Ideal expansion for heat engine? 1

[altalab]

I am considering using an a/c rotary vane compressor as an expander in a solar generating project. The working fluid is R134a refrigerant. From volume measurements I've determined it takes a 12cc space to a 36cc space before dumping, or 1:3. From what I've heard it should be more like 1:9. I've considered using a smaller and larger unit in a compound configuration or installing a cutoff device at the inlet. My questions are:
1) What is the ideal expansion ratio for efficiency regardless of resulting power/speed output?

2) What is the correct sizing steps for double and triple compound expansion?

3) Does throttling behave in any way like the results from a admission percentage cutoff device?

Thanks

 

[ElwoodGreen]

If I understand, you are using solar energy to heat the working fluid (I assume you will evaporate a liquid), then you expand through a "turbine" to create useful power. The ideal condition would probably be to expand until the fluid condenses. At that point, you have taken out all the useful heat energy, and you don't need an extra condensing heat exchanger which is usually fan driven (parasitic load) for a stationary application.

If the working fluid is always gaseous, then you expand to the lowest achievable temperature (usually ambient conditions). Ideal efficiencies are constrained by the high temp and low temp reserviors for any heat engine--review Carnot. Your situation almost seems like a Brayton cycle, but I would have to crack open a book to know for sure.

 

[altalab]

A month has transpired since I posted this question and I determined that I am limited in possible expansion due to mechanical reasons. I am now going to put two of the same expanders in series with the second one running twice as fast as the primary. They will be coupled via belt. This should give 1:6 expansion. The leftover heat will be used to preheat the feed"water", trying to minimize losses. Do you now how I can calculate the exit temp and pressure? The R134a working fluid will be delivered at 585 psi @ 210F max and expand 6 times volume.
Thanks for your response.

 

[ElwoodGreen]

I have no thermodynamic properties for R134a. If you have access to a thermo or other book that describes the Rankine & Brayton cycles, it would help your calculations. For both, the idealized expansion process is constant pressure. In reality, you will have internal losses. Use ideal gas law with P2 = P1 to estimate your exit temperature (T2). Do a few iterations with various volume changes to find the ideal expansion ratio. Build a test unit and measure the T's & P's to find losses. Then re-calculate to see if the system can be further optimized.

In steam cycles (Rankine) they caution against forming excessive liquid droplets prior to turbine exit. The droplets can lead to erosion of turbine blades. I don't know if this would be a problem for your application.

I am glad I found your question, because I need the review as well. I may not have the right cycle, but the expansion process sounds about right for your application.