深井换热技术原理及其换热量评估

深井换热又称套管换热,是在深井中通过同轴套管进行单井内部流体循环,基于热传导的方式与地层换热,从而以"取热不取水"形式开发地热能的技术.本文详细介绍了深井换热技术特点,评述其在国外的应用进展.国外的实践表明,深井换热延米功率均在200 W以下,多不超过100 W,换热量远小于基于热对流的取热方式,比如深井热水开采技术.本文针对我国北方典型地区地热地质条件,分别采用Beier解析法和双重连续介质数值模拟法(基于OpenGeoSys模拟平台)计算了短期(4个月)采热和长期(30年)采热情景下的换热量.解析法和数值法的结果均表明,延米换热功率上限不超过150W.在间断采热,即每天供热12个小时,停止12个小时的情景下,延米换热功率可以翻倍,但是总换热量基本不变,且水温在一天内的波动明显变大.对数值模型进行敏感性分析发现,在地温梯度一定的条件下,井深对延米换热功率影响不大,而地层热导率对其影响较为明显.最后指出,提高深井换热技术换热量的主要手段是增加井周围地层中的热对流,或者说,增加循环水与岩石的接触面积.

Principle and capacity quantification of deep-borehole heat exchangers

Deep-Borehole Heat Exchanger (DBHE) is generally installed as a co-axial pipe in deep boreholes. The DBHE can obtain heat without extracting groundwater. Literature review shows that the specific heat extraction rate of DBHE is less than 200 W·m^-1, which is much smaller than that by conventional hydrothermal exploitation systems. Using both analytical and numerical methods, we evaluated the capacity of DBHE with representative parameters in northern China. Three modeling scenarios were constructed using the simulator OpenGeoSys, including (1) short-term (4 months) continuous heat extraction; (2) short-term heat extraction with half-day intervals and (3) long-term (30 years) continuous heat extraction. Results show that the sustainable heat extraction rates in all continuous extraction scenarios are less than 150 W·m^-1. While in the intermittent scenario, the rate could be doubled, but will lead to larger fluctuations in the outflow temperature. Sensitivity analysis indicates that the depth of borehole is not a significant influencing factor to the heat extraction rate, while the heat conductivity of surrounding formation is critical. Finally, we point out that the basic need in order to increase the specific heat extraction rate of a DBHE is to increase the convection in the surrounding formation, or in other words, the contact area between circulation water and formation.