Design method for laying length of heat preservation material of middle-deep layer buried pipe heat exchanger

Abstract

The invention belongs to the technical field of geothermal energy development, relates to a design method for laying length of a thermal insulation material of a middle-deep layer buried pipe heat exchanger, and aims at optimizing the laying length of the thermal insulation material to improve heat exchange efficiency and reduce engineering cost. The design method specifically comprises the steps that ground temperature field distribution data of a target area is obtained, and heat flow distribution of the outer wall of a buried pipe is calculated; determining the critical depth by combining the specific heat capacity, the mass flow rate and the system design parameters of the working medium; the laying length of the thermal insulation material is optimized through mathematical modeling, and an engineering economic coefficient is introduced for dynamic adjustment; verifying a design result through numerical simulation; according to the method, the relationship among the ground temperature gradient, the heat flux density, the thermal resistance of the thermal insulation material and the economical efficiency can be scientifically quantified, the balance between the thermal efficiency and the construction cost is achieved, and the method is suitable for design of medium-deep layer buried pipe heat exchanger systems under different geological conditions and has wide engineering application value.

Description

Technical Field

[0001] This invention belongs to the field of geothermal energy development technology and relates to a design method for the laying length of insulation material in a medium-deep buried pipe heat exchanger. Background Technology

[0002] As one of the five major non-carbon energy sources, geothermal energy is characterized by its vast reserves, wide distribution, and stable performance. Medium-deep buried pipe geothermal heating technology refers to a new type of geothermal technology that uses medium-deep buried pipe heat exchangers with a depth of 2000 meters or more to extract heat from underground soil and rock through a closed-loop circulation of medium within the pipe casing. A heat pump is then used to enhance the heat quality for building heating. In medium-deep buried pipe geothermal heating systems, the length of the insulation material directly affects heat exchange efficiency and system operating costs. On the one hand, excessively long insulation material significantly increases construction costs and may even lead to unnecessary resource waste; on the other hand, insufficient length may result in excessive heat loss, reducing the heat extraction efficiency of the buried pipe heat exchanger. Therefore, a scientific design method is needed to determine the optimal length of the insulation material to achieve a balance between efficiency and cost.

[0003] However, existing design methods for the insulation material laying length of medium-deep buried pipe heat exchangers have the following shortcomings: Firstly, most current design methods for buried pipe insulation layers are based on empirical values ​​from shallow buried pipe heat exchangers, lacking dedicated design methods for medium-deep buried pipe heat exchangers. Therefore, the design results often lead to high heat loss and low heat extraction efficiency, or high cost and poor cost-effectiveness, making it difficult to achieve a good balance. Secondly, in terms of applicability, due to significant differences in geological conditions across different regions, existing design methods are difficult to widely adapt to the complex geological conditions of various areas.

[0004] Therefore, a design method is needed to reduce costs and minimize heat loss while ensuring the heat extraction efficiency of medium-deep buried pipe heat exchangers in order to solve the above-mentioned technical problems. Summary of the Invention

[0005] The technical solution adopted by this invention to solve the technical problem is: a method for designing the laying length of insulation material for medium-deep buried pipe heat exchangers, comprising the following steps:

[0006] Step 1: Obtaining geothermal field distribution data; By conducting field measurements or using existing geological data, combined with regional geological exploration data, obtain the geothermal geological parameters of the calculation area and establish a mathematical model of the geothermal field; Use temperature probes or thermistors to measure temperature data at different depths in the target area to obtain the temperature at different depths, and calculate the average geothermal gradient by fitting the data.

[0007] Step 2: Analysis of heat flux density and heat transfer performance of buried pipe; Based on the geothermal field mathematical model established in Step 1 and the geothermal geological parameters obtained, calculate the heat flux density at different depths of the outer wall of the buried pipe, evaluate the heat transfer performance of the buried pipe based on the heat flux density, calculate the heat loss of the buried pipe at different depths, and find the depth range where the heat flux density decreases significantly.

[0008] Step 3: Determining the critical depth; The critical depth y is determined by combining the heat flux density distribution and engineering economic coefficients. c Sensitivity analysis was performed on the critical depth to ensure that it met the actual operating conditions.

[0009] Step 4: Determining the length of the insulation material; based on the critical depth y determined in Step 3. c Based on the heat flux density distribution calculated in step 2, the laying length L of the insulation material is obtained. in Adjustments were made based on actual construction conditions.

[0010] Step 5: Numerical simulation verification; use numerical simulation tools to dynamically analyze the length of the insulation material laying, and finally determine the design scheme that meets the engineering requirements.

[0011] Preferably, in step 1, the geothermal gradient is:

[0012]

[0013] In equation (1), G represents the geothermal gradient. Δy represents the bottom hole temperature and the surface temperature, respectively, and Δy represents the well depth.

[0014] The temperature distribution of the soil and rock is as follows:

[0015]

[0016] In equation (2), This represents the temperature at depth y.

[0017] More preferably, in step 1, when the data on the temperature distribution of soil and rock is fitted with the measurement data, the control error is less than 2%.

[0018] Preferably, in step 2, the heat flux density is:

[0019]

[0020] In equation (3), λ represents the thermal conductivity of the rock and soil. δ represents the temperature of the soil and rock at depth y and the temperature of the pipe wall, and δ represents the thickness of the insulation material.

[0021] The heat exchange between the pipe wall and the soil / rock is:

[0022]

[0023] The heat exchange efficiency of the buried pipe is:

[0024]

[0025] In equations (4) and (5), η represents the heat exchange efficiency, Q represents the heat taken by the buried pipe, A represents the heat exchange area of ​​the pipe wall, and Q S This indicates the heat exchanged through the pipe wall.

[0026] Preferably, in step 3, the critical depth y c for:

[0027]

[0028] In equation (6), ΔT represents the temperature difference between the inside and outside of the buried pipe, C p q represents the specific heat at constant pressure of a fluid. m This indicates the mass flow rate of the working fluid.

[0029] Preferably, in step 4, the laying length L in for:

[0030]

[0031] In equation (8), U represents the total thermal resistance of the outer wall of the heat exchanger, and A represents the heat exchange area of ​​the tube wall.

[0032] Preferably, in step 5, the cost and efficiency comprehensive analysis is as follows:

[0033] C total =C material +C energy (9)

[0034] In equation (9), C total C represents the total cost. material C represents the cost of materials. energy This indicates energy consumption cost.

[0035] Preferably, in step 1, the temperature probe or thermistor is a high-precision temperature probe or thermistor.

[0036] Preferably, in step 5, the numerical simulation verification specifically includes: heat extraction simulation, simulating the amount of heat extracted by the buried pipe at different laying lengths to ensure that heat extraction is guaranteed while reducing heat loss; cost analysis, calculating the total cost by combining the cost of insulation materials and the cost of operating energy consumption; and dynamic adjustment, through multiple rounds of simulation and optimization.

[0037] The beneficial effects of this invention are:

[0038] 1. This invention overcomes the problem of lack of deep thermophysical data support in traditional empirical design methods by comprehensively analyzing the distribution law of the medium-deep geothermal field. By calculating the heat flux density distribution, it dynamically reflects the heat extraction and release performance of buried pipes at different depths, making up for the shortcomings of the simplified treatment of geothermal distribution in traditional design. Therefore, this invention provides a complete method for optimizing the laying length of insulation material for medium-deep buried pipe heat exchangers, filling the gap in the prior art and having significant promotional value.

[0039] 2. This invention divides the design process into five stages: geothermal field data acquisition, heat flux density analysis, critical depth determination, insulation material laying length optimization, and numerical simulation verification. Through mathematical optimization models, the critical depth and optimal laying length are scientifically calculated, the insulation material laying length is optimized, and construction costs are effectively reduced. Attached Figure Description

[0040] Figure 1 This is a flowchart illustrating the design method for the laying length of insulation material in a medium-deep buried pipe heat exchanger according to the present invention.

[0041] Figure 2 It is a schematic diagram of the overall structure of the present invention.

[0042] In the diagram, 1 is the inner tube; 2 is the annular cavity; 3 is the insulation layer; and 4 is the soil and rock mass. Detailed Implementation

[0043] The related technologies of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0044] refer to Figures 1-2 The following is an embodiment of the design method for the laying length of insulation material in a medium-deep buried pipe heat exchanger according to the present invention. This method reduces costs and minimizes heat loss while ensuring the heat extraction efficiency of the medium-deep buried pipe heat exchanger.

[0045] 1. Acquisition of geothermal field distribution data

[0046] By utilizing field measurements or existing geological data, combined with regional geological exploration data, geothermal geological parameters of the calculation area are obtained, and a mathematical model of the geothermal field is established. This model accurately reflects the temperature gradient around the buried pipe, thereby enabling subsequent calculations of heat flux density and optimization of insulation length. High-precision temperature probes or thermistors are used to measure temperature data at different depths within the target area, obtaining temperatures at different depths. The average geothermal gradient is calculated through data fitting.

[0047] 2. Analysis of heat flux density and heat transfer performance of buried pipes

[0048] Based on the geothermal field model and geothermal geological parameters, the heat flux density q at different depths of the outer wall of the buried pipe is calculated. The heat exchange performance of the buried pipe is evaluated based on the calculated heat flux density, the heat loss of the buried pipe at different depths is calculated, and the depth range where the heat flux density decreases significantly is identified.

[0049] 3. Determination of critical depth

[0050] A scientific method for determining the critical depth y is proposed. c The method involves determining the critical depth, which is the maximum depth at which insulation material needs to be laid on the buried pipe. Heat loss below the critical depth has a relatively small impact on system performance, so an insulation layer may not be necessary. The critical depth is determined by combining heat flux density distribution and engineering economic coefficients, and a sensitivity analysis is performed on the critical depth to ensure it meets actual operating conditions.

[0051] 4. Determining the length of insulation material laying

[0052] Based on the critical depth and heat flux density distribution, the laying length L of the thermal insulation material is obtained. in To achieve a balance between thermal efficiency and economy, adjustments are made based on actual construction conditions.

[0053] 5. Numerical simulation verification

[0054] To verify the scientific validity and applicability of the design results, numerical simulation tools were used to dynamically analyze the insulation material laying length. This included heat extraction simulation, which simulated the amount of heat extracted by the buried pipe at different laying lengths to ensure heat extraction while minimizing heat loss; cost analysis, which calculated the total cost by combining the cost of insulation materials and operating energy consumption; and dynamic adjustment, which determined the final design scheme that met the engineering requirements through multiple rounds of simulation and optimization.

[0055] Example

[0056] To illustrate the effectiveness and feasibility of the invention, an embodiment of the design method for the laying length of insulation material in a medium-deep buried pipe heat exchanger according to the present invention is given below:

[0057] 1. Temperature distribution in soil and rock

[0058] First, by consulting local geothermal geological parameters, relevant thermal properties parameters, including specific heat capacity and density, of the soil, buried pipes, and operating working fluid are obtained.

[0059] The temperature distribution of soil and rock is fundamental to optimizing the laying length of thermal insulation materials. Measurement points are set up in the target area according to geological conditions. Temperature probes, high-precision thermistors, or geothermal survey instruments are used. The equipment accuracy is ensured to be ±0.1℃, and temperature data T(z) at different depths is recorded. The geothermal gradient is then calculated using the following formula:

[0060]

[0061] in:

[0062] G represents the geothermal gradient (°C / m).

[0063] These represent the bottom hole temperature and the surface temperature (°C), respectively.

[0064] Δy represents the well depth (m).

[0065] A relevant formula for soil and rock temperature distribution was established, and the obtained data was fitted with the measured data to control the error within the allowable range (<2%).

[0066]

[0067] in:

[0068] Temperature at depth y (°C)

[0069] 2. Heat exchange between the buried pipe wall and the soil / rock

[0070] The heat flux density of buried pipes represents the heat transfer capacity per unit area. Its calculation formula is:

[0071]

[0072] in:

[0073] Heat flux at depth y of the pipe wall (W / m) 2 )

[0074] λ: Thermal conductivity of soil and rock (W / (m·K))

[0075] Temperature of soil and rock at depth y and temperature of pipe wall (°C)

[0076] Therefore, the heat exchange between the pipe wall and the soil is:

[0077]

[0078] Calculate the heat exchange efficiency of the buried pipe based on the heat flux density:

[0079] in:

[0080]

[0081] η: Heat exchange efficiency

[0082] A: Heat exchange area of ​​the pipe wall (m²) 2 );

[0083] Q: Heat extraction via underground pipes (W)

[0084] Q s Heat exchange through the tube wall (W)

[0085] 3. Determination of critical depth

[0086] Figure 1 The overall structure of this invention is shown in the schematic diagram. The method for determining the critical depth is as follows: First, based on the premise of satisfying η, the corresponding Q is deduced from (5). s Therefore, the corresponding critical depth can be calculated according to the following formula.

[0087]

[0088] in:

[0089] ΔT: Temperature difference between the inside and outside of the buried pipe (°C);

[0090] c p Specific heat at constant pressure of a fluid (J·kg⁻¹) -1 ·K -1 );

[0091] q m Working fluid mass flow rate (W / m³) 2 )

[0092] 4. Optimization of insulation material laying length

[0093] Total thermal resistance of the heat exchanger outer wall:

[0094]

[0095] in:

[0096] h: Convection heat transfer coefficient of the outer wall of the buried pipe (W / (m²)) 2 ·K));

[0097] δ: Insulation material thickness (m);

[0098] λ m Thermal conductivity of insulation material (W / (m·K)).

[0099] Calculate the laying length by combining the thermal resistance U and the heat flux q of the pipe wall:

[0100]

[0101] The design is calculated layer by layer using relevant numerical calculation methods to ensure that it meets the thermal efficiency requirements.

[0102] 5. Numerical simulation verification and optimization

[0103] A comprehensive cost and efficiency analysis is conducted using formula (9).

[0104] C total =C material +C energy (9)

[0105] in:

[0106] C total Total cost (RMB)

[0107] C material Material cost (RMB)

[0108] C energy Energy consumption cost (yuan)

[0109] In relevant commercial software (such as ANSYS and COMSOL), a corresponding underground pipe heat extraction model is constructed by inputting the ground temperature field, heat flux density, and insulation material parameters. The influence of different laying lengths on the system heat loss is simulated, the corresponding insulation layer laying length is recorded, and the optimal balance between cost and efficiency is found by repeatedly adjusting the laying length and material parameters.

[0110] In summary, this invention overcomes the problem of lack of deep thermophysical data support in traditional empirical design methods by comprehensively analyzing the distribution law of the medium-deep geothermal field. By calculating the heat flux density distribution, it dynamically reflects the heat extraction and release performance of buried pipes at different depths, making up for the shortcomings of the simplified treatment of geothermal distribution in traditional design. Therefore, this invention provides a complete method for optimizing the laying length of insulation material for medium-deep buried pipe heat exchangers, filling the gap in the prior art.

[0111] It should be emphasized that the above are merely preferred embodiments of the present invention and are not intended to limit the present invention in any way. Any simple modifications, equivalent changes and alterations made to the above embodiments based on the technical essence of the present invention shall still fall within the scope of the technical solution of the present invention.

Claims

1. A method for designing the laying length of insulation materials for medium-deep underground heat exchangers, characterized in that: The following steps are involved: Step 1: Acquisition of geothermal field distribution data; through field measurements or existing geological data, combined with regional geological exploration data, the geothermal geological parameters of the calculation area are obtained, and a mathematical model of the geothermal field is established; temperature probes or thermistors are used to measure temperature data at different depths in the target area to obtain temperatures at different depths, and the average geothermal gradient is calculated by data fitting; Step 2, heat flux density and heat exchange performance analysis of the buried pipe: based on the geothermal field mathematical model established in step 1 and the geothermal geological parameters obtained, the heat flux density at different depths of the outer wall of the buried pipe is calculated, and the heat exchange performance of the buried pipe is evaluated according to the heat flux density, and the heat loss of the buried pipe at different depths is calculated to find the depth range where the heat flux density decreases significantly; Step 3: Determine the critical depth; Determine the critical depth y by combining the heat flux density distribution and engineering economic coefficient c , and conduct sensitivity analysis on the critical depth to ensure that it meets the actual operating conditions; Step 4: Determine the length of the insulation material to be laid; based on the critical depth y determined in step 3 c And the heat flux density distribution calculated in step 2, the laying length L of the insulation material is obtained in , and make adjustments based on actual construction conditions; Step 5: Numerical simulation verification: Use numerical simulation tools to dynamically analyze the laying length of the insulation material and ultimately determine a design solution that meets engineering requirements.

2. The method for designing the laying length of insulation material for a medium-deep underground heat exchanger according to claim 1 is characterized in that: In step 1, the geothermal gradient is: In formula (1), G represents the geothermal gradient, represent the bottom hole temperature and the surface temperature respectively, and Δy represents the well depth; The rock and soil temperature distribution is: In formula (2), represents the temperature at depth y.

3. The method for designing the laying length of insulation material for a medium-deep underground heat exchanger according to claim 2 is characterized in that: In the step 1, when the obtained data of the rock and soil temperature distribution are fitted with the measured data, the control error is less than 2%.

4. The method for designing the laying length of insulation material for a medium-deep underground heat exchanger according to claim 1 is characterized in that: In step 2, the heat flux is: In formula (3), λ represents the thermal conductivity of rock and soil, represents the rock and soil temperature and pipe wall temperature at depth y, and δ represents the thickness of the insulation material; The heat exchange between the pipe wall and the rock is: The heat transfer efficiency of the buried pipe is: In formulas (4) and (5), η represents the heat transfer efficiency, Q represents the heat taken by the buried pipe, A represents the heat transfer area of ​​the pipe wall, and Q S Represents the heat transfer of the tube wall.

5. The method for designing the laying length of insulation material for medium-deep underground heat exchanger according to claim 1 is characterized in that: In step 3, the critical depth y c for: In formula (6), ΔT represents the temperature difference between the inside and outside of the buried pipe, C p represents the constant pressure specific heat of the flowing medium, q m Indicates the mass flow rate of the working fluid.

6. The method for designing the laying length of insulation material for a medium-deep underground heat exchanger according to claim 1 is characterized in that: In step 4, the laying length L in for: In formula (8), U represents the total thermal resistance of the outer wall of the heat exchanger, and A represents the heat transfer area of ​​the tube wall.

7. The method for designing the laying length of insulation material for a medium-deep underground heat exchanger according to claim 1 is characterized in that: In step 5, the comprehensive analysis of cost and efficiency is as follows: C total =C material +C energy (9) In formula (9), C total represents the total cost, C material represents the material cost, C energy Represents the energy cost.

8. The method for designing the laying length of insulation material for a medium-deep underground heat exchanger according to claim 1 is characterized in that: In step 1, the temperature probe or thermistor uses a high-precision temperature probe or thermistor.

9. The method for designing the laying length of insulation material for a medium-deep underground heat exchanger according to claim 1, characterized in that: In step 5, the numerical simulation verification specifically includes: heat extraction simulation, simulating the heat extraction of the buried pipes at different laying lengths to ensure that heat loss is reduced while ensuring heat extraction; cost analysis, calculating the total cost by combining the insulation material cost and the operating energy consumption cost; dynamic adjustment, through multiple rounds of simulation and optimization.

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