Converting a commercial scroll compressor into an expander: Experimental and analytical performance evaluation

The development of low cost small scale Organic Rankine Cycle (ORC) has a very interesting potential in generating electricity using low temperature waste heat sources. Moreover, HVAC companies could significantly extend their market if a commercial scroll compressor can be converted into an expander using similar units. Therefore, this work reports experimental test funded by an Italian HVAC company on a scroll compressor modified to work as scroll expander in a non-regenerative cycle and a subcritical fluid regime, aimed at reducing system cost and complexity. The scroll expander has been tested with its fluid R410A in a ORC cycle in order to obtain the isentropic efficiency of the scroll expander (0.5) and the pump (0.4). On the basis of the experimental tests, a model accomplished by means of MATLAB/CoolProp has been set up to evaluate the performance of the ORC group to achieve 10 kWe as target power output. Four operative fluids have been simulated, i.e. R245fa, R134a, R1234yf, R1234ze, fixing 100°C as evaporating temperature and considering the condenser temperature in the range 20-50°C. The results have showed that R245fa is the most promising working fluid since there is a higher expansion ratio within lower pressure values. As a consequence, not only a lower mass flow rate is necessary, but overall a lower pump consumption is needed, reaching greater overall conversion efficiency (about 6.5% with condensing temperature of 20°C) and power. Thus, a commercial heat pump scroll compressor can be effectively converted into an expander. The fluid selection shows that the most common ORC fluid can be used with relative low performance but at low cost and easy management.