EXPERIMENTAL TESTING OF A SMALL-SCALE SUPERCRITICAL ORC AT LOW-TEMPERATURE AND VARIABLE CONDITIONS

asme-orc2015 Tracking Number 126

The detailed experimental investigation of an organic Rankine cycle (ORC) is presented, which is designed to operate at supercritical conditions. The net capacity of this engine is almost 3 kW and the temperature of the hot water is always lower than 100 oC. The laboratory testing of the engine includes the variation of the heat input and of the hot water temperature. The maximum heat input is 48 kW, while the hot water temperature ranges from 65 up to 100 oC. The tests are conducted at the laboratory and the heat source is a controllable electric heater, which can keep the hot water temperature constant, by switching on/off its electrical resistances. The expansion machine is a modified scroll compressor with major conversions, in order to be able to operate with safety at high pressure (max. pressure around 40 bar). The ORC engine is equipped with a dedicated heat exchanger of helical coil design, suitable for such applications. The speeds of the expander and ORC pump are regulated with frequency inverters, in order to control the cycle top pressure and heat input. The performance of all components is evaluated, while special attention is given on the supercritical heat exchanger and the scroll expander. The performance tests examined here are the ones for hot water temperature of 95 oC, with the aim to examine the engine performance at the design conditions, as well as at off-design ones. Especially the latter are very important, since this engine will be coupled with solar collectors at the final configuration, where the available heat is varied to a great extent. The engine has been measured at the laboratory, where a thermal efficiency of almost 6% has been achieved, while supercritical operation did not show superior performance as expected, due to the oversized expander. A smaller expander would allow operation at even higher pressures for higher speed with increased electric efficiency, which would probably reveal the full potential of the supercritical operation.