated as rates of reduction relative to a conventional system. The conventional system was assumed to be a combination of electricity purchased from an external grid and hot water supplied by a natural gas boiler. The CO2 emission intensities of electricity from the external grid and natural gas were assumed to be 0.555 kg-CO2/kWh and 0.051 kg-CO2/MJ, respectively. Hydrogen was assumed to be reformed from natural gas at an efficiency of 80%. Primary energy consumption and CO2 emission were reduced by 2e12% and 7e19%, respectively. The reduction rates varied by season; they were large in the spring and fall, when the balance of electricity and hot water demands is appropriate. The reduction was small in the summer and winter, because the contribution of fuel cells was small (see Section 4.2). The annual average reductions in primary energy and CO2 emission were 6 and 11%, respectively.
5.
Fig. 8 e Load duration curves of electricity demand and fuel cells. (a) Detached house case. (b) Apartment building case.
Conclusion
A demonstration has been conducted to evaluate our proposal of an energy system based on fuel cells and energy networks of electricity, hot water, and hydrogen in residential areas. The system saves primary energy consumption by 6% and mitigates CO2 emission by 11% if the pipe layout is optimized
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for practical application. Three small fuel cells are sufficient to cover most of the end-use energy demands of six homes by networking the homes and systems. Our results demonstrate the effectiveness of a hydrogen-based energy system that is technologically available at present.
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[11] Peacock A, Newborough M. Effect of heat-saving measures on the CO2 savings attributable to micro-combined heat and power (mCHP) systems in UK dwellings. Energy 2008;33(4): 601e12. [12] Marba´n G, Valde´s-Solı´s T. Towards the hydrogen economy? International Journal of Hydrogen Energy 2007;32(12): 1625e37. [13] Aki H, Yamamoto S, Ishikawa Y, Kondoh J, Maeda T, Yamaguchi H, et al. Operational strategies of networked fuel cells in residential homes. IEEE Transactions on Power Systems 2006;21(3):1405e14. [14] Aki H, Yamamoto S, Kondoh J, Maeda T, Yamaguchi H, Murata A, et al. Fuel cells and energy networks of electricity, heat, and hydrogen in residential areas. International Journal of Hydrogen Energy 2006;31(8):967e80. [15] Sammes NM, Boersma R. Small-scale fuel cells for residential applications. Journal of Power Sources 2000; 86(1e2):98e110. [16] Maeda T, Hasegawa Y, Ito H, Shimakawa N, Matsumoto K. The measurements and analyses of energy consumed by residents of an apartment house in Kyushu district (in Japanese). In: The 27th Conference on Energy, Economy, and Environment. Tokyo: 2004. p. 383e386. [17] Wakui T, Yokoyama R, Shimizu K. Suitable operational strategy for power interchange operation using multiple residential SOFC (solid oxide fuel cell) cogeneration systems. Energy 2010;35(2):740e50. [18] Liso V, Zhao Y, Brandon N, Nielsen MP, Kær SK. Analysis of the impact of heat-to-power ratio for a SOFC-based mCHP system for residential application under different climate regions in Europe. International Journal of Hydrogen Energy; 2011. [19] Erdmann G. Future economics of the fuel cell housing market. International Journal of Hydrogen Energy 2003;28(7): 685e94. [20] Sugimoto I, Yamamoto S, Aki H, Ishida M. Energy network designs and a hydrogen energy community growth scenario based on networking stationary fuel cells. In: Hydrogen and Fuel Cells 2011. Vancouver: 2011. [21] Farret FA, Simo˜es MG. Power plants with fuel cells, Integration of alternative sources of energy. John Wiley & Sons, Inc; 2006. 159e197. [22] Shiba T. Experimentations at experimental complex NEXT21 (in Japanese). Energy and Resources 2007;28(5):342e5.
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Fuel cells and energy networks of electricity, heat, and hydrogen: A demonstration in hydrogen-fueled apartments Hirohisa Aki a,*, Yukinobu Taniguchi a, Itaru Tamura b, Akeshi Kegasa b, Hideki Hayakawa b, Yoshiro Ishikawa c, Shigeo Yamamoto c, Ichiro Sugimoto d a
National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japan Osaka Gas Co., Ltd., 6-19-9, Torishima, Konohana-ku, Osaka City 554-0051, Japan c KRI Inc., 6-19-9, Torishima, Konohana-ku, Osaka City 554-0051, Japan d Advanced Cogeneration and Energy Utilization Center Japan, 1-16-4, Toranomon, Minato-ku, Tokyo 105-0001, Japan b
article info
abstract
Article history:
A demonstration was performed to evaluate our proposal of a residential energy system
Received 13 August 2011
based on fuel cells and energy networks of electricity, hot water, and hydrogen. The
Received in revised form
demonstration was conducted from April 2007 to March 2009 in a small apartment building
7 October 2011
constructed for experimental purposes in Osaka City. Three small proton exchange
Accepted 8 October 2011
membrane fuel cells were installed, and the electricity and hot water from the fuel cells
Available online 5 November 2011
were shared among 6 units via an internal electricity grid and hot water pipe. A hydrogen production facility, a small storage device, and a hydrogen pipe were installed to supply
Keywords:
hydrogen to the fuel cells. Six families went about their normal daily lives using this
Hydrogen system
system. The energy flow from hydrogen production to consumption was demonstrated.
Energy network
The results of fuel cell operation, energy supply, and energy demand, as well as an analysis
Fuel cell
of primary energy saving and CO2 emission mitigation are presented. Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
Distributed generation Residential
combined
heat
and
reserved.
power system Demonstration
1.
Introduction
Combined heat and power (CHP) systems based on fuel cells are promising applications of fuel cell technology [1,2]. Residential fuel cells have been commercially available in Japan since 2009. As of 2010, more than 10,000 units have been installed in general homes, in addition to another 3307 installed through a government demonstration project in 2005e2008 [3]. Current fuel cell systems involve proton exchange membrane fuel cells (PEMFCs) or polymer electrolyte fuel cells (PEFCs). Systems based on solid-oxide fuel cells
(SOFCs), which are more attractive because they have a higher efficiency of electricity generation [4,5], are likely to be commercialized in the near future [6]. Commercialized systems have built-in fuel processors that generate hydrogen from natural gas/kerosene so that the systems can be used without a hydrogen supply. These systems face challenges against broad commercial penetration in terms of the performance or energy demand characteristics of homes. Hence, despite the many studies already conducted on the performance improvement and cost reduction of fuel cells
* Corresponding author. Tel.: þ81 29 861 4194; fax: þ81 29 861 5754. E-mail address: [email protected] (H. Aki). 0360-3199/$ e see front matter Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2011.10.021
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[7e9], providing solutions and knowledge on the operational strategies and optimum system integration that can make full use of the advantages of fuel cells is needed to accelerate penetration [10,11]. More generally, the penetration of fuel cells is an important step toward a hydrogen economy [12]. However, there are no concrete scenarios in which hydrogen networks can be arranged to reach homes or offices. The concept of interconnecting a group of residential homes by energy networks involving electricity, heat (hot water), and hydrogen, with some of the homes possessing fuel cells, has previously been proposed as a solution to these problems [13,14] Such networks that interconnect fuel cell stacks, fuel processors, and other equipment are expected to contribute to the mitigation of associated environmental impacts through flexible and efficient utilization of equipment devices. Starting with small-scale systems, a gradual expansion to larger systems can be achieved by consistently using conventional energy systems. This can also help to avoid large initial investments. An energy interchange system has also been proposed for apartments. The combination of two different types of distributed generation (DG)dPEMFC and gas engines/ SOFCdallows variations in the ratio of heat to electricity supply and enables the load matching necessary for homes. Mathematical models, a PC-based simulator, and an experimental system have been developed to evaluate the proposed system. The effects of the system have been analyzed and verified by the simulator and experimental systems, respectively. As the next step towards practical application, a demonstration has been conducted in existing residential homes to evaluate the proposal. The demonstration was performed between April 2007 and March 2009, in a small apartment building with 18 units constructed for experimental purposes in Osaka City. Two homes on the 3rd floor and four homes on the 4th floor were involved in the demonstration. The six families went about their daily lives as usual using the system. Three small PEM fuel cells with hot water tanks were installed, and the electricity and hot water from the fuel cells were shared via an internal electricity grid and a hot water pipe. A hydrogen production facility, a small storage system, and a hydrogen pipe were installed as infrastructure to supply hydrogen to the fuel cells. The energy flow among the homes was demonstrated, from hydrogen production to consumption. Two cases were examined: a system for detached houses, and a system for apartment buildings.
2.
Fuel cells and energy networks
The first challenge for commercialized residential fuel cells relates to the demand characteristics of residential homes [15]. The installation of fuel cells with capacities that equal or exceed the peak demand is economically unjustified. Therefore, the capacity of commercialized residential fuel cells is set to 700e1000 W. The fuel cells are operated in a griddependent mode and not intended to supply the peak demand. The energy demand in residential homes generally varies. The electricity base load of Japanese homes is usually
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250e500 W with a peak load of 3e5 kW [16]. Their electricity demands have “demand spikes”, which are large loads in a short period such as those caused by the use of microwaves. The load factors of the electricity demands are usually low because of the demand spikes. The maximum value of the combined load of all homes is considerably smaller than the sum of the peak load values of each home by smoothing effect. For a group of homes, combining the electricity load of all homes and considering the group as a single consumer reduces the required total capacity of the equipment including the fuel cells [16]. The implementation of energy networks for a group enables the equipment and energy demand of the homes to be interconnected. Thus, coordinated and flexible operation of the equipment can be achieved [17]. Fuel processors that produce hydrogen from natural gas (methane) or kerosene are installed in the fuel cell systems so that no hydrogen infrastructure is necessary. However, the shortcoming of this scheme is that the response to demand is slow [15]. Fuel processors, which are based on chemical processes, have four different catalytic reactions that must be kept at different temperatures. Thus, fuel processors cannot respond rapidly to changes in load, whereas fuel cell stacks can respond quickly and immediately to spikes in electricity demand. In addition to their slow response, fuel processors are less efficient at partial load operation and require 1 h or more of preheating for the catalytic reactions at start-up. These shortcomings cancel out the advantages of fuel cell stacks. Residential fuel cells are used as CHP systems that provide high total efficiencies by heat recovery. To make full use of this advantage, it is critical to match the supply and demand for both electricity and heat [18,19]. The ratio between electricity generation and hot water generation is fixed at approximately 1.25e1.41, depending on the characteristics of the system [20]. On the other hand, the ratio between electricity demand and hot water demand always varies. This is because hot water consumption varies by season; for example, the hot water consumption in winter is five times that in summer. Based on the considerations above, the authors have designed and proposed typical system configurations for detached houses and apartment buildings. An example of the system configurations for a group of eight detached houses is shown in Fig. 1. The electricity generated from the fuel cells installed in residential homes can be shared because the fuel cells are interconnected with a grid. Therefore, the homes share virtually all of the fuel cells. CHP systems should be installed near points of hot water consumption to avoid heat loss. Fuel cells, especially PEMFCs, can be scaled flexibly because of their structure [21]. Therefore, it is preferable to install small fuel cells in each home than to install a single large fuel cell in one place, especially for detached houses because high heat isolation of hot water pipe is impossible. Hot water from the heat exchangers of fuel cells should primarily be used in homes where fuel cells are installed, after which the rest should be transported to other homes. Fig. 2 shows an example of an apartment building with 100 apartment units (10 10). In the case of an apartment building, it is possible to install larger systems in public rooms
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Fig. 1 e Example of the proposed system for eight detached houses.
such as the electric room. A hybrid generation system, which combines two or more types of generation systems (e.g., PEMFCs and SOFCs), enables variations in the ratio of heat to electricity supply depending on the demand. The authors propose the installation of PEMFCs into some of the homes along with gas engines or SOFCs. A centralized hydrogen production system with purifiers [pressure swing adsorption (PSA) units] is chosen for the apartment buildings. It can provide pure hydrogen and is better suited for the operation of fuel cell stacks; however, it is unavailable for detached houses owing to technical constraints in PSA production.
In these apartment buildings, the hot water is circulated horizontally and shared between all units on the same floor. Sharing hot water between all units is another option; this depends on the trade-off between the electricity consumption involved in pumping up the water and its utilization.
3.
Demonstration project
A demonstration project was initiated to evaluate a previous proposal. Its aims were as follows: 1) to evaluate the technological feasibility of a hydrogen-based system, 2) to improve
Fig. 2 e Example of the proposed system for an apartment building (10 3 10 [ 100).
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the operational algorithms to adapt the system to more practical applications, and 3) to verify the effects of the proposed system on reducing primary energy consumption and CO2 emission. The demonstration was conducted at the Experimental Complex NEXT21 (Fig. 3) in Osaka, Japan. NEXT21 is an apartment building in which various experiments and demonstrations can be conducted [22]. The seven-story building includes a basement with 18 families living on the 3rd to 6th floors. The allocated rooms are completely separated from one another to simulate detached houses. In addition, the plans of all rooms are completely different. Each floor has a large space to install pipes for hot water and gas as well as electric cables beneath the floor. The large space enables complete modifications of these pipes if necessary.
3.1.
System configuration
Two homes on 3rd floor and four homes on 4th floors were involved in the demonstration project. A hydrogen production facility, which consists of three sets of a fuel processor (steam reforming of methane, 1.5 Nm3/h each) and a PSA, and a small hydrogen tank (200 L) were installed on the roof of the building. Hydrogen pipes were installed for the entire area involved in the demonstration; in addition to electricity and water, hydrogen was supplied as part of the basic infrastructure. In this demonstration, the operation of the hydrogen production facility was beyond the scope of evaluation. One and two PEMFCs were installed on 3rd and 4th floors, respectively. Hot water tanks were installed next to the fuel cell stacks to use the recovered heat to produce hot water, which was transported through a pipe. Auxiliary boilers were provided to all six homes as a backup. Each PEMFC consisted of a fuel cell stack, a heat recovery system, and an inverter for grid connection. The most significant departure from commercialized residential fuel cell
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systems was that the PEMFC was not equipped with a fuel processor to make hydrogen from hydrocarbon fuels. The design specifications of the installed PEMFCs are shown in Table 1. Note that the electricity generation efficiency is the system output efficiency, which depends not only on the characteristics of the fuel cell stack but also on inverter performance. The actual heat recovery efficiency was approximately 40% according to our measurements. Two types of demonstrations were performed. Fig. 4 represents the implemented energy systems. Detached house case: A simulation of the system for a group of detached homes. Four homes on 4th floor were involved. Two of the four homes had PEMFCs. Electricity and hot water were interchanged or shared. The interchange of hot water was possible between two tanks. Apartment building case: A simulation of the system for an apartment building. The PEMFCs and hot water tanks were shared. Two hot water tanks on the 4th floor were connected sequentially and used as a single tank. Electricity was shared among all six homes, and hot water was circulated and shared among two/four homes on each floor.
3.2.
Instrumental and control system
Monitoring and control servers were installed underground, and an Ethernet-based private network was installed in the entire area used for the demonstration. The signals from instruments such as flow meters and thermocouples were converted to TCP/IP by signal converters and collected by the monitoring server. The fuel cells were equipped with TCP/IP communication function. Each signal converter or fuel cell was assigned a unique IP address. The energy demands of homes (e.g., electricity consumption for air conditioning and lighting, hot water consumption
Fig. 3 e Demonstration site.
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Table 1 e Design specifications of fuel cells and hot water tanks. Item Type Fuel Electric output
Heat recovery Dimensions
Design specifications Polymer Electrolyte Membrane fuel cell (PEM-FC) 99.99% hydrogen Rated output: 700 W Efficiency at rated operation: 40% (HHV) Output: AC 200 V Efficiency at rated operation: 30% (HHV)a Temperature: 60e70 C Generation units: 300 W 1250 H 440 D (m) Hot water tanks: 370 L 1 (3rd floor), 200 L 2 (4th floor)
a According to the authors’ measurements, the actual efficiency is 40%.
flow. The flow always varied and often caused errors, and small errors, e.g., 2e3 C in the measured temperature were critical for evaluations of the hot water system efficiency. Although the complex state-of-the-art boiler (Fig. 5) provided more comfort to inhabitants, it also posed difficulties for investigators. Fig. 5 shows the hot water system of each apartment. The mixers always regulated the flows of the tap water and the hot water from the tanks to maintain the temperature of the hot water supplied to the apartment units. Each unit was equipped with an auxiliary boiler as a backup, which was automatically turned on when the hot water temperature was low. When the valves were opened, the cold water in hot water pipe flowed into the mixer. The cold water was heated by the auxiliary boiler.
3.3. for bathing, etc.), as well as the states of the equipment and energy flow, were measured every 2 s and stored on a server via Ethernet. The major measurement points are summarized in Table 2. Accurate measurement of the hot water system was challenging because heat values could not be measured directly and were calculated from the temperature and water
System operation
The fuel cells were operated to match the electricity load unless the hot water tanks became full. Details of the operational strategies are given below. Detached house case: The two fuel cells shared the sum of electricity load of four units equally when the sum of electricity loads was smaller than the rated output of the fuel cells.
Fig. 4 e Energy systems of the demonstration. (a) Detached house case (boilers for emergency backup only). (b) Apartment house case (boilers for emergency backup only).
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Fuel cells, hot water tanks, others Hydrogen consumption Electricity generation (AC)
Apartments Electricity consumption Electricity consumption of air conditioner Natural gas consumption
Recovered heat (hot water) temperature Hot water tank level
Natural gas consumption of aux. boiler Cold/hot water flow & temperature to/from aux. boiler Hot water flow & temperature from hot water tank
Hot water flow and temperature
The rest of the time, the fuel cells were operated at the rated output (Fig. 6). The hot water was stored in the hot water tanks. A fuel cell was stopped when the adjacent hot water tank became full. Hot water was supplied to two units from a hot water tank. The two hot water tanks were interconnected by an interchange pipe. When one hot water tank became empty while the other had sufficient hot water, the valve on the interchange pipe was opened to supply hot water to the empty tank. Apartment building case: The three fuel cells shared the sum of the electricity load of six units equally. The hot water tanks of the 4th floor were sequentially connected to single tank. The hot water was circulated on each floor. The circulation pumps were not always operated but turned on every hour only when the temperature of the circulation pipe was low. The hydrogen production facility was turned on or off depending on the pressure of the hydrogen tank. It was independent of the other components of the demonstration system. The demonstrations were conducted for three weeks each season (spring, summer, fall, winter) in the following order: 1) energy demand measurements without fuel cells to establish a baseline, 2) the detached house case, and 3) the apartment building case.
4.
Results
The entire system functioned well; no major faults or troubles occurred during the demonstration. This indicates that that
Aux. boiler Kitchen, others
Load, ooutput (kW W)
Table 2 e Measurement points.
Bath tab
(Gas boiler)
Shower kitchen
Mixer
Measurement point
Natural gas Tap water
Hot water (from tank) Fig. 5 e Details of hot water system for each apartment and measurement system.
Sum of electricity demand
Rated output of fuel cells FC#2
FC#1 Time
Fig. 6 e Fuel cell operation (Detached house case).
the hydrogen-based system is technologically available to residential homes at the present time.
4.1.
Operational results
The operational results in fiscal years (FY) 2007 and 2008 are summarized in Table 3. The average of five days (three weekdays and two days of the weekend) and the average of three weekdays are shown for FY2007 and FY2008, respectively. Note that the number of units is four in the Detached house case and six in Apartment building case. The conditions of the columns are different, e.g., the behavior and energy demands of residents vary day by day. “Heat loss” was calculated as “Fuel cells” plus “Aux. boilers” minus “Demand,” because it cannot be measured directly. The contribution of the fuel cells to electricity demand was small in the summer. Fuel cells occasionally stopped because hot water demand was small and the hot water tanks became full. Although the installed system reduced such problems by sharing the hot water, they could not be reduced completely. The majority of hot water demand was supplied by fuel cells except in the winter. The demand was too large to be supplied only by fuel cells, and required compensation by the auxiliary boilers in the winter. The installed systemda CHP system with auxiliary boilersdis suitable for residential dwellings with large variations in hot water demand. The demand in the winter was 3e5 times that in the summer. In the fall of 2007, the hot water demand of the Apartment building case was significantly larger than that of the Detached house case, because the ambient temperature during the Apartment building case was low. The heat loss in the tanks, circulation, and branch pipes between the main circulation pipe and each apartment was substantial. Contrary to expectations, the heat loss was large in the summer and exceeded the demand. Little hot water was consumed. The hot water often stayed in the pipe and became cold because of the low demand. The majority of the external energy supply was hydrogen. That is, the units depended on hydrogen for most of their enduse energy.
4.2.
System operations
The fuel cell operation, energy supply, and energy demand are shown in Fig. 7.
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Table 3 e Operational summary of average energy supply and demand. Spring
(a) Detached house case Energy balance Electricity (kWh) Demand Fuel cells Grid Fuel cells contributed Hot water (MJ) Demand Fuel cells Loss Aux. boilers Fuel cells contributed External energy supply (MJ) Electricity Natural gas Hydrogen Covered by hydrogen Misc. H.W./electricity demands Avg. temp. of tap water (b) Apartment building case Energy balance Electricity (kWh) Demand Fuel cells Grid Fuel cells contributed Hot water (MJ) Demand Fuel cells Loss Aux. boilers Fuel cells contributed External energy supply (MJ) Electricity Natural gas Hydrogen Covered by hydrogen Misc. H.W./electricity demands Avg. temp. of tap water
Summer
Autumn
2007
2008
2007
2008
2007
41.6 27.9 13.7 67.0%
50.7 31.8 18.9 62.7%
53.1 23.3 29.8 43.9%
63.6 24.1 39.5 37.9%
95.9 98.5 42.8 40.3 58.0%
84.7 115.9 48.1 16.9 80.0%
47.3 81.5 51.2 17.1 64.0%
49.5 41.7 242.8 72.7%
68.0 24.8 295.6 76.1%
0.64 20.8
2007
2008
37.4 29.7 7.7 79.5%
47.0 28.5 18.5 60.6%
48.8 30.2 18.6 61.8%
42.4 93.6 64.1 12.9 69.6%
67.2 106.7 52.5 13.1 80.5%
200.5 89.4 30.1 141.2 29.6%
200.2 118.7 52.2 133.7 33.2%
107.3 15.8 200.6 62.0%
142.1 11.4 215.0 58.4%
27.6 21.3 267.6 84.5%
66.7 199.8 254.2 48.8%
67.0 199.6 287.8 51.9%
0.46 22.6
0.25 26.4
0.19 30.3
0.50 20.9
1.18 9.7
1.14 12.0
66.1 43.2 23.2 65.0%
74.8 49.1 25.7 65.6%
81.3 42.2 39.1 51.9%
86.3 37.9 48.5 43.9%
70.4 45.3 25.2 64.2%
88.3
75.6 48.7 26.8 64.5%
111.4 151.6 85.8 45.5 59.1%
155.0 159.1 70.6 66.6 57.0%
63.9 144.5 102.9 22.4 65.0%
59.9 139.0 104.4 25.4 57.6%
185.4 152.6 72.5 105.3 43.2%
309.8
272.2 184.1 81.5 169.6 37.7%
83.4 52.7 372.9 73.3%
92.5 100.5 448.1 69.9%
140.8 27.0 362.6 68.4%
174.4 34.1 336.3 61.7%
90.8 163.2 401.5 61.3%
47% 23.4
58% 22.0
22% 28.4
19% 29.8
73% 18.3
Fig. 7(a) shows an example of the supply and demand of electricity, including fuel cell operation in the Apartment building case of a weekday in February 2009. The figure shows the total electricity demand, electricity from a grid of six units, and total electricity generation of the fuel cells. The majority of electricity demand was supplied by the fuel cells. The fuel cells followed the demand variations during lowdemand hours (2:00e4:00, 4:30e6:00, 10:00e14:00), and kept the rated output during other hours. This shows that PEMFCs without fuel processor have enough load following capability. The hot water supply and demand varies by season. Fig. 7(b) shows the hot water supply and demand on each floor of the Apartment building case of a weekday in February 2009. The figure shows the hot water tank level (estimated by thermocouples), total hot water demand, operations of
2008
Winter
96.6 257.0 455.0 56.3% 97% 9.2
100% 11.3
auxiliary boilers of units, and hot water from fuel cell(s). The hot water tank level varied as hot water was consumed. The variation in the tank level on the 3rd floor is simpler than that of the 4th floor because there are only two units on the 3rd floor. The auxiliary boilers often used to heat up the water in the branch pipe, because this water was not circulated and its temperature decreased rapidly in the winter. In peak hours, the hot water tank became empty on the 4th floor. The fuel cells could not cover all of the demand, without assistance from the auxiliary boilers. The supply and demand for electricity and hot water in the Detached house case of a weekday in July 2008 is shown in Fig. 7(c). The hot water demand was low, and the hot water tank level reached full capacity at around 17:30. The fuel cell then stopped for 1.5 h until the tank level decreased. Although the heat of the reaction in the fuel cell stack must be removed
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Fig. 7 e Operations of the system. (a) Apartment building case in winter. (b) Hot water supply and demand. (c) Detached house case in summer.
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by heat recovery system, it cannot be removed if the tank becomes full. Therefore, the fuel cell must be stopped. The load duration curves of electricity demand and electricity generation by fuel cells are shown in Fig. 8. Most of the base demand was supplied by the fuel cells. The load factors of the fuel cells were high. If fuel cells are installed in all homes at a high initial cost, more demand would be covered, but load factor will become low. The average capacity of fuel cells per home is only 350 W. However, most of demand was covered. This is an effect of networking.
4.3.
Analysis
The effects of the proposed system on primary energy conservation and CO2 mitigation were analyzed. The length of the hot water pipe was too long, since the layout of equipment could not be optimized. As mentioned above, this caused extensive heat loss. This heat loss should be considered when evaluating the effects for practical applications. The amount of heat loss generally depends on the length of the pipe from the tank. The longest distance from the tank to the units is 67.6 m on the 4th floor in the Apartment building case. This could be halved if the system is optimized and installed in a general apartment building. The heat loss could be halved as well. The operation of auxiliary boilers will decrease if the heat loss is reduced. The external energy supply in Table 3 was recalculated based on these assumptions. Fig. 9 plots the CO2 Fig. 9 e Evaluation of primary energy saving and CO2 mitigation. (a) Detached house case. (b) Apartment building case.
emission and primary energy consumption calculated as rates of reduction relative to a conventional system. The conventional system was assumed to be a combination of electricity purchased from an external grid and hot water supplied by a natural gas boiler. The CO2 emission intensities of electricity from the external grid and natural gas were assumed to be 0.555 kg-CO2/kWh and 0.051 kg-CO2/MJ, respectively. Hydrogen was assumed to be reformed from natural gas at an efficiency of 80%. Primary energy consumption and CO2 emission were reduced by 2e12% and 7e19%, respectively. The reduction rates varied by season; they were large in the spring and fall, when the balance of electricity and hot water demands is appropriate. The reduction was small in the summer and winter, because the contribution of fuel cells was small (see Section 4.2). The annual average reductions in primary energy and CO2 emission were 6 and 11%, respectively.
5.
Fig. 8 e Load duration curves of electricity demand and fuel cells. (a) Detached house case. (b) Apartment building case.
Conclusion
A demonstration has been conducted to evaluate our proposal of an energy system based on fuel cells and energy networks of electricity, hot water, and hydrogen in residential areas. The system saves primary energy consumption by 6% and mitigates CO2 emission by 11% if the pipe layout is optimized
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 1 2 0 4 e1 2 1 3
for practical application. Three small fuel cells are sufficient to cover most of the end-use energy demands of six homes by networking the homes and systems. Our results demonstrate the effectiveness of a hydrogen-based energy system that is technologically available at present.
references
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