Applied Thermal Engineering ( IF 6.4 ) Pub Date : 2020-11-21 , DOI: 10.1016/j.applthermaleng.2020.116335 Xiong Zhang
Two important problems are often encountered in the application of middle-deep geothermal heat pump (MD-GHP) systems: (1) large deviations between the operating and design conditions and (2) the lack of a theoretical basis for parameter control during actual operations. Accurate heat transfer and energy conversion models of MD-GHP must be established to solve these problems.
The novelty of this work lies in its establishment of a complete unsteady heat transfer model of a ground heat exchanger (GHE) and the coupling of this model with the energy conversion model of a heat pump. The influences of key design and control parameters on the coefficient of performance (COP) of MD-GHP are then studied. The use of vacuum insulated tubes as central pipes can increase the COP of the system by approximately 10%. When the well depth increases from 2000 m to 3000 m, the maximum heating load increases nearly linearly from 500 kW to 960 kW. At the bottom 20% of the well, insulation of the central pipe does not obviously increase the COP of the system. An optimal circulation rate occurs in MD-GHP, and this rate increases with the well depth. When the well depth increases from 2000 m to 3000 m, the optimal circulation rate increases from 6.7 kg/s to 9.9 kg/s. When the average heating load is kept constant, the intermittent heating mode slightly affects the average COP of the system. The thermal conductivity of the cement sheath exerts minimal effects on the COP of the system.
The overall merit of this work is that a mathematical relationship between the heating load and formation temperature distribution is established to predict the energy efficiency of the whole system. This study provides important theoretical guidance for the design and control of MD-GHP.
中文翻译:
地热交换与热泵系统耦合地热区域供热的数值研究
在中深层地热热泵(MD-GHP)系统的应用中经常遇到两个重要问题:(1)运行和设计条件之间的偏差大;(2)缺乏实际运行中参数控制的理论基础。必须建立MD-GHP的精确传热和能量转换模型来解决这些问题。
这项工作的新颖之处在于它建立了地面热交换器(GHE)的完整非稳态传热模型,并将该模型与热泵的能量转换模型耦合。然后研究了关键设计和控制参数对MD-GHP性能系数(COP)的影响。使用真空隔热管作为中心管可以使系统的COP大约提高10%。当井深从2000 m增加到3000 m时,最大加热负荷从500 kW线性增加到960 kW。在井底20%处,中央管道的隔热不会明显提高系统的COP。MD-GHP中出现最佳循环速率,并且该速率随井深而增加。当井深从2000 m增加到3000 m时,最佳循环速率从6.7 kg / s增加到9.9 kg / s。当平均加热负载保持恒定时,间歇加热模式会稍微影响系统的平均COP。水泥护套的导热性对系统的COP的影响最小。
这项工作的总体优点是,在加热负荷和地层温度分布之间建立了数学关系,以预测整个系统的能源效率。该研究为MD-GHP的设计和控制提供了重要的理论指导。