Performance of HDPE and Vacuum-Insulated Central Pipes for Coaxial Heat Exchangers in Geothermal Systems

Abstract

The thermal performance of a deep borehole coaxial heat exchanger with High-Density Polyethylene (HDPE) and vacuum-insulated central tubing (VIT) is presented in this paper. The analysis was performed for a wellbore equipped with a casing steel pipe of external diameter 198.52 mm and 10.36 mm thick. The central pipe consists of an HDPE tube 139.7 mm in outside diameter and 25.7 mm thick or two coaxial steel tubes, the first having an outside diameter of 139.7 mm and 10.5 mm thick, with the inner tube having an outside diameter of 101.6 mm and a thickness of 6.65 mm giving the same inner diameter of 88.3 mm as the HDPE tube. The gap between the steel pipes was maintained at different partial vacuum pressures. The depth was up to 5 km. Water was used as the working fluid. It was found that the VIT outperforms the HDPE for deep wells with the advantage diminishing for more shallow heat exchangers. A partial vacuum of 10 Pa does not demonstrate any relevant improvements over a gap maintained at normal atmospheric pressure, while the yield can be significantly improved when the pressure is reduced to 1 and 0.1 Pa.