Quantification and Optimal Control of District Heating Network Flexibility

In the evolution to a more sustainable energy system, large shares of renewable and residual energy sources are required. However, their intermittency calls for a more flexible energy system as well. One of the ways to create energy flexibility is through temporary energy storage. By storing excess energy at one point in time, an energy shortage can be compensated at another time.

District heating systems contain many places to store thermal energy and are hence promising in this context. Besides dedicated storage services, such as water storage tanks, aquifers and pits, there is storage inherent to the system as well. For example, the water contained in the network pipes represents thermal capacity than, when used well, can aid in creating energy flexibility. More specifically, by temporarily increasing/decreasing the network temperatures, the network can charged/discharged. 

However, the energy flexibiliy created with network storage, called network flexibility from now on, is difficult to quantify and control. It is these challenges that this PhD tackles. The PhD proceeds in different steps. Firstly, network flexibility is characterized, i.e. does a general behaviour exist? Secondly, network flexibility is quantified  such that different activations of network flexibility can be compared to each other and to other types of energy flexbility. Thirdly, models specific for network flexibility are developed, presenting a trade-off between accuracy and computational complexity. Finally, an optimal control tool for network flexibility is developed and applied to different cases. These cases show that network flexibility is only interesting for operation heat pump optimisation giiven variable electricity prices when the price is volatile and negatove at times. However, network flexibility is shown to be an effective and efficient means to achieve peak shaving.