The application of Solar Heat
Despite its small size, the solar district heating system in Lodi, close to Milan, is one of few in Italy to feed surplus heat into a local network. Since March 2017, heat produced by a 192 m² solar thermal installation is used as a heating source. The local district heating, or DH, network is 25 kilometres long and provides about 200 buildings with thermal energy, which corresponds to around 60 GWh of distributed heat per year. Heat sources include a gas- and biomass-fired CHP unit and a solar thermal system installed at the sports centre’s swimming pool.
The idea to connect the solar field to the local district heating network was born when the efficiency and annual energy yield of the system dropped to unsatisfactory levels. This happened after major changes had been made to the pool buildings and the installation turned out to be oversized for the new layout.
Despite the connection, the energy yield turned out to be lower than expected. In cooperation with the Energy Department of the Politecnico di Milano, the district heating company Linea conducted an analysis of the monitoring data available for the entire period since March 2017 to better understand why performance was so poor and to suggest a course of action. For instance, between May and June last year, the solar system supplied the network with around 13,600 kWh. This means efficiency was at 33%, partly because of long pipelines, which generate a loss of about 10%.
Based on a TRNSYS simulation model, the researchers investigated the possibility of optimising current strategies without generating additional costs. The idea was to reduce the above-described flow rate until it matches the one on the demand side, which would prevent hot water coming from the DH return pipe from being mixed with the flows exiting HXS1 and HXS2. It would also lower the inlet temperature of the HXS. In the simulated environment, solar field efficiency rose from 33% to 43% and energy output by about 30% (see the chart below).