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AS the inevitable exhaustion of fossil fuels looms ever closer, the exploration of alternative energy sources becomes increasingly important. One appealing option for low-power applications is the organic Rankine cycle engine (ORCE). Similar to the large steamRankinecycleenginesusedinelectricpowerplants, anORCE utilizes heavy working uids and has superior effi ciency compared to steam Rankine cycle engines for heat source temperatures below around 900 K. An ORCE typically requires only a single-stage expander, making the ORCE much simpler than multistage steam systems. Such simplicity also gives the ORCE an obvious advantage over other types of small engines that are more mechanically complex, for example, typical two-or four-stroke engines as well as the Stirling engine.

For the past several decades, thousands of ORCEs have been developed and used for remote terrestrial applications with …