Development of Analytical Model for a Vertical Single U-Tube Ground-Coupled Heat Pump System

Abstract

An analytical model was built to study the thermal design of a single vertical U-tube coupled heat pump under steady-state conditions. It was based on the philosophy of U-tube replacement by an equivalent thermal resistance situated between the heat transfer medium that flows inside the tube and the borehole boundary. An obstruction factor was introduced to account for the reduction of heat flow from or to a tube in the borehole due to the presence of the second leg of the U-tube. Two Copper U-tubes with wall factors of (12.5) and (14.29) were implemented to comprise several borehole configurations to verify the present work. The shank spacing was ranged between (2) and (4) times the U-tube outside diameter producing shank spacing to borehole diameter ratio range of (0.29-0.59). The model was utilized for the assessment of DX ground heat exchangers works as a condenser for cooling purposes. Reducing of the tube spacing to tube outside diameter ratio from (3.3) to (2) for both tube wall factors showed a rise for the borehole thermal resistance in the range of (22-54)% and (26.5-28)% predicted at wall factors of (12.5) and (14.29) respectively.