Re-designing district heating networks through innovative CO2 solutions

One of the main problems of current district heating networks is the significant heat losses that occur due to the high temperature of the heat distribution fluid (usually, water in the Italian climate conditions). Moreover, in ORC- or steam-turbine-based cogeneration power plants, the heat distribution fluid is heated directly by the condenser of the ORC cycle resulting in low efficiency of the power production section of the system. Considerable power is also employed for pumping the district heating flow rate through a long distribution ring. In this work, the effect of replacing water with low-temperature CO2 as a working fluid in a district heating network is analyzed. Specifically, a real district heating installation (the Biogenera-Calenzano network) providing at design conditions 850 kW of electricity and 1.7 MW (average) of heat is assessed using an experimental data set, simulating the equivalent working conditions with CO2 distributed through the network. The proposed innovative solution continues to receive the heat from the condenser of an ORC cycle, but the temperature of the working fluid is significantly lower providing a relevant increase in the power production because of the reduction of the condenser temperature (determining an increase of the expansion ratio of the turbine). The increased power production of the turbine exceeds the power needed by local heat pumps which should be installed near the utilities of the district heating network to provide flexible heat on demand.