IEA Task 42, Compact Thermal Energy Storage - 4. year of participation

There has been carried out theoretical and experimental studies of how heat storage modules for compact seasonal heat storage, based on a stable of cooled saltwater mixture of sodium acetate and water, is designed best. A solar system with such compact seasonal heat storage can completely cover the annual heat demand for low-energy houses in Denmark. Based on the studies, it is recommended to design the modules as sandwich constructions with flat bottoms with brine mixture surrounded by a heat exchanger vessel above and below the brine tank. There will bee used a saltwater mixture, consisting of 58% (% by weight) and 42% of water. It is also recommended to use steel as container material and that the saltwater tank capacity is approx. 250-500 l, the saltwater container height is approx. 5 cm, and the heat container consists of a number of parallel channels with a height of approx. 2 mm. In addition, it is recommended that the inner surfaces of the saltwater tank is as smooth as possible and that saltwater tank is equipped with a fill plug, designed so that all the salt hydrate crystals can be melted by heating using heat exchangers. Finally, we recommend that module be supplied with a mini tank in good thermal contact with the salt container surface. Mini container can be supplied liquid CO2 under a pressure of 5 bar when solidification of the supercooled saltwater mix has to start, when there is a heat demand to be met from the heat store. There will then be immediately transported heat from the brine tank for the liquid CO2, which will boil and thus cool a small portion of brine mixture at such low temperature the solidification will starts.