evidence proof that events are getting ahead of me: GE has a SOFC fuel-cell stack which hits 49% efficiency. The GE stack is estimated to cost $254/kW for a 5.4kW system (under $1400 for 5.4 kW), exclusive of fuel reformer and electronics.
It's not clear if the efficiency is for the stack alone (probably) or for the complete system, or what the cost and efficiency loss of fuel preparation would be. If the cells are aimed at use in coal plants, it seems likely that they will take scrubbed syngas (CO + H2 + inert gases) as their fuel and require no further reformation. If fuels like methane and propane require no preparation and the 49%-efficient stack is followed by a 95%-efficient converter, the fuel-to-juice efficiency would be 46.6%. That's not half bad.
The system cost for cells such as these is not easily pinned down , but if we assume $600/kW we probably won't be far off. Last, we need a total system efficiency. Condensing furnaces exceed 95%, so that seems like a completely reasonable assumption.
If we assume 46.6% electrical efficiency and 95% total efficiency (48.4% output as heat, 5% losses) here's what we'd get out of a therm of natural gas, assuming a price of 65¢/therm for gas and $.68/therm for the produced heat:
Looks like the unit would just about double your money. If you had a 5 kW unit running at 25% capacity factor for the year (providing both space heat and DHW), it would consume 802 therms of gas at a cost of $521 and yield heat and electricity worth $1139. The gross profit before depreciation and maintenance would be $618/year, sufficient to pay off a $3000 unit in 5 years.
Most households don't use 5 kW of electricity during the heating season except for electric stoves or heat pumps. I haven't looked at the net efficiency of the fuel cell plus heat pump in at least a week, so it's time to scribble on another envelope. 5 kW into a heat pump with an EER of 12 would yield 60,000 BTU/hr. The example fuel cell system producing 5 kW would burn 0.366 therm/hour of gas and yield 17,700 BTU/hr of heat; together, the two could produce 77,700 BTU/hr of heat from 36,600 BTU/hr of gas. This is an EER of 212%, cutting fuel requirements by 54% compared to a 98%-efficient condensing furnace.
I'll come back and look at the combo of FC/heat pump plus wind power and PHEV later. For now, it looks like a replacement of gas furnaces with FC/heat pump combos could offset quite a few years of natural gas depletion, and make it even cheaper to adopt alternative energy supplies when we develop them (so long as they're SOFC-compatible). Samples suitable for these purposes are in testing at the Department of Energy, and the projected price is right.
 I found Assessment of Solid Oxide Fuel Cells in Building Applications which specifies the cost of an inverter as $400/kW and so forth, but the exponents for scaling most of these costs with size are specified as zero. I can't tell if the authors assumed that the cost would be roughly the same across the 1-5 kW power range (which seems likely for elements such as the water heater), or if this is a typo. This report is also 7 years old and way out of date. The SECA report 10 kW SOFC Power System Commercialization SECA Semi-Annual Report 41244R04 states a goal of $400/kW system cost for the third and final phase. It is far more recent, and probably more trustworthy. (back)
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