title: Object-oriented modelling of solar district heating grids with underground thermal energy storage
Creators: name: Formhals, Julian
Publisher: name: UNSPECIFIED
Year: 2022

description: The transformation of the heating sector towards renewable energy sources is a key element for the mitigation of man-made climate change. In this regard, solar thermal energy is a particularly well-suited solution, as it is a simple, cost-efficient and proven technology. A main barrier for a more widespread use, is the seasonal mismatch of heat demands and solar yields, which usually limits the solar share on the overall heat supply of district energy systems to about 20%. It is therefore necessary to store the abundant solar energy supply during summer for several months to be able to use it in winter. Underground thermal energy storage (UTES) is currently the most promising technology for such applications, as it shows a high maturity level in comparison to other technologies and facilitates storage of thermal energy on a district scale. Integration of UTES into solar district heating (SDH) systems is commonly accompanied by further technologies, such as geothermal energy, industrial waste heat or power-to-heat applications, resulting in complex energy systems. These SDH-UTES systems require a thorough design of component dimensions, system layouts and control strategies to ensure security of supply, while avoiding costly overdimensioning of generation capacities. Therefore, dynamic system simulations are used for system design, as they consider the temporal distribution of heat supplies and demands as well as the strong interactions between components. The modelling language Modelica constitutes a powerful conceptual approach for modelling and simulation of thermal energy systems and is therefore applied increasingly. However, to exploit Modelica’s numerous advantages for the simulation of SDH-UTES systems and reach a large number of users, adequate model libraries are required. These should be accurate in their representation of physical components, easy to use and have a low numerical effort. The presented cumulative dissertation and the corresponding publications in scientific journals demonstrate the development of such a model library called MoSDH (Modelica Solar Dis-trict Heating). The library consists of components for the accurate, efficient, user friendly and robust simulation of such systems, including models for UTES technologies which were previously not implemented for Modelica. Selected models and aspects were already presented and demonstrated in case studies in the above-mentioned journal papers. The presented thesis contains a comprehensive description of the model components as well as the general system modelling concept. Furthermore, several case studies are used to highlight certain key functionalities and demonstrate the accurate representation of the physical systems in a numerically efficient way. The models can be used for extensive optimization studies as well as detailed investigations of certain specific aspects. In addition to that, the object-oriented modelling approach facilitates the easy adaption and reuse of model components. Finally, MoSDH is used to investigate the transition of a sub-grid of the TU Darmstadt university district heating (DH) system into a SDH-UTES system, demonstrating the possibility of those systems to satisfy the universities emission saving goals in a cost-efficient way.