The use of a thermal accumulator can help to resolve the gap between heat production and consumption, for refrigeration and air conditioning purposes. The heat accumulator can store heat as both latent heat and sensitive heat. For sensitive storage, stored energy raises the temperature of the material, while for latent heat storage, the material changes phase at constant temperature by releasing the heat.
The main advantages of latent heat storage on sensitive storage are:
Thus, for the same capacity of 180 kWh, the volume occupied by sensitive storage is 7 times larger than that used by latent storage.
Thermal losses due to the difference between the temperature inside the bin and the outside temperature are much greater for sensitive storage than for latent storage, due to a much larger transfer area. Thus, for a capacity of 180 kWh, thermal losses are more than three times greater for sensitive storage than for latent storage.
In addition, additional thermal losses are to be considered for sensitive storage, due to the existence of a mixture layer. Indeed the hot water on the return is mixed directly with cold water at the start. The result is a significant thermal loss, with the establishment of a mixture layer with a thickness of several tens of centimeters. Because of their large volumes, sensitive storages are placed outside while latent storages are placed inside. This results in an additional thermal loss for latent storages due to solar radiation.
For operating costs, the cost varies depending on whether the sensitive storage has a free area or not. If it has a free surface, it is necessary to add the electrical consumption of the pump necessary to hoist the water back to the top of the tank. For example, a 150W pump is required for a storage capacity of 180 kWh. Finally, the use of latent storage allows for a very low temperature, close to 0oC, which allows the cold to be transported over long distances, especially in the case of a cooling district.
In conclusion, latent storage frees you from the many constraints inherent in sensitive storage. While latent storage has traditionally been for water storage in the form of ice, the current rise in PCMs or phase-changing materials now allows heat to be stored at multiple temperatures and further expands the field of opportunities offered by latent storage.