Assessment of available thermal capacity of district heating systems for increased medium-and short-term flexibility of multi-modal power systems

Abstract

In theory, district heating systems (DHS) offer an opportunity to increase the flexibility of combined heat-and-power (CHP) plants by working as short- and medium-term thermal storage. Using this capacity, it is possible to dispatch CHP units more flexibly and thus offer, e.g., operating reserve for the electric grid without any additional investments in thermal storages. State-of-the-art optimization studies use this flexibility, but do not show the lever for further tweaks. This work's contribution is to analyze the actual extent of flexibility and the reasons for current limitations in detail. To this end, a model of the propagation of temperature fronts is suggested and applied, and the general applicability of pre-charging a DHS is proved by simulation. A thorough study of design and operation parameter changes is then conducted. Results show that some system variables (e.g., the thermal demand) must be forecast precisely to determine the absorbable thermal power, and that a smart design of system parameters (e.g., supply temperatures) can increase the flexibility.