The energy sector is undergoing a fundamental transformation, with a significant investment in low-carbon technologies to replace fossil-based systems. In densely populated urban areas, deep boreholes offer an alternative over shallow geothermal systems, which demand extensive surface areas to attain large-scale heat production. This paper presents numerical calculations of the thermal energy that can be extracted from the medium-deep borehole heat exchangers in the low-enthalpy geothermal setting at depths ranging from 600 to 3000 m. We applied the thermogeological parameters of three locations across Finland and tested two types of coaxial borehole heat exchangers to understand better the variables that affect heat production in low-permeability crystalline rocks. For each depth, location, and heat collector type, we used a range of fluid flow rates to examine the correlation between thermal energy production and resulting outlet temperature. Our results indicate a trade-off between thermal energy production and outlet fluid temperature depending on the fluid flow rate, and that the vacuum-insulated tubing outperforms a high-density polyethylene pipe in energy and temperature production. In addition, the results suggest that the local thermogeological factors impact heat production. Maximum energy production from a 600-m-deep well achieved 170 MWh/a, increasing to 330 MWh/a from a 1000-m-deep well, 980 MWh/a from a 2-km-deep well, and up to 1880 MWh/a from a 3-km-deep well. We demonstrate that understanding the interplay of the local geology, heat exchanger materials, and fluid circulation rates is necessary to maximize the potential of medium-deep geothermal boreholes as a reliable long-term baseload energy source.

中文翻译：

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越深越好吗？低焓结晶岩中深埋管换热器的热地质分析

能源部门正在经历一场根本性的转变，对低碳技术进行了大量投资以取代基于化石的系统。在人口稠密的城市地区，深钻孔为浅层地热系统提供了替代方案，后者需要大面积的表面积才能实现大规模的热生产。本文介绍了在 600 至 3000 m 深度范围内的低焓地热环境中可从中深埋管换热器中提取的热能的数值计算。我们应用了芬兰三个地点的热地质参数，并测试了两种同轴埋管换热器，以更好地了解影响低渗透率结晶岩产热的变量。对于每个深度、位置和集热器类型，我们使用一系列流体流速来检查热能产生与最终出口温度之间的相关性。我们的结果表明热能产生和出口流体温度之间的权衡取决于流体流速，并且真空绝缘管在能量和温度产生方面优于高密度聚乙烯管。此外，结果表明，当地的热地质因素会影响产热。600 米深井的最大能源产量达到 170 兆瓦时/年，从 1000 米深的井增加到 330 兆瓦时/年，从 2 公里深的井增加到 980 兆瓦时/年，最高可达 1880 兆瓦时/a 来自一口 3 公里深的井。我们证明了了解当地地质、热交换器材料、