Experimental investigation on the use of CuO/water nanofluid in horizontal spiral-coil ground heat exchanger

The thermal performance of a ground heat exchanger (GHE) largely depends on the heat transfer fluid circulating in the system. In this study, a laboratory test apparatus for a horizontal spiral-coil ground heat exchanger (HSGHE) was designed and constructed to investigate the effects of different volume fractions of CuO/water nanofluid on the energy efficiency of the HSGHE. The results showed that, at a volumetric fraction of 1%, the CuO/water nanofluid led to a 9.4% increase in the heat exchange rate of the HSGHE, compared to water. In addition, the heat transfer enhancement effect at a lower volume fraction of the CuO/water nanofluid was greater than the effect of its viscosity on the performance of the HSGHE. The CuO/water nanofluid with a volumetric fraction of 1% was not recommended for use in the HSGHE owing to its performance efficiency coefficient (PEC) of less than 1. The optimal volume fraction was 0.5%, at which the PEC was the highest (1.025). Therefore, it was determined that the utilization of a low volume fraction of CuO/water nanofluid in HSGHE provides energy efficiency enhancements independent of the GHE design.