## 1 Introduction  
The Ground Source Heat Pump (GSHP) is a heating and cooling system that is usually coupled with the Ground Heat Exchanger (GHEX) to utilize geothermal energy. The GHEX allows the circulation of a working fluid through heat exchange pipes, which are buried underground with various arrangements, to induce heat exchange between the fluid and the surrounding ground. In general, a closed-loop vertical GHEX is a conventional type of GHEX, and is most popular in practice. However, the conventional closed-loop vertical GHEX incurs high initial investment costs because of a need for additional borehole drilling and extra construction area [1]. In particular, the drilling cost occupies more than 50% of the total construction cost. Therefore, to reduce the construction cost, novel types of GHEXs have been developed, which are fabricated in underground structure elements. For example,an energy textile (or tunnel) encases heat exchange pipes in a tunnel lining with a horizontal layout. [PERSON] et al. (2012) and [PERSON] et al. (2014) conducted experimental studies on an energy textile with various arrangements [2; 3], and compared the thermal performance indirectly with the conventional closed-loop vertical GHEX [2]. [PERSON] et al. (2013) developed an analytical solution for the energy textile, and verified their solution with field measurements [4]. An energy pile is another example that encases heat exchange pipes in a foundation pile [5; 6; 7; 8; 9; 10; 11; 12; 13]. Several studies have been conducted on thermal performance of energy piles [5; 6; 7], and a design method in the form of engineering charts has recently been developed [8]. Finally, an energy slab is installed as one component of floor slab layers to utilize a building structure as a hybrid energy exchanger [9]. Some experimental studies have been performed on the thermal performance of energy slabs [10; 11; 12]. If energy slabs are installed in a building along with conventional GHEXs, the construction cost can be economized by reducing the length of boring depth and the number of GHEXs. However, to the best of our knowledge, no comprehensive experimental studies have been conducted for enhancing the thermal performance of energy slabs or evaluating the effect of system parameters. In general, increasing the thermal conductivity of surrounding media (such as ground formations, concrete slabs and heat exchange pipes) tends to improve the thermal performance of GHEXs. For example, increasing the thermal conductivity of grouting materials is suggested as an effective way to enhance the thermal performance of GHEXs [14; 15]. However, in the case of energy slabs, it is difficult to change the material properties of concrete slabs, which encase heat exchange pipes, because it has to satisfy the structural requirements as a priority. Moreover, the thermal properties of ground formations should be predetermined by the construction condition. Therefore, increasing the thermal conductivity of heat exchange pipes can be an effective way to improve the thermal performance of energy slabs [16]. An energy slab is usually installed in the basement floor of buildings because such an underground environment may enhance their thermal performance, along with a relatively constant temperature throughout the year and the high thermal conductivity of ground formations. Moreover, in the underground space, the energy slab can be constructed not only on the floor, but also on the side wall in a basement, which can utilize the maximum thermal capacity of the ground and underground space more effectively. Even though an energy slab is constructed in the underground structure, the heat exchange pipes embedded in the concrete slab are closely adjacent to ambient air in the underground space, which causes thermal interference to the heat exchanger. Therefore, it is necessary to consider a thermal insulation layer in the energy slab to reduce thermal interference induced by ambient air temperature.