The objective of this research is physical evaluation of a heat engine concept for converting solar energy or waste heat to electric power. This type of heat engine could be useful for distributed power applications delivering power at the rate of three to seven kilowatts. The concept calls for a Rankine cycle using R123 for the working fluid. It uses a positive-displacement expander coupled to an electric generator for electric output. In the laboratory, the energy input comes from steam.
The evaluation includes characterizing the behaviors of available positive-displacement expanders for use in the heat engine: scroll and gerotor. Laboratory tests determine the isentropic efficiency and energy conversion efficiency of the expanders at different operating conditions. The system performance is determined by combining the measured expander performance with a thermodynamic model of a complete system.
A parametric study shows the relationships between operating conditions and system performance. This information can be used to select the appropriate heat exchangers and liquid pump to build a heat engine. Both the scroll and gerotor expanders are suitable for use in a low-temperature heat engine. The predicted performance for a system operating between 350°F and 70°F using one of these rotary positive-displacement expanders is 5kW power output with 14% thermal efficiency.