Feasibilities of utilizing thermal inertia of district heating networks to improve system flexibility

The use of district heating network inertia (DHNI) has been regarded as an efficient and cost-saving method to improve the flexibility of the energy systems. However, the acclaimed benefits in most studies are found in specific cases with traditional middle-temperature systems. The cases where DHNI is not feasible are unclear and the applicability of DHNI in future low-temperature district heating (LTDH) systems requires further investigations. Therefore, this study applied a top-down methodology where the practical storage potentials of DH networks are evaluated based on field investigations of 134 Swedish DH networks and 25 Chinese DH networks with various sizes and demand densities. Empirical relationships between the heat density and storage potentials are established and analyzed. Then, bottom-level analysis from technical and economical aspects are conducted on a variety of application scenarios for DHNI, including different temperature levels, heat sources, control strategies and renewable energy profiles. It is found that in LTDH system, by raising the network temperatures to actively use the DHNI, the heat source efficiency is reduced regardless the size and density of the network and, thereby, making the DHNI infeasible. This implies that the DHNI is only applicable in middle-temperature systems with combined heat and power plant (CHP) as a heat source in the extraction mode. Furthermore, the back-pressure mode is not economically attractive. In summary, the results from a multi-scenario analysis identified limited benefits of the DHNI, implying a proper consideration of its roles in future works.