Object-oriented modelling of solar district heating grids with underground thermal energy storage

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

A Modelica Toolbox for the Simulation of Borehole Thermal Energy Storage Systems

Borehole thermal energy storage (BTES) systems facilitate the subsurface seasonal storage of thermal energy on district heating scales. These systems’ performances are strongly dependent on operational conditions like temperature levels or hydraulic circuitry. Preliminary numerical system simulations improve comprehension of the storage performance and its interdependencies with other system components, but require both accurate and computationally efficient models. This study presents a toolbox for the simulation of borehole thermal energy storage systems in Modelica. The storage model is divided into a borehole heat exchanger (BHE), a local, and a global sub-model. For each sub-model, different modeling approaches can be deployed. To assess the overall performance of the model, two studies are carried out: One compares the model results to those of 3D finite element method (FEM) models to investigate the model’s validity over a large range of parameters. In a second study, the accuracies of the implemented model variants are assessed by comparing their results to monitoring data from an existing BTES system. Both studies prove the validity of the modeling approaches under investigation. Although the differences in accuracy for the compared variants are small, the proper model choice can significantly reduce the computational effort

Optimized Layouts of Borehole Thermal Energy Storage Systems in 4th Generation Grids

Borehole thermal energy storage (BTES) systems are a viable option to meet the increasing cooling demand and to increase the sustainability of low-temperature district heating and cooling (DHC) grids. They are able to store the rejected heat of cooling cycles on a seasonal basis and deliver this heat during the heating season. However, their efficient practical implementation requires a thorough analysis from technical, economic and environmental points of view. In this comparative study, a dynamic exergoeconomic assessment is adopted to evaluate various options for integrating such a storage system into 4th generation DHC grids in heating dominated regions. For this purpose, different layouts are modeled and parameterized. Multi-objective optimization is conducted, varying the most important design variables in order to maximize exergetic efficiency and to minimize levelized cost of energy (LCOE). A comparison of the optimal designs of the different layouts reveals that passive cooling together with maximizing the heating temperature shift, accomplished by a heat pump, lead to optimal designs. Component-wise exergy and cost analysis of the most efficient designs highlights that heat pumps are responsible for the highest share in inefficiency while the installation of BTES has a high impact in the LCOE. BTES and buffer storage tanks have the lowest exergy destruction for all layouts and increasing the BTES volume results in more efficient DHC grids