Design aspects of a reversible heat pump - Organic rankine cycle pilot plant for energy storage

Combining a heat pump cycle (HP) and an organic Rankine cycle (ORC) in one reversible cycle and adding a thermal storage enables an innovative and reversible energy storage concept. This paper describes preliminary considerations, design methods and the development of a HP-ORC-pilot-plant. The storage temperatures are critical factors, as they determine the storage capacity and influence the power-to-power-efficiency. Methods of multicriteria decision-making, such as Pareto optimization, identify favorable combinations. On this basis a pilot plant is designed for 90 degrees C lower and 120 degrees C upper storage temperature aiming at a power-to-power-efficiency of 59% and an electrical storage capacity of 3,6 kWh/t. The selection of the working fluid has also a strong impact on the reversible process, as apparatus properties alter, in some cases effecting significant increase in costs. In the considered case the refrigerant R1233zd(E) shows the best combination of performance, costs and safety. The realization of the compressor of the heat pump and the expander of the organic Rankine cycle in one single machine is another complex task to be considered. Especially the lubrication system has to fulfill the demands of both modes, thus innovative designs of the oil supply and separation are shown, like parallel and serial layouts. (C) 2020 Elsevier Ltd. All rights reserved.