Model-based design and experimental analysis of a compact PCM thermal storage for integration in heat pump cycles

Air source heat pumps (HPs) with integrated phase change material (PCM) enhanced thermal storage have a high space-saving potential compared with the state-of-the-art using sensible heat storage. To be competitive, they must be manufactured at low cost and achieve a sufficiently high thermal efficiency. This contribution considers the optimal design of a compact and cost-efficient storage module. The storage can be used specifically as a desuperheater or generally as a storage located between the refrigeration circuit and the water circuit. It uses aluminum micro port extrusion (MPE) tubes which are brazed together with aluminum fins and are immersed in PCM. If used as desuperheater, the setup allows to use the high temperature of the refrigerant’s hot gas phase for Domestic Hot Water (DHW) generation. Reduced numerical Modelica models are developed for the model-based design of the storage internal heat exchanger (HEX) configuration. Refrigerant, water and PCM layers are transformed to equivalent lumped and distributed elements which are further discretized in the fluid flow direction. Predicted heat transfer rates, charging and discharging times, storage internal temperatures as well as stored energies have been analyzed using detailed Computational Fluid Dynamics (CFD) models as well as experimental data from a HP test rig. Experiments were performed using a small propane heat pump (250 g refrigerant) with a PCM storage module working as a desuperheater using the commercial PCM RT55. The results indicate that the reduced Modelica model gives reasonably accurate predictions. Furthermore, for a small volume test case, the reduced model was about 33 000 times faster on one processor core compared to the CFD simulation on 16 processor cores. Hence, the reduced models open the way for fully automated design optimization. In a case study, an optimized design was found for a storage module acting as a booster for DHW production in an R32-HP, which allowed high heat transfer rates with a minimum number of Micro Port Extrusion (MPE) tubes. It turns out that the module reaches a degree of compactness of 75% which means a significant improvement compared with previously analyzed modules based on plate and bar geometry, which only achieved 48%.