The volume of the seasonal storage tank necessary for a maximum solar fraction is calculated from the energy density of magnesium sulphate (2.52 kWh. m-3) and the part of annual heat demand which is not provided by the solar energy in the reference case. But Fig. 1 shows that a large amount of heat collected in summer by the vacuum tubes is wasted. The seasonal TESS thus enables to complete or replace the thermal energy production of the auxiliary electric heater during the winter. The solar fraction is defined here as the part of the annual heat demand covered by the solar thermal energy. The storage volumes of material are presented in the Table 1.

Storage volumes ranging from 0.2 to 0.9 m3 are expected, depending on the type of climate and the achieved solar fraction. The excess solar energy available during the summer, used for the regeneration of the material, limit the storage capacity ; the solar fraction is therefore limited to a value of 57.3 % in Paris, whereas 100 % solar fraction is achievable in the sunnier region of Marseille. The summer regeneration of the material will be studied in detail in future works. It will lead to the re-sizing of the solar collectors area.

The corresponding stoechiometric amounts of water vapour, which reacts with MgSO4 during the phase of heat production, vary from 0.5 to 2.5 tons. The reactor is an open system and the water is taken from the ambient spoilt air exiting the house, which could be completed by external humid air or by an extra external air humidifier if natural humid air is not sufficient. The model is being optimised in order to get more accurate and meaningful values of the solar collector area, the volume of the tank and the control parameters with respect to the climate and the targeted solar fraction.

Two parameters are important when designing a TESS : the quantity of heat that can be stored per given volume and mass (the energy density) and the rate at which this energy can be delivered (the power density). A very high energy density is of no use if the heat is released very slowly, resulting in very low temperature lifts and unworkable thermal energy. Despite a high theoretical energy density, the practical use of pure magnesium sulphate is quite difficult. Under real operating conditions, the theoretical energy density of the magnesium sulphate cannot be reached at usable power densities. The material needs to be dispersed to react at a suitable rate, which decreases the energy density. For instance, the storage volume increases up to 2 m3 if 50 % of the theoretical energy density of the dense salt is achieved. One of the purposes of the experiments is to find a proper porous matrix to disperse the magnesium sulphate in order to reach the maximum energy density of the salt.