## 2 GSHP District Sharing System  
According to the Department of Energy and Climate Change (DECC) definition, a district heating network consists of two or more distinct buildings connected to a single heat source or a building in which there are more than 10 individual customers connected to a single heat source. For domestic buildings in the UK, by connecting with at least one neighbor, the eligible grant category can switch from the Domestic Renewable Heat Incentive (RHI) to Non-Domestic RHI. The Domestic RHI is intended for domestic applications of ground source heat pumps, and the payment is at a rate of 20.46 p/kWh of renewable heat generated for 7 years. The Non-Domestic RHI, also known as the Commercial RHI, deals mainly with commercial applications or \"district\" heating schemes in domestic properties, and the payment is at a rate of 9.36 p/kWh on the first 1314 h of eligible energy produced, followed by 2.79 p/kWh on any additional heat produced for 20 years [19]. The network size is also defined on the basis of the number of domestic and non-domestic customers connected to the heat source. There are large networks (500 or more residential properties and/or more than 10 non-domestic users), medium networks (between 100 and 500 residential properties and/or between three and 10 non-domestic users and small networks (less than 100 residential properties and/or less than three non-domestic users). Currently, in the UK, the smaller-sized networks are predominant, representing 75% of the 1765 individual district heating networks. The average number of dwellings per network is 35. Medium and large-sized networks make up the rest of the total, at 20% and 5%, respectively [20]. The size of GSHP district heating systems can range from micro to macro scales. By linking systems together, the system capacity can be enhanced by drilling deeper boreholes and making borehole locations more flexible [10]. As commercial buildings often have different heat demand timings compared with residential buildings, this temporal heterogeneity can benefit the energy use efficiency of a shared system. With respect to the available space for borehole installation, residential houses can share their surplus geothermal energy with commercial buildings and urban infrastructure at a large scale to achieve a low-carbon city. In addition, residential houses can benefit from more generous tariff benefits by switching to a district network, because the eligible incentive scheme can change from domestic to non-domestic. However, in practical situations, buildings within a certain district generally belong to different landlords, and it is therefore not always feasible to have a central system for a whole district. In this case, it is also recommended to design individual systems for buildings but allow systems to share shallow geothermal energy underground spatially, meaning to share their land with each other to install borehole heat exchangers. The landlords can make a contract to confirm a rental fee as a part of the initial cost and run their own heat pump systems separately to avoid operation conflicts. In this way, arguments in the negotiation of the operation management can be reduced, which can be considered to enhance the feasibility of implementing the district system. However, as there is no limitation to installing closed-loop GSHP systems in the UK, the subjective opinions of landlords are always a dominant factor in the feasibility of building a district sharing system and vary by case. Therefore, in this study, two sharing strategies were considered for a district GSHP system: (i) individual systems with borehole sharing (Figure 1b) and (ii) a central district system (Figure 1c). For the individual system shown in Figure 1a, the dotted line shows that the residential buildings have surplus space for borehole installation.