Increasing the energy flexibility of existing district heating networks through flow rate variations

The possibility of recovering waste and renewable heat in urban areas has made district heating networks one of the key infrastructures for decarbonising the heating sector in Europe. The ability to shift the heat production over time is an important asset to improve the competitiveness of both new and existing networks. To this purpose, the present study evaluates the flexibility potential provided by the water volume enclosed in the network pipelines using the district heating network of Verona (Italy) as a case study. Given the users heat demand in two representative weeks, computer simulations were carried out to calculate the network's thermal response to circulating flow rate variations. The latter were designed to cause an early or late shift in the daily peak thermal load. The local temperature drop at the users substations compared to the current situation was used as a relative discomfort indicator. The simulations were carried out using the model NeMo, which has been attached as supplementary material to this publication. The simulation results show that the network pre-charge strategy achieves significant benefits in terms of both peak shaving and load shifting with a limited impact on discomfort for the users. Furthermore, the paper shows that the available storage capacity of the network is significantly influenced by the duration of the flow adjustment, by the average velocity of the heat carrier fluid and by the daily heat demand pattern. Finally, it is shown that the energy flexibility of the network also depends on its topology. In this regard, the article provides preliminary indications on the number and position of the bypass pipes needed to implement the proposed flow rate variations. In conclusion, the article shows that the thermal inertia of the heat carrier fluid is an interesting source of flexibility for existing networks, and the results highlight its potential and limitations.