Design, construction, and commissioning of a 500 kW high-temperature heat pump plant for the district heating network of Bochum, Germany

An innovative thermal pilot plant at the Fraunhofer IEG location in Bochum is a proof of concept for the technical use of high-temperature heat pumps (HTHPs) with seasonal mine thermal energy storages (MTESs), as a heat source, to supply conventional district heating grids (DHGs), significantly lowering CO2 emissions from fossil fuel employment. In this study, we present the steps involved in plant development, from preparation and planning to construction, testing and heat injection, which were essential for achieving successful operation and advancing the plant from TRL 4 to TRL 8. The implementation phases enabled the injection of heat into the DH grid, currently allowing to save annually up to 4800 tons of CO2 emissions, and represent a solid base for the construction of analogous experimental systems, which will be increasingly required due to greenhouse gas emissions reduction and renewable energy implementation. An important technological accomplishment of the project is the first-time achievement of the HTHP, which features a cascaded system of two units with ammonia and butane as refrigerants, of a supply temperature of 120 degrees C. The plant technology can be transferred and scaled to various applications, including industrial processes and low-temperature DHGs, and locations. Given the abundance of unused mining facilities and high thermal energy needs in these regions, the system can be implemented in former mining areas and paired with other geothermal sources. Depending on the available MTES, various HTHP sizes can be considered, ranging from tens of MW to smaller than the installed one.