Feasibility and pipe length prediction Method of the soil direct cooling system for small data centers

Small data centers are numerous and their cooling systems are inefficient. Air/Water-based free cooling technology can enhance cooling system efficiency, but some application issues limit its promotion. Targeting the feature of rich surrounding soil resources, this paper proposes the soil direct cooling system with strong adaptability and high energy-saving potential for small data centers. To study this cooling system feasibility under the continuous heat release impact of small data centers and the influence of key parameters on the system performance, an accurate three-dimensional simulation model for heat exchange between ground heat exchangers and soil is established. Results indicate that when the length of ground heat exchangers is designed reasonably, the soil direct cooling system can ensure the long-term stable operation of small data centers, and the coefficient of performance of system can reach up to 25.78 ∼ 37.85. The pipe length considering soil internal moisture transfer increases by about 5.41 % compared with the pure heat conduction condition. For every 0.75 W/(m·K) increase in soil thermal conductivity, 200 J/(kg·K) increase in soil specific heat capacity, 0.5 m increase in borehole spacing and 0.5 °C increase in initial soil temperature, the pipe length decreases by 15.93 %∼19.23 %, 11.54 %∼13.59 %, 14.55 %∼17.02 % and increases by 13.95 %∼17.54 %, the coefficient of performance of system increases by 2.30 ∼ 2.71, 1.54 ∼ 1.93, 2.08 ∼ 2.45 and decreases by 1.46 ∼ 1.99. A multiple regression analysis on the heat exchange rate per unit buried depth of ground heat exchangers is conducted. According to the regression result and cooling load of data centers, the pipe length can be predicted, which can provide guidance for the practical engineering application of this system.