Exergy Analysis of Scroll-Based Rankine Cycles with Various Working Fluids

Abstract

In this study the possibility of converting scroll compressor into expander is investigated. Refrigeration equipment manufacturers produce scroll compressors massively for refrigeration and air conditioning applications. It is shown here that, through appropriate modeling, catalog data of scroll compressors can be used to predict the operation in reverse, as expanders. The modification of the geometry with respect to rolling angle and involute angles are necessary to use scroll compressor as expanders in heat engines. If no modifications are made to the scroll compressor, the efficiency of the Rankine cycle will result low because the built-in volume ratio is not adapted to the cycle configuration for the same pressure and temperature levels in the expanders. A low capacity scroll compressor is selected from a refrigeration equipment manufacturer and using the equations for modeling of positive displacement compressors and the compressor manufacturer data for nominal operation, isentropic efficiency, built-in volume ratio and the flow coefficient of the scroll machine are determined. After these determinations, the expander model has been used to predict the operation of the same scroll machine in reverse as it without and modification of the geometry. The resulting Rankine cycle is non-realizable with a low exergy efficiency of 50 % since the sink temperature for the cycle is far below the normal environmental temperature. In order to run a feasible Rankine cycle with the selected expander, without changing the scroll geometry and the working fluid, the upper pressure and temperature must be increased. It is found that by increasing the pressure and temperature at the expander intake to supercritical value, that is 68 bar and 264 °C, the cycle becomes realizable and achieves an exergy efficiency of 61 %, respectively.