Experimental analysis of a reversible high-temperature heat pump/ORC test rig for geothermal CHP applications

Reversible high-temperature heat pumps (HTHP) are a promising technology for the application in geothermal combined heat and power (CHP) plants. The HTHP operation enables clean peak load coverage of an attached district heating network (DHN) during the winter, replacing fossil boilers, while reversible operation as an Organic Rankine Cycle (ORC) generates electricity from otherwise unused geothermal heat during the summer. This work presents the experimental characterisation of a novel fully reversible HTHP/ORC test rig, designed for the application in geothermal CHP plants. The test rig is supplied by a 200 kW hot water heating circuit as a heat source and uses a fully reversible 20 kW twin-screw machine as compressor and expander. A closed-loop intermediate cooling circuit allows simulating a DHN with varying temperature levels and mass flows during HTHP operation. The HTHP is capable of thermal loads up to 110 kW at heat sink outlet temperatures of up to 120 ∘ C , making it well suited for geothermal CHP applications. For investigated temperature lifts between 25 K and 57 K, COPs between 6.4 and 3.0 are observed. During ORC operation, net electric thermal efficiencies of 5-6 % and maximum net electric system efficiencies of 2.5 % are achieved for various heat source mass flow rates and temperatures between 110 ∘ C and 130 ∘ C . The oil separator is identified as a major limitation in both operational modes and should be replaced by a larger version to enhance efficiency and plant capacity. Furthermore, the oil delivery to the compressor needs to be optimised during HTHP operation in further studies. • Novel reversible high-temperature heat pump/ORC test rig for geothermal applications • Experimental characterisation under realistic operation conditions for geothermal CHP plants • Heat pump duties of up to 110kWth at heat sink outlet temperatures up to 120 °C • COP values between 6.4 and 3.0 at heat pump temperature lifts between 25K and 57K • Net ORC efficiencies between 5% and 6% for various heat source conditions.