Abstract
Shallow geothermal energy is a renewable energy source used to reduce electric demand to produce cooling and heating of buildings. The temperature at a specific ground level is constant year-round depends on the geographic region. It can be utilized by exchanging heat in the hot weather (cooling) or cold weather (heating) using Ground Source Heat Exchangers GSHE. Many attempts have been proposed to investigate the GSHE controlling factors with a lack of interconnection effects of mutual inclusive parameters.
The current work investigates the interconnection relation of seven factors; three geometrical factors, two thermophysical factors, and two operational and environmental factors. The studied geometrical factors are the wellbore diameter and length and the tube diameter. The thermal conductivities of the wellbore grout and soil are the studied two thermophysical properties. The two studied operational and environmental factors are the circulating fluid flow rate, circulating fluid input temperature difference with the soil temperature.
A 2D axisymmetric CFD model is built to investigate the effect of the controlling parameters on the targeted output saved energy per tube length. Third-order surface response of the main output is achieved using a hybrid Box-Behnken Central-Composite design of experiments methods DOE. The Box-Behnken method concerns the mid of extremes, and the Central-Composite method concerns the rotatable variable interconnections. Although both methods are designed for second-order response surfaces, the proposed hybrid method can accurately predict third-order correlation using the Stepwise regression method on 136 design points. The nonlinear correlation is verified using another 100 random verification points, showing a root mean squared error of less than 1.5 [W/m].
The significance of each parameter on the target normalized saved energy is presented and discussed. The pipe diameter, grout conductivity, soil conductivity, and temperature difference are the most significant parameters controlling the GSHE performance. The water mass flow rate is lesser significant, while the grout diameter is insignificant. The response surface study has shown high normalized saved energy of 100 [W/m] of the pipe length for the investigated domains.
