Design of a seasonal storage for a solar district heating in Florence

Abstract The energy demand in residential sector constitutes an important fraction of the entire energy consumption (40% in EU). Solar district heating (SDH) is a key strategy to reduce use of fossil fuels in buildings. Inside the European project REPLICATE, financed by Horizon 2020 SCC1 Smart Cities and Communities, a SDH with a seasonal storage (STES) has been designed to be realized in the city of Florence. It is the first example of solar district heating with a STES in Italy. The design phase has aimed to size properly the extension of solar field and the volume of seasonal storage based on several parameters such as number of dwellings to feed, heat demand, solar resource, geology of the location and economic reasons. The paper deals with the model that has been realized through TRNSYS to describe the energy fluxes of heating plant andtheir optimization process. The computational model depicts the possible operating conditions and leads to define the control strategies of solar field and seasonal storage, integrated with commercial components that complete theplant such as gas boiler, heat pump and overall circuit. Hot water tank TES has been selected as the appropriate typology of storage for this application based on geological considerations. The thickness of insulation material and various layers have been determined. The numerical analysis fixes the volume of TES to be 3800 m3 and a solar field of about 1000 m2