# Nanotechnology Applications in Ground Heat Exchanger Pipes: A Review
## 1 Introduction  
GSHPs are considered to be the best available technology for the air conditioning of residential or commercial spaces [1]. A GSHP is defined as a heating and/or cooling system designed to transfer heat to or from the ground, groundwater, or a surface water as a heat source or sink. A GSHP uses these different energy sources all the time, without any intermittent availability, as a heat source (in the winter) or a heat sink (in the summer). The principal advantages of this technology are the high electrical efficiency, low maintenance, reduction in fossil resources and low-carbon emissions. Attempts have also been undertaken to use water in such a way to minimize the pipe network length [2, 3]. The GSHP has three main components. The first component is the GHE, which uses the subsoil as an energy source. The GHE moves fluid using a pump to transfer heat to the space to be conditioned as a source (heating in winter) or a heat sink (cooling in summer) through a pipe network buried under the subsoil (horizontal or vertical). Second, the heat pump (HP) raises the heat collected to a useful temperature, and transfers it to the space to be conditioned. Finally, the third component is a distribution system composed of a duct-work as an air-forced duct system. Typically, a GHE uses a pipe network with different configurations to capture and/or dissipate heat from the ground, pond or lake (see Figure 1). This paper reviews the main developments in using GSHP pipes as a GHE and the new fluids used to improve the heat exchange. The most used and available material for pipes in the GHE is high-density polyethylene (HDPE). HDPE has a low thermal conductivity, affecting the heat transfer between the ground and the space to be conditioned. This implies an increased GHE installation cost. Therefore, the installation of GSHPs becomes expensive due to the high costs of the GHE and its pipes, as mentioned by [4]. The vertical GHEs installed in boreholes are more expensive than horizontal installations in trenches. The first alternative was the use of thermally enhanced grout to improve thermal conductivity. For example, in [5], a thermally enhanced grout is used that reduces borehole thermal resistance. It minimizes the length and the number of boreholes for the GHE, reducing the installation costs. Recently, researchers have striven to develop technologies to improve heat transfer in GSHP systems. This can be achieved by (1) increasing the thermal conductivity of the soil, improving the properties of the pipe material; (2) using different fluids; and/or (3) modifying a geometrical GHE's configurations pipes. A new research area to solve these problems is the use of nanotechnology for different purposes. Nanotechnology is defined as an applied science, capable of manipulating matter on an atomic and molecular scale to solve different problems [6]. It has been used in many fields such as engineering, computing, and medicine. Nowadays, nanotechnology is deployed in distinct energy applications such as fuel cell, hydrogen, nuclear, photovoltaic, tidal, wind and geothermic technologies. An exhaustive review of these applications (including theoretical and experimental works) of nanotechnology in renewable energy systems can be found in [7]. Figure 1: Different configurations of Ground Heat Exchanger: (**a**) closed horizontal loop, (**b**) closed slinky loop, (**c**) closed vertical loop, (**d**) open wells loop, (**e**) closed water loop and (**f**) open-circuit well and surface body. In nanotechnology, a particle is defined as a small object that behaves as a whole unit with respect to its transport and properties. Nanoparticles are particles between 1 and 100 nanometers (nm) in size with a surrounding interfacial layer. The interfacial layer is an integral part of nanoscale matter, affecting all of its properties. The interfacial layer typically consists of ions and inorganic and organic molecules.