ORC, solar, geothermal, single-screw compressor
This paper describes the behavior of an Organic Rankine Cycle (ORC) fed by a heat source with adaptable temperature and mass flow. For a suitable choice of working fluid, the setting of its evaporation pressure is crucial for the performance of an ORC installation. The higher the evaporation pressure, the higher the cycle efficiency on the one hand, but the lower the energy recovered from the heat source due to a higher outlet temperature on the other hand. An optimum has to be found to achieve maximum net power production. Heat source temperature profiles typical for geothermal and solar applications are simulated with an electric thermal oil boiler. An optimal control strategy, based on the installation empirical model is developed and verified on an 11 kWe ORC test rig. The installation consists of a reversed single-screw compressor acting as expander, plate heat exchangers acting as evaporator, internal heat exchanger, and condenser and a variable speed multistage centrifugal feed pump. Data acquisition and visualization are realized in a LabView application, which also includes thermodynamic parameters that cannot be measured directly, but are calculated in real time by means of thermophysical property library, Coolprop (Bell et al., 2014). The test rig can be charged with different working fluids. This paper deals with the results obtained during test runs with SES36 (Solkatherm) and R245fa, which are commonly used fluids in low temperature heat recovery applications. The oil temperature at the evaporator inlet is limited to 125°C because of the expander design: the average temperature may not exceed 120°C and the maximum pressure is 12 bar. Subcooling caused by non-condensable gases present in the working fluid, typically the case for installations with sub-ambient pressure in the condenser (in particular Solkatherm-filled), is taken into account.