Geothermal energy is one of the well-known types of renewable energies that can be considered as a promising alternative to fossil fuels in order to mitigate the CO2 emission in atmosphere. Geothermal energy has recently been used for heating and cooling systems in many countries of the world. Therefore, studying such kind of energy seems important and necessary. This paper studies the heat transfer processes in a borehole heat exchanger (BHE). The flow of fluid in a BHE has been considered with different geometries and heat transfer processes are modeled numerically and in a finite volume method through convection and conductivity. First, a single U-shaped BHE was modeled and its thermal properties were investigated and then other geometries were studied to obtain the best geometry and heat transfer efficiency. In order to increase the heat transfer in the inner part of the pipes, the fin with the given specifications was used and the modeling was carried out and the thermal parameters were compared for three BHE’s with different geometries. In the final part, optimization results are presented, and the interaction of parameters on the Nusselt number and friction coefficient have been investigated. Also, the desirability of optimization and reliability is given to the results presented in percentage terms. Comparing the results of this article with the results of previous research shows a very good agreement and therefore the applied method is reliable and accurate.

Introduction

Fossil fuels mortality, diversification into energy sources, sustainable development and energy security and environmental problems caused by the use of fossil fuels on the one hand and clean and renewable sources of new energy such as the sun, wind and geothermal on the other hand has caused the world’s attention to develop and expand use of renewable energy and increase the share of those resources in the global energy basket. Geothermal energy means energy from the internal origin of the earth. This energy emanates from the inner part of the earth, in the form of tangible heat and there are rocks and water in the gaps and pores inside the rock in the earth’s crust. Over the life of the earth, this internal heat is slowly produced and kept within the ground. This has made it an important source of energy and is now being considered as a new energy source. In geothermal systems, geothermal wells and heat pumps are commonly known as geothermal Borehole Heat Exchanger (BHE). To design geothermal BHEs, the thermal conductivity of the earth and the thermal resistance of the well are required. Usually, the above parameters are estimated using the experimental Thermal Response Test (TRT) which for first time was proposed by Mogensen [1]. Subsequently, supplementary guides of the TRT are suggested by Ashrae [2] and Sanner et al. [3]. The topic of geothermal heat pumps has attracted much attention as a renewable energy technology and has been used for heating and cooling [4–۶]. Recent studies have been conducted to study the efficiency of geothermal heat pump systems [7–۹]. Li et al. [10] experimentally investigated the efficiency of a U-shaped vertical BHE in which, temperature changes and thermal equilibrium of the system have been numerically simulated and analyzed. Those results indicated that the geothermal source could be used as a fountain or thermal well for the geothermal heat pump system to achieve higher returns and more energy storage. Karabacak et al. [11] investigated the cooling performance of the geothermal heat pump system in Denizli, Turkey experimentally.