Passive Solar Stirling Hydraulic Free Piston, is this possible?

  • #1

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OK very crude first sketch let me try to explain and you tell me if it sounds remotely workeable.
Likemany other eco nuts after the holy grail of energy, I'm trying to brainstorm a low cost solar CHP system. I'm of the opinion that passive solar is always going to win on a BTU/$ collected and that if the metric of success in an engine is $/kWhr over a lifetime that efficiency may not be as important as ease of maintenance, and upfront capital cost.

That being said I'm proposing building a low temperature differential stirling that runs on a hybrid passive solar collector. The hybridization component is the mirror wings, which under sunny conditions will increase temp and overall efficiencies, however unlike a pure concentrative machine it'll still OK on moderately cloudy days.

Since the differential pressures will be low and heat transfer is an issue I am thinking that a large air impermeable fabric accordion cylinder with a metal bottom could be coupled to a hydraulic pump to operate as a free piston hydraulic engine.

The dynamics of the engine could be controlled through hydraulic actuated valves. When the working piston is at the top of the stroke the inlet valve is opened, and the two exhaust valves are closed and the pressurized air expands the working piston downwards until either the stroke limit or lack of forward progress is reached. The the inlet valve is then closed, the two exhausts valves are opened and the LP reservoir in the hydraulic pump pushes the cylinder back until either stroke limit is reached or progress is stopped. Then the cycle begins again with the inlet valve opening and exhaust valves closing.

The stroke length would be controlled through regulating the low pressure reservoir and the high pressure reservoir pressures.

Does this cycle seems possible and more importantly remotely feasible?

Also yes I understand overall cycle efficiencies at this low of delta T with hydraulic loss will probably run around 5-7%. This may be OK if the overall cost of construction is low enough.
 

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  • #2
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Engineers work with numbers. You gave a long verbal description of your engine with no numbers at all. That means, our answers must be very general.

The efficiency of any heat engine may not exceed that of a Carnot Cycle. That cycle is characterized by the temperature differences. So start there.

Determine the maximum temperature of your working fluid, calculate the Carnot efficiency. That is a maximum. In real life it may be hard to achieve 1/2 of that.
 

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