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Solar Engine

Concentrated Sun Engine
  [vote for,

This idea is half baked, since I lack the training to prove it before building. It's a closed cycle heat engine that uses concentrated sunlight as the heat source, but is more simple than a stirling engine.

A Stirling moves the working gas from the hot space to the cold space using a displacer. In my engine the gas is forced through the regenerator into the cold space during the compression stroke by the power piston itself. Since the gas is cooled by the cold heat exchanger during compression and heated during expansion, more work is applied to the piston during expansion than used during compression.

To make leakage around the piston less critical and reduce the flywheel requirement, I want to go with a double acting piston, essentially resulting in a 2 cylinder engine in a single cylinder with only one moving part.

Construction would be simple with the engine cylinder at the focal point of a solar trough. The piston would be centered in the cylinder with a regenerator at each end of the stroke. The cold spaces would be at each end adjacent to each regenerator mesh. Like in a Stirling, the regenerator captures heat as the gas is pushed to the cold side, to be released as the gas flows back to the hot side.

Using butane, pentane or a refrigerant gas as the working gas may be a good idea to increase the pressure effects of heating and cooling with a boiling point within the temperature differential.

HotAirGuy, Sep 04 2007

Simple Solar Engine drawing http://firstlines.net/SimpleEngine.JPG
Drawing of the simple engine [HotAirGuy, Sep 04 2007]


       I think you need to draw a cylinder diagram showing what happens on each stroke.   

       To me it seems that the cold sides are the ends of the cylinder and the hot point is in the middle where the piston is. This is the cycle as I see it at the moment:   

       1) POWER - Gas expands and pushes the piston.   

       2) EXHAUST - The piston returns and pushes the hot gas through a heat exchanger (which must have a valve to stop this happening during stroke 1) before arriving at a cold end. Meanwhile, the other end of the cylinder is doing step 1).   

       3) INTAKE - The cooled gas is now moved back past the heat exchange to the hot area (perhaps by linking the 2 cold sides) as the other end of the cylinder is doing step 2.   

       4) COMPRESSION? - The gas is now heated and pushing on the cylinder which is already fully extended. The engine stops.
marklar, Sep 04 2007

       What could be more simple than a Stirling? I mean, shoot, there's only five or six moving parts on the simplest rotating models - I think the free-piston ones only have two or three.   

       I'm not saying yours is a bad idea, but it sounds as if you'd rather not call it a Stirling, and I don't think there's anything in your description that clearly points it out as anything _but_ a Stirling.
elhigh, Sep 04 2007


       There is no exhaust or intake. The engine is sealed to the outside with only one seal on the piston rod. Power comes from more heat added to each side during that side's expansion and more heat rejected during compression.   

       It will not be self starting, so a starter is needed on the flywheel not shown.   

       The pressure of compression will exceed the pressure change resulting from the change in temperature.   

       The process I see is as follows: At top dead center for one side, the gas will be compressed in the regenerator and cold space. The regenerator mesh is hottest at that point. As the pressure of compression pushes the piston off of TDC, cool gas flows out of the cold space, recovering heat from the regenerator and further heated by the hot cylinder. The opposite is happening on the other side of the piston, but because the heat released from the regenerator mostly occurs early in the expansion stroke (where the regenerator is hottest and more molecules flow through), the work available during expansion is greater than is used for compression of the other side. The flywheel helps carry the piston through the negative portion of the stroke.   

       My main concern is thermal lag of the heat exchangers, but I believe the regenerator will offset this lag, especially since the pressure of compression should make the regenerator even more effective than in a stirling engine.   

HotAirGuy, Sep 04 2007


       A flywheel will carry it through the stroke, but with a double acting piston it definitely won't get struck at the end of the expansion, since the pressure on the other side of the piston will be greater due to compression on the opposing side.   

       Regarding thermal lag, in a stirling the power piston lags behind the displacer movement. Thermal lag will shoot me in the foot here if the gas is heated more after the expansion stroke is complete. Hopefully the early regeneration and increasing temperature of compression will ensure that more heat is added during the expansion part of the stroke.
HotAirGuy, Sep 04 2007

       Your diagram is almost identical to the one I sketched when I was trying to understand how it works (except I added a pipe joining the 2 cold parts). However, your diagram does not show a complete cycle and gives no indication of how the engine works.   

       I used EXHAUST etc, as they are terms people are familiar with. What I meant by the term was then explained.   

       I will try to explain again using the pressure gradient between each side: At engine start, piston head is in the centre and all is cold with equal pressure. Starter moves head to far left, gas on left has high pressure and slight temp increase, gas on right has low pressure and slight temp decrease but starts to heat in hot area. Right gas heats & expands so the starter has to overcome that force to push the piston to the right, left gas expands into heating area. Right gas cools as it heats the regenerator, left gas heats & expands. The starter has to overcome the pressure from the left to move the piston to the left.   

       At what point can the starter stop working?   

       // the work available during expansion is greater than is used for compression of the other side // This is not true. The energy available during expansion is taken from the compression phase, the regenerator would need to be more than 100% efficient.
marklar, Sep 04 2007


       I'm trying to come up with something more simple than a stirling. The question is how do I move the gas from the hot side to the cold side without a displacer. The answer I came up with is to use the power piston to move the gas.   

       My thought process was this: If I assume no friction, no heat exchange, and no temperature gradient, then the engine would continue to run with a sufficient flywheel. If I add heat during expansion and reject heat during compression, shouldn't that get me net power, and with a sufficient temperature differential to overcome friction for usable power out?   

       The idea started as an idea to improve the little test tube Lamina engine. With just a single acting piston, the flywheel would be very large, so I made it double acting to make things easier for construction. Look at the 2 sides as separate.   

       An analogy that makes me think it will work. If I take a lubricated syringe and plug the end, when I push the plunger down and release it, it returns to the starting point (no net work). Now if I heat it up before plugging the end, and cool it as I push the plunger, it takes less work to push the plunger. Then as I add heat as the plunger is released, it returns all the way just like it would if there was no temperature change, but it requires less work to compress to the same point.   

       The temperature/pressure analysis of the cycle is beyond my abilities, but I think it does more than just move heat through the engine, though I may be only "half baked".
HotAirGuy, Sep 04 2007


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