I am working on a project to design an organic Rankine cycle to convert low temperature thermal energy (waste heat or geothermal) into electrical energy. We are trying to select an expander to extract work from the pressure differential in the refrigerant. The refrigerant (ammonia or R134a) will be heated to about 120 F in the evaporator and cooled to about 80 F in the condenser. For both of these refrigerants there will be about a 1.6-1.7 pressure ratio between the expander inlet and exit. We don't really want to use a turbo expander For this small application and are trying to keep this from being an extremely sophisticated system (more to show the viability of a "cheap solution" than to use state of the art technology). We have been considering using a modified compressor (e.g. reciprocating, radial screw, rotary vane) running in reverse to extract the work. How feasible would this be to accomplish? Which would be the best for the application? Could you expect similar efficiencies as the compressor operation? Also which could we expect the highest efficiency from? I know this isn't information that people probably know off the top of their head but any help would be appreciated.