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Fuel Cell basics
Solid Oxide Fuel Cell (SOFC) basics
Molten Carbonate Fuel Cell (MCFC) basics
SARA DCFC research (MCFC approach)
CellTech DCFC research (SOFC with a twist)

In all cases, DCFCs operate at between 700 and 850 degrees centigrade (reaction temperature). A reasonably sized residential cell would be intended for combined heat and power (CHP). Because there's no thermal cycle, per se, just direct chemical conversion, you can pretty much sneer arrogantly at the Carnot limit. In all cases, theoretical efficiency is ~70% for the overall system (compare to conventional coal at ~25-40%). The input fuel can vary from graphite powder to carbon black to methanol (ground diamond could theoretically work). The waste product is CO2, but it's easily captured if you care about that (I don't). The listed temperature suggests that these might be excellent candidates for thermoelectric conversion if electricity is more greatly desired than heat, and partial thermoelectric conversion might improve the overall efficiency in any case. Most of the papers I've read were written prior to the recent revolution in thermoelectrics, so that's worth looking into. Further, I can easily envision a situation utilizing all the waste from such a cell to heat simultaneously heat a greenhouse and supply it with extra CO2 to stimulate plant growth -- nearly ideal for an arctic or antarctic enclosed farm. Or Michigan in Winter, say.

This sort of cell could be scaled for use in a car, but the high temperature might be a very serious issue, which is likely why Honda went with the methanol-reforming cells for their super-expensive fuel cell concept car. They run at a much lower operating temperature (250-300 degrees centigrade, comparable to an internal combustion engine), and the waste from them is H2O rather than CO2 (H2O is actually the stronger greenhouse gas, but it can be caught and condensed at some cost to efficiency). However, you have to carry around a cell full of methanol as a hydrogen source, and the efficiency of such cells is much lower (25-40%). This is another area where thermoelectric insulation could make a very serious difference in the utility of such a cell for this very desirable application.

I shall continue to ponder. Both the companies I listed above did literally start as garage shops. I'm not likely to beat them to commercialization, but it would be fun to build a cell as a project. Wouldn't mind heating my house with graphite or carbon black, for that matter.

Posted by: leoncaruthers at 07:40 AM | No Comments | Add Comment
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