Energy efficiency analysis of Organic Rankine Cycles with scroll expanders for cogenerative applications

Small scale Organic Rankine Cycle (ORC) systems has been the object of a large number of studies in the last decade,because of their suitability for energy recovery and cogenerative applications. The paper presents an ORC numerical model and its applications to two different case studies; the code has been obtained by combining a one-dimensional model of a scroll machine and a thermodynamic model of a whole ORC system. Series production components,such as scroll compressors,from HVAC field,have been first considered in order to reduce costs,because this is a critical issue for small scale energy recovery and cogeneration systems. The detailed model of the scroll machine is capable to calculate the performances of both a compressor and an expander,as function of the geometry of the device and of the working fluid. The model has been first tested and validated by comparing its outputs with experimental tests on a commercial scroll compressor,then used to calculate the working curves of commercial scroll machines originally designed as compressors in the HVAC field,but operating as expanders. The model of the expander has been then integrated in the thermodynamic model of the ORC system. A series of comparisons have been carried out in order to valuate how the performances are influenced by cycle parameters,scroll geometry and working fluid for different applications. The results confirm the feasibility of small scale CHP systems with acceptable electrical efficiency,taking into account the low temperature thermal source,the small power output and the low-cost series production components employed.