IEA Task 42 Compact Thermal Energy Storage: Material Development and System Integration

Investigations of compact seasonal heat storage based on stable supercooling of a sodium acetate water mixture have been carried out. The heat storage module concept is based on the advantage of stable supercooling. By using this concept the heat storage module will have no heat loss for a long period making seasonal heat storage possible. If a sodium acetate water mixture consisting of 58% (weight%) sodium acetate and 42% water, which has a melting point of 58°C, has been fully melted during the sunny summer, it can cool down in its liquid phase to the surrounding temperature and still preserve the latent heat related to the heat of fusion. The heat storage module can be left in this state with no heat loss until a heat demand occurs in the house in the winter, in which case solidification is activated, the heat of fusion is released, and the heat storage temperature increases almost immediately to the melting point.

Based on the investigations and experience gained with differently designed modules the following recommendations for the design of heat storage modules can be given:

Each module of a heat storage can be designed as a sandwich with a flat salt water mixture room in the middle and with flat heat exchanger rooms below and above the salt water mixture room. A height of about 5 cm of the salt water mixture room of a heat storage module is suitable. Heat exchangers above and below the salt water mixture room with water rooms with a height of 2 mm and with parallel channels, through which water is flowing, are suitable. The heat exchangers must be point welded to the outer surfaces of the salt water room to make a durable construction. Steel is suitable as the module material.

The inner part of the salt water room must be smooth, if possible without any “equipment” to stabilize the construction. The holes used to fill the salt water mixture into the salt water room must be placed at the end of the module, so that the module can be completely filled in a vertical position. The holes must be designed, so that no crystals can be placed outside the dimensions of the salt water mixture room. Finally, it is recommended to attach a mini tank in good thermal contact to the outer surface of the salt water mixture room. The mini tank, which has a pressure of 5 bar, can be filled with liquid CO₂. The boiling point of the CO₂ is thus -78°C. Solidification of the salt water mixture can be started by cooling down a small part of the supercooled salt water mixture to -16°C by boiling a small amount of CO₂ in the mini tank