[chuckm]

Just a slightly different track to suggest. Instead of using water as your working fluid, could you use refrigerant (R-134a)? The lower specific heat and boiling point will help your efficiency (even though the heat energy extracted may be the same as with water, turbines and the like run via temperature/pressure differences). Then, with an aftercooler condenser, you may be able to use the phase change to extract energy more efficiently by generating a higher pressure diffential. The circuit would be muffler/exchanger --> turbine or other expander --> aftercooler condenser (probably just an automotive condenser unit) --> check valve --> muffler/exchanger.