Nowadays, utilizing shallow geothermal energy for heating and cooling buildings has received increased interest in the building sector. Among different technologies, large borehole heat exchanger arrays are widely employed to supply heat to various types of buildings. Recently, a 16-borehole array was constructed to extract shallow geothermal energy to provide heat to a newly-developed public building in Berlin. To guarantee the quality of the numerical model and reveal its sensitivity to different subsurface conditions, model simulations were conducted for 25 years with two finite element simulators, namely the open-source code OpenGeoSys and the widely applied commercial software FEFLOW. Given proper numerical settings, the simulation results from OpenGeoSys and FEFLOW are in good agreement. However, further analysis reveals differences with respect to borehole inflow temperature calculation implemented in the two software. It is found that FEFLOW intrinsically uses the outflow temperature from the previous time step to determine the current inflow temperature, which makes it capable of much faster simulation by avoiding iterations within a single time step. In comparison, OpenGeoSys always updates the inflow and outflow temperature based on their current time step values. Because the updates are performed after each iteration, it delivers more accurate results with the expense of longer simulation time. Based on this case study, OpenGeoSys is a valid alternative to FEFLOW for modeling ground source heat pump systems. For modellers working in this field, it is thus recommended to adopt small enough time step size, so that potential numerical error can be avoided.

中文翻译：

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模拟邻域尺度浅层地热能利用：柏林的案例研究

如今，利用浅层地热能为建筑物供暖和制冷已经引起了建筑部门的越来越大的兴趣。在不同的技术中，大型埋管换热器阵列被广泛用于向各种类型的建筑物供热。最近，建造了一个 16 孔阵列，以提取浅层地热能，为柏林新开发的公共建筑提供热量。为了保证数值模型的质量并揭示其对不同地下条件的敏感性，使用开源代码 OpenGeoSys 和广泛应用的商业软件 FEFLOW 两个有限元模拟器进行了 25 年的模型模拟。给定适当的数值设置，OpenGeoSys 和 FEFLOW 的模拟结果非常一致。然而，进一步的分析揭示了在两个软件中执行的钻孔流入温度计算方面的差异。发现 FEFLOW 本质上使用前一个时间步的流出温度来确定当前流入温度，这使得它能够通过避免在单个时间步内进行迭代来更快地模拟。相比之下，OpenGeoSys 总是根据当前时间步长值更新流入和流出温度。因为更新是在每次迭代之后执行的，所以它可以提供更准确的结果，但会花费更长的仿真时间。基于此案例研究，OpenGeoSys 是 FEFLOW 用于模拟地源热泵系统的有效替代方案。对于在该领域工作的建模者，因此建议采用足够小的时间步长，