In the EU-funded R&D project COMTES, three technologies for the seasonal storage of solar thermal energy are being developed. The aim of the project is to arrive at storage systems that need less volume than comparable systems working with water as the sensible storage material. In one development line, the Swiss research institutes EMPA and SPF work together with the UK company Kingspan Renewables on a system based on the liquid sorption technology, using sodium hydroxide as storage material. The principle of this technology is that with charging the storage, heat is used to change the diluted lye into a concentrated lye. The challenges with liquid sorption are maintaining a low pressure, that is needed to have effective evaporation of the water from the lye at reduced temperatures, and maximising the concentration of the lye, in order to have the highest storage density, without crystal formation.
A second technology, based on the supercooling effect of sodium acetate trihydrate, is developed by DTU and the Nilan company from Denmark, the Austrian TU of Graz and Kingspan Renewables. Here, the principle of storage is based on supercooled phase change material. In effect, the storage material can cool down to temperatures below the solidification temperature without solidifying. Therefore, the material can be stored for very long periods of time without heat losses. When the supercooled liquid is triggered, solidification starts and heat becomes available. The challenges with this technology are maintaining a stable supercooling and finding techniques to have a controlled solidification.
In the third development line, AEE INTEC from Austria, ITW, TH Wildau and the company Vaillant from Germany collaborate on the development of a solid sorption thermal storage. In an earlier stage, a comparison was made between an open and a closed sorption system. The choice has been made for a closed system, evacuated to work at low water vapour pressures with a number of storage modules that consist of a vessel with a fixed bed of sorption material and an embedded heat exchanger for charging and discharging. The aim is to have a test system ready in summer 2014 that will be monitored for a period of one year. In the preparation of this system, several prototype modules were built and tested at ITW and at AEE INTEC. The goal of these tests is to study and understand the dynamic behaviour of a fixed bed of sorption material, in order to optimise the modules used for the test system.