Integration of heat pumps into thermal plants for creation of large-scale electricity storage capacities

Within Thermo-Electric Energy Storage (TEES) concepts, thermal plants are conceivable for reconversion of stored heat into electricity. By this means, new areas of application for existing thermal plants are established and the costs of the storage system are reduced. A promising TEES approach is Pumped-Heat-Electricity-Storage (PHES). In the present study, the thermodynamic potentials of the new concept of integrating PHES systems into different types of thermal plants for the creation of large-scale electricity storage units are assessed – based on exergetic quantities – including the discussion of technical aspects. Using the environment as the heat source, recuperated heat pump designs are investigated with regards to the achievable efficiencies for different working fluids (CO2, air and Argon) and the related processes (trans-critical/super-critical). The investigated maximum heat pump temperature range is between 50 °C and 700 °C. The heat pump designs are individually optimized concerning their remaining degrees of freedom. Finally, a combined characteristic diagram is provided, which allows to identify the most reasonable heat pump working fluid and process configuration referring to the boundaries of a specific storage concept. Electric heaters as a simpler method for power-to-heat conversion are assessed as well. The results show that exergetic heat pump efficiencies of above 70% can be achieved if the maximum temperature of the provided heat is in the range of 300–600 °C while the minimum temperature is elevated. It is also shown that trans-critical cycle designs are not generally superior to super-critical cycle designs at these boundaries. Based on the results of the heat pump analysis, the round-trip-efficiencies of different heat integration options into different types of thermal plants are estimated – the reachable efficiencies are roughly in the range of 50–60%. Finally, the application of heat pumps and electric heaters in series connection is assessed. Then, the round-trip-efficiencies of the storage concepts drop by a few percent points (2–5%) but the technical challenges of designing high temperature heat pumps are reduced.