Background

More than 65% of the energy demand in the built environment is for space and hot water heating. According to the Dutch Climate Agreement (Klimaatakkoord), this demand should be halved by 2020. The steps needed to accomplish this target have been elaborated in a research program by Building Future, a research collaboration by ECN and TNO regarding energy in the built environment. One technology that is essential in reaching this target is the availability of compact seasonal storage of solar heat.|

Neither water nor phase change materials are capable of storing the summer’s abundance of solar heat for use in winter: only thermochemical materials can be used for efficient long-term heat storage. In theory, a storage system based on thermochemical materials can store five to ten times more heat in a given volume than a similar storage system based on water. Moreover, thermochemical storage does not suffer from substantial heat losses.

In thermochemical storage, heat is stored in a reversible chemical reaction A + B → C + heat. In summer, the storage is charged by separating material C into its two components A and B under the addition of heat; in winter, the stored heat is released when A and B are brought together. In many cases, a hydration reaction is used, where A is a solid, dehydrated salt hydrate, B is water vapour, and C is the hydrated salt.

Material research

ECN’s research on thermochemical materials is aimed at the development of a material that can store heat with an energy storage density of more than 1 GJ/m3 (more than 275 kWh/m3), and that can be charged using solar thermal collectors, i.e. at maximum temperatures of  150°C.
In earlier research, a number of promising materials have been found, including magnesium sulphate heptahydrate (MgSO4•7H2O), magnesium chloride hexahydrate (MgCl2•6H2O), and calcium chloride dihydrate (CaCl2•2H2O). However, even though these materials have very promising energy storage densities, they are also difficult to handle in repeated charging and discharging cycles.

Reactor development

The aim of ECN’s research on thermochemical reactors is to develop a reactor for one of the materials mentioned above, to be used for domestic space and tap water heating, and to be charged by solar thermal collectors with a maximum operating temperature of 150°C.
Several reactor concepts have been investigated, including both atmospheric and low-pressure reactors, and including designs with both separate and integrated reactor and storage. Based on a first techno-economical evaluation, it was decided to focus current research on the development of an atmospheric, integrated reactor concept. Currently, a prototype reactor with a volume of ~0.5 liter is being tested under laboratory conditions.

System integration

No technology can be developed without thorough knowledge of the application it is being developed for. Because of the impact of thermochemical storage on a building and heating system - both in terms of physical size and in terms of novelty in systems approach - it is important to pay attention to building and systems integration aspects early on in the development.