An experimental investigation of CuO/water nanoﬂuid heat transfer in geothermal heat exchanger Ruiqing Du a,b , DanDan Jiang a,b , Yong Wang a,b,⇑ , Kwok Wei Shah c a National Centre for International Research of Low-Carbon and Green Buildings, Ministry of Science & Technology, Chongqing University, Chongqing 400045, China b Joint International Research Laboratory of Green Buildings and Built Environments, Ministry of Education, Chongqing University, Chongqing 400045, China c Department of Building, School of Design and Environment, National University of Singapore, Singapore 117566, Singapore article info Article history: Received 14 May 2020 Revised 2 August 2020 Accepted 18 August 2020 Available online 24 August 2020 Keywords: Nanoﬂuid Geothermal heat exchanger Performance efﬁciency coefﬁcient Nanoparticle motion abstract The energy efﬁciency of geothermal heat exchanger (GHE) systems can be signiﬁcantly impacted by the thermal performance of heat transfer ﬂuid inside. In the present study, nanoﬂuid (CuO/water) was exper- imentally used in the GHE to investigate its thermal performance as circuit ﬂuid. The main part of exper- imental system consisted of a 0.8 m  0.5 m  0.58 m box, two double U-tubes, thermocouples and thermal resistors. A three-dimensional discrete phase model was built to simulate the ﬂow process. By using nanoﬂuid, the heat transfer rate and pumping power consumption of the GHE system increased by 39.84% and 16.75%. Moreover, the heat load-to-pumping power ratio had an enhancement of 20.2%. Furthermore, the previous literature showed that the nanoﬂuids, which had a signiﬁcant effect on the heat transfer of other types of heat exchangers, had an insigniﬁcant effect (less than 5%) on the energy efﬁciency of GHEs. The simulation result showed that the special structure of traditional GHEs may be the main reason for the lower possibility of collision between nanoparticles and the insigniﬁcant effect of nanoﬂuid on its heat transfer enhancement. Therefore, the length of every straight tube segment should be restricted to achieve the better thermal performance of GHEs using nanoﬂuids as heat transfer ﬂuid. Ó 2020 Elsevier B.V. All rights reserved. 1. Introduction The energy crisis has become a challenge to global economic development. Since the buildings are responsible for approxi- mately 40% of the world’s energy consumption annually, there are many efforts focusing on renewable energy in buildings such as geothermal energy. The geothermal energy has been widely uti- lized around the world in recent decades [1], and the ground source heat pump (GSHP) systems account for most of (55.2% in the year 2014) its application [2]. The GSHP systems utilize geothermal energy from the shallow ground to supply heating or cooling energy for the buildings. In a GSHP system, the thermal performance of the ground heat exchangers (GHEs) has directly relevant to the energy efﬁciency of the system [3]. Nanoﬂuids, which consist of nanoparticles and base ﬂuids, may be one of the promising ways to improve the efﬁciency of the GSHP systems. The nanoparticles enhance the thermophysical properties of the ﬂuids passing through GHEs. Numerous studies have inves- tigated the effect of nanoﬂuid on the thermal performance of GHEs. Narei et al. [4] tried to theoretically ﬁnd the optimum parameters of thermal conductivity and viscosity for nanoﬂuid using the Multi- objective Flower Pollination algorithm. The results indicated that Al 2 O 3 -water nanoﬂuid contributed to a reduction of no more than 1.3% in the bore length, in comparison to pure water. Diglio et al. [5] numerically studied the effect of different types of nanoﬂuids on the performance of GHEs. They found that the Cu-based nano- ﬂuid reduced 3.8% of the thermal resistance of the borehole when the concentration was 1%. Xiao-Hui Sun et al. [6] simulated the suspension and migration of nanoparticles in GHEs considering the effects of forces such as Brownian motion, gravity, turbulent eddy diffusivity, etc. The results showed that a pulsed ﬂow and optimized structure of the bottom borehole can signiﬁcantly enhance the durability of the nanoﬂuid and improve its thermal performance. Dan Sui et al. [7] also conducted a numerical study on the heat transfer rate of nanoﬂuid in the geothermal double pipe heat exchanger, and the nanoﬂuid extracted 11.24% more energy than water. Sergio Bobbo et al. [8] theoretically and prelim- inarily analyzed the Mouromtseff number of Al 2 O 3 /water nanoﬂuid for four different concentrations (3%, 5%, 30%, and 40%) in the GSHP https://doi.org/10.1016/j.enbuild.2020.110402 0378-7788/Ó 2020 Elsevier B.V. All rights reserved. ⇑ Corresponding author at: National Centre for International Research of Low- Carbon and Green Buildings, Ministry of Science & Technology, Chongqing University, Chongqing 400045, China. E-mail address: cqwangyong@cqu.edu.cn (Y. Wang). Energy & Buildings 227 (2020) 110402 Contents lists available at ScienceDirect Energy & Buildings journal homepage: www.elsevier.com/locate/enb