In this report an overview of heat storage systems is presented. Usually, storage systems are insulated tanks filled with water. Small tanks are used in combination with solar collector systems and large tanks in greenhouses. For a better stratification vertical systems are preferred to horizontal tanks. Disadvantages are the heat loss and the requirement of a large temperature difference to unload the system.
Examples are salt hydrates and paraffines.
A great deal of research has been carried out to improve the heat conduction and heat transfer. To avoid oxidation of the salt and loss of material, some PCMs are encapsulated. These macro-encapsulated PCMs can be used in water tanks or a water bag to increase the storage density. For passive systems in buildings it is also possible to use organic or micro-encapsulated PCM in building materials like wallboard and plasters. These building materials are not commercially available yet.