Design and operation of Borehole Thermal Energy Storage systems

在井眼热能储存（BTES）系统中，目标是将地面温度提高到正常值以上，并长期储存能量。BTES系统尚未得到广泛研究，需要科学发展来更好地设计和操作这些系统。该研究计划的长期目标是开发知识和模型，以更好地了解BTES系统的性能，以便工程界可以广泛使用它们，充分利用可再生能源的潜力。用于预测钻孔传热的传热系数通常是根据标准相关性计算的。然而，对于减少流量的条件，目前的自然对流和混合对流相关性不足以用于井眼。提出了开发一种热流密度传感器，以确定钻孔管内的局部传热系数。试验将在一个小规模的钻孔试验设施中进行，以开发适用于钻孔的一般传热系数相关性。更先进的BTES系统已被提出，其中电热泵在夜间运行，此时电网的温室气体排放量最低，用于将热量从外环钻孔传递到中心核心钻孔。在这些系统中，将在单个井眼上进行热量注入/收集，以快速提高（或降低）BTES某些区域的储存温度。这将为建筑物提供灵活性，使其可以直接从中心核心加热或从外环冷却。为了提高计算效率，本文将开发一种新的载荷聚合方案来处理每个井眼热历史的时间叠加。业界需要制定指导方针和工具来评估BTES的设计和操作。首先，工程师需要知道BTES的最大传热速率。提出了用变热交换器效率作为BTES充电状态函数的概念来表示钻孔传热。工程师面临的第二个设计问题是确定最佳BTES工作温度，以最大限度地节约能源和减少温室气体排放。这个最佳温度是在足够高的温度直接提供空间加热和足够低的温度之间的折衷，以限制BTES热损失和由此产生的热泵温度提升。因此，需要进行参数研究，以找到适合特定应用的最佳参数集。随着我们向社区规划迈进，存储资源共享以实现能源的灵活性和弹性，BTES系统将发挥更大的作用。这里提出的项目将增强我们对BTES热行为的理解，并为更好地设计这些系统提供严格的计算方法。

In a Borehole Thermal Energy Storage (BTES) system, the goal is to increase the ground temperature above its normal value and to store energy over long periods. BTES systems have not been studied extensively and scientific developments are required to better design and operate these systems. The long-term objective of the research program is to develop knowledge and models to better understand the performance of BTES systems so that the engineering community can use them extensively to fully utilise the potential of renewable energy. Heat transfer coefficients used to predict borehole heat transfer are typically calculated based on standard correlations. However, for reduced flow conditions, current natural and mixed-convection correlations are inadequate for boreholes. It is proposed to develop a heat flux sensor that will enable the determination of local heat transfer coefficients inside borehole tubes. Tests will be performed in a small-scale borehole test facility to develop general heat transfer coefficient correlations applicable to boreholes. More advanced BTES systems have been proposed where electric heat pumps, operating at night when greenhouse gas emissions from the grid are the lowest, are used to transfer heat from an outer ring of boreholes to the central core boreholes. In these systems, injection/collection of heat will be performed on individual boreholes to rapidly increase (or decrease) the storage temperature in certain regions of the BTES. This will provide flexibility for buildings to operate on direct space heating from the central core or cooling from the outer ring. A new load aggregation scheme will be develop to handle temporal superposition of the thermal history of each borehole to improve computational efficiency. There is an industry need to develop guidelines and tools to asses the design and operation of BTES. First, engineers need to know the maximum heat transfer rate that can be expected from a BTES. It is proposed to represent borehole heat transfer using the concept of variable heat exchanger efficiency as a function of the BTES state-of-charge. The second design problem facing engineers is the determination of the optimum BTES operating temperature for maximum energy savings and minimum greenhouse gas emissions. This optimum is a compromise between a high enough temperature to provide space heating directly and a low enough temperature to limit BTES heat losses and the resulting temperature lift imposed on the heat pump. Thus, parametric studies will be needed to find the best set of parameters for particular applications.  As we advanced towards community planning where storage resources are shared for energy flexibility and resilience, BTES systems will play a larger role. The projects proposed here will enhance our understanding of the thermal behaviour of BTES and provide rigorous calculation methods to better design these systems.