Soil Nature Effect Investigation on the Ground-to-Air Heat Exchanger for the Passive Cooling of Rooms

The building sector consumes much energy either for cooling or heating and is associated to greenhouse gas emissions. To meet energy and environmental challenges, the use of ground-to-air heat exchangers for preheating and cooling buildings has recently received considerable attention. They provide substantial energy savings and contribute to the improvement of thermal comfort in buildings. For these systems, the ground temperature plays the main role. The present work aims to investigate numerically the influence of the nature of soil on the thermal behavior of the ground-to-air heat exchanger used for building passive cooling. We have taken into account in this work the influence of the soil nature by considering three types of dry soil: clay soil, sandy-clay soil and sandy soil. The mixed convection equations governing the heat transfers in the earth-to-air heat exchanger have been presented and discretized using the finite difference method with an Alternate Direction Implicit (ADI) scheme. The resulting algebraic equations are then solved using the algorithm of Thomas combined with an iterative Gauss-Seidel procedure. The results show that the flow is dominated by forced convection. The examination of the sensitivity of the model to the type of soil shows that the distributions of contours of streamlines, isotherms, isovalues of moisture are less affected by the variations of the nature of soil through the variation of the diffusivity of the soil. However, it is observed that the temperature values obtained for the clay soil are higher while the sandy soil shows lower temperature values. The values of the ground-to-air heat exchanger efficiency are only slightly influenced by the nature of the soil. Nevertheless, we note a slightly better efficiency for the sandy soil than for the sandy-clayey silt and clayey soils. This result shows that a sandy soil would be more suitable for geothermal system installations.

Simulation of embedded heat exchangers of solar aided ground source heat pump system

Aimed at unbalance of soil temperature field of ground source heat pump system, solar aided energy storage system was established. In solar assisted ground-source heat pump (SAGSHP) system with soil storage, solar energy collected in three seasons was stored in the soil by vertical U type soil exchangers. The heat abstracted by the ground-source heat pump and collected by the solar collector was employed to heating. Some of the soil heat exchangers were used to store solar energy in the soil so as to be used in next winter after this heating period; and the others were used to extract cooling energy directly in the soil by circulation pump for air conditioning in summer. After that solar energy began to be stored in the soil and ended before heating period. Three dimensional dynamic numerical simulations were built for soil and soil heat exchanger through finite element method. Simulation was done in different strata month by month. Variation and restoration of soil temperature were studied. Economy and reliability of long term SAGSHP system were revealed. It can be seen that soil temperature is about 3 ℃ higher than the original one after one year's running. It is beneficial for the system to operate for long period.

Research on Performance of Ground-Source Heat Pump Double U Underground Pipe Heat Exchange

This paper uses FLUENT software building the three-dimensional unsteady state model of ground source heat pump single U and double U underground pipe to study on heat exchange of underground pipe system in the condition of unsteady state long-term continuous running, analyzes the change of soil temperature filed around underground pipe and performance of underground pipe heat exchange between single U and double U pipe system. The results show that double U pipe system is better than single U system, which can improve unit depth heat exchange efficiency, reduce the number of wells and reduce the initial investment.

Achieving Cooler Soil as an Effective Heat Sink for Earth-to-Air Heat Exchanger (EAHE) Cooling Technology in Malaysia Tropical Climate

This research is intended to explore the capacity of Malaysia soil in becoming a more effective heat sink for the application of Earth-to-Air Heat Exchanger (EAHE) Cooling Technology in Malaysia. EAHE Cooling Technology consists of buried pipes underground where the ambient air is channeled through from the pipe inlet and produces cooler air at its outlet. Within the buried pipes, heat exchange process occurs between the air and the soil that surrounding the pipe. This building cooling technology has been applied in many countries, mostly in temperate or hot and arid climate where the diurnal temperature is large. However, minimal resources were found on the study of EAHE application to buildings in Malaysia, hence there is room to develop. A parametric study on EAHE cooling application in Malaysia was done through field experiment and concluded that among many parameters affecting the technology performance, the soil temperature which surrounded the pipe was the most influential factor. The study recommended to further reduce the soil temperature to achieve a cooler outlet temperature. In response to that, this research conducted a parametric study of soil temperature under three different soil surface conditions: bare, shaded with timber pallettes and insulated with used tyres at 1.0 m and 1.5 m underground. The data was logged for a month and the result has shown significant reduction in the soil temperature underground below the shaded and insulated soil surface as compared to below bare soil surface condition. The insulated soil surface produced the best result where the soil temperature was reduced up to 26.9°C. The main contribution of this paper is to highlight that the soil surface treatment can be used to reduce solar heat gain within the soil underground and thus improving the performance of EAHE Cooling Technology particularly for the application in Malaysia tropical climate.

坡地面栽培对日光温室土壤温度环境的影响

以平地面温室为对照,对坡地面温室土壤白天蓄热期间的温度变化和蓄热量变化进行研究,以期为老旧日光温室的改造提供参考。结果表明:坡地面温室明显改善了日光温室土壤温度环境,与平地面温室相比,坡地面温室日提高土壤温度0.1~1.7℃,日最高土壤温度提高1.0~1.9℃,日降低冠层上部空气温度1.9~4.0℃,日增大土壤蓄热量8.44%~44.57%。晴天土壤温度和蓄热量的增加幅度大于阴天。

间歇运行下地埋管周围土壤热湿特性实验研究

搭建地源热泵地埋管周围土壤热湿传递的试验台,对初始含水率、防水层、保温层以及相变回填材料等因素对土壤热量和水分传递的影响进行探究。沙箱实验结果表明:竖直埋管的热量会引起周围土壤的温度传递和水分迁移,土壤初始含水率的提高有利于温度恢复和地埋管换热,系统运行期间的温度峰值随初始含水率的增加而降低,防水层、保温层和相变回填材料也会对温度峰值和温度恢复产生影响。温度梯度和水力梯度是引起水分迁移的主要因素,在15%初始含水率时,温度梯度占据主导,驱动水分沿径向向远处扩散;在25%初始含水率时,水力梯度占据主导,水分沿深度向下渗透。

基于TRNSYS模拟的多能源时序互补供暖系统研究

针对典型气候区(郑州和哈尔滨)不同建筑类型的负荷特点,提出多能源时序互补集中式供暖系统,并对蓄热场的蓄取规律进行模拟分析。通过TRNSYS模拟,对多能源时序互补集中式供暖系统在不同建筑中运行时的蓄热场土壤平均温度、系统运行策略、能效提升率进行分析。结果表明:土壤平均温度处于动态平衡,公建系统土壤平均温度最高可达65℃;公建及寒冷地区居建系统运行策略均以太阳能直供和蓄热场直供为主要供热方式,地源热泵为辅,严寒地区居建系统运行策略是以地源热泵为主要供热方式,太阳能直供和蓄热场直供为辅;多能源时序互补集中式供暖系统与地源热泵系统相比能效提升明显,公建系统能效可提升90%以上,严寒地区居建系统能效提升59.12%。

跨季节土壤蓄冷-土壤源热泵系统长期运行特性实验研究

为解决土壤源热泵在长期运行条件下土壤热失衡导致的运行效率下降问题,针对夏热冬冷地区的特点,提出一种利用室外风机盘管将冬季或过渡季室外空气中的冷量存储于土壤中的跨季节蓄冷-土壤源热泵系统。利用地埋管换热系统的相似模型实验台,进行了系统运行3年的供冷、供暖、蓄冷工况实验,综合分析了埋管进出口水温、气温、土壤温度、土壤及埋管换热量、单位埋深换热量等参数,研究系统的长期运行特性。研究显示:3年的土壤平均温度分别上升0.14℃、0.22℃、0.20℃,总体来说升高幅度较小;蓄冷时的室外气温越低、土壤与室外空气的温差越大,蓄冷效果越好;埋管进出口平均温差、土壤平均温度变化、土壤换热量、埋管换热量、单位埋深换热量的大小关系一致。研究结果表明,跨季节蓄冷维持了土壤的长期热平衡,系统的长期运行效果较好。

长沙市某区浅层地埋管岩土体热响应试验性能研究

长沙市某区为高新技术发展示范基地,能源冷热需求量大,该片区地质条件复杂,邻近少有地源热泵热响应测试数据,通过性能研究,揭露研究区地层以第四系覆盖层及元古界泥岩、砂岩、板岩为主,10~40 m存在上层少量上层滞水;通过线热源模型热响应试验,测试岩土体综合平均导热系数为1.78~1.95 W/(m·K),钻孔热阻值为0.18~0.23(m·K)/W,并与钻孔取芯和室内热物性测试进行对比,吻合度较好。考虑长沙地区气候特征,在不考虑大量钻孔换热过程中存在热干扰和长时间持续运行情况下,通过换热计算,夏季每延米换热量最大可达51~53 W/m,冬季每延米换热量最大取值为47~49 W/m。

EPC项目中地源热泵系统的设计优化与施工技术

浅层地热能是一种重要的新型能源,开发浅层地热能主要作用是为建筑物提供供暖制冷,目前利用浅层地热能作为冷、热源的地埋管地源热泵系统在工程中得到广泛应用。对芜湖某EPC公建项目地源热泵系统进行了研究,介绍了项目场地区域岩土热响应试验情况,根据相关规范要求进行了地埋管地源热泵系统的设计优化,阐述了地埋管地源热泵的施工技术。由岩土热响应试验得出芜湖地区适合做地源热泵系统,利用规范中热量平衡原理,优化了初步设计中地源热泵的数量和节点处理措施,缩短了施工工期并节省了工程造价,针对地源热泵施工过程中的技术难点提出应对措施,对工程实际具有参考意义。

冷凝热回收地源热泵系统运行仿真研究

根据经验模型与样本数据在TRNSYS平台上建立开发了冷凝热回收地源热泵系统仿真模型,以夏热冬冷地区的上海市宾馆建筑为例,利用开发模型研究了冷凝热回收地源热泵系统长期运行的效益。仿真计算结果表明,与传统地源热泵+燃气锅炉系统相比,冷凝热回收地源热泵系统中长期运行能保持土壤温度稳定,土壤温升减少了76%,运行20年一次能源标准煤消耗量减少10%,冷凝热回收改造动态投资回收期为4年,具有更好的稳定运行与环保节能效益。

减缓集群地埋管冷/热量累积效应的分区运行方法

针对冬夏季负荷不平衡时集群竖埋管地埋管换热器区域地下土壤冷/热量累积问题,提出低负荷季节只运行较为中心区域埋管的分区运行方法以减缓冷/热量累积效应。以夏季负荷大于冬季负荷为例进行分析,发现只运行较为中心区域部分的埋管时,土壤最高温度和平均温度均较未分区运行时显著下降,可有效抑制地下热量累积效应。分析还表明当土壤导热系数较小时,采用分区运行方法对抑制和减缓地下冷/热量累积效应更为有效。上述结论同样适用于冬季负荷大于夏季负荷的情况。

地源热泵土壤温度恢复特性研究

根据某实际工程的换热器管群的布置情况和地源热泵系统的运行情况,对典型区域典型年和运行5年的土壤温度变化情况进行了数值模拟,并对模拟结果进行了深入分析,所得结论可为地源热泵系统的优化设计提供参考。

水平埋管周围土壤温度场数值模拟研究

建立了应用于地源热泵系统的水平埋管的传热数学模型,并采用 ANSYS 软件模拟了埋深为 3 m 的水平埋管在冬夏运行工况下周围土壤的温度场变化。根据选取的特定时间比较了单管和双管水平埋管在相同工况下周围土壤温度场,得出各自的影响半径,为水平埋管换热器的埋设提供了参考。

湿土壤含湿特性对传热影响研究

针对于地源热泵地能换热系统在具有热源条件下土壤热湿迁移现象,进行了初步分析和实验,并对模拟计算应用可行性进行了认证。土壤含水静态传热实验,目的在于探讨土壤含湿程度对传热能力的影响,以及地下含水程度对换热区域换热性能的影响。研究土壤不同含水率情况下的传热性能,从而推断含水率在工程中的影响程度。

土壤蓄冷与释冷过程的模拟与试验验证

结合土壤耦合热泵技术与冰蓄冷技术的特点,并在土壤蓄冷与土壤耦合热泵集成系统构想的基础上,建立了考虑土壤周期性冻融相变的土壤蓄冷、释冷过程的数学模型,并采用数值方法进行求解。通过建立模拟土壤蓄冷、释冷过程的砂箱试验台并由试验测试结果表明:所建模型的计算结果与试验数据吻合较好,二者偏差在10%以内。应用本文所建数学模型可对土壤蓄冷系统的蓄冷、释冷运行特性进行计算与分析,并为该集成系统的应用提供理论支持与技术储备。

土壤蓄冷与土壤耦合热泵集成系统

以平衡电网负荷、削峰填谷及利用可再生浅层地热能为基本出发点,提出了土壤蓄冷与土壤耦合热泵集成系统的新设想。该集成系统将蓄冷装置与地下埋管换热器合二为一,在夏季空调工况时,利用电力低谷时段的廉价电力,将冷量全部或部分储存到土壤中,以供白天用电高峰期空调使用;在需供冷的过渡季,系统按土壤耦合热泵系统的供冷工况运行,将系统的冷凝热排至土壤中,为冬季供暖储备热量;供暖季节,系统按土壤耦合热泵系统的供暖工况运行,土壤作为热泵系统的低位热源。介绍了该集成系统的形式、技术特点及核心研究内容,并在前期研究工作的基础上对影响集成系统运行特性、冷量损失的因素进行了较全面的总结。

土壤温度场对竖直U形地埋管换热性能的影响

对广州地区地下50m以内土壤温度分布进行了模拟,研究了土壤温度的分布特性和变化规律,模拟了冬季工况下埋管区域土壤温度场的分布情况,分析了动态负荷分布对地埋管换热性能的影响。

日光温室地中热水管加温对土壤温度的影响

为了明确日光温室地中埋热水管对土壤加温效果的影响,利用Ansys有限元分析方法,对日光温室内地埋热水管在不同进回水平均温度(30,40,50℃)、水流速度(0.05,0.10,0.25,0.50m·s-1)、管道埋深(30,40,50,80cm)、埋管间距(20,40,50,60,80,100cm)和管道直径(25,32,40,50,63mm)下的土壤温度分布进行研究。结果表明:水温对地表温度影响显著,管道内水流速≥0.10m·s-1时地表温度变化不大,在长期平稳条件下,埋管深度、管径对地表温度影响不大,而管间距对土壤温度分布影响显著。选择适宜条件:进回水温度40℃,水流流速0.10m·s-1,管道埋深40cm,管间距20cm,管径25mm进行试验,与相应条件下的模拟结果对比,实测值与模拟值相关程度较高,可为地埋管道加温方式的布设和运行提供理论参考。