1、4667 Considerations in the Design and Application of Solid Oxide Fuel Cell Energy Systems in Res i dent i al Markets Robert J. Braun, Ph.D. Sanford A. Klein, Ph.D. Fellow ASHRAE Douglas T. Reindl, Ph.D., P.E. Member ASHRAE ABSTRACT This paper examines aspects offuel cell system design for applicatio
2、n in stationary residential markets. The develop- ment offuel cell systems for sub-1 O kwstationary applications involves consideration of sizing, fuel processing, operating point selection, fuel cell operating capabilities, system inte- gration, and load management strategies. Each of these conside
3、rations is discussed, and strategies are presented for matching the electrical and thermal energy demands of a resi- dence with a solid oxide fuel cell power system. Eficiency considerations for conjguring fuel cell, DC-to-AC inverter, and electrical energy storage components for conditioning of DC
4、power generated by the fuel cell stack are also given. Recommendations are made on the potential opportunities for solid oxide fuel cells in small-scale stationary power applica- tions. INTRODUCTION In the U.S., residential and commercial sectors together are responsible for over 35% of the total an
5、nual energy consumption (EIA 1996). Of this fraction, over 50% is used for low-efficiency space heating, domestic hot water, air- conditioning, and refrigeration (EIA 1995). Modem residen- tial fumaces operate with second law efficiencies of less than 1 STO, leaving substantial room for improvement.
6、 Nearly all energy conversion technologies in the various end-use sectors (transportation, industrial, and utility) attain higher efficien- cies than residential heating applications. The low cost of heat- ing fuels (natural gas, propane, and fuel oil) has allowed continued use of inefficient direct
7、-fired heating systems. However, increasing national and international pressure to reduce greenhouse gas emissions (primarily COz) coupled with concerns of finite energy resources are providing renewed impetus toward improving fuel conversion efficien- cies (Grubb 1999; Oberthuer and Ott 1999). Addi
8、tionally, electric utilities and independent power producers nationwide are studying ways to meet the increasing energy demands in a competitive environment through the use of distributed gener- ation resources. Research and development in the area of fuel cell tech- nology has gained momentum durin
9、g the past decade. Ongo- ing efforts in this area offer a timely opportunity to achieve significant improvements in energy conversion efficiency and reduction of energy-related emissions. Although fuel cells themselves have been studied extensively, primarily from materials and electrochemical viewp
10、oints, a considerable gap exists in the area of application techniques to maximize bene- fits of fuel cell systems for both electrical energy generation and thermal energy utilization. In this paper, we present design and operating approaches that will achieve optimal performance for solid oxide fue
11、l cell (SOFC) systems in small-scale (1-1 O kW) stationary applica- tions, with particular focus on single-family detached dwell- ings. The paper begins by discussing application requirements for single-family residential dwellings. Next, design consid- erations for SOFC systems that provide residen
12、tial heat and power are examined. The effect of design cell voltage on fuel cell thermal-to-electric ratio, cost of electricity, and value of thermal energy are quantified and reported. The paper concludes with recommendations to achieve variable thermal- to-electric ratios for grid-connected reside
13、ntial end-use and a short commentary on the outlook of fuel cell technology. Robert J. Braun is a senior systems engineer at UTC Fuel Cells, a United Technologies Company, South Windsor, Conn. Sanford A. Klein is Bascom Ouweneel Professor of Mechanical Engineering at the University of Wisconsin-Madi
14、son. Douglas T. Reindl is an associate professor of engineering professional development at the University of Wisconsin-Madison and is the director of the Industrial Refrigeration Consortium, Madison, Wisc. 14 02004 ASHRAE. Madison, Wisconsin USA (January Day) 9 ., o 4.5 g 48 Y 3.5 8 9 B ! - 3 2.5 !
15、 W - 2m 1.5 0.6 0.625 0.65 0.675 0.7 0.7215 0.75 0.775 0.8 Design Cell Voltage volt Figure 7 Influence of design cell voltage on cost of electricity. 7.4 Wh. The unit system capital cost3 associated with a 0.70 V design cell voltage is about 1500 $/kW. Interestingly, as the design cell voltage is in
16、creased, the electric-only COE decreases more rapidly than the cogenera- tion COE, reaching an optimal value of 0.76 V. The optimum for either cost is established by the same mechanism of competing fuel and capital costs; however, the location of the optimum is altered as el savings for recuperated
17、thermal energy from the system are not realized. Since the cost estimates remain uncertain for fuel cell systems, a capital equipment cost uncertainty of *30% was applied to the present analysis. The COE resulted in a fixed value of 7.4h1.3 $/kWh. The operating costs were estimated to contribute 56%
18、 of the cost of electricity at the 0.7 V design condition. Of this percentage, annual fuel cost accounted for 5 1% and operation and maintenance for 5%. The remaining 44% of the COE was distributed among the BOP (32%) and the SOFC cell stack (12%) capital costs. The fuel cell capital cost estimates
19、were given for a high-volume production scenario (i.e., mature) and have a 33% salvage value at the end- of-life. For the mature mass production situation, fuel and balance-of-plant (BOP) costs dominate the total system life- cycle costs. Figure 7 also presents the value of thermal energy recov- ere
20、d, as defined in Equation 11, as a function of design cell voltage. The plot shows a nearly linear decreasing value of thermal energy with increasing design cell voltage. The higher the cell voltage, the higher the system electric efficiency and the less thermal energy is available for DHW. Figure 8
21、 illustrates the economy of scale associated with balance of plant hardware that can be realized when varying the system power rating for a cell stack operating at 800C (1422F) with an average cell voltage of 0.735 V and a system 3. The unit system capital cost does not include installation, ship- p
22、ing, or contingency fees. 20 ASHRAE Transactions: Research F 9.5 x9 o 8.5 s Y 28 o 7.5 u7 .- L a3 O - ic 06 I! 6-5LLLx%4 O 2000 4000 6000 8000 10000 System AC Power Rating wl Figure8 Influence of system power rating on life-cycle costs. fuel utilization of 77.5%. As the size of the SOFC system incre
23、ases, the cost of electricity decreases from a COE of 9.4 $kWh at 1 kW to 6.1 $kWh at 10 kW. Clearly, SOFC system capacities are more economical at 5-10 kW than at 1 kW, suggesting that competitive application of the technology would be in multiple-family dwellings rather than single- family, detach
24、ed dwellings. It should be noted that Figure 8 only includes the economy-of-scale associated with the balance-of-plant hardware and not with the costs of the solid oxide fuel cell stack, which are more sensitive to economies of production. DESIGN STRATEGIES FOR VARIABLE THERMAL-TO-ELECTRIC RATIOS Th
25、e most significant application requirements for resi- dential fuel cell power systems are the required transient response, magnitude of the electrical loads, and the thermal- to-electric ratio (TER). The transient response and the magni- tude of the electrical loads are not as significant for grid-
26、connected SOFC systems where short-term load transients (and peak demands) can be met by the electric grid. Since resi- dential applications require flexibility in operating modes, designing a system with variable TERs is a desirable objective. Solid oxide fuel cell systems have difficulty in follow
27、ing the dynamic electrical load due to both the response time of the fuel delivery system (seconds) and cell-stack thermal response (minutes). The SOFC will eventually modulate up or down in power output in a relatively slowly changing manner, while the instantaneous power demand is served by the el
28、ectric grid (or battery in stand-alone systems). Thus, the most difficult resi- dential energy demand characteristic to meet with a fuel cell system is the high thermal/low electrical load condition. With this in mind, the following engineering design strategies have been conceived to address meetin
29、g the quasi-instantaneous thermal (hot water)-to-electric load ratios. Strategy I-Net Metering Many states in the U.S. offer the possibility to sell elec- tricity produced from a home power generation system back to the utility through a net metering program. In such a case, the SOFC system could be
30、 designed to produce a fixed electric power with excess power sold back to the utility. The fuel cell TER could then have flexibility in matching the residential (or other end-use) TER. However, most net metering programs only buy back power at retail rates as long as the net kwh of the residence, a
31、s registered on the utility meter, are greater than zero for the billing cycle. The merit of this design and operating strategy from the customer perspective is explored further in Braun et al. (2004b). Strategy 2-Hot Water Storage Tank System This strategy makes use of the off-peak thermal energy r
32、ecuperation by storing it to serve peak demands at another time. It is the simplest of the available options to meet demand and may be integrated with a conventional hot water heater if demands cannot be met solely by waste heat recovery. An SOFC system with a two-tank hot water system is simulated
33、and results are presented in Braun et al. (2004b). Strategy 3-Use Excess Fuel Cell Electrical Energy Generation for Electric Water Heating The electrical demand of the application could be increased by use of a combination of electric and thermal energy recuperative hot water heating. In this scenar
34、io, both the fuel cell electrical and thermal output could increase, with only the resistive heating element as the additional system capital cost. However, higher SOFC load factors need to be weighed against cell life and durability issues, as well as the impact on operating costs. Strategy 4. Acce
35、ptable voltage degradation over the lifetime of the cell stack is typically targeted at 10.5% mV drop per 1000 hours operation or 5 20% drop in operating cell voltage from beginning of life to end of life (Hirschenhofer et al. 1998). 22 ASHRAE Transactions: Research however, they are more efficient
36、when operating off hydrocar- bons than hydrogen, primarily due to improved thermal management when using internal reforming (see Braun 2004a), and relatively tolerant to fuel impurities. The ultimate measure of success for fuel cell systems will be whether the technology can compete with other power
37、 generation technologies in terms of total cost, total life perfor- mance, and reliability. In many instances, the perceived “value” of the system is not only related to first cost but to avoided cost; for example, where expansion of the transmis- sion and distribution infrastructure can make the al
38、ternative, distributed generation technology more attractive. The “value” of fuel cell technology may also be enhanced when compared to conventional generating technology in CHP applications. It has been noted (see Ellis and Gunes 2002) that widespread application of CHP systems in buildings has bee
39、n limited because conventional technology tends to (I) be most efficient in large sizes and when operating near full-load, (2) require a larger and more skilled maintenance staff, and (3) be limited in new plant siting due to environmental restrictions on noise and emissions. Fuel cell technology ha
40、s the potential to overcome all of these limitations while offering enhanced efficiencies and lower chemical and acoustic emissions than conventional power producing and heating equipment. ACKNOWLEDGMENTS The authors would like to thank ASHRAE for a Grant-in- Aid Award to R.J. Braun and the Edwin A.
41、 Link Energy Foun- dation and the Energy Center of Wisconsin for financial support. REFERENCES Achenbach, E. 1995. Response of a solid oxide fuel cell to load change. J. Power Sources Vol. 57, pp. 105-109. ADL. 2001. Conceptual design of POX / SOFC 5 kW net system. Prepared for the U.S. Department o
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