by Wim van Helden, Group Leader, Thermal Energy Storage
Solar thermal energy has the potential to cover 100 % of the heat demand of households through completely renewable energy. To achieve this, the surplus of solar heat from summer needs to be stored by a seasonal thermal energy storage to cover the demand in winter.
To meet the special requirements for a solar seasonal thermal energy storage used in households, which are mainly a high energy density (low space requirement) and low losses, an adequate technology has to be found.
Main features of the project
The EU-funded project COMTES, with a research grant of 4.7 Mio €, was divided in three different development lines that all work on a different technology for seasonal thermal energy storage. In development Line A, AEE INTEC, ITW Stuttgart, Vaillant and TH Wildau collaborated on the development of a solid sorption thermal storage based on the working pair zeolite 13XBF and water vapour.
The aim of the project was to develop and demonstrate a solar seasonal thermal energy storage system which has a significantly higher energy storage density than sensible water storages. To this end, full scale prototypes were developed, tested and improved and integrated in a complete storage system.
The sorption storage stores the heat gained by the evacuated tube collectors and release the heat in case of heat demand (domestic hot water or space heating), see the system diagram below. The low temperature heat source supplies the necessary heat to evaporate the water coming from the water reservoir. The generated vapour flows to the sorption storage and the adsorption (discharging) takes place. Reversely, during desorption (charging) the solar heat is used to drive the water vapour out of the zeolite and the released vapour is condensed at the evaporator/condenser heat exchanger again and the heat of condensation is transferred to the low temperature heat sink in this case. The storage system is under vacuum and therefore a low temperature level is sufficient to evaporate the water.
The testing period was separated in two steps. First, explicit system experiments were carried out to test the system under given boundary conditions and to determine the main parameters of the system. The second part was the completely automated operation of the system over a heating period under realistic conditions at the AEE INTEC laboratory in Gleisdorf, Austria.
At first, the system was tested to gain knowledge about the energy density under different boundary conditions. Therefore, the desorption, adsorption, evaporation and condensation temperatures were varied.
The results show that under good, but realistic conditions a storage energy density of 178 kWh/m³ is achievable. This is about three times more than sensible water storages can store, with the additional advantage that this amount of energy can be stored over months without losses. This high energy density could be achieved due to the application of the novel charge boost method, which is used to further increase the storage capacity of the material.
In the second step the heating capacity of the system was tested in automated operation for a longer period. To this end, the system was integrated in a "hardware-in-the-loop” system, where a building simulation calculated the heat demand of a low-energy single family house according to the actual weather conditions at the test site in real time. This way a heating period could be demonstrated and the feasibility of the technology could be proven. The result was a solar fraction of 83 % which could be achieved with the demonstrated system over the heating period.
The results of the experiments and the demonstration period prove that this technology is promising for application as seasonal thermal energy storage, but also other applications are promising e.g. for temperature conditioning of electric batteries of electrical or hybrid cars, for storing waste heat from industrial processes or for using surplus electricity from PV or wind.
Wim van Helden
Thermal Energy Storage
AEE - Institut für Nachhaltige Technologien
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