District heating and cooling systems transition : Evaluation of current challenges and future possibilities

Abstract: District heating and cooling systems offer a cost-effective and secure supply of heating and cooling to connected buildings. They are widely recognised as a key solution to decarbonise the building sector due to their high potential of integrating renewable energy sources. The systems traditionally involved generating heating and cooling centrally with distribution through separate pipes before delivery to end-customers. Nowadays, advanced concepts of district heating and cooling feature, for example, integration of low-enthalpy renewable energy sources such as shallow geothermal, simultaneous supply of heating and cooling through the same piping network, and combining district and heat pump technologies. Despite their high potential in achieving carbon neutrality, such advanced concepts have major technical challenges, and their practical implications need to be thoroughly evaluated.This thesis aims to analyse, contribute, and improve district heating and cooling systems transition by linking theory with practice. It is divided into four main parts. First, the thesis presents the development and implementation of a multi-domain model for the design and analysis of advanced district heating and cooling technologies. Second, it describes a new analytical method for modelling network hydraulics in district systems with bidirectional mass and energy flows. Third, the thesis applies and validates the developed models using three real-world systems, a built lab test bench, and a set of predefined comparative tests. Finally, the thesis examines the implications of district heating and cooling systems transition from technical, social, and political aspects.The thesis adds to the existing body of knowledge on district heating and cooling in several dimensions. The application of the developed model on the analysed systems reveals several benefits for new technologies of district heating and cooling including but not limited to, electrification of thermal networks using decentralised heat pumps, sharing energy flows between connected buildings, and reducing network heat losses in combined heating and cooling systems. Moreover, the model application shows that the newly developed analytical method yields a fast, yet accurate, modelling approach to evaluate the hydraulic states in bidirectional networks with any number of connected prosumers. The thesis also analyses the technical, social, and political implications of district heating and cooling systems transition based on the opinion of middle agents representing system planners, district heating companies, and heat pump experts. The thesis reports that heat pumps have a vital role in fully decarbonising district heating and in providing flexibility to the power grid by offering services such as frequency regulation. Main development efforts for heat pumps should be placed on offering products with low temperature lifts and shifting the use to natural refrigerants. With regard to the social implications, advanced district heating and cooling with the concept of shared energy communities can help alleviate energy poverty. This is achieved by providing a secure supply of heating and cooling to all connected buildings and by reducing the financial burden on tenants who experience rent increase when energy retrofit projects are carried out. Finally, and in order to accelerate systems transition, it is necessary to establish industry standards to propose best practices and to fill the gap in skills shortage.