5th Generation District Heating and Cooling : A High-Level Simulation Model of a Novel District Energy Network

Abstract: 5th generation district heating and cooling is a novel approach to district heating and cooling networks. Instead of a centralized energy supply, the technology relies on multiple building-level energy centers, equipped with heat pumps and chillers. The energy centers are connected to a low-temperature district energy network which allows for energy exchange by rejecting the waste heat and coolth produced by the chillers and heat pumps. A growing interest in 5th generation district heating and cooling has spurred new research on the topic but there are still many unanswered questions regarding the viability of the concept, both from a technical and economical perspective. This thesis aims to increase the understanding of these types of networks by creating a simple model that can be used to evaluate the performance of a potential 5th generation district heating and cooling network, based primarily on the hourly heating and cooling demand of different building types. The model was implemented on 2 theoretical building clusters located in Bristol, UK. Cluster 1 was made up of offices, retail establishments, and hotels while cluster 2 consisted of residential buildings, hospitals, and data centers. The network was modelled after a 2-pipe closed loop system. Cluster 2 was able to limit the ratio of external heat and coolth addition to the system to 30% while cluster 1 required 53%. The low-temperature network enabled the connected heat pumps to reach a seasonal coefficient of performance of 3.4 for both clusters. The cooling equipment (free cooling heat exchanger and chiller) managed to reach a seasonal coefficient of performance of 17 in the case of cluster 1 and 18.2 for cluster 2. Distribution losses in the network amounted to about 3.2% for the warm pipe while the cold pipe gained 1.4% of coolth from the ambient. The results of the simulations indicate that the seasonality of the heating and cooling demand is one of the main factors to account for when designing a 5th generation district heating and cooling network. Highly seasonal heating and cooling demands lead to less possibilities for energy exchange on the network and will thus require more energy addition or increased energy storage capacity. The findings of this thesis seem to suggest that the advantages of 5th generation district heating and cooling networks are highly dependent on the demand profile of the connected buildings. Future research should aim to establish under what conditions these networks are economically viable. This will depend on several factors but arguably the most important ones are the amount of energy required to balance the network, cost of decentralized substations, and availability of cheap low-temperature heat and seasonal storage.