title: 5th generation district heating and cooling for holistic energy system decarbonisation: Novel system designs and detailed techno-economic assessment
Creators: name: Angelidis, Orestis
Publisher: name: University of Glasgow
Year: 2025

description: To limit climate breakdown, decarbonising the heating and cooling sector in a cost effective manner represents a crucial challenge. Holistic energy system decarbonisation is recognised as a key solution, relying on sector coupling, energy reuse and energy storage. 5th Generation District Heating and Cooling (5GDHC) presents a promising pathway for holistic decarbonisation. It utilises an ambient temperature water network with decentralised heat pumps to exploit synergies between heating and cooling, to harness low temperature waste heat and allow sector coupling with the electricity grid. However, questions remain on 5GDHC’s hydraulic design, operational stability, economic viability, and overall performance against alternative supply options.

This PhD thesis addresses these gaps by developing and experimentally validating a novel hydraulic design and operational methodology for 5GDHC systems. This design focuses on decentralised variable speed pumping and a centralised passive balancing unit to ensure system stability. Detailed simulation models for the proposed design are also developed and experimentally validated, which are made open access. Additionally, a technoeconomic assessment tool, the Centralisation Analysis Tool for Heat Pump Systems (CATHeaPS), is created to evaluate the economic viability of 5GDHC against other thermal supply options. CATHeaPS is also made open access.

To complete these tasks, 

1. A systematic literature review is conducted and integrated with views from leading industry and academic professionals in the field, collected during bespoke interviews on the opportunities and challenges of 5GDHC for holistic decarbonisation. It is found that further research is required on quantifying the range of operational boundaries that make 5GDHC more techno-economically viable, hydraulic design and operation standardisation as well as a business structure redefinition. 

2. A novel hydraulic design and suitable control philosophies are developed and experimentally validated. The experiments showed stable operation over a period of 20 hours for all demand regimes without control instabilities. A discussion is conducted on generalisability of findings and full-scale implementations.

3. Bespoke Modelica models are created and validated by building a digital twin of the experiment. A stepwise validation of the assembled digital twin demonstrates that the developed Modelica components effectively capture the behaviour of 5GDHC, with a Coefficient of Determination (R²) for the primary pump flowrates exceeding 0.88 in all cases. The similarity of the results is also seen through an analysis of the overall system behaviour. 

4. For the techno-economic comparison of 5GDHC against alternative energy supply strategies for decarbonising heating and cooling, CATHeaPS is employed. CATHeaPS is verified using a direct comparison of outputs and the theoretical results/analysis verification approaches. Economic and environmental analyses are conducted highlighting the impact of cooling and access to waste heat sources. When there is no cooling, 5GDHC performs worse than 4th Generation District Heating (4GDH) for any network topology and consumer class configuration. An annual cooling to heating demand share of 30% (with a demand overlap coefficient of 16%) is required for 5GDHC to start performing better for a higher number of connections per property. 

5. A real-world case study with heating and cooling loads and a waste heat source/sink during the winter/summer is used to demonstrate the application of the developed models and methodologies. It quantifies the techno-economic performance of 5GDHC against alternative supply options for the project area. 5GDHC shows a better economic performance than 4GDH with individual Air Condition units for cooling (4GDH&amp;AC) with 227£/MWh compared to 257£/MWh. It is less energy efficient than 4GDH&amp;AC, having an additional 81 tonnes of CO2 over a 40-year period. However, having reversible BHP units at each prosumer allows for a smaller electric capacity requirement, reducing the impact on the electricity grid. A discussion is also present on space constraints, operational complexity, and expansion flexibility.

Altogether, this research delivers a comprehensive framework for designing, simulating, and assessing 5GDHC systems. It facilitates their understanding and ultimately assists the efforts for a holistic energy system decarbonisation.