Due to various factors that influence underground heat exchange in ground-source heat pump systems—such as design complexity and the lack of fundamental data—improper parameter selection can lead to unacceptably high project costs and hinder the technology's widespread adoption. It wasn't until the late 1980s that this system began to be widely used in building HVAC applications. In recent years, numerous studies have highlighted the progress made abroad [1].
With its environmental benefits and energy efficiency, the ground-source heat pump air conditioning system has gained increasing attention in China. Recently, practical projects involving this system have started to emerge. However, detailed design guidelines and matching data with traditional building systems remain limited. Field investigations and operational experiences are crucial for advancing research on ground-source heat pumps globally.
A key challenge in ground-source heat pump technology is the study of geothermal heat exchangers, which serve as the core of the system and form the basis for its application. Most existing design methods are based on experimental studies conducted in the U.S. and Europe. Domestic research has primarily focused on these experiments, but due to insufficient understanding of the heat transfer mechanisms in soil, the conclusions drawn are often limited in scope and lack strong theoretical support. This makes it difficult to apply them directly to real-world engineering designs. Therefore, developing a more realistic heat transfer model for geothermal heat exchangers is a critical area of current research.
The performance of a ground-source heat pump system is largely determined by two aspects: the length and configuration of the geothermal heat exchanger, and the compatibility between the heat pump unit and the system. Once the heat exchanger configuration is set, the system’s performance becomes a central concern in engineering practice. Thus, another focus of this paper is to develop a dynamic model of the geothermal heat exchanger and the heat pump unit, and validate its accuracy through experiments.
1. Overview of Geothermal Heat Exchanger Models
Geothermal heat exchangers can be classified into horizontal and vertical types based on their layout. Vertical systems typically use U-shaped pipes buried in boreholes, offering high heat transfer efficiency and compact space usage. These systems are widely applied in engineering, and this paper focuses on the most commonly used single U-tube configuration. Due to the complexity of heat transfer processes, no universally accepted models exist. Current international models fall into two categories: analytical solutions based on thermal resistance concepts and numerical solutions using discrete calculations. The former, while simple and easy to apply, relies on many simplifications and struggles to account for time-varying loads and imbalances. The latter offers greater accuracy but requires significant computational resources, making it unsuitable for large-scale or real-time engineering applications.
2. Establishment of a Vertical Single U-Tube Geothermal Heat Exchanger Model
2.1 Quasi-Three-Dimensional Borehole Model
Previous one- and two-dimensional models oversimplified the borehole structure, assuming constant fluid temperature in the U-tube and ignoring depth variations and thermal short circuits. To improve accuracy, a quasi-three-dimensional model was developed, incorporating axial convection heat transfer while maintaining simplicity by neglecting axial thermal conductivity within the borehole. This model allows for more realistic simulation of fluid temperature changes along the tube.
2.2 Transient Temperature Field Analysis of Boreholes
To analyze the transient temperature field of drilled holes, the Green’s function method was applied to calculate the temperature response of finite-length heat sources in a semi-infinite medium. This approach balances accuracy and computational efficiency, addressing limitations of traditional infinite-length models.
3. Water-Water Heat Pump Unit Model
Foreign heat pump models are often based on manufacturer-provided data, while Chinese models tend to use component-based approaches. This paper constructs a model for the compressor, condenser, evaporator, and expansion valve using both distributed and centralized parameter methods. A triple-iteration process is employed to simulate steady-state operation, focusing on superheat control. By iteratively adjusting variables such as evaporation temperature and refrigerant dryness, the model accurately predicts system performance.
4. Ground-Source Heat Pump System Model
The system comprises three loops: the underground loop, the refrigerant loop, and the user-side water loop. The model integrates the geothermal heat exchanger, heat pump, and load models through mass and energy balance equations. Simulation steps include inputting system parameters, calculating initial cooling capacity, and iterating over time to predict system behavior under varying conditions.
5. System Model Verification
Field measurements were used to verify the model. Results showed good agreement between simulated and measured chilled water temperatures, with a maximum error of 0.5°C. The geothermal heat exchanger outlet temperature showed larger initial errors, which decreased over time as heat transfer stabilized. Compressor power simulations had an error margin of less than 5%, confirming the model's reliability for engineering applications.
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