Due to many factors that affect the underground heat exchange of ground-source heat pumps, such as the difficulty in design and the lack of basic data, the improper selection of some parameters will make the project cost unacceptable and limit the technology, so it was not until the late 1980s , Civil air-conditioning system used in buildings. In recent years, a large number of reports have reflected the work carried out abroad and the achievements made [1].

Due to its environmental protection and energy saving features, ground source heat pump air conditioning system is receiving more and more attention in China. In recent years, especially, the actual project of ground source heat pump air conditioning system has started to exist in China. Therefore, the design details of ground-source heat pumps and their matching data with traditional building systems are scarce. The investigation and experience of ground-source heat pump projects are an important aspect of the research on ground-source heat pumps in the world.

In the ground source heat pump system, the research of geothermal heat exchanger has been the difficult point of ground source heat pump technology, and it is also the core of the technology research and the basis of its application. The existing design methods of geothermal heat exchangers are mostly based on the experimental study of geothermal heat exchangers in the United States and Europe. The domestic research on ground-source heat pumps are all focused on the experimental study of geothermal heat exchangers, and the related experimental results are given respectively. Due to the lack of in-depth study on the heat transfer mechanism of the heat exchanger in the soil, the conclusions obtained are only applicable to a specific experimental system, the theory is poor, and the basic data provided are less, so it is difficult to guide the actual engineering design . Therefore, one of the contents of current research is to establish a geothermal heat exchanger heat transfer model that is closer to the actual situation.

As we all know, the characteristics of ground source heat pump system is mainly determined by two parts: First, the length and configuration of geothermal heat exchangers, and second, matching the performance of heat pump units. Therefore, in the geothermal heat exchanger configuration has been set, the performance of the ground source heat pump system is the most concerned about the current engineering problems. Therefore, another study of this paper is to establish a dynamic model of geothermal heat exchanger and heat pump units, and verify the accuracy of the model by experiments.

1. Geothermal heat exchanger model overview

According to the different layout, closed-loop geothermal heat exchangers can be divided into horizontal buried pipe and vertical buried heat exchanger two broad categories. The vertical buried geothermal heat exchanger is also a geothermal heat exchanger with underground buried pipes in several vertical boreholes, usually in the form of U-shaped buried pipes. U-shaped geothermal heat exchanger is also a hole in the layout of U-shaped pipe, together with the backfill material, and the surrounding soil form a whole. A hole can be set in a single group of U-shaped tube, you can also set two U-shaped tube. Vertical buried pipe covers an area of ​​small, high heat transfer efficiency, has been widely used in engineering, this article mainly to project the most widely used single U-tube as an example 1. Due to the complexity of the heat transfer process involved in geothermal heat exchangers, the heat transfer model of geothermal heat exchangers is still the focus of research work on closed-loop ground-source heat pump systems at home and abroad. Geothermal heat exchangers for heat transfer, so far there is no universally accepted models and norms. The current international heat transfer model can be divided into two broad categories. The first type is the analytical solution model based on the concept of thermal resistance. The second type is the numerical solution model based on discrete numerical calculation. The first model uses Kelvin's linear heat source model or an infinite cylinder model [2]. The concept of such semi-empirical methods is simple and clear, easy to accept for engineering and technical personnel, so get some applications in the project. The disadvantage is that a large number of simplifying assumptions are made in the calculation of the thermal resistances [3]. The model is too simple and can be considered for a limited number of factors. In particular, it is difficult to consider the change of the cold and heat load with time, Imbalances and other more complex factors. In the second method, the heat transfer model based on discrete numerical calculation can be considered in the near-real situation. The finite element method or finite difference method is used to solve the underground temperature response and analyze the heat transfer. However, due to the problem of heat transfer in geothermal heat exchangers involving large spatial range, complicated geometrical configuration and load changing with time, the time span is more than ten years. Therefore, if this analysis method is used to solve practical engineering problems based on three-dimensional unsteady state problems It would be almost impossible to spend a lot of computer time solving the engineering problem directly under the current computing conditions. At present, this method is only suitable for the parameter analysis in the research under a certain simplified condition, but not suitable for the heat transfer simulation of the large-scale multi-borehole geothermal heat exchanger, and is not suitable for engineering design And optimized.

2. Vertical single U-tube geothermal heat exchanger model establishment

2.1 bored quasi-three-dimensional model of the establishment

When studying the performance of ground source heat pump system, the thermal properties of backfill material and the geometric dimensions of borehole have a significant impact on the performance of ground source heat pump system due to the relatively small time span. In the past one-dimensional model and two-dimensional model, due to the simplified structure of the borehole, the two U-shaped tubes are simplified into one, and assuming that the temperature of the fluid in the U-shaped tube is constant, the fluid temperature in the borehole can not be obtained Varies with the depth of the borehole and the thermal short circuit between the two U-tubes. Therefore, there is a certain difference between the model and the actual situation, resulting in a larger model prediction error.

In recent years, the research group has conducted some innovative research on the heat transfer model of geothermal heat exchangers. Based on the two-dimensional model [4], the change of the fluid temperature in the depth direction and the axial convection heat transfer must be given consider. In order to keep the model concise, the axial thermal conductivity of the solid part inside the borehole is still negligible. We call this model a quasi-three-dimensional model. For the heat balance analysis of the single U-tube borehole, the solution of the dimensionless form of the fluid temperature in the U-tube is solved according to the energy balance equation of the fluid flowing downward and upward in the U-tube [5,6]:

Where c is the specific heat of the fluid, M is the mass flow rate of the fluid in the U-tube, R11 is the thermal resistance of the U-tube to the borehole wall, R12 is the thermal resistance between the two U-tubes [5] , Tb is borehole wall temperature, H is the depth of the borehole and is the fluid inlet temperature.

2.2 Borehole transient temperature field analysis

Drilled holes buried with tubes and exchanging heat with the soil can usually be approximated as line heat sources placed in a semi-infinite medium for heat transfer analysis to determine the temperature of the borehole wall. The formally recommended model for extra-borehole thermal resistance calculation is mainly the infinite-length heat source model [2,3], which is a one-dimensional model that ignores the limited depth of borehole and the influence of the ground surface as a boundary. Heat transfer problems will cause greater error. We use Green's function method to obtain the temperature response of the finite long-term heat source in the semi-infinite medium for the first time and solve the contradiction between the accuracy of solution and the calculation time. The Green's function method can be used to obtain the temperature response in semi-infinite media [7]:

3. Water - water source heat pump unit model

Foreign models of heat pump units are mostly based on the data provided by the manufacturer's product samples and established. In China, most samples provide only the performance parameters of the rated conditions, even if a few products provide operating performance parameters, the reliability of the data given is also difficult to guarantee. Therefore, a complete model based on sample data can not be achieved. Domestic research on heat pump unit to use more parts model method, that is, to establish a model for each component, unit model by the various components of the model through the appropriate interface parameters connected.

Water - water heat pump unit mainly by the compressor, condenser, expansion valve, evaporator four components. In this paper, the model of bushing condenser and evaporator is established by the distributed parameter method. The model of compressor and thermal expansion valve is established by using the centralized parameter method, and then the components are connected through a certain iterative relation. After guessing a set of initial values, start with the innermost loop, and the other variables are based on these assumptions. If the convergence condition is not satisfied, the assumed value is replaced by the new value after modification. This completes the loop calculation from the inside to the outside layers.

There are many control methods for heat pump units, and the most widely used method is still to control superheat. This article mainly studies the simulation algorithm of the heat pump unit that controls the superheat. The purpose of unit simulation is to set the initial value of the variable and to determine the actual operating conditions of the unit by constantly iterating and changing the set value of the variable under the premise of certain error. The steady-state heat pump unit simulation mainly consists of triple iterative process, the main steps are as follows:

(1) set the evaporator outlet refrigerant superheat â–³ ts.

(2) Enter the known quantities, including the structural parameters of the evaporator and condenser, the refrigerant charge and the operating parameters.

(3) Set the initial value of evaporation temperature Te, condensation temperature Tc and evaporator inlet refrigerant dryness x.

(4) Call the compressor model, calculate the refrigerant mass flow and compressor inlet point 1 state parameters.

(5) Call the evaporator model to calculate the heat transfer area Ae of the evaporator and compare with the actual heat transfer area Aeo of the evaporator. If> ε, go ​​to 3) Reset the evaporation temperature until it is satisfied. This is the first iteration.

(6) The expansion valve model is invoked to calculate the state parameters of compressor exit point 2, condenser exit point 3, and expansion valve exit point 4.

(7) Call the condenser model to calculate the heat transfer area Ac of the condenser and compare it with the actual heat transfer area Aco of the condenser. If> ε, go ​​to 3) Set the condensation temperature again until it is satisfied. This is the second cycle.

(8) Calculate the mass M of the refrigerant in the entire system, where. If> ε, then go to 3), reset the evaporator inlet refrigerant dryness x, until it is satisfied. This is the third cycle.

(9) The various performance parameters of the computer group, such as the coefficient of performance, compressor power, cooling capacity, etc., output the parameters.

For the heat pump unit model, the outlet temperature of cooling water and chilled water and various performance parameters of the unit can be simulated by inputting the inlet temperature and flow rate of the cooling water and chilled water when the unit structural parameters are known.

4. Ground source heat pump system model

Ground source heat pump system consists of three loops, namely underground antifreeze or water loop, heat pump unit within the refrigerant loop and the user side of the water loop, so the system model is geothermal heat exchanger model, heat pump model and user load Model through the balance of mass and energy conservation link made.

In the geothermal heat exchanger length and configuration of a certain circumstances, ground source heat pump system performance simulation steps are as follows:

(1) Enter the known parameters, including these parameters

Geothermal heat exchanger structural parameters, geothermal heat exchanger length, underground geotechnical and plastic pipe thermal physical properties;

Heat pump unit compressor, condenser, evaporator, and expansion valve structure parameters;

Cooling water initial inlet temperature Tf0, flow Mex, Cpx specific heat;

Chilled water initial inlet temperature Tw0, flow Me, CP specific heat;

Indoor cooling load at any moment.

(2) call the heat pump unit model, calculate the initial cooling capacity of the unit, the heat release, chilled water and cooling water outlet temperature.

(3) Taking the heat flow of the heat pump unit as a known variable of the geothermal heat exchanger, the geothermal heat exchanger model is invoked to calculate the geothermal heat exchanger fluid outlet temperature Tfou for the first time.

(4) call the indoor load model, calculate the first moment of the cold load.

(5) Taking the outlet temperature of the chilled water in the initial time as a known variable, the water loop model on the user side is invoked to obtain the chilled water return temperature Tw2 at that moment.

(6) Chilled water temperatures Tw2 and Tfou calculated at the first time

As a known variable, the heat pump unit model is invoked to calculate the cooling capacity, heat release, unit performance coefficient, chilled water temperature Tw2, Tfou and the like at this moment.

(7) Take the chilled water temperatures Tw2 and Tfou calculated in (6) as known variables and then go to (2) and calculate the performance parameters of the unit at the next moment until the total simulation time is reached.

5. System Model Verification

In order to verify the effectiveness of the system model, the water temperature and the water temperature of the ground source heat pump system were measured. Based on the system simulation software and the measured water volume and the user's return water temperature as known parameters, The source heat pump system was simulated.

The results show that the simulation of the chilled water temperature and the measured results are in good agreement, the measured temperature and the maximum error of the simulated value of 0.5 ℃; geothermal heat exchanger outlet temperature measured values ​​and simulated values ​​at the start of the error is large, running about 3 hours , The error decreases gradually and the maximum error does not exceed 0.5 ℃, which is mainly due to the heat transfer in the borehole by the geothermal heat exchanger is approximately caused by the stable heat transfer. The relative error of compressor power simulation is not in operation Over 5%.

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