There are different sorption principles that can be used for heat storage. An absorption technique is used in a commercial device that can provide 20 kW of heat and 10 kW of cooling from a solar source, the TCA from company Climatewell in Sweden. Extensively monitored during Task 32, it is more a chemical heat pump than a true storage.

It can however store energy for a few hours and convert hot water to cooling and heating. The principle is that when using one tank containing water (evaporator), and an other one containing an hygroscopic salt (reactor), water will evaporate to the salt that absorbs the water. When the confine /

space is in a state of vacuum the water transport will be so high that the water will start boiling in order to produce vapour at the same speed as it is absorbed by the salt. Such evaporation requires energy. If the energy is not supplied from outside the system it will be taken from the water itself, which as a consequence gets colder

Other techniques based on adsorption principles tried to use silicagel or zeolite as a storage medium. After extensice search in laboratory in Switzerland and in an experimental house in Austria, both media showed limitations in terms of usable temperature lifts during discharge when used as particles in beds. A new way to use zeolite has being developed in Germany within Task 32. An extruded solid zeolite with air channels showed interesting properties at laboratory level. Simulation showed that a 8 m3 storage volume would be enough for 70% of the heating load of a low energy house. A laboratory prototype will deliver more information soon.

Fig. 7. The Monosorp storage concept from ITW, Germany is still a laboratory prototype but could lead to a dense seasonal storage solution for one family house.

At Empa in Switzerland, a prototype of storage unit based on the NaOH desorption at 150 C principle has been set up. First results show that the material can dry in summertime even better than anticipated (65% concentration reached) reducing the needed charging temperature to 120 C. The pilote installation will be monitored during 2 years and modelling will enlarge the scope of the results.