Performance comparison of a vertical direct expansion geothermal evaporator: PART II, multiple U-pipes

Abstract Part I showed that Carbon Dioxide (CO2) is a promising candidate for use in vertical direct expansion ground evaporators, as it requires smaller pipe diameters within the geothermal heat exchanger and lower power for refrigerant circulation compared to conventional refrigerants. Furthermore, pressure change along the geothermal evaporator influences marginally the evaporation temperature leading to quasi-isothermal evaporation of CO2. However, in single U-pipe configurations, small pipe diameter means lower pipe surface area and therefore higher thermal resistance. In Part II, all these advantages are taken into consideration toward proposing new geothermal evaporator configurations with multiple U-pipes (2 to 4 U-pipes in one borehole). R410A is also compared against CO2 for the new configurations. Operating parameters of all configurations were chosen so that CO2 and R410A are both subjected to a complete evaporation for equal total pipe surface area and equal heat extraction rate. Although all configurations have same total pipe surface area, it is shown that adding U-pipes and reducing pipe diameters diminishes borehole thermal resistance significantly. A novel configuration is also proposed to lower pressure drop of CO2 in the 4-U borehole configuration by insulating a portion of the downward U-pipe legs. Decreasing pipe dimensions by adding U-pipes while keeping total pipe surface area constant offers an opportunity to bundle pipes closer to each other and make the evaporator more compact as seen in the proposed Polymer Block configuration.