The increasing electricity demand and cost, in addition to the increasing global energy demands, are the main motivations in seeking for sustainable new technologies in the ﬁeld of energy conversion and utilization. Various thermodynamic cycles, such as organic Rankine cycle (ORC), supercritical Rankine cycle, supercritical Brayton cycle, Kalina cycle and trilateral ﬂash cycle, have been proposed to convert the low-grade heat sources into electricity. ORC is extremely popular for generating electricity from a wide variety of heat sources, such as industrial waste heat, solar energy, geothermal energy, waste heat from internal combustion engine, etc. For large systems (> 5 MW), typically axial turbine expanders are the preferred design. However successful implementation of any of these cycles requires availability of systems less than 5 MW scale and definitely at less than 1 MW. The critical component in these small scale system is a positive displacement expander. Examples of positive displacement expanders are linear piston expanders, rotary wankel engine type expanders, etc. The development of a cost effective positive displacement expander that is not only efficient but also inexpensive is critical for the success of small scale electricity generation systems.