The sizing of energy storages or how valuable is the last step?

ABSTRACT This paper presents the electric energy self-sufficiency and the grid independence rate of a residential building equipped with a photovoltaic (PV) roof, as a function of battery capacity. Thermal energy self-sufficiency is enlarged by driving a heat pump (HP) with the electricity surplus of the PV system. The size of the thermal storage tank was varied from 0.3 up to 50 m3. Further on, the effect of using the building mass as a thermal storage was investigated. The considerations deal with theoretical investigations of the sizing of both electrical- and thermal storage systems in order to reduce the load on the supply grids under economic conditions. The investigation of storage capacities is based on thermal-energetic building and plant simulations. A new cost-benefit assessment for storage systems, which considers calendrical lifespan and charging cycles, is presented. The simulation results show that small to middle sized decentralized electrical and thermal storage are an economic way to keep the power grid stable during day cycle, while reducing CO2-emissions by using more renewable energies. Here the thermal mass of the building can also be used as a short-term storage. Running seasonal storage that guaranties 100% grid independence economical is almost impossible today.