Experimental characterization of single screw expander performance under different testing conditions and working fluids

During the last years, one of the most popular ways to recover low-grade waste heat is the organic Rankine cycle (ORC). This technology is widely studied and continuously optimized and, as a result, there are many efficient installations available on the market utilizing heat with stable parameters such as from geothermal sources or from the biomass combustion process. However, if a variable hot source in terms of either temperature or flow rate is introduced, the expansion devices have to work at non-optimal conditions, which decrease the global efficiency of ORC installations, e.g. in the case of waste heat recovery. In order to characterize the performance of a positive displacement expander close enough to the optimum, the influence of pressure ratios, filling factor, and working fluid properties on power output is studied. In this paper, experimental results obtained on a small-scale ORC test setup based on an 11 kWe single-screw expander are presented. Two working fluids are used during the tests, i.e. R245fa and SES36 (Solkatherm). These working fluids are common for ORC installations exploiting low-temperature waste heat. The waste heat source is simulated by an electrically heated thermal oil loop with adjustable temperature and flow rate. Various waste heat inlet flow rates are considered in order to find an optimal evaporation pressure and to maximize the power output with different heat source profiles. Based on the experimental data, the expander model is developed. For each working fluid, optimal working conditions are determined. In most cases, there is under-expansion due to a relatively small built-in volume ratio, causing certain losses. By means of the model, the ideal expansion process is simulated and compared with the real one obtained experimentally to quantify these losses and conclusions can be drawn whether significant benefits can be offered by using an optimized expander instead of an ”off-the-shelf” reversed compressor.

ISBN:978-2-9600059-2-9

Publication year:2015

Accessibility:Open