Performance analysis of deep borehole heat exchangers for decarbonization of heating systems

Meeting the climate change mitigation targets will require a substantial shift from fossil to clean fuels in the heating sector.Heat pumps with deep borehole exchangers are a promising solution to reduce emissions.Here the thermal behavior of deep borehole exchangers(DBHEs)ranging from 1 to 2 km was analyzed for various heat flow profiles.A strong correlation between thermal energy extraction and power output from DBHEs was found,also influenced by the heating profile employed.Longer operating time over the year typically resulted in higher energy production,while shorter one yielded higher average thermal power output,highlighting the importance of the choice of heating strategy and system design for optimal performance of DBHEs.Short breaks in operation for regenerating the borehole,for example,with waste heat,proved to be favorable for the performance yielding an overall heat output close to the same as with continuous extraction of heat.The results demonstrate the usefulness of deep boreholes for dense urban areas with less available space.As the heat production from a single DBHE in Finnish conditions ranges from half up to even a few GWh a year,the technology is best suitable for larger heat loads.

An Analytical Model for the Thermal Assessment of a Vertical Double U-Tube Ground-Coupled Heat Pump System in Steady-State Conditions

An analytical model was built to predict the thermal resistance of a vertical double U-tube ground-coupled heat pump that operates under steady-state conditions.It included a geometry obstruction factor for heat transfer throughout the backfill medium due to the presence of the second loop.The verification of the model was achieved by the implementation of five different borehole configurations and a comparison with other correlations in the available literature.The model considered a U-tube spacing range between(2)and(4)times the U-tube outside diameter producing a geometry configuration factor range of(0.29-0.6).The results of the model were utilized for the assessment of the DX ground heat exchanger coupled heat pump system.For similar geometrical configurations,the borehole thermal resistance experienced a decrease as the geometry factor increased.The single U-tube borehole thermal resistance was higher than that of the double U-tube heat exchanger by(10-27)%for the examined geometry configurations.The borehole thermal resistance at tube spacing of twice the tube diameter was higher than the predicted value at the triple diameter and fell in the range of(18-34)%.

Energy saving analysis of a hybrid ground-coupled heat pump project

A hybrid ground-coupled heat pump（HGCHP）project in Nanjing,China is chosen to analyze the building energy-consumption properties in terms of different control strategies,building envelope and the terminal air-conditioning system.The HGCHP uses a supplemental heat rejecter to dissipate extra thermal energy to guarantee underground soil heat balance.The software EnergyPlus is employed to simulate the project and design the heat flow of the cooling tower and the borehole heat exchanger（BHE）.Then two feasible control strategies for the cooling tower and the borehole heat exchanger are proposed.The energy-saving potential of the building envelope is analyzed in terms of the surface color of the wall/roof.With the same terminal system,it is found that in the cooling season the heat flow of the insulated building with black wall/roof is 1.2 times more than that with white wall/roof.With the same insulated building and gray wall/roof,it is concluded that the heat pump units for a primary air fan-coil system show an annual energy consumption increase of 44.7 GJ compared with a radiant floor system.

地源热泵竖埋管换热器热工参数反演方法研究

基于线热源模型,研究地源热泵竖埋管换热器钻孔热阻、岩土导热系数及体积比热等热工参数的4种反演方法,基于实测数据进行反演计算。结果表明:只利用埋管进出口水温数据,采用寻优方法可得3个参数,但钻孔热阻及体积比热可能偏离物理实际。只利用水温、补充钻孔热阻或体积比热的反演方法得到的结果都有一定的不确定性。利用水温及壁温计算钻孔热阻、利用壁温反演岩土导热系数及体积比热,所得结果更符合物理实际。建议热响应测试同时记录水温及壁温,孔壁温度可在埋管上部、中部以及底部设置测点。

单U、双U型埋管换热器换热性能与经济性研究

结合实际工程建立单U型与双U型埋管换热器实验系统,进行夏季排热和冬季取热实验。以单位井深换热量为评价指标对不同埋管方式的换热性能进行分析比较,得出排热和取热工况下,双U型埋管单位井深换热量均高于单U型。同时,分析了单U与双U型埋管换热器的经济成本构成,采用投资成本指标、静态投资成本差值和投资成本比数等参数,从换热性能和经济成本两方面对地下埋管换热器进行综合评价。

垂直埋管式地源热泵土壤温度分布研究

以某办公建筑双U垂直埋管式地源热泵系统作为研究对象，对其连续3a的运行进行土壤温度监测及分析。结果表明：在换热器夏季累积换热量为366．5MJ，冬季累积换热量为233．8MJ的情况下，距换热器5．0m范围内土壤温度共升高2．8℃，系统运行30a后，土壤温度超过35℃；土壤在与工程井径向换热的同时进行深度范围内的轴向换热；另外土壤的蓄热性使土壤温度变化出现2-6个月的时间滞后。

竖直埋管换热器热响应半径计算方法

随着能源压力的日益增大,世界各国都十分重视可再生能源的利用与开发,地源热泵技术作为一种清洁、高效的可再生能源,近年得到了较快的发展。该文利用无限长线热源传热计算模型,讨论了介质内过余温度场的分布特性。结果表明:介质内温度响应在孔壁处最大,随离孔壁距离的增加呈指数衰减,随时间的增加而增大;热传播区域随时间的增加而增大,随介质的热扩散系数的增加而增大。针对工程中群埋管换热器情况,利用叠加原理计算群埋管的孔壁温度,定义换热器的热响应半径为其他钻孔引起的过余温度影响系数≤5%时相邻钻孔中心线之间的垂直距离。在大量计算分析基础上,提出了竖直埋管换热器热响应半径计算方法。计算结果表明该文方法具有较好的计算精度,竖直埋管换热器的热响应半径随岩土热扩散系数增大而增大,随持续运行时间增加而增大,随钻孔排数增加而增大,随着钻孔孔径增大而增大;钻孔布置方式不同对钻孔热响应半径的影响较明显,相同布置方式下钻孔直径对其热响应半径的影响较小。针对工程中常见的115和135 mm 2种孔径,绘制了不同岩土介质下钻孔单排、双排和三排以上布置时热响应半径-运行时间的关系曲线。工程算例表明该文方法简单方便,为工程设计提供了便利。

分层渗流地层中竖直地埋管的换热计算模型

假定实际地层为水平分层且部分地层中有稳定的水平渗流,建立竖直地埋管换热器的计算模型。该模型采用解析和数值计算相结合的方法,采用一个迭代公式便能方便地得到任意时刻换热器和地层中的温度分布,且能直接用于变负荷条件下换热器工作性能的分析。应用该模型对一简单分层渗流地层中换热器的工作性能进行计算,结果表明:稳定的地下水渗流能有效增强换热器的工作性能;渗流层所对应孔壁的温度明显低于其他岩土层,且渗流层的单位深度换热量高于其他岩土层;渗流对换热器性能的影响需要一定时间才能反映出来,渗流速度越小所需时间越长。比较该模型与现有解析模型表明,现有解析模型中采用整个地层具有稳定渗流的假定高估了换热器的工作性能。

上海某垂直单U地埋管钻孔换热能力试验研究

通过对上海某典型地质钻孔垂直单U地埋管换热能力进行现场试验,得出了夏季散热及冬季取热模式下单孔的换热能力。结果表明,在本测试工况条件下,冬季工况每延米取热量约为夏季工况散热量的65%,冬、夏季钻孔取放热能力明显不平衡。试验结果能为类似地质条件下地源热泵系统的设计及施工提供有益参考。

商用土壤源热泵设计模型的实用对比

通过武汉清江花园小区工程来具体说明类三维计算模型在土壤源热泵中央空调系统中的实际应用,从计算比较中得出,地下换热器的尺寸选择对整个系统的经济和高效起着基础的作用。为了使计算结果比较接近使用需求,应对一些不确定环境因素进行分析。

小型竖直埋管地源热泵空调系统实验研究

为了探讨上海地区小型竖直埋管地源热泵空调系统的性能及其适用性,笔者建立了一小型竖直埋管地源热泵空调系统实验台。本文分别在上海典型供冷工况和供热工况下,对该系统地埋管换热器的进出口水温、换热能力及热泵机组的性能进行了实验和分析。所得结果可为上海地区竖直埋管地源热泵空调系统的工程设计和理论研究提供参考。