Flexibility provision in 5th gen district heating systems

Motivation: To reach climate neutrality in 2045 in Germany, a major transformation is needed to decarbonize heating systems in the building sector. Next to decentral electrical heating systems, district heating networks are meant to play a vital role in that transformation [1]. With district heating systems evolving and buildings’ energy efficiency improving, system temperatures go down, with 5th gen heating networks allowing flow temperatures of e.g. 20°C or less, boosting efficiency and enabling the use of e.g. low temperature waste heat. In such systems, heating and cooling demands can be supplied and balanced out at the same time, typically incorporating heat pumps and compression chillers. These technologies provide the possibility of sector coupling, together with storages they allow flexible operation, according to the needs of a power grid dominated by renewable generation. Thus, they are well suited for the integration in future renewable energy systems. However, systems of this type have high investment costs and the cost competitiveness of district heating networks might not always be given, depending on local factors. Establishing business models using that flexibility can play a crucial role in decreasing the overall costs of these systems.
Methods: The communal energy system modelling tool KomMod [2] is used to represent the energy system of an example district in Herne, Germany, incorporating a 5th gen district heating network supplying a quarter characterized by seasonal heating demand and all-year cooling demand. KomMod is a linear, cost-optimizing, techno-economic energy system model. It yields a cost optimal energy system including the installed capacities and operation in hourly resolution of all technologies. Thermal networks are represented in the form of energy differences, which are solved on different temperature levels. The considered district consists of various multistory-buildings with different demand patterns. The heart of its energy system is the heating network with a hot water pipe with 22°C and a cold water pipe with 12 °C. Heating demands are supplied on different temperature levels by decentral heat pumps, cooling demands are supplied passively by the cold water pipe or compression chillers. In this study different possibilities of providing flexibilities are assessed, namely the reduction of peak power and the provision of balancing services. As power grids have to be designed for peak loads, the reduction of peak power is of great interest to the grid providers. Balancing services are needed to maintain a stable frequency in power grids, control reserves are retained to be activated in case of frequency deviation. For the representation of offering flexibility by participating in the control reserve energy market, a two-stage optimization approach is developed. The prices for control reserve can be quite volatile and their future development remains uncertain, as renewable generation forecasts improve and more flexible generators and consumers emerge [3, 4]. Thus, instead of applying current price levels, a price sensitivity analysis is carried out, yielding the critical points at which system behavior and design is altered by the optimizer to provide flexibilities.
Results: The results show that providing balancing services with the described system is economically attractive, even for low payment levels of 5% to 25% of current prices. Without changing the system design, which is optimized for operation without flexibility provision, the costs of the energy system can be reduced by only adjusting its operation. By increasing thermal storage capacities, the amount of reserve energy delivered can be significantly increased and costs of heating are further reduced. This means, that overdimensioning of thermal storages in the planning process of 5th gen district heating systems can be an option to reduce system costs when flexibilities are offered. However, flexibilities can only be offered when there is a demand for heating or cooling. Due to the all-year cooling demand flexibilities can be offered all year long, in a system without these cooling demands, flexibilities could only be offered during heating season. This would reduce the economic appeal of building larger storages for the provision of balancing services. However, it can still present an attractive business model. Reducing peak load is less attractive in the considered system, as the potential is limited with electricity demands of heat generation making up only a minor part of the electrical load.