Design of a solar district heating system with seasonal storage in Italy

The residential sector is responsible for 26% of final energy consumption in the European Union. A key strategy to reduce household fossil fuel use is solar district heating with seasonal thermal energy storage. Although this technology has been widely applied in Northern Europe (Sweden, Denmark, and Germany), it has not been implemented in Italy. This research presents a new numerical tool, and applies it to the REPLICATE project in the Italian city of Florence, which is financed under the Horizon 2020 Smart Cities and Communities initiative. Our novel tool, which is based on a dynamic model coupled with the finite element method, has been developed to guide the design of the district heating plant and obtain a reliable estimation of performance, notably storage heat losses. The overall aim is to reduce the prediction inaccuracies that have characterized past projects. The final dynamic model is implemented in TRNSYS, and makes it possible to select the main plant parameters and define control strategies. It is linked to a detailed heat transfer model developed in COMSOL Multiphysics®, which can calculate storage heat losses and determine the optimal thickness of insulation material. Our in-depth parametric study determined the optimal volume of the hot water tank to be 3800 m3, and the size of the solar field to be 1000 m2. We also evaluated the effectiveness of the addition of a water–water heat pump. This analysis found that it is a crucial component as it can increase storage capacity and improve the performance of solar collectors by up to 124 MWh. Our results indicate that with an optimized configuration, the solar fraction of the system can reach up to 44%