Heat transfer analysis of U-type deep borehole heat exchangers of geothermal energy

Abstract

This paper describes a U-type deep borehole heat exchanger (DBHE) containing both two vertical boreholes and one horizontal boreholes for the application of medium-deep geothermal energy, and presents the heat exchanger’s characteristics and the working principles. Compared with the single DBHE, the ability of the U-type DBHE to extract underground energy is better, and therefore a higher heating load of buildings can be assumed. First, the preconditions are explained, and the governing equations of heat transfer are established. Second, the numerical finite difference methods are employed to build the node equations for both the fluids in the heat transfer tubes and the underground medium. In addition, the pursuit method is taken into consideration to obtain the temperatures used in the node equations. Accordingly, the temperatures of the different locations in both the tubes and the underground medium can be acquired. Finally, one actual project is studied based on the models and the corresponding equations; the nominal heat absorption is analyzed, and the factors that exert an influence on it are discussed; the inlet and outlet temperatures of the circulating fluid in vertical and horizontal boreholes are investigated, and then, the temperatures of the underground medium at typical locations near the DBHEs are explored. The research results for the U-type DBHEs are beneficial to the improvement of heat transfer models of medium-deep geothermal energy, and they promote the development of heating technology used in geothermal energy applications.