# Nanotechnology Applications in Ground Heat Exchanger Pipes: A Review
## 1 Introduction
This paper will discuss two important aspects of nanotechnology in the sector of geothermic technologies for GSHP: nanomaterials and nanofluids. On the one hand, scientists have not unanimously settled on a precise definition of nanomaterials, but agree that nanomaterials are identified by their very small size, around a few nanometers. These nanosized particles differ from coarser particles in their tendency to form agglomerates which are macroscopically perceived as one particle [6]. On the other hand, a nanofluid is a nanotechnology-based colloidal dispersion prepared to disperse some nanoparticles in conventional liquids. These advanced fluids have the capacity to enhance the performance of the conventional heat transfer fluids according to [8]. The design of new heat exchanger configurations is an important aspect to improve energy capture. This presents a multivariate problem, requiring the use of computer tools to simulate the heat exchanger before it is manufactured. Additionally, the simulation analyzes the behavior of certain mixtures of fluids and materials in order to develop an optimized design. Some similar reviews analyzing the nanotechnology in GHSP have been biased, in that they have only focused on specific aspects. For example, the work of [9] presents a review only covering nanofluids. The authors of [10] reported a review about the improvements in the thermal properties of a fluid, with the soil and the material individually affecting the heat exchanger in the geothermal energy pipes. The research work of [11] presents some experimental studies in the laboratory using nanofluids. Finally, in [7], all the possible applications of nanotechnology to any renewable sources addressing the GSHP as a subclass are reported. All these works are relevant. However, they are focused on some particular research focus. A global vision involving all the main aspects of the GSHP technology for the researchers is omitted. A more integral and complete review that summarizes the principal nanomaterials and nanofluids approaches in a single article could therefore be useful. Thus, the main contributions of this work are summarized in the following points:  
* This paper presents an analysis of theoretical, numerical, and experimental studies, conducted in recent years, addressing the use of nanotechnology to improve heat transfer in a GSHP. * To the best of our knowledge, a similar integral review has not been published in the literature. * This work breaks down research on nanofluids and nanomaterials in the composition, designs, and configurations of heat pipes, including the software used to simulate these systems with the objective to enhance the heat transfer efficiency and reduce the installation cost of GHE. * This review presents some tables to visualize the main contributions of the reviewed works. The methods or models found in the literature are highlighted in these tables. * A summary of all these works is presented, including a reflection and discussion of the future trends of nanotechnology applied to GHE pipes for GSHP systems. * Finally, this review clarity to several areas, and can not only be beneficial to those working in nanotechnology, but rather to members of the general public who may be interested in these issues.