## 1 Introduction  
In response to global warming, sustainable energy technologies are being promoted worldwide to reduce carbon emission. The Chinese government has announced that China will reduce carbon dioxide emissions per unit of GDP by 40-45% in 2020 when compared to the 2005 baseline [1]. The EU Energy Efficiency Directive (2012/27/EU) and the amending directive (2018/2002) have announced a series of binding measures to achieve a 20% energy efficiency target by 2020 [2]. To reach these targets, great efforts must be made for space conditioning, as building comfort consumes a large proportion of energy; in particular, this amounts to around half of the expended energy in the EU [3]. In the UK, the method by which heat is generated, distributed and used in buildings has to be changed to achieve a supply of affordable, secure and low-carbon energy for all consumers [4]. Heat has traditionally been generated on-site in individual buildings, with the most common sources being electric heaters, gas boilers and oil boilers. District heating, also called a heat network, involves multi-building heating, in which heat is transported by circulating either hot water or low-pressure steam through a network of insulated pipes [5; 6]. According to IEA energy balances, Russia, China and the European Union were responsible for 85% of district heating activities globally [7]. The main user categories of district heating are industries and residential buildings [8]. The government's Strategic Framework for Low-Carbon Heating in the UK identified in 2016 that heat networks represented an important element to achieve the decarbonization of heat in buildings. However, to realize significant emissions reduction with district heating, the heat in the networks must be provided from low-carbon sources [5]. This is also a global trend, with renewable energy usage showing an increasingly large share in the district heating system market, especially in the European Union (27%) and the USA (13%) [5]. Among the possible renewable sources of energy, vertical-closed ground source heat pump (GSHP) systems can be an effective low-carbon solution that is compatible with district heating. A buried borehole heat exchanger is used to extract shallow geothermal energy from the ground. According to [PERSON] et al. [9], careful consideration should be given to the availability of nearby energy sources. The greatest advantage of this system is that there are no location restrictions for this technology, and underground energy sources are stable for long-term use. The benefits of GSHP systems also include their low running costs, minimal maintenance costs and long-life expectancy. A report from Kensa Heat Pumps indicates that heat pumps can be expected to have a lifetime of 20-25 years, and a borehole can be used for as long as 100 years [10]. In order to encourage the growth of this technology, the Renewable Heat Incentive (RHI) classified GSHP district heating as a non-domestic tariff stream; thus, users of this system are eligible to receive 9.36 pence per kWh heating consumed for as long as 20 years [11]. Combining cooling with a district heating system can make district energy more economically attractive, especially in areas with a mix of land use buildings [4; 5; 12]; furthermore, it is considered to be more efficient for energy resource utilization [13]. GSHP systems are perfectly suited to match this requirement, as cooling supply is a basic function of a GSHP system. In addition, the combination of heating and cooling together can benefit system efficiency and the temperature balance underground. The utilization of shallow geothermal energy as an energy source for a district heating system is currently concentrated in the hybrid systems [14], which means systems which integrate a GSHP in a heat network to provide part of the heating load. In Europe, over 50 successful schemes of this type have been running for over a decade [5].