System performance of a deep borehole heat exchanger |
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Authors: | Thomas Kohl Renzo Brenni Walter Eugster |
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Affiliation: | 1. Institute of Geophysics, ETH Hönggerberg, 8093 Zurich, Switzerland;2. Polydynamics Engineering Zurich, Malojaweg 19, 8048 Zurich, Switzerland;1. School of Engineering and Sustainable Development, De Montfort University, The Gateway, Leicester, LE1 9BH, UK;2. School of Civil Engineering, University of Leeds, Woodhouse Lane, Leeds, LS2 9JT, UK;1. EA 4592 ENSEGID, 1 allée F. Daguin, 33607 Pessac Cedex, France;2. FONROCHE Géothermie, Technopôle Hélioparc, 2 av. P. Angot, 64053 Pau Cedex 9, France;1. Department of Mechanical Engineering Technology, Oklahoma State University, Stillwater, OK 74078, United States;2. Department of Energy Technology, Royal Institute of Technology (KTH), Brinellvägen 68, 100 44 Stockholm, Sweden;3. Palne Mogensen AB, Emblavägen 29, 182 67 Djursholm, Sweden;1. School of Human Settlements and Civil Engineering, Xi’an Jiaotong University, Xi’an, Shaanxi 710049, China;2. Sustainable Buildings Research Centre (SBRC), University of Wollongong, Wollongong 2522, NSW, Australia;3. Department of Energy Engineering, Guangdong University of Technology, Guangzhou, Guangdong 510006, China;1. State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum, Beijing, Beijing, 102249, China;2. Sinopec Star Petroleum Co. Ltd, Beijing, 100083, China |
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Abstract: | Deep borehole heat exchanger (BHE) systems, installed in abandoned boreholes, have been operative in Switzerland for several years now. The operational conditions of the 2302 m deep BHE plant at Weggis have been monitored continuously since 1994. In the first operational phase, lasting from October 1994 to May 1996, the plant was severely underused, as shown by the high production temperatures (40 °C). This behaviour was investigated by a numerical model accounting for the heat transport in the rock matrix and along the different tubing systems, with special emphasis on the heat transfer in a multi-layer insulated central pipe. Lacking detailed logging data or undisturbed temperature profiles, an axis-symmetrical model had to be used, assuming uniform rock parameters. Sensitivity studies highlighted the effect of varying flow rate or operation/recovery cycle lengths and helped to develop a strategy that allowed us to make an accurate calculation of the long-term Weggis production history. The initial model assumptions, based on this detailed treatment of the tubing system, could explain the operational data. By means of slight model variations that account only for the minor effects of metallic sleeves, the long-term production temperature history of the Weggis plant could be accurately fitted. These findings were confirmed by a detailed analysis of the May 1996 data. Due to the low degree of utilization, only numerical sensitivity analyses were able to highlight the potential of the deep BHE plant at Weggis. The results indicate that the low utilisation of 40 kW during the first operational phase could be increased to over 200 kW. The specific yield of deep systems is much higher than in conventional shallow BHE systems. Our simulation procedure proves that the heat transfer in a deep BHE system is well understood. |
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Keywords: | Heat flow Borehole heat exchanger Well casing Numerical modelling Weggis Switzerland |
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