Construction and Validation of a Lab-scaleBorehole Thermal Energy Storage Model

Abstract: Borehole heat exchangers are widely used in heat pumps of residential buildings and industrialsystems. It is known as one of the most energy ecient technologies which provides heatingand cooling by using sustainable geothermal energy. The life time of borehole heat exchangerslasts more than 50 years which is longer than combustion boilers. Therefore, designing abore eld with accurate sizing is important for its future applications. Due to the large volumeof the ground, the transient heat transfer process of the bore eld lasts for a long time span. Because of this, only a few of the heat transfer models for borehole ground heat exchangersare validated by experiments. Besides, experimental validation in a real scale borehole can bedicult because of the uncertainty of the composition and thermal properties of the ground. A solution to faster experimental validation is to scale down the size of the borehole andground. This report presents the construction process of a lab-scale model simulating a 4x4 bore eldof 300 m depth vertical boreholes. The process of experimental construction is describedin detail, including ground set up, conductivity test, construction of hydraulic system anddata acquisition system. The pressure drop of hydraulics system is around 2.8 bar under the a flow rate of 200 ml/min and corresponding pump speed is around 2900 to 3100 rpm. The property of the sand has been investigated through a series of conductivity tests, which shows an average thermal conductivity of 1.75 W / (m • K) and average thermal diffusivity of 8.14x10-7 m2/s. Numerical simulation (via COMSOL) is carried out for preliminary validation. Comparison of experimental and simulation results shows discrepancies and one possible reason can be: the actual heat injection rate in experiment is lower than simulation due to heat losses of hydraulic system; uncertainty of ground (saturated sand) conductivity and thermal diffusivity.