Zeng Zhixiong

South China University of Technology Architectural Design and Research Institute

Abstract: This paper analyzes the current representative model of underground borehole heat exchanger, vertical U-tube heat exchanger model, heat transfer model of single well recharged heat exchanger and horizontal buried pipe model of road and bridge melting. According to the current research and application, the research direction of heat transfer in underground heat exchangers is given.

Key words: groundwater heat pump system, underground heat exchanger, heat transfer model

The Ground Source Heat Pump System is an air conditioning system that uses low-grade energy sources such as soil sources and groundwater sources. The research and application of ground source heat pump system in foreign countries started earlier, mainly focusing on heat transfer research and analysis, system design method, installation technology and operation condition test of underground buried heat exchanger, and there are a large number of engineering examples. With the development of computer simulation technology, research on ground source heat pump system and its components has become a common research method. To establish an accurate and efficient ground source heat pump system simulation model, it is necessary to change the underground buried pipe. The heat transfer mechanism and model of the heat exchanger are analyzed, and the corresponding calculation methods are proposed. Therefore, the research and analysis of the heat transfer model of the underground heat exchanger has become one of the current research hotspots.

1 Classification of heat transfer models for underground heat exchangers

The heat transfer model of the underground borehole heat exchanger can be divided into:

1. Semi-empirical design calculation formula. Based on the concept of thermal resistance, this model estimates the length of the buried heat exchanger of the underground heat exchanger according to the cold and heat load. However, when the calculation formula is used for calculation, the thermal resistance is simplified, which is inconsistent with the actual operation. Large deviation.

Based on the discretization numerical calculation, the finite element method or finite difference method is used to solve the underground temperature response and conduct heat transfer analysis.

Based on the concept of thermal resistance, the analytical expression of the thermal resistance of the single heat transfer link of the underground heat exchanger is obtained. The superposition principle is used to process the complex multi-heat transfer link. At the same time, the relevant correction coefficient of the analytical model is determined by experiments and other means. Improve the calculation accuracy of the model.

2 Research and development of underground heat exchanger model

In the design and installation of the whole ground source heat pump system, the design, installation and commissioning of the underground buried heat exchanger account for a large proportion of the entire project investment. In the design and operation phase, the computer simulation is used to evaluate the system energy consumption and operation. The method of passing. According to the system optimization design requirements, the calculation efficiency and calculation accuracy of the simulation model are put forward higher requirements.

2.1 Analytical solution model for underground borehole heat exchanger

In 1986, Hart and Couvillison calculated the soil temperature distribution around the line heat source based on the closed-source analysis of the line source theory, and proposed the concept of the far-end radius (the far-end radius refers to the soil temperature around the borehole under the heat of the online heat source). The radius of the affected area, beyond this radius, the temperature of the soil will not change, and in the area within this radius, the soil temperature will change with the heat of the line heat source.

The IGSHPA Approach is the standard method for determining the size of underground borehole heat exchangers in North America.

Cylindrical Source Mode l1. In 1947, Carslaw and Jaeger first proposed a theoretical model of cylindrical source; later developed a cylindrical source analysis solution under constant heat flow, and obtained the surrounding of the buried heat exchanger through this theoretical model. The temperature distribution of the soil. From the theoretical research point of view, the cylindrical heat source heat transfer model in infinite area is more advantageous and research value than the line heat source heat transfer model.

2.2 Numerical solution model of underground borehole heat exchanger

The numerical solution model of underground borehole heat exchanger mainly includes numerical solution model based on finite-length heat source, transient heat transfer model of vertical tube-type heat exchanger based on energy conservation, and densely buried underground for simulating seasonal heat energy storage. Energy storage performance model of vertical heat exchangers, etc.

In 1993, T.-K.Lei used the finite difference method to establish a simulation model of the vertical U-shaped buried pipe. The model considered the intermittent operation conditions of the heat pump unit and the convective heat transfer inside the pipe, without considering the surface, grouting material and more. Thermal interference between the drill holes. In 1997, Rottmayer et al. established a two-dimensional transient heat transfer model for a single vertical U-shaped buried tube in an infinite medium based on the explicit finite difference method.

In the early 1990s, China began to conduct exploratory research on ground source heat pumps. The research focuses on the experimental research of geothermal heat exchangers. The theoretical research is mainly based on foreign research, mainly focusing on underground heat transfer. Heat transfer model, simulation study of heat transfer calculation of underground heat exchangers and other related fields. In 2003, Fang Shuhong and Yan Nairen from Shandong University of Architecture [2] obtained an analytical solution model for the heat source temperature response of an infinite medium with a seepage flow through the establishment of a heat transfer energy equation in a porous medium. The model was used to analyze the groundwater temperature. The effect of seepage on the heat transfer of geothermal borehole heat exchanger.

In 2007, Yang Weibo [3] of Southeast University proposed a variable heat flow line heat source model, and based on the superposition principle and step theory, the heat source model of the variable heat flow line was invented and verified.

2.3 Single Well Recharge (Standing Column Wells) Heat Exchanger Model

The single well recharge ground source heat pump system is a new type of system. According to the ASHRAE Handbook: HVAC Applications (1995) classification, the single well recharge ground source heat pump belongs to the fourth category system, which is extracted and recharged into the same well. Groundwater for heat exchange. Because of the radial and longitudinal heat and mass transfer of groundwater and the complexity of underground aquifers, it is a complex three-dimensional seepage process. The study of heat transfer models of this system at home and abroad is as follows: In 1979, Bose et al. In the single-well recharge system of the “Geo-thermal Well” system, later generations made in-depth research in this field: In 1986, Tan and Kush used the research on a single well recirculation system with a diameter of 152 mm and a depth of 189 m. The line heat source theory has obtained an analytical solution that can approximate the temperature of the rock around the wellbore during long-term operation. From 1993 to 1995, Mikler and Yuill analyzed the seepage and heat transfer processes of single well recharge by numerical methods. A large amount of experimental data; In 1999, Orio based on Kelvin's line heat source theory to analyze the heat transfer process of a single well recharge system; in 2004, Rees, Spiller, Deng Z [4~5] and others established a two-dimensional, non- A steady-state numerical model is developed on the basis of which a one-dimensional simplified numerical model is developed for practical engineering calculations. In 2007, Yan Nairen, Li Wei and others obtained an analytical model of the outflow of a single well recharged geothermal heat exchanger in a confined aquifer under certain simplified conditions.

2.4 Horizontal buried pipe model of road and bridge melting snow

By laying a horizontal buried pipe under the ground and maintaining the surface temperature of the road by circulating hot water, it can achieve the purpose of melting snow and ice, mainly used for airport runway pavement and bridge deck snow melting. This model needs to take into account factors such as real-time snowfall, surface snowmelt speed, and outdoor temperature. In 1970, Schnurr and Roger first published an article on ASHRAE, based on the line source theory to establish a steady-state model of buried snow.

From 1972 to 1973, M.Leal and NMSchnurr established the instantaneous model of snow melting, but did not consider the accumulation of undissolved snow on the road surface. In 2000, Chiasson et al. established a two-dimensional snow melting model, and Rees et a (lJ.D. Spitler) developed the model to dynamically calculate the entire snow melting according to external temperature and snow conditions. The process, as well as experimental verification of the model.

3 Research direction and application prospects

At present, the application of ground source heat pump system is increasing at a rate of about 10% per year. In the future, research on this system will focus on high efficiency and low investment. Future research on ground source heat pump systems will focus on the following areas:

1. Simulation study of ground source heat pump system

The means of analyzing the energy consumption, design and control of the ground source heat pump system through simulation technology has become one of the important methods for research, and the ground-loop heat exchanger is the ground source heat pump system. The important part of its heat transfer is the focus of research. Therefore, for the simulation of ground source heat pump system, the main research direction focuses on the establishment and optimization of the underground heat exchanger model.

2. Research on control strategy of ground source heat pump system

For ground source heat pump systems, how to more effectively perform long-term stable cooling or heating is the criterion for judging the merits of the system. The heat and cold load of the building and the heat dissipation of the underground heat exchanger to the soil are uneven, the heat exchange capacity of the underground heat exchanger is affected by the groundwater seepage, and the integration of various cold and heat sources in the air conditioning system Utilization has become more and more common, so it is particularly important to study the control strategy of underground heat exchanger system.

3. Optimization of filling of underground buried heat exchanger

The optimization study of the filler in the borehole of the underground heat exchanger is beneficial to improve the heat exchange between the underground heat exchanger and the soil and improve the efficiency of the system.

4. Research on soil thermal conductivity testing technology

The heat transfer process of underground heat exchangers is complicated and involves many factors. Therefore, the underground heat transfer model is established and improved to make it have better adaptability and calculation accuracy. It is the design of underground heat exchangers. The theoretical basis of the determination of soil thermal properties will certainly become the focus of research work. At the same time, in the construction of the system, how to quickly and effectively pass the test and simulation to obtain the soil physical parameters is one of the main technical problems concerned in the actual engineering.

5. Consideration of various influencing factors and research on unusual heat exchangers such as spiral buried pipes

The existing underground borehole heat exchanger model is mostly based on the simulation model of the vertical underground borehole heat exchanger, while the modeling research on the horizontal buried pipe, the inclined buried pipe and the spiral buried pipe is relatively lacking. With the continuous development of computer simulation technology, taking into account the influence of tube group, the influence of soil freezing and thawing, groundwater seepage and other related factors, it is necessary to increase the investment in the underground buried converter simulation model. (Editor: Zhuo Jianben)

references

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