Combining an Organic Rankine Cycle and a Heat Pump Cycle in a Test Plant for Reversible Storage of Energy

orc2019 Tracking Number 11

The heat pump cycle is a well-known concept to lift the temperature of a medium by using electricity and waste heat. The ORC-process on the other side offers the possibility to gain electrical energy out of thermal energy. Combining these two processes and adding a thermal storage enables an innovative and reversible energy storage concept called “Combined Heat Pump – Organic Rankine Cycle (HP-ORC)” [1]. As both processes use similar components, it is a consequent step to combine these in one single cycle with two operation modes. The combination on the one hand, reduces the investment costs, as the most expensive components (e.g. heat exchangers) pare down to the minimum. On the other, it poses difficulties, which have to be countered by finding the best compromises in process properties and apparatus designs. Crucial factors are the upper and lower storage temperature as they determine the storage capacity and influence the overall efficiency (power-to-power-efficiency). Methods of multi-criteria decision-making, such as Pareto-Optimization, identify favourable combinations of upper and lower storage temperature. The selection of the working fluid has also a critical influence on the combined process, as apparatus properties alter, in some cases effecting significant increase in costs The biggest challenge of the combined cycle is to realise the compressor of the heat pump and the expander of the ORC in one single machine. A lot of preliminary planning was done to find the best working principle for this diverse task. As the temperatures differ according to the mode (ORC or HP), the mechanical load situation is complex. Additionally the lubrication system has to fulfil the demands of both modes, requiring an innovative design of the oil supply and separation. To evaluate the concept, a pilot plant is built at the Institute of Process Machinery and Systems Engineering at FAU. This paper describes preliminary considerations, design methods and the development of a HP-ORC-pilot-plant. [1] Staub, S.; Bazan, P.; Braimakis, K.; Müller, D.; Regensburger, C.; Scharrer, D.; Schmitt, B.; Steger, D.; German, R.; Karellas, S.; Pruckner, M.; Schlücker, E.; Will, S. & Karl, J. Reversible heat pump-organic rankine cycle systems for the storage of renewable electricity, Energies, 2018, 11