Thermal-hydraulic coupling model construction method of high-temperature superconducting cable cooling system

Abstract

The invention discloses a thermal-hydraulic coupling model construction method for a high-temperature superconducting cable cooling system. The thermal-hydraulic coupling model construction method comprises the following steps: 1, calculating a cooling load; 2, hydraulic loss calculation; 3, judging whether the height difference during cable construction is lower than the theoretical calculation result of the maximum height difference of cable construction; 4, determining the relationship between the liquid nitrogen flow and the maximum temperature difference of the liquid nitrogen; 5, determining the temperature rise limit according to the relationship between the liquid nitrogen flow and the maximum temperature difference of the liquid nitrogen, and judging the range of the liquid nitrogen flow to select the type of the low-temperature circulating pump preferentially; and 6, the refrigerating unit meeting the refrigerating temperature and having the refrigerating capacity matched with the cold load is selected. According to the efficient cooling system, the cooling process can be dynamically adjusted and controlled, so that the service life of the high-temperature superconducting cable is prolonged, and the operation cost is reduced.

Description

Technical Field

[0001] The invention relates to the technical field of high-temperature superconducting cable cooling system design, and in particular to a method for constructing a thermal-hydraulic coupling model of a high-temperature superconducting cable cooling system. Background Art

[0002] High-temperature superconducting cable is one of the most important applications of superconducting technology in the electric power industry. It has the advantages of small size, light weight, low loss, large transmission capacity and environmental friendliness. It can be used in short-distance transmission of large currents such as from generator to transformer, new energy power supply transmission, substation to urban power grid port, power plant and substation, as well as large or super-large urban power transmission occasions, to achieve low-loss, high-efficiency and large-capacity power transmission.

[0003] In order to ensure that the high-temperature superconducting cable maintains its superconducting state, the cable needs to be cooled below its critical temperature, which requires an efficient and stable low-temperature cooling system. The cooling system of the high-temperature superconducting cable is a key link to ensure the safe and efficient operation of the superconducting cable. By rationally designing the cooling system and selecting appropriate refrigeration equipment, the service life of the superconducting cable can be effectively extended and its stable operation can be ensured.

[0004] Accurate evaluation of the cooling load and hydraulic loss of the high-temperature superconducting cable system is the basis for formulating cooling strategies. Among them, the cooling load represents the cooling power required to maintain the low-temperature environment of the high-temperature superconducting cable system and is the core indicator of the refrigeration system design. The system cooling load directly affects the capacity and specification selection of the refrigeration equipment to ensure that the refrigeration system can operate stably. In addition, accurate calculation of the cooling load can avoid the waste of cooling power and reduce the cost of the system, thereby improving the overall operating efficiency and economic performance of the system. Hydraulic loss refers to the pressure drop caused by friction and local resistance when liquid nitrogen flows in the pipeline. The size of the hydraulic loss directly affects the selection and energy consumption of the circulation pump. If the hydraulic loss is higher than the head of the circulation pump, the load of the pump will increase, thereby reducing the efficiency of the system and even failing to complete the operation of the system. If the hydraulic loss is lower than the head of the circulation pump, the economy of the system will be reduced. Accurate calculation of hydraulic loss helps to optimize the design of pipelines and circulation loops, ensure that the cooling medium can circulate efficiently and stably, and ensure the reliability of the cooling system and the stability of long-term operation. In short, the accurate calculation of cooling load and hydraulic loss is not only the basis for the design of superconducting cable refrigeration system, but also an important factor in determining system cost, performance and reliability.

[0005] In the prior art, the thermal-hydraulic coupling model is mainly used to calculate the cooling load, and the hydraulic coupling model is used to calculate the hydraulic loss. However, for the operation process of the liquid nitrogen cooling system of the high-temperature superconducting cable, it is necessary to comprehensively consider the core parameters such as the cooling load, liquid nitrogen temperature rise, liquid nitrogen flow rate and hydraulic loss of the liquid nitrogen cooling system to support the formulation of the cooling plan and the selection of the cooling equipment. In the prior art, there is no thermal-hydraulic coupling model based on the operation process of the liquid nitrogen cooling system of the high-temperature superconducting cable.

[0006] Therefore, how to achieve dynamic regulation and control of the cooling process while ensuring stable operation of the cooling system through precise thermal and hydraulic calculations, as well as the coupling relationship between the pump flow and the temperature rise of liquid nitrogen, so as to extend the service life of high-temperature superconducting cables and reduce operating costs has become a technical problem that technical personnel in this field urgently need to solve. Summary of the invention

[0007] In view of the above-mentioned defects of the prior art, the present invention provides a method for constructing a thermodynamic-hydraulic coupling model of a high-temperature superconducting cable cooling system. The purpose of the present invention is to provide an efficient cooling system capable of dynamically adjusting and controlling the cooling process while ensuring the stable operation of the cooling system through accurate thermodynamic and hydraulic calculations combined with the coupling relationship between the pump flow rate and the temperature rise of liquid nitrogen, so as to extend the service life of the high-temperature superconducting cable and reduce the operating cost.

[0008] To achieve the above object, the present invention discloses a method for constructing a thermal-hydraulic coupling model of a high-temperature superconducting cable cooling system, comprising the following steps:

[0009] Step 1: Calculate cooling load;

[0010] Step 2: Calculate hydraulic loss;

[0011] Step 3, judging whether the height difference during cable construction is lower than the theoretical calculation result of the maximum height difference during cable construction;

[0012] If it is satisfied, the hydraulic loss is used as the basis for selecting the pump head;

[0013] If not, the height difference during cable construction in the specific project shall be used as the basis for calculating and selecting the circulation pump head;

[0014] Step 4, determining the relationship between the liquid nitrogen flow rate and the maximum temperature difference of the liquid nitrogen;

[0015] Step 5: Determine the temperature rise limit based on the relationship between the liquid nitrogen flow rate and the maximum temperature difference of the liquid nitrogen, and determine the range of the liquid nitrogen flow rate to select the optimal type of the cryogenic circulation pump;

[0016] Step 6: Select a refrigeration unit that meets the cooling temperature and has a cooling capacity that matches the cooling load.

[0017] Preferably, in step 1, the cooling load is calculated by calculating the local cooling load of the insulated pipe per unit length, and the specific formula is as follows:

[0018]

[0019] Where Q is the cooling load per unit length of the insulated pipe, in W / m; k is the effective thermal conductivity of the insulated pipe, in W / (m·K); T ∞ is the ambient temperature, unit: K; is the working liquid nitrogen temperature, unit: K; D out D is the outer diameter of the insulation pipe, in mm; in It is the inner diameter of the insulation pipe, unit: mm.

[0020] More preferably, in step 2, the hydraulic loss calculation includes the hydraulic loss along the way H f and local hydraulic loss H L .

[0021] More preferably, the hydraulic loss along the way H f The calculation formula is as follows:

[0022]

[0023] Where f is the Darcy drag coefficient; L is the length of the bellows, in meters; D h is the hydraulic diameter of the liquid nitrogen channel, in m; v is the liquid nitrogen flow rate, in m / s; g is the gravitational acceleration, in m / s 2 .

[0024] More preferably, when calculating the hydraulic loss along the bellows, the Reynolds number is 10 4 Until April 10 4 The Darcy drag coefficient f of the bellows is only related to the bellows structure, and the expression is as follows:

[0025]

[0026] Where, e is the wave height of the bellows, in m; is the wave distance, in m; D h is the hydraulic diameter of the liquid nitrogen bellows channel in the cable, and the calculation formula is as follows:

[0027]

[0028] Among them, 3D out Is the outer diameter of the cable, unit: m.

[0029] More preferably, the local hydraulic loss H L The calculation formula is as follows:

[0030]

[0031] Where K is the local resistance coefficient of a specific accessory; v is the liquid nitrogen flow rate, in m / s; g is the acceleration due to gravity, in m / s 2 .

[0032] More preferably, in step 3, the calculation formula of the theoretical calculation result of the maximum height difference of the cable construction is as follows:

[0033]

[0034] Where Δh max is the theoretical calculation result of the maximum height difference of the cable construction;

[0035] P inlet is the inlet pressure of liquid nitrogen, unit: Pa;

[0036] P inlet =P outlet +ΔP friction (L)+ρgΔh;

[0037] P outlet is the outlet pressure of liquid nitrogen, unit: Pa;

[0038] Δh is the height difference of the cable during the construction process, unit: m;

[0039] ΔP friction (L) is the pressure loss when the cable output distance is L, the unit is: Pa, the calculation formula is as follows:

[0040]

[0041] Where f is the Darcy resistance coefficient of the liquid nitrogen flow pipeline; L is the cable output distance, unit: m; D h is the hydraulic diameter of the liquid nitrogen bellows channel in the cable, in m; ρ is the density of liquid nitrogen, in kg / m 3 ; v is the liquid nitrogen flow rate, unit: m / s.

[0042] P boil (T) is the maximum height difference allowed for construction, and the corresponding output pressure of liquid nitrogen is equal to the saturated vapor pressure at the corresponding temperature, in Pa, and the calculation formula is as follows:

[0043] P boil (T) = P outlet .

[0044] More preferably, in step 4, the limit operating temperature range of liquid nitrogen is determined to be between the triple point of liquid nitrogen 63.4K and the boiling point of liquid nitrogen at normal pressure 77K; then the limit temperature difference is 13.6K;

[0045] Then, the liquid nitrogen cooling temperature of 65K±10% with a maximum temperature difference of 12K±10% from the boiling point of liquid nitrogen at normal pressure of 77K is taken as the starting temperature of liquid nitrogen, and 80% of the maximum temperature difference, i.e. 9.6K±10%, is taken as the standard for determining whether the liquid nitrogen flow rate meets the thermodynamic limit. The formula for the relationship between the liquid nitrogen flow rate and the maximum temperature difference of liquid nitrogen is as follows:

[0046]

[0047] Where, L is the distance of the high-temperature superconducting cable, in meters; Q1 is the terminal unit loss, in W / unit; N1 is the number of terminals; Q2 is the joint unit loss, in W / unit; N2 is the number of joints; Q3 is the loss of other pipelines in the system, in W; ρ is the density of liquid nitrogen, in kg / m 3 ; is the volume flow rate of liquid nitrogen, unit: m 3 / s;c p is the constant pressure specific heat capacity of liquid nitrogen, unit: J / (kg·K).

[0048] More preferably, step 6 is as follows:

[0049] Step 6.1, select the best working condition of the circulating pump, that is, the circulating pump flow rate with the highest pump efficiency;

[0050] Step 6.2, calculating the temperature difference according to the circulation pump flow rate using the temperature rise formula;

[0051] Step 6.3, judging whether the temperature difference is satisfied;

[0052] If satisfied, the output includes a refrigeration solution for a cryogenic refrigerator and a cryogenic circulation pump;

[0053] If not, increase the circulation pump flow rate until the temperature rise of the liquid nitrogen satisfies the condition that it is less than the maximum temperature difference, and then output a matching refrigeration solution including a cryogenic refrigerator and a cryogenic circulation pump.

[0054] Beneficial effects of the present invention:

[0055] The present invention uses precise thermal and hydraulic calculations, combined with the coupling relationship between pump flow and liquid nitrogen temperature rise, to further provide a high-efficiency cooling system capable of dynamically adjusting and controlling the cooling process while ensuring stable operation of the cooling system, so as to extend the service life of the high-temperature superconducting cable and reduce the operating cost.

[0056] The concept, specific structure and technical effects of the present invention will be further described below in conjunction with the accompanying drawings to fully understand the purpose, characteristics and effects of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0057] Figure 1 A flow chart showing an embodiment of the present invention is shown.

[0058] Figure 2 A schematic diagram of the internal structure of a superconducting cable in one embodiment of the present invention is shown. DETAILED DESCRIPTION

[0059] Example

[0060] like Figure 1 As shown, the method for constructing a thermal-hydraulic coupling model of a high-temperature superconducting cable cooling system includes the following steps:

[0061] Step 1: Calculate cooling load;

[0062] Step 2: Calculate hydraulic loss;

[0063] Step 3, judging whether the height difference during cable construction is lower than the theoretical calculation result of the maximum height difference during cable construction;

[0064] In practical applications, the above judgment can avoid the problem of vaporization caused by the liquid nitrogen pressure being lower than the saturated vapor pressure during the filling process.

[0065] If it is satisfied, the hydraulic loss is used as the basis for selecting the pump head;

[0066] If not, the height difference during cable construction in the specific project shall be used as the basis for calculating and selecting the circulation pump head;

[0067] Step 4, determining the relationship between the liquid nitrogen flow rate and the maximum temperature difference of the liquid nitrogen;

[0068] Step 5: Determine the temperature rise limit based on the relationship between the liquid nitrogen flow rate and the maximum temperature difference of the liquid nitrogen, and determine the range of the liquid nitrogen flow rate to select the optimal type of the cryogenic circulation pump;

[0069] Step 6: Select a refrigeration unit that meets the cooling temperature and has a cooling capacity that matches the cooling load.

[0070] The present invention can simulate the estimation of the cooling load of the high-temperature superconducting cable in the liquid nitrogen cooling system, and utilize the limiting coupling of the liquid nitrogen flow rate and the liquid nitrogen temperature rise to select the optimal type of the low-temperature circulation pump, thereby providing a solid theoretical basis and optimization direction for the formulation of liquid nitrogen cooling schemes and the selection of equipment, and ensuring the long-term, efficient and stable operation of the high-temperature superconducting cable.

[0071] In some embodiments, in step 1, the cooling load calculation is to calculate the local cooling load of the insulated pipe per unit length, and the specific formula is as follows:

[0072]

[0073] Where Q is the cooling load per unit length of the insulated pipe, in W / m; k is the effective thermal conductivity of the insulated pipe, in W / (m·K); T ∞ is the ambient temperature, unit: K; is the working liquid nitrogen temperature, unit: K; D out D is the outer diameter of the insulation pipe, in mm; in It is the inner diameter of the insulation pipe, unit: mm.

[0074] The cooling load of high-temperature superconducting cables depends on the insulation performance of the insulation pipes used in the system. The insulation pipes of high-temperature superconducting cables are usually corrugated flexible double-wall structures, that is, with multiple layers of vacuum interlayers such as Figure 2 As shown in Figure 2, the corrugated geometry of the tube wall can be widely used in high-temperature superconducting cable insulation systems because the heat load is usually higher than that of rigid insulation systems.

[0075] In some embodiments, in step 2, the hydraulic loss calculation includes the hydraulic loss H along the way. f and local hydraulic loss H L .

[0076] The hydraulic loss in the liquid nitrogen cooling system of high-temperature superconducting cables is the core indicator for selecting circulating pumps, and the hydraulic loss directly determines the head range of the circulating pump. The hydraulic loss inside the high-temperature superconducting cable cooling system mainly includes hydraulic loss along the way and local hydraulic loss.

[0077] In some embodiments, the hydraulic loss H along the way f The calculation formula is as follows:

[0078]

[0079] Where f is the Darcy drag coefficient; L is the length of the bellows, in meters; D h is the hydraulic diameter of the liquid nitrogen channel, in m; v is the liquid nitrogen flow rate, in m / s; g is the gravitational acceleration, in m / s 2 .

[0080] The hydraulic loss along the pipeline is the resistance loss caused by pipeline friction when liquid nitrogen flows in the pipeline.

[0081] In some embodiments, such as when calculating the hydraulic loss along the bellows, the Reynolds number is 10 4 Until April 10 4 The Darcy drag coefficient f of the bellows is only related to the bellows structure, and the expression is as follows:

[0082]

[0083] Where, e is the wave height of the bellows, in m; is the wave distance, in m; D h is the hydraulic diameter of the liquid nitrogen bellows channel in the cable, and the calculation formula is as follows:

[0084]

[0085] Among them, 3D out Is the outer diameter of the cable, unit: m.

[0086] In certain embodiments, the local hydraulic loss H L The calculation formula is as follows:

[0087]

[0088] Where K is the local resistance coefficient of a specific accessory; v is the liquid nitrogen flow rate, in m / s; g is the acceleration due to gravity, in m / s 2 .

[0089] In some embodiments, in step 3, the calculation formula of the theoretical calculation result of the maximum height difference of the cable construction is specifically as follows:

[0090]

[0091] Where Δh max is the theoretical calculation result of the maximum height difference of cable construction;

[0092] P inlet is the inlet pressure of liquid nitrogen, unit: Pa;

[0093] P inlet =P outlet +ΔP friction (L)+ρgΔh;

[0094] P outlet is the outlet pressure of liquid nitrogen, unit: Pa;

[0095] Δh is the height difference of the cable during the construction process, unit: m;

[0096] ΔP friction (L) is the pressure loss when the cable output distance is L, the unit is: Pa, the calculation formula is as follows:

[0097]

[0098] Where f is the Darcy resistance coefficient of the liquid nitrogen flow pipeline; L is the cable output distance, unit: m; D his the hydraulic diameter of the liquid nitrogen bellows channel in the cable, in m; ρ is the density of liquid nitrogen, in kg / m 3 ; v is the liquid nitrogen flow rate, unit: m / s.

[0099] P boil (T) is the maximum height difference allowed for construction, and the corresponding output pressure of liquid nitrogen is equal to the saturated vapor pressure at the corresponding temperature, in Pa, and the calculation formula is as follows:

[0100] P boil (T) = P outlet .

[0101] In some embodiments, in step 4, the limit operating temperature range of liquid nitrogen is determined to be between the triple point of liquid nitrogen 63.4K and the boiling point of liquid nitrogen at normal pressure 77K; then the limit temperature difference is 13.6K;

[0102] Then, the liquid nitrogen cooling temperature of 65K±10% with a maximum temperature difference of 12K±10% from the boiling point of liquid nitrogen at normal pressure of 77K is taken as the starting temperature of liquid nitrogen, and 80% of the maximum temperature difference, i.e. 9.6K±10%, is taken as the standard for determining whether the liquid nitrogen flow rate meets the thermodynamic limit. The formula for the relationship between the liquid nitrogen flow rate and the maximum temperature difference of liquid nitrogen is as follows:

[0103]

[0104] Where, L is the distance of the high-temperature superconducting cable, in meters; Q1 is the terminal unit loss, in W / unit; N1 is the number of terminals; Q2 is the joint unit loss, in W / unit; N2 is the number of joints; Q3 is the loss of other pipelines in the system, in W; ρ is the density of liquid nitrogen, in kg / m 3 ; is the volume flow rate of liquid nitrogen, unit: m 3 / s;c p is the constant pressure specific heat capacity of liquid nitrogen, unit: J / (kg·K).

[0105] Liquid nitrogen flow rate is an important indicator for selecting a cryogenic circulating pump (i.e., pump flow rate), and the change in liquid nitrogen flow rate is negatively correlated with the head of the circulating pump. Therefore, the above formula can determine the range of liquid nitrogen flow rate according to the temperature rise limit for the optimal selection of the cryogenic circulating pump.

[0106] In some embodiments, step 6 is as follows:

[0107] Step 6.1, select the best working condition of the circulating pump, that is, the circulating pump flow rate with the highest pump efficiency;

[0108] Step 6.2, calculate the temperature difference according to the circulation pump flow rate using the temperature rise formula;

[0109] Step 6.3, determine whether the temperature difference is satisfied;

[0110] If satisfied, the output includes a refrigeration solution for a cryogenic refrigerator and a cryogenic circulation pump;

[0111] If not, increase the circulation pump flow rate until the temperature rise of liquid nitrogen meets the condition of being less than the maximum temperature difference, and then output a matching refrigeration solution including a cryogenic refrigerator and a cryogenic circulation pump.

[0112] The preferred specific embodiments of the present invention are described in detail above. It should be understood that a person skilled in the art can make many modifications and changes based on the concept of the present invention without creative work. Therefore, any technical solution that can be obtained by a person skilled in the art through logical analysis, reasoning or limited experiments based on the concept of the present invention on the basis of the prior art should be within the scope of protection determined by the claims.

Claims

1. A method for constructing a thermal-hydraulic coupling model of a high-temperature superconducting cable cooling system; characterized in that: The steps include: Step 1: Calculate cooling load; Step 2: Calculate hydraulic loss; Step 3, determining whether the height difference during cable construction is lower than the theoretical calculation result of the maximum height difference during cable construction; If it is satisfied, the hydraulic loss is used as the basis for selecting the pump head; If not, the height difference during cable construction in the specific project shall be used as the basis for calculating and selecting the circulation pump head; Step 4, determining the relationship between the liquid nitrogen flow rate and the maximum temperature difference of the liquid nitrogen; Step 5: Determine the temperature rise limit based on the relationship between the liquid nitrogen flow rate and the maximum temperature difference of the liquid nitrogen, and determine the range of the liquid nitrogen flow rate to select the optimal type of the cryogenic circulation pump; Step 6: Select a refrigeration unit that meets the cooling temperature and has a cooling capacity that matches the cooling load.

2. The method for constructing a thermal-hydraulic coupling model of a high-temperature superconducting cable cooling system according to claim 1, characterized in that: In step 1, the cooling load calculation is to calculate the local cooling load of the insulated pipe per unit length, and the specific formula is as follows: Where Q is the cooling load per unit length of the insulated pipe, in W / m; k is the effective thermal conductivity of the insulated pipe, in W / m / K; T ∞ is the ambient temperature, unit: K; is the working liquid nitrogen temperature, unit: K; D out D is the outer diameter of the insulation pipe, in mm; in It is the inner diameter of the insulation pipe, unit: mm.

3. The method for constructing a thermal-hydraulic coupling model of a high-temperature superconducting cable cooling system according to claim 2, characterized in that: In step 2, the hydraulic loss calculation includes the hydraulic loss along the way H f and local hydraulic loss H L .

4. The method for constructing a thermal-hydraulic coupling model of a high-temperature superconducting cable cooling system according to claim 3, characterized in that: The hydraulic loss along the way H f The calculation formula is as follows: Where f is the Darcy drag coefficient; L is the length of the bellows, in meters; D h is the hydraulic diameter of the liquid nitrogen channel, in m; v is the liquid nitrogen flow rate, in m / s; g is the gravitational acceleration, in m / s 2 .

5. The method for constructing a thermal-hydraulic coupling model of a high-temperature superconducting cable cooling system according to claim 4, characterized in that: For example, when calculating the hydraulic loss along the bellows, the Reynolds number is 10 4 Until April 10 4 The Darcy drag coefficient f of the bellows is only related to the bellows structure, and the expression is as follows: Where, e is the wave height of the bellows, in m; is the wave distance, in m; D h is the hydraulic diameter of the liquid nitrogen bellows channel in the cable, and the calculation formula is as follows: Among them, 3D out Is the outer diameter of the cable, unit: m.

6. The method for constructing a thermal-hydraulic coupling model of a high-temperature superconducting cable cooling system according to claim 3, characterized in that: The local hydraulic loss H L The calculation formula is as follows: Where K is the local resistance coefficient of a specific accessory; v is the liquid nitrogen flow rate, in m / s; g is the acceleration due to gravity, in m / s 2 .

7. The method for constructing a thermal-hydraulic coupling model of a high-temperature superconducting cable cooling system according to claim 3, characterized in that: In step 3, the calculation formula of the theoretical calculation result of the maximum height difference of the cable construction is as follows: Where Δh max is the theoretical calculation result of the maximum height difference of the cable construction; P inlet is the inlet pressure of liquid nitrogen, unit: Pa; P inlet =P outlet +ΔP friction (L)+ρgΔh; P outlet is the outlet pressure of liquid nitrogen, unit: Pa; Δh is the height difference of the cable during the construction process, unit: m; ΔP friction (L) is the pressure loss when the cable output distance is L, the unit is: Pa, the calculation formula is as follows: P boil (T) is the maximum height difference allowed for construction, and the corresponding output pressure of liquid nitrogen is equal to the saturated vapor pressure at the corresponding temperature, in Pa, and the calculation formula is as follows: P boil (T)=P outlet 。 8. The method for constructing a thermal-hydraulic coupling model of a high-temperature superconducting cable cooling system according to claim 3, characterized in that: In step 4, it is determined that the limit operating temperature range of liquid nitrogen is between the triple point of liquid nitrogen 63.4K and the boiling point of liquid nitrogen at normal pressure 77K; then the limit temperature difference is 13.6K; Then, the liquid nitrogen cooling temperature of 65K±10% with a maximum temperature difference of 12K±10% from the boiling point of liquid nitrogen at normal pressure of 77K is taken as the starting temperature of liquid nitrogen, and 80% of the maximum temperature difference, i.e. 9.6K±10%, is taken as the standard for determining whether the liquid nitrogen flow rate meets the thermodynamic limit. The formula for the relationship between the liquid nitrogen flow rate and the maximum temperature difference of liquid nitrogen is as follows: Where, L is the distance of the high-temperature superconducting cable, in meters; Q1 is the terminal unit loss, in W / unit; N1 is the number of terminals; Q2 is the joint unit loss, in W / unit; N2 is the number of joints; Q3 is the loss of other pipelines in the system, in W; ρ is the density of liquid nitrogen, in kg / m 3 ; is the volume flow rate of liquid nitrogen, unit: m 3 / s;c p is the constant pressure specific heat capacity of liquid nitrogen, unit: J / (kg·K).

9. The method for constructing a thermal-hydraulic coupling model of a high-temperature superconducting cable cooling system according to claim 3, characterized in that: Step 6 is as follows: Step 6.1, select the best working condition of the circulating pump, that is, the circulating pump flow rate with the highest pump efficiency; Step 6.2, calculating the temperature difference according to the circulation pump flow rate using the temperature rise formula; Step 6.3, judging whether the temperature difference is satisfied; If satisfied, the output includes a refrigeration solution for a cryogenic refrigerator and a cryogenic circulation pump; If not, increase the circulation pump flow rate until the temperature rise of the liquid nitrogen satisfies the condition that it is less than the maximum temperature difference, and then output a matching refrigeration solution including a cryogenic refrigerator and a cryogenic circulation pump.

Images