Performance of the absorption process in a seasonal sorption heat storage prototype

Seasonal heat storage has great potential to enable renewable heating and seasonal load shifting. Nevertheless, it remains an on going challenge today. The benefits of sorption heat storage are fonud in the potentially lossless storage, when not including charging and discharging processes, and the prospective of greater volumetric energy densities compared to water. In sorption heat storage not sensible heat is stored, but the potential to regain heat. In the framework of the EU funded project COMTES a closed sorption heat storage demonstrator based on sodium hydroxide as sorbent and water as sorbate was developed and built. The built demonstrator operates on a hybrid basis. Heat is stored in sensible hot water tanks for diurnal storage and in the sorption heat storage system for seasonal storage. This grants the possibility to utilise the benefits of both systems, namely low charging and discharging losses in sensible storage and low heat losses during storage time in sorption storage. The complete system is built into a 7 m long shipping container. A solar collector field with an active area of 18 m 2 mounted on the container serves to cover the total heat demand. The central component of the sorption system is the interconnected absorber and desorber (AD) and evaporator and condenser (EC) unit. Both units are built as tube bundle falling film heat and mass exchangers similar to a solar thermal chiller. In initial operation with water, 6 kW of water vapour equivalent could be transported from desorber to condenser with a temperature difference between desorber and condenser of approximately 30 K. Nevertheless, absorption tests with sodium hydroxide at a concentration of 50 wt% showed much lower power output below 1 kW. It was found that even though absorption occurred, the process of water vapour absorption is slower than expected. The sodium hydroxide flows over the tube bundle with little water gained, thus releasing little heat. Results from this work and further testing of the speed of absorption have shown that novel heat and mass exchangers are required for sorption heat storage applications.