Long-Term Heat Storage with NaOH
Abstract
To reach high solar energy fractions for building heat supply, several seasonal thermal storage techniques have been developed so far. Besides ground storage techniques, thermo-chemical techniques with high heat storage capacity and virtually no heat losses in the storage state are most promising. The paper deals with closed sorption systems and focuses on the concept with sodium hydroxide (NaOH) - water as the working pair. In an experimental prototype set up, heat charge and discharge processes of the storage under low pressure conditions were analysed and verified. The storage capacity is limited by the temperature levels of the produced heat and by the solidification of the NaOH lye. Simulation results show that for charging the storage, solar heat can be supplied at 100°C, and that, compared to conventional water storage, the system volume related heat capacity could be increased by a factor of 6 for low temperature space heating (40°C) and by a factor of 3 for domestic hot water supply (65°C). During the last months, the laboratory prototype was extended with a double-stage heat exchanger. Currently measurements to identify system parameters are carried out. Future systems shall be built in one integral vacuum container, containing solution tanks, heat exchangers, piping and pumps.