Modelling of the temperature change in vertical ground heat exchangers with single U-tube installation Dr. LÁSZLÓ GARBAI, Prof. Department of Building Services and Process Engineering Budapest University of Technology and Economics 1111 Budapest, Mőegyetem rkp. 9 HUNGARY SZABOLCS MÉHES, PhD STUDENT Department of Building Services and Process Engineering Budapest University of Technology and Economics 1111 Budapest, Mőegyetem rkp. 9 HUNGARY Abstract: - One of the major problems of ensuring optimal working of ground source heat pump systems is a heat transfer around vertical ground heat exchanger. The working of vertical U-tube can be understood as a heat exchanger between the ground and the heat carrying medium. In our case this heat carrying medium is fluid, which transfers the extracted heat from the ground to the heat pump. In winter we extract heat from the ground and in summer we transfer heat to the ground. In our paper we propose a simple calculation model to calculate the temperature change in vertical ground heat exchangers with single U-tube installation. Key-words: - heat pump, U-tube, heat transfer, heat flow, thermal resistance, system theory. 1 Introduction In the Carpathian basin, but mainly on the territory of Hungary the crust of the earth is thinner than the average; therefore its geothermal features are very good. Under the ground surface in the earth core levels from the decomposition of radioactive isotopes heat is produced. Its flow directed towards the surface is geothermal energy. The global average of the geothermal gradient is 33 m/°C, while in Hungary it is only 18-22 m/°C. The average value of the heat flow from the inner core of the ground is 80-100 mW/m 2 according to the heat flow map of Hungary, which is almost the double of the average value measured on the mainland [1]. The primary side heat sources of heat pumps operating with water-water sources are the following: underground waters and heat of the earth (geothermal energy). The geothermal energy is extracted from the ground by ground heat exchangers with U-tubes. The installation of U-tubes can be vertical and horizontal. In our paper we deal with heat extraction of vertically installed single U-tubes. For the vertical U-tubes boreholes are made with a diameter of 75 – 150 mm, with a depth range 40 – 200 m. In these boreholes single and double U-tubes are installed. (Fig. 1) After the installation the ground heat exchanger is filled with bentonite grout. This grout ensures better heat transfer and it blocks the underground waters [2]. Fig. 1: Location of double (a) and single (b) U-tubes in boreholes 2 Review of heat transfer modelling in the case of U-tubes In a descending and ascending branch of the U-tubes, the fluid gets warm and forwards the heat to the heat pump through a heat exchanger. The modelling of this heat 6th IASME/WSEAS International Conference on HEAT TRANSFER, THERMAL ENGINEERING and ENVIRONMENT (HTE'08) Rhodes, Greece, August 20-22, 2008 ISSN: 1790-5095 288 ISBN: 978-960-6766-97-8