A STUDY ON A SUPPLY-DEMAND SIMULATION MODEL FOR THE STAND-ALONE HYBRID POWER SUPPLY CONSISTING OF FUEL CELL AND ENERGY CAPACITOR SYSTEMS

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1 A STUDY ON A SUPPLY-DEMAND SIMULATION MODEL FOR THE STAND-ALONE HYBRID POWER SUPPLY CONSISTING OF FUEL CELL AND ENERGY CAPACITOR SYSTEMS Katsuji Mitsui 1 Masahiko Shimizu1 Kazuaki Bogaki Dr.2 1 Development Department, Power Systems Co., Ltd., 4475 Ikebe-cho, Tsuzuki-ku, Yokohama, Kanagawa , Japan 2 Building Research Institute, 1 Tachihara,Tsukuba,Ibaraki , Japan Keywords: Double Layer Capacitor, EDLC, Fuel Cell, Hybrid Power Supply Summary Generally, a stand-alone power supply cannot respond to a transient load change. On the other hand, setting up a power supply which can overcome peak power consumption will need both large expenditure and large space, which will lead to deterioration of the efficiency and the ratio of building volume. Among a large number of storage devices of electric energy our newly developed system, named ECaSS (Energy Capacitor Systems), will provide an excellent solution. If we use a hybrid power supply with the ECaSS, which is a kind of physical battery consists of an electric double layer capacitor and electronic circuits, the power supply can provide stable electricity. In this paper, we design a new model of the hybrid power supply which consists of a fuel cell (FC) as a constant power supply and the ECaSS as a power storage device. We also create a simulation program of it. We tried to test the program using actual sample power consumption patterns of the ordinary households. We analyzed the optimal combination of the hybrid system consisting of the FC and the ECaSS through the simulation model study 1. Introduction The source of energy consumed in Japan has been mostly fossil fuels such as petroleum and coal, and nuclear and hydro power. However, the supply of fossil fuels is not stable because most of them are imported from foreign countries and subject to the world situation. As people s concerns for environmental problems have risen in recent years, expectation for the fuel cell (FC) has been growing. However, when we use FC as a stand-alone power supply, some electric storage systems are necessary because FC output cannot respond easily to a transient load change. However the conventional electric power storage systems comprised of lead-acid batteries, nickel metal-hydride batteries, etc., have a lot of problems such as pollution caused by the hazardous substances in the discarded batteries, short cycle lifetime, and low efficiency of charge and recharge ratio. In this paper, we apply the new electric power storage system, ECaSS, a combination of electric double layer capacitor (EDLC) and electronic circuits, to FC, and simulate a hybrid power supply, FC-ECaSS, to meet the fluctuant domestic power consumption. The technology of ECaSS has already been established [1], [2], and as an applied research, in recent years ECaSS has been aggressively connected to the grid with natural energy [3]. 2. Energy consumption of a household in Japan All-electrified housing is introduced in Japan to improve amenities and correspond to various lifestyles, and the demand for power per household is increasing. Therefore Building Research Institute investigated

2 energy consumption patterns in several ordinary households located in the Kanto area in Japan during 2001 to 2004 [4]. We selected five samples among them and applied each power consumption pattern to our simulation model. The electric power consumption patterns in five sampled households during a year are shown in Figure 1. Figure 1 Power Consumption Pattern

3 The amount of electric energy consumption in the samples U, M, and S is larger than K and N, because U, M, and S use air conditioners for house heating in winter. Household Table 1 Maximum power hour [W] Power consumption data in each household Maximum electric energy day [Wh/day] Average electric energy day [Wh/day] Average power hour [W] U K M N S The power and electric energy consumption data in each household are shown in Table1. We obtained the following from the data: - The average electric energy day of S (18.5kWh/day) is twice as much as that of K (7.5kWh/day). - The maximum power consumption during the year of M (6.6kW) is three times as much as that of K (2.2kW). - In the sample M, the maximum power consumption during the year (6.6kW) is eleven times as much as the average power consumption during the year (0.6kW). This indicates that a strong storage device is necessary to cut the peak. If we apply a stand-alone power supply only by FC to the household (M), the required output power of FC is 6.6kW to meet the maximum power consumption during the year. However, the average power consumption during the year is only 0.6kW (M). The required output power of FC is eleven times as much as the average power consumption of a household to meet the peak demand of the household. More than 80% of the year, FC works at a power less than half of its maximum power. It is considered uneconomical to design the number of FC stacks to meet the maximum power consumption. A certain remedy is required to reduce the number to operate FC stacks more efficiently. In this paper, we apply ECaSS to FC as a method to optimize the number of FC stacks. We named this combination FC-ECaSS. 3. FC-ECaSS simulation model The block diagram of a stand-alone hybrid power supply comprising of FC and ECaSS is shown in Figure 2. All loads such as air conditioner, TV, etc. are operated only by the power generated in the FC, because this system has no connection with the grid line.

4 ECaSS Air conditioner Fuel Cell Stack Charge r Inverter TV IH Heater Etc. EDLC Modules Load AC100V Figure 2 Block Diagram of FC-ECsSS system The algorithm of the system is shown in Figure 3. Input Initial Pfc_max, Uedlc_max Pfc: FC Output Power Uedlc: Energy of EDLC modules Pload: Load Power Pfc_max<Pload? Uedlc=Uedlc_max? EDLC:Discharge Pfc=Pfc_max Pdischarge=Pload-Pfc Pfc=Pload EDLC:Charge Pfc=Pfc_max Pcharge=Pfc-Pload Figure 3 Flow Chart of FC-ECaSS When FC maximum output power (Pfc_max.) is smaller than the load power (Pload), the FC outputs its maximum power and EDLC is discharged and complements the difference. On the contrary, when Pfc_max is greater than Pload, the excess power (Pfc_max. Pload) is utilized to charge EDLC as long as EDLC is not fully charged. In case EDLC is fully charged, Pfc is automatically adjusted to be equal to Pload. Based on the algorithm above, we calculate how much ECaSS can reduce the required output power of FC.

5 4. Simulation results We perform the simulation of FC-ECaSS model in the following four cases and estimate the respective required capacity of ECaSS in each case. Pfc_max = 770W (maximum of Average power consumption during the year) 1.63kW (average power on the Maximum electric energy day) 2kW (about one third of maximum of Maximum power day) 3kW (about one half of maximum of Maximum power day) The results of the simulation are shown in Table2. Table 2 Simulation results Uedlc_max. of ECaSS[Wh] Household Pfc_max.=770W Pfc_max.=1630W Pfc_max.=2000W Pfc_max.=3000W U K M N S While the required capacity of ECaSS is 2kWh in case of K when the FC output is 770W, more than 20 kwh is required in the sample cases of U and M. As the energy density of the existing EDLC is still relatively small at 10Wh/litter, a huge size EDLC of 2.4m 3 is necessary for storage of 24 kilowatt-hours (in case of U for FC=770W of Table2), which is not realistic for installation in a household. Currently, research and development to enhance the energy density of EDLC is being aggressively carried out and we expect that the energy density of the next generation EDLC will increase to as much as 30Wh/litter [5]. If it is realized, the size of EDLC will be drastically reduced from 2.4m 3 to 0.8m 3 (in case of U for FC=770W of Table2), almost the same size as a household refrigerator. A stable supply and cost reduction of EDLC is another concern for many customers. We are now on the road to realizing them and EDLC shall become commercially available in the near future. EDLC will also realize a low environmental burden power supply with high efficiency. 5. Conclusion We established a simulation model of FC-ECaSS as a stand-alone hybrid power supply installed in households with various power consumption patterns. As the result of the simulation, we found that in the case of FC-ECaSS hybrid the maximum output power of FC could be reduced by a tenth of that in the case of all FC power supply systems. In this paper, we conclude that FC-ECaSS hybrid power supply is an efficient method for a stand-alone power supply system. Considering the contribution of FC s heat generation capability, the design of FC-ECaSS hybrid could be optimized, and the size of FC could also be further reduced, because thermal energy generated by FC is applicable to a heat source in winter. We continue to study in this respect.

6 References [1] M. Okamura, A Basic Study on Power Storage Systems, Electrical Engineering in Japan, 115, No.3 (1996) [2] M. Okamura, A New Capacitor-Electronics Power Storage, Proc. of EVS-13, 6H-01, (1996) [3] Romny Om et al., Design and Performance Evaluation of Grid Connected PV-ECS System with Load Leveling Function, IEE of Japan, Vol.121-B, No.9, Sep., 2001 [4] BRI R&D Project, Development of support systems for autonomous and renewable energy and resource type housing during 2001 to 2004, Building Research Institute [5] News story of Nikkan Kougyou Shinbun, 19th Jan., 2005