The sun is a huge energy source with great potential of providing energy to the heating of homes and other buildings in an environmentally sustainable manner. In order to provide buildings with energy from the sun it is necessary to transfer the energy supply over time to when the demand arises. By storing the heat in a seasonal storage, solar energy from the summer can be used in the winter when the demand for heating is greatest.

Today's existing plants are mainly in Europe and particularly in Germany. These facilities are designed to supply heat demands greater than 400 MWh and covers about 40-50 % of this need which consists of energy for space heating and domestic hot water. How much of the heat demand that is covered, the solar fraction, is partly due to losses from the storage which in turn is connected to the surface area of the storage. The bigger a storage, the smaller the losses because of the decreasing relationship between surface area and storage volume. Looking at the size of the seasonal storages that are currently in operation, the question if seasonal storage is also suitable for installations designed for heat demands smaller than 400 MWh arises.

Jonas Haglund at the architect office Skanark AB in Karlstad is planning an accommodation of 40 flats and hopes that seasonal stored solar energy can serve as the main energy source for space heating and hot water. In order to make housing more attractive he is considering the idea of adding other features, like a pool and an atrium, that also require heating but with lower temperature requirements. Haglund would like to investigate whether the efficiency of the solar collector increases when the extra energy demands are added and if the energy cost, for those demands, in this way can be reduced.

The purpose of this study is to investigate the possibility of covering a large fraction of a small-scale annual heat demand corresponding to about 40 newly built apartments. The study shall present the solar fractions that can be achieved with different storage concepts when storage size and collector area is varied. The study will also answer how the solar fraction will change if a heating demand with lower temperature requirements and varied character is added to the basic domestic heating and if the added energy demand to some extent can be free. These questions are answered by calculations and simulations with the simulation software COMSOL Multiphysics.

The results show that it is possible to obtain solar fractions above 80% with sufficient collector area. Suitable storage volume varies depending on the specific storage concept. Simulations of seasonal storage in a tank show that a storage volume of 13 m³/MWh is an appropriate size, while the corresponding figure for duct storage in clay is 20 m³/MWh. An added heating demand of low temperature character increases the efficiency of the solar panels and creates, so called, free energy.