Heat transfer and rock-soil temperature distribution characteristics in whole life cycle for different structure sizes of medium-deep U-shaped butted well

Abstract

Medium-deep U-shaped butted well (MUBW) is one of the most potential geothermal-heating way. However, the thermal interference may happen during long-term geothermal extraction period, which results in a heating performance attenuation. In this work, the heat transfer performance and surrounding rock-soil temperature distribution characteristics of MUBW in whole life cycle were investigated. The three-dimension full-size heat transfer model for long-term thermal extraction was proposed and the different structure sizes were considered. The results showed that the thermal interference is more likely to happen in a high heating load condition for the conventional structure size. By increasing horizontal length from 200 m to 600 m, the average specific heating load could increase by 26.1 %. When vertical depth increases to 3000 m, the heating load increases by 47.0 % at least. It is noted that the annual average inlet and outlet temperatures decrease by 5.1 °C in the whole life cycle, which should be considered in design on system operation. Additionally, the horizontal length of 600 m is large enough to avoid thermal interference in whole life cycle for MUBW with depth of 2000∼3000 m. This study has a practical meaning for ensuring a sustainable thermal extraction of MUBW.

Keywords

• Medium-deep
• Heat transfer
• Geothermal energy
• Rock-soil
• Thermal interference