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Posted by David Sherman [72.47.9.228] on Wednesday, February 23, 2011 at 18:56:16 :
In Reply to: Wise Guy Question O ? T posted by junior copey [184.151.63.145] on Wednesday, February 23, 2011 at 16:33:36 :
First, Stirling engines have been a wonderful idea for a very long time, but nobody has put them to practical use. I'm not sure why that is, but there's probably a good reason. Their beauty is their simplicity and the fact that they don't need a tight sliding seal. On the other hand they aren't very efficient. Perhaps that's the only reason they've never been used seriously.
Second, your way of looking at it is the opposite of what they do in "cogeneration" plants. You're looking at using exhaust from the heating unit to generate power, but the usual way of doing it is to use exhaust heat from a power unit to do space heating. For example, you might built yourself a regular old-fashioned steam engine, which is only 10-15% efficient, and then use the condenser cooling water for space heating since the "waste" heat from the steam engine is still close to the boiling point at atmospheric pressure. A lot of industrial plants do something like this, only they're using steam turbines that burn some waste product like hog fuel or sugar cane. For home use, some people have set up a natural-gas powered generator set so that the engine coolant heat's the house's hot water rather than being wasted in a radiator.
In thermodynamics, there's the concept of the quality of heat. A given amount of heat is more valuable if it's at a higher temperature, because it can be used for more things. Spinning a turbine or a piston engine requires high quality heat. Heating hot water requires lower-quality heat. Space heating is even less demanding, and warming a catfish pond is even lower down on the list. To make optimum use of the energy, you start with something like generating high-pressure steam to drive an engine of some sort. The condensate from that can be used for hot water or space heating, and the water that's too cool even for those uses can be used for a fish pond or de-icing a driveway.
Even within the engine, a good design uses the same steam several times and progressively lower temperatures. A turbine accomplishes it by having larger blades further down along the line as the steam cools off and loses pressure. The old triple-expansion marine engines did it by having different size pistons and cylinders. The steam first expands in a small high-pressure cylinder, and then goes to progressively larger, lower-pressure cylinders all coupled to the same crankshaft and sized such that each piston's force on the crankshaft is the same as the others despite them having different steam pressure.