Properties of some salt hydrates for latent heat storage

Abstract

For the utilization of low-grade heat the latent storage of thermal energy is of great advantage because the heat can be preserved at a constant temperature perfectly matched to the special purpose of application. Investigations on the heat capacities, enthalpies of fusion, densities, crystallization behaviour and other chemical and physical properties have shown that the following salt hydrates are especially suitable media for storing low-grade heat. The eutectic mixture of water and 3.92% by weight of sodium fluoride, melting point (MP) = - 3.5°C, is extremely convenient and cheap for refrigerating or other cooling purposes. Lithium chlorate trihydrate, LiClO3. 3H2O, MP = +8.1°C has an extremely high storage capacity and other advantageous properties as a storage medium in cooling systems, but a very high price will limit its application. Calcium chloride hexahydrate, CaCl2. 6H2O, MP = + 29.2°C, is a suitable and cheap storage medium for heating purposes. For the same application disodium hydrogen phosphate dodecahydrate, Na2HPO4. 12H2O, MP = + 35.2°C, is even better because of the larger storage capacity per unit volume and other advantages which largely compensate the higher material cost. the unique properties of potassium fluoride tetrahydrate, KF. 4H2O, MP = +18.5°C, make it especially suitable for storing low-grade heat. It can directly function as an energy sink and as an energy reservoir in heat collecting and consuming systems. Examples of the practical applicability for residential heating, temperature levelling and cooling are described.