Optimization of the hydraulic performance and integration of a heat storage in the geothermal and waste-to-energy district heating system of Ferrara

District heating is an effective way to increase the share of renewables in the thermal energy sector. The district heating network (DHN) of the municipality of Ferrara in northern Italy uses a geothermal reservoir at a depth of 1 km supplying hot water at 100 C. Besides geothermal energy, the overall annual thermal demand of 170 GWh is covered by a combined heat and power (CHP) plant fed by municipal solid waste (MSW), and by auxiliary natural gas boilers. A short-term thermal energy storage having a useful capacity of 45MWh decouples the heat production plants from the distribution network. An advanced monitoring and control system has been recently implemented to supply a high-level service to the customers connected to the network, searching for the maximum electricity saving by reducing the supply pressure of the pumping station, and targeting the maximum natural gas saving by a proper production programming of the heat supply plants. The paper describes the main design and operational features of the district heating system, and shows how the improved operation was successful in reducing the electricity and natural gas consumption. The new pressure management consists in monitoring the differential pressure in many substations, especially those located at the periphery of the network, and adjusting the water supply pressure in order to fulfil the minimum differential pressure at the most critical substation. The modified production scheduling consists in keeping using the geothermal energy and waste-to-energy plants in periods of low thermal demand and low electricity prices to daily charge the storage system. Experimental data measured in the first months of improved operation show a high saving potential of electric power for pumping equal to 400 MWh/year, which represents a saving of 30% compared to the previous “manual” optimization of the hydraulic performance. Moreover, the enhanced storage operation enables a saving of 7000MWh/year of energy from natural gas, which finally contributes for only 11% to the annual heat demand. The calculated payback time of only a few years shows the high cost saving potential of these energy conservation measures in such systems.