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Unread 01-12-2001, 14:14
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I did some research into stirling cycles today... Thought I'd share the main ideas here. The text posted supposedly from DEKA is completely accurate, although I thought DEKA had already developed a small stirling engine many years ago? Either way if they've developed a practical implementation of this cycle it could mean a lot for the future of electrical generators.

Anyway, a "hypothetical" stirling engine could be made as a piston-cylinder assembly, with two pistons in the cylinder and a regenerator in the middle. Basically the regenerator is a fixed piece of metal in the middle, or another material good at holding heat over time. The regenerator is porous so that gas can flow through the regenerator, but if it does, the regenerator will either (a) take heat from the gas if the gas is at a higher temperature than regenerator, or (b) give heat to the gas if the gas is at a lower temperature.

During a cycle, initially there is some gas on one side of the regenerator, and the piston is right against the regenerator on the other side, so we can say there is volume to the left and none to the right of the regenerator. The gas on the left is first heated (possibly from a combustion source), which results in the volume on the left increasing and the left piston moving left. Next, both pistons are moved right to force all the gas through the regenerator. The regenerator absorbs lots of the heat from the gas, so the gas is now all on the right and at a lower temperature. Next, heat is extracted from the gas which results in the volume decreasing and the temperature remaining the same. Finally, both pistons move left and the low temperature gas moves through the regenerator, picking up the heat the regenerator had earlier stored. This completes the cycle.

Basically, there are two external heat sources, one at a high temp and one at a low temp. These sources alone are used for the heat addition and heat rejection in the cycle. From these two sources alone, the stirling engine can produce mechanical piston work, which is easily converted to other types of work or electricity. The stirling engine is much better than other types of heat engines because it is perfectly ideal, i.e. no energy is lost to the environment, and it is all converted to mechanical energy. Of course, some heat will travel out of the piston-cylinder assembly in reality. I think (and could be wrong) the point is that there are no stages of the cycle that waste energy. As an example a combustion engine (based on the ideas behind an "Otto Cycle") requires work input to start the cycle (combustion). A stirling engine is started simply by taking heat from the high temperature heat source, which moves the first piston.

The advantes of stirling cycles are that there is no internal combustion involved, however external combustion may be required (or another source of external thermal energy). With external combustion you can use a wide range of different combustion sources, and it is much easier to find one that does not harm the environment in any significant way (e.g. hydrogen or helium).

The stirling engine has yet to leave the realm of theory because of the regenerator. The problem is that the regenerator would require an infinate surface area to completely transfer all the thermal energy from the regenerator to the gas, or vice versa. Or, you'd need a really really good thermoconductive material. If he's figured a clever way to solve the problem of storing this heat (or another implementation of the stirling cycle without this problem at all), then indeed he's made an awesome discovery.

Patrick

Last edited by patrickrd : 01-12-2001 at 14:21.
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