Efficient heat exchange type energy storage pile and bearing capacity calculation method thereof

Abstract

The invention discloses an efficient heat exchange type energy storage pile and a bearing capacity calculation method thereof. The energy storage pile comprises an energy storage pile body, a heat exchange pipeline system and a phase change energy storage system. The bearing capacity calculation method comprises the steps of collection of calculation parameters, calculation of pile body additional thermal stress, correction of a thermal degradation effect, calculation of pile side friction resistance and pile end resistance under thermal-mechanical coupling, checking calculation of a bearing capacity limit state and checking calculation of a pile body axial compressive strength limit state. The unique structure of the energy storage pile body forms a continuous and efficient heat diffusion channel, and the heat exchange capacity of the pile body and the surrounding soil body is greatly enhanced. Surplus heat during intermittent operation of the heat pump is dynamically stored, and the problem of supply and demand imbalance caused by intermittency of renewable energy sources is effectively solved; a bearing capacity calculation method considering a temperature field, thermal stress, soil mechanical parameter thermal degradation, pile-soil interface contact pressure change and a long-term cycle attenuation effect is provided for the first time.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of pile foundation, and relates to a high-efficiency heat exchange type energy storage pile and a bearing capacity calculation method thereof. BACKGROUND

[0002] The energy pile technology is a new energy-saving technology combining a ground source heat pump system with an engineering pile foundation. By embedding heat exchange pipelines in the pile foundation, the pile foundation simultaneously bears the functions of transferring structural load and exchanging heat with the rock-soil body, which is one of effective technologies for fully utilizing underground space and realizing energy saving and carbon reduction.

[0003] However, the existing conventional energy pile technology is essentially more focused on the function of a "heat exchanger". In long-term operation, it has the following inherent limitations, which restrict its performance improvement and large-scale safe application: 1) bottleneck of heat exchange and energy storage efficiency: the traditional energy pile usually adopts ordinary reinforced concrete materials with low thermal conductivity, which hinders the efficient transfer of heat between the heat exchange pipelines and the soil around the pile. The internal heat exchange pipelines mostly adopt simple U-shaped or W-shaped arrangement, the effective heat exchange area is limited, and there is a lack of heat transfer strengthening structure, which makes it difficult to further improve the heat exchange capacity and heat storage capacity of a single pile, and cannot meet the demand of large and medium-sized buildings for high-power and high-density energy storage; 2) significant thermal stress caused by uneven axial heat exchange of the pile body: the existing spiral pipe type energy pile mostly adopts a fixed pitch design. Due to the change of underground temperature field and rock-soil thermal properties with depth, the constant pitch leads to uneven heat exchange per unit length of the pile body in the shallow, middle and deep parts. This uneven heat extraction and injection will cause non-uniform temperature field and thermal stress in the pile body and the surrounding soil. Long-term heat circulation may have adverse effects on the mechanical properties of the pile-soil interface and the long-term safety of the structure; 3) lack of bearing capacity evaluation method under thermal-mechanical coupling: most of the existing design methods only consider the bearing capacity of the pile foundation under normal temperature conditions, and seriously ignore the thermal-mechanical coupling environment that the energy pile bears in the actual working state. The mechanical parameters of the pile body concrete and the surrounding soil and the behavior of the pile-soil interface will change significantly with temperature, and there is a thermal degradation effect. The existing method cannot quantitatively evaluate the cumulative damage of temperature change and long-term heat circulation on the bearing capacity, resulting in unsafe or overly conservative design, and lacking of theoretical basis to ensure the safe operation of the energy pile in the whole life cycle.

[0004] In summary, although the existing energy pile technology has made preliminary progress in functional integration, it still has obvious shortcomings in heat exchange and energy storage efficiency, long-term reliability and safety design theory. Therefore, there is an urgent need in the field for a comprehensive innovative solution from materials, structure to design method to promote the evolution of the energy pile technology from a single "heat exchanger" to a high-efficiency, safe and intelligent "energy storage pile" system. SUMMARY

[0005] In view of the problems of low heat exchange efficiency, small energy storage density, large structural risk caused by thermal stress and lack of bearing capacity evaluation method of existing energy pile, the application provides a high-efficiency heat exchange type energy storage pile and a bearing capacity calculation method thereof, breaks through the heat conduction bottleneck of ordinary concrete energy pile to improve the heat exchange capacity per unit volume, and establishes a bearing capacity calculation model under the condition of thermal coupling to ensure long-term structural safety.

[0006] A high-efficiency heat exchange type energy storage pile, comprising an energy storage pile body, a heat exchange pipeline system and a phase change energy storage system. The energy storage pile body is poured by using high-thermal-conductivity modified concrete, the modified concrete takes ordinary portland cement as a matrix, and high-thermal-conductivity fillers with a mass fraction of 5% to 15% are added, so that the overall thermal conductivity of the modified concrete is not less than 3.0 W / (m·k); 4 to 6 axial grooves are uniformly formed on the outer side of the energy storage pile body along the circumference, the depth of the grooves is 30 to 50 mm, and the width is 40 to 60 mm; high-thermal-conductivity flexible heat-conducting sheets are laid in the grooves and keep contact with the soil around the pile, so as to enhance the heat transfer efficiency and guide the flow of underground water to form active convection heat exchange. The heat exchange pipeline system comprises a main heat exchange pipe and an outer wall reinforcing structure; the main heat exchange pipe is arranged in a reserved hole in the pile body and is made of a single or multiple high-strength copper spiral pipes, the pitch of the spiral pipes P changes with the depth h to meet the requirement of uniform heat exchange along the depth, and the expression of the pitch P is as follows:

[0007] wherein, P 0 is the initial pitch at the top of the pile and the value range is 150 to 300 mm; β p is the attenuation coefficient of the pitch and the value is 0.001 to 0.005 m -1 , which needs to be calibrated according to the ground temperature gradient and the in-situ thermal property of rock and soil; The outer wall reinforcing structure is an integrally formed high-thermal-conductivity annular copper fin arranged on the outer wall of the main heat exchange pipe, the fin has a thickness of 2 to 5 mm and a thermal conductivity not less than 380 W / (m·k); the fin is connected with the grooves on the outer side of the pile body to form a continuous heat transfer channel and strengthen the heat diffusion efficiency. The phase change energy storage system is arranged above the energy storage pile body and adopts an encapsulated phase change material module, the module shell is made of stainless steel and filled with paraffin-based composite phase change material, which is used for storing heat during intermittent operation of the heat pump and improving the energy storage density of the system.

[0008] Preferably, the high thermal conductivity filler is steel fiber or carbon nanotube, wherein the steel fiber has a length of 10 to 20 mm and a diameter of 0.5 to 1 mm, and the carbon nanotube has a diameter of 10 to 50 nm and a length of 1-5 μm.

[0009] Preferably, the high-efficiency heat exchange energy storage pile also includes an intelligent control system; the intelligent control system includes temperature sensors arranged in the pile body and the surrounding soil, a flow sensor arranged at the inlet end of the main heat exchange pipe, an electric proportional flow control valve, a two-way circulation pump and a PLC controller; the temperature sensor is arranged every 2 to 3 meters along the depth to monitor the temperature of the pile body and the soil in real time; the flow sensor is used to monitor the flow at the inlet end of the main heat exchange pipe; the PLC controller controls the electric proportional flow control valve and the two-way circulation pump according to the monitoring data to achieve dynamic adjustment of the fluid flow and flow direction.

[0010] A method for calculating the bearing capacity of a high-efficiency heat exchange energy storage pile comprises the following steps: S1. Collection of calculation parameters: Collect pile design parameters, soil parameters, thermal cycle parameters and load parameters; the pile design parameters include pile diameter D , pile length L , pile equivalent cross-sectional area A p , the geometric dimensions of the groove, the density of the pile p c , Pile equivalent thermal conductivity l c Specific heat capacity of pile B c , thermal expansion coefficient of pile α c , pile elastic modulus E c and room temperature axial compressive strength f c0 ; Among them, the equivalent cross-sectional area of ​​the pile A p The area occupied by the internal reserved channel has been deducted, and the equivalent thermal conductivity of the pile body is l c It is obtained by weighted average calculation of thermal conductivity of modified concrete, fins and thermal conductive sheets; the soil parameters include thermal conductivity of each layer of soil l s , soil density p s Specific heat capacity of soil B s , soil cohesion at room temperature C s0 、 Internal friction angle of soil at room temperature f s0, soil normal temperature shear modulus G 0 , soil normal temperature pile tip resistance characteristic value q p0 ; the heat cycle working condition parameter is according to the historical operation data of intelligent control system in at least 1 complete heat load cycle, extracts the measured temperature distribution along the depth direction of pile body h , T 1 ( h, t ), heat transfer fluid inlet and outlet temperature T 2 , heat charging or heat releasing mode switching period t 3 ( h ); the load parameter includes vertical static load transmitted to pile top by upper structure N ; S2, calculation of additional thermal stress of pile body: with pile body central axis as z axis, radial as r axis, establish heat conduction control model under column coordinate system, as shown in the following formula:

[0011] wherein, when calculating pile body temperature field, p = p c , B = B c , l = l c ; when calculating soil temperature field, p = p s , B = B s , l = l s ; T actual temperature, ℃; t time, s; input the parameters collected in step S1 to finite element analysis software, establish model and calculate to obtain temperature variation of any point of pile body r , z , t ) ( AT , r , z , t ); wherein, AT = T - T 0 , T 0 reference temperature and take T 0 = 20 ℃; Considering that the vertical bearing capacity of the pile body is mainly controlled by the axial stress, the axial additional thermal stress of the pile body s th The expression of the axial additional thermal stress of the pile body

[0012] is as follows: s th The positive value of the axial additional thermal stress of the pile body S3, correction of thermal degradation effect: The influence of the soil cohesion C s ( T ), the soil internal friction angle f s ( T ) and the soil shear modulus G 0 ( T ) is considered, and the expressions of the indoor direct shear test and the shear modulus test at different temperatures, C s ( T ), f s ( T ) and G 0 ( T ) are as follows:

[0013] wherein, T is the actual temperature, ℃; T 0 is the reference temperature and is taken as T 0 = 20℃; β c , β φ , β G respectively are the temperature influence coefficients of the cohesion, the internal friction angle and the shear modulus, which need to be calibrated according to the in-situ soil sample, and the typical value range is β c ∈[-0.005,-0.002]℃ -1 , β φ ∈[-0.003,-0.001]℃ -1 , β G ∈[-0.004,-0.002]℃ -1 ; the negative value of the temperature influence coefficient indicates that the temperature rise has a degradation effect on the mechanical properties of the soil; S4, calculation of the side friction and the tip resistance under thermal coupling: The side friction is provided by the cohesion and the internal friction angle of the pile-soil interface, and the thermal degradation effect of the soil and the change of the contact pressure caused by the thermal expansion and contraction of the pile-soil interface need to be considered simultaneously. h The side friction characteristic value of the pile at a depth T and a temperature f s T h is shown in the following formula:

[0014] wherein, K is the lateral pressure coefficient of the soil; k sf is the thermal correction coefficient of the pile-soil interface, reflecting the influence of the temperature change AT on the contact pressure of the pile-soil interface, which is obtained by fitting in the thermal cyclic shear test of the energy storage pile body for more than 1000 times, and the fitting expression is shown in the following formula:

[0015] wherein, β k1 is the thermal correction coefficient of the pile-soil interface k sf temperature influence coefficient, which is calibrated by indoor test, and the typical value range is β k1 ∈ [-0.02, -0.01]℃ -1 ; The side friction characteristic value of the pile in the depth direction is integrated to obtain the side friction of the pile under thermal coupling R s T , which is shown in the following expression:

[0016] The tip resistance is affected by the mechanical parameter attenuation and the thermal compressibility caused by the thermal degradation of the soil at the pile tip, and the tip resistance R p T satisfies the following expression:

[0017] wherein, q p0 is the tip resistance characteristic value of the soil at normal temperature; S5, limit state checking of bearing capacity: Considering the pile-soil interface degradation caused by long-term repeated thermal cycles, the total vertical bearing capacity of the pile under thermal coupling R is shown in the following expression:​​​​T ) is expressed as follows:

[0018] in, N is the vertical static load transferred from the superstructure to the pile top, kN; k r is the safety factor, which is taken according to the Code for Design of Building Foundations. k r =1.25; s is the long-term attenuation coefficient, based on the number of thermal cycles N cycle Calibration, the expression is:

[0019] in, β η is the attenuation coefficient of the long-term attenuation coefficient, which is calibrated by indoor tests and has a typical value range of β η ∈[0.01,0.03]; S6. Verification of ultimate limit state of pile axial compressive strength: The energy storage pile mainly bears vertical loads. The axial compressive strength of the pile concrete under the combination of static load and thermal load is verified to see whether it meets the requirements, as shown in the following expression:

[0020] in, s max is the maximum axial compressive stress of the pile, kPa; f c ( T ) is the temperature-corrected axial compressive strength of the pile concrete, which satisfies the following expression:

[0021] in, β f is the temperature influence coefficient of the design value of the axial compressive strength of the pile concrete, which is calibrated through indoor tests and the typical value range is β f ∈[-0.02,-0.01]℃ -1 .

[0022] In summary, compared with the prior art, the beneficial effects of the present application are: in view of the problems of low heat exchange efficiency, small energy storage density, large structural risk caused by thermal stress and lack of bearing capacity evaluation method of existing energy piles, the present application proposes a high-efficiency heat exchange type energy storage pile and a bearing capacity calculation method thereof, the energy storage pile comprises an energy storage pile body, a heat exchange pipeline system and a phase change energy storage system, the bearing capacity calculation method comprises collection of calculation parameters, calculation of additional thermal stress of the pile body, correction of thermal degradation effect, calculation of pile side friction resistance and pile end resistance under thermal coupling, bearing capacity limit state checking, and pile body axial compressive strength limit state checking, realizing the following beneficial effects of breakthrough improvement: 1) The heat exchange efficiency is significantly improved: the high-thermal-conductivity modified concrete, the groove on the outer side of the pile, the flexible heat-conducting sheet structure and the copper fin design integrated with the outer wall of the main heat exchange pipe together constitute a continuous and efficient heat diffusion channel, which greatly enhances the heat exchange capacity of the pile body and the surrounding soil; 2) The supply-demand imbalance problem is solved by energy storage: through the encapsulated phase change energy storage module, the surplus heat during intermittent operation of the heat pump is dynamically stored, the load fluctuation is smoothed, and the supply-demand imbalance problem caused by the intermittency of renewable energy is effectively solved; 3) Eliminate axial thermal stress concentration: the spiral heat exchange pipe with variable pitch design can design the heat exchange intensity according to the change of ground temperature gradient, ensure that the heat exchange capacity per unit length of the pile body in shallow, medium and deep parts tends to be uniform, and avoid the risk of concrete cracking caused by non-uniform thermal expansion; 4) Establish a scientific and reliable thermal coupling bearing capacity evaluation system: for the first time, a bearing capacity calculation method considering temperature field, thermal stress, thermal degradation of soil mechanical parameters, change of pile-soil interface contact pressure and long-term cyclic attenuation effect is systematically proposed; the soil parameters are dynamically corrected by introducing the temperature influence coefficient, and the thermal correction coefficient and the long-term attenuation coefficient are used to quantify the cumulative damage effect of thermal cycling, so that the bearing capacity calculation result is closer to the real working state of the energy pile, overcoming the serious shortcomings of the traditional method which only considers the normal temperature working condition, and providing accurate theoretical basis for the long-term safety design and evaluation of the energy pile; 5) Intelligent dynamic regulation: the intelligent control system can monitor the key parameters such as pile body and soil temperature, fluid flow rate in real time, and dynamically adjust the flow rate and flow direction through the PLC controller, which not only can optimize the heat exchange performance and avoid damage caused by extreme temperature, but also can provide real and continuous input parameters for the bearing capacity calculation model, ensuring that the system always operates in an efficient and safe state. BRIEF DESCRIPTION OF DRAWINGS

[0023] Fig. 1 is a structural schematic diagram of the high-efficiency heat exchange type energy storage pile of the present application; Fig. 2 is a flow chart of the bearing capacity calculation method of the high-efficiency heat exchange type energy storage pile of the present application; Wherein, 1-energy storage pile body, 11-modified concrete, 12-groove, 2-heat exchange pipeline system, 21-main heat exchange pipe, 22-outer wall reinforcement structure, 3-phase change energy storage system, 4-intelligent control system, 41-temperature sensor, 42-flow sensor, 43-electric proportional flow regulating valve, 44-bidirectional circulating pump. DETAILED DESCRIPTION

[0024] The specific embodiments of the present application are described in detail below with reference to the accompanying drawings. It should be understood that the specific embodiments given here are only for illustration and explanation of the present application, and cannot be used to limit the present application.

[0025] The first aspect of the present application discloses a high-efficiency heat exchange type energy storage pile as shown in Fig. 1 The high-efficiency heat exchange type energy storage pile comprises an energy storage pile body 1, a heat exchange pipeline system 2, a phase change energy storage system 3 and an intelligent control system 4.

[0026] The energy storage pile body 1 is poured by using high-thermal-conductivity modified concrete 11, the modified concrete 11 takes ordinary portland cement as a matrix, and a high-thermal-conductivity filler with a mass fraction of 5% to 15% is added, so that the overall thermal conductivity of the modified concrete 11 is not less than 3.0 W / (m·k); in specific implementation, the high-thermal-conductivity filler uses steel fibers or carbon nanotubes, wherein the length of the steel fibers is 10 to 20 mm, and the diameter is 0.5 to 1 mm, and the diameter of the carbon nanotubes is 10 to 50 nm, and the length is 1-5 μm; 4 to 6 axial grooves 12 are uniformly arranged on the outer side of the energy storage pile body 1 along the circumference, the depth of the groove 12 is 40 mm, and the width is 50 mm; high-thermal-conductivity flexible heat-conducting sheets are laid in the grooves 12 and keep contact with the soil around the pile, so as to enhance the heat transfer efficiency and guide the flow of underground water to form active convection heat exchange.

[0027] The heat exchange pipeline system 2 comprises a main heat exchange pipe 21 and an outer wall reinforcement structure 22; the main heat exchange pipe 21 is arranged in a reserved hole in the pile body, and uses a single or multiple high-strength copper spiral pipes, the pitch P of which changes with depth h to meet the requirement of uniform heat exchange along the depth, and the expression of the pitch P is as follows: (1) wherein, P 0 is the initial pitch at the top of the pile, and the value range is 150 to 300 mm; β p is the attenuation coefficient of the pitch, and the value is 0.001 to 0.005 m -1 , which needs to be calibrated according to the ground temperature gradient and the in-situ rock-soil thermal properties; in specific implementation, the initial pitch P 0 at the top of the pile is 200 mm, and the attenuation coefficient of the pitch β is 0.002 m.p 0.003m -1 , pile length L is 30m, then P ( h= 10m)=194mm, P ( h= 20m)=188mm.

[0028] The outer wall reinforcement structure 22 is an integrally formed high-thermal conductivity annular copper fin provided on the outer wall of the main heat exchange tube 21. The fin thickness is 2 to 5 mm and the thermal conductivity coefficient is not less than 380 W / (m·k). The fin is connected to the groove 12 on the outside of the pile body to form a continuous heat transfer channel and enhance the heat diffusion efficiency.

[0029] The phase change energy storage system 3 is arranged above the energy storage pile body and adopts an encapsulated phase change material module. The module shell is made of stainless steel and is filled with paraffin-based composite phase change material. It is used to store heat during intermittent operation of the heat pump and improve the system energy storage density.

[0030] The intelligent control system 4 includes a temperature sensor 41 arranged in the pile body and the surrounding soil, a flow sensor 42 arranged at the inlet end of the main heat exchange pipe 21, an electric proportional flow control valve 43, a two-way circulation pump 44 and a PLC controller; the temperature sensor 41 is arranged every 2 to 3 meters along the depth to monitor the temperature of the pile body and the soil in real time; the flow sensor 42 is used to monitor the flow at the inlet end of the main heat exchange pipe 21; the PLC controller controls the electric proportional flow control valve 43 and the two-way circulation pump 44 according to the monitoring data to achieve dynamic adjustment of the fluid flow and flow direction.

[0031] The second aspect of this application discloses Fig. 2 The method for calculating the bearing capacity of a high-efficiency heat exchange energy storage pile shown includes the following steps: S1. Collection of calculation parameters: Collection of pile design parameters, soil parameters, thermal cycle parameters and load parameters; the pile design parameters include pile diameter D , pile length L , pile equivalent cross-sectional area A p , the geometric dimensions of the groove, the density of the pile p c , Pile equivalent thermal conductivity l c Specific heat capacity of pile B c , thermal expansion coefficient of pile α c , pile elastic modulus E c and room temperature axial compressive strength fc0 ; wherein the equivalent cross-sectional area of the pile body A p The equivalent thermal conductivity of the pile body has deducted the area occupied by the internal reserved channel l c The thermal conductivity of the modified concrete, the fin and the heat-conducting sheet is obtained by volume-weighted average calculation; the soil body parameters include the thermal conductivity of each layer of soil body l s , the soil body density p s , the specific heat capacity of the soil body B s , the cohesion of the soil body at normal temperature C s0 、 the internal friction angle of the soil body at normal temperature f s0 , the shear modulus of the soil body at normal temperature G 0 , the characteristic value of the pile end resistance of the soil body at normal temperature q p0 ; the thermal cycle working condition parameters are obtained according to the historical operation data of the intelligent control system in at least one complete thermal load cycle, and include the measured temperature distribution along the depth direction of the pile body h T 1 ( h, t ), the inlet and outlet temperatures of the heat exchange fluid T 2 , the charging or discharging mode switching period t 3 ( h ); the load parameters include the vertical static load transmitted by the upper structure to the pile top N .

[0032] S2, calculation of the additional thermal stress of the pile body: taking the central axis of the pile body as the z axis and the radial direction as the r axis, a heat conduction control model in the column coordinate system is established, as shown in the following formula: (2) wherein, when calculating the temperature field of the pile body, p = p c , B = B c , l = l c ; when calculating the temperature field of the soil body, p = p s , B = B s , l = l s ; T the actual temperature, ℃ t ​For time, s; The parameters collected in step S1 are input into finite element analysis software to establish a model and calculate the temperature variation of the pile body at any point r , z , t ) (temperature variation AT ( r , z , t ) of the pile body) ; wherein, AT = T - T 0 , T 0 is a reference temperature and is taken as T 0 = 20℃; Considering that the vertical bearing capacity of the pile body is mainly controlled by axial stress, the expression of the axial additional thermal stress s th of the pile body is as follows: (3) Wherein, the value of the axial additional thermal stress s th of the pile body is a positive value indicating tensile stress or a negative value indicating compressive stress; in a specific implementation, for a typical point at a depth z =15m, the elastic modulus E c of the pile body is 35000MPa, the temperature variation AT ( r , z , t ) is 8℃, and the thermal expansion coefficient α c of the pile body is 1.2×10 -5 ℃ -1 , then the calculation of the axial additional thermal stress s th of the pile body is as follows: (4) That is, the axial additional thermal stress s th of the pile body at the typical point is 3.36MPa, and the expansion causes the pile body to be subjected to compressive stress because AT >0.

[0033] S3, correction of thermal degradation effect: The cohesion C s ( T ) of the soil, the internal friction angle f s ( T ) of the soil, and the shear modulusG 0 ( T ) of the soil, the expressions of the influence of temperature on the cohesion, the internal friction angle and the shear modulus are as follows: C s ( T )、 f s ( T ) and G 0 ( T ) are as follows: (5) wherein, T is the actual temperature, ℃; T 0 is the reference temperature and takes T 0 = 20℃; β c 、 β φ 、 β G respectively are the temperature influence coefficients of the cohesion, the internal friction angle and the shear modulus, which need to be calibrated according to the in-situ soil sample, and the typical value range is β c ∈[-0.005,-0.002]℃ -1 、 β φ ∈[-0.003,-0.001]℃ -1 、 β G ∈[-0.004,-0.002]℃ -1 ; the temperature influence coefficient is negative, which indicates that the temperature rise has a deteriorating effect on the mechanical properties of the soil.

[0034] S4, calculation of the pile side friction and the pile tip resistance under thermal coupling: The pile side friction is provided by the cohesion and the internal friction angle of the pile-soil interface, and the thermal degradation effect of the soil and the change of the contact pressure caused by the thermal expansion and contraction of the pile-soil interface need to be considered; the expression of the characteristic value h of the pile side friction at the depth T and the temperature f s ( T , h ) is as follows: (6) wherein, K is the lateral pressure coefficient of the soil; k sf is the thermal correction coefficient of the pile-soil interface, which reflects the change of the contact pressure caused by the thermal expansion and contraction of the pile-soil interface.AT The influence of the pile-soil interface contact pressure is fitted according to the energy storage pile in more than 1000 times of thermal cycle shear test, and the fitting expression is as shown in the following formula: (7) Wherein, β k1 is the pile-soil interface thermal correction coefficient k sf is the temperature influence coefficient, which is calibrated by indoor test, and the typical value range is β k1 ∈[-0.02,-0.01]℃ -1 ; In the specific implementation, for a typical point at a depth z =15m, the lateral pressure coefficient of the soil body K is 0.5, the temperature change AT is 8℃, and the pile-soil interface thermal correction coefficient k sf is the temperature influence coefficient β k1 is-0.015, then the pile-soil interface thermal correction coefficient k sf is calculated as shown in the following formula: (8) The soil cohesion C s ( T ) is 13.58kPa, and the soil internal friction angle f s ( T ) is 31.5°, then the pile side friction characteristic value f s ( T , h ) is calculated as shown in the following formula: (9) That is, the pile side friction characteristic value f s ( T , h ) at the typical point is 20.2 kPa; The pile side friction characteristic value in the depth direction is integrated to obtain the pile side friction under thermal force coupling R s ( T ), as shown in the following expression: (10) The pile tip resistance is affected by the mechanical parameter decay caused by thermal degradation of the soil at the pile tip and thermal compressibility. The pile tip resistance R p ( T ) satisfies the following expression: (11) wherein, q p0 is the characteristic value of the pile tip resistance of the soil at normal temperature.

[0035] S5, limit state checking of bearing capacity: Considering the pile-soil interface degradation caused by long-term repeated thermal cycles, the expression of the total vertical bearing capacity of the pile under thermal coupling R ( T ) is as follows: (12) wherein, N is the vertical static load transmitted to the pile top by the superstructure, kN; k r is the safety factor, taken as k r =1.25 according to the “Code for Design of Building Foundation”; s is the long-term decay coefficient, calibrated based on the number of thermal cycles N cycle , and the expression is: (13) wherein, β η is the decay coefficient of the long-term decay coefficient, calibrated through indoor tests, and the typical value range is β η ∈[0.01,0.03]; In specific implementation, the decay coefficient of the long-term decay coefficient β η is 0.02, when the number of thermal cycles N cycle is 1000 times, the calculated long-term decay coefficient s is 0.86; the pile tip resistance R p ( T ) is 2595 kN, the pile side friction resistance R s ( T ) is 1520 kN, the safety factor k r is 1.25, the vertical static load transmitted to the pile top by the superstructure N is 1000 kN, and the equivalent cross-sectional area of the pile A p0.48 m 2 Then the total vertical bearing capacity of the pile body under thermal coupling R ( T )According to formula (12), the following formula is verified: (14) That is, the bearing capacity limit state verification is met.

[0036] S6, axial compressive strength limit state verification of pile body: The energy storage pile mainly bears vertical load, and whether the axial compressive strength of the pile body concrete under the combination of static load and thermal load meets the requirements is verified, which is expressed as follows: (15) Wherein, AT max The maximum axial compressive stress of the pile body is kPa; f c ( T )The temperature-corrected axial compressive strength of the pile body concrete meets the following expression: (16) Wherein, β f The temperature influence coefficient of the axial compressive strength design value of the pile body concrete is calibrated by indoor test, and the typical value range is β f ∈ [-0.02, -0.01] ℃ -1 ; In specific implementation, the temperature influence coefficient of the axial compressive strength design value of the pile body concrete β f -0.015 ℃ -1 , the normal temperature axial compressive strength f c0 25 MPa, then the temperature-corrected axial compressive strength of the pile body concrete f c ( T ) is 22 MPa; The axial compressive strength of the pile body concrete under the combination of static load and thermal load is verified according to formula (15) as follows: (17) That is, the axial compressive strength limit state verification of the pile body is met.

[0037] The above is one or more embodiments of the present application, which is described more specifically and in detail, but can not be understood as a limitation on the scope of the patent of the present application. It should be noted that for ordinary skilled in the art, without departing from the concept of the present application, a number of modifications and improvements can be made, which are within the scope of protection of the present application. Therefore, the scope of protection of the patent of the present application should be subject to the appended claims.

Claims

1. A high-efficiency heat exchange type energy storage pile, characterized in that, The energy storage pile body, the heat exchange pipeline system and the phase change energy storage system are included. The energy storage pile body is made of modified concrete with high thermal conductivity, the modified concrete takes ordinary Portland cement as a matrix, and high thermal conductivity fillers with a mass fraction of 5% to 15% are added, so that the overall thermal conductivity of the modified concrete is not less than 3.0 W / (m·k); 4 to 6 axial grooves are evenly arranged on the outer side of the energy storage pile body along the circumference, the depth of the grooves is 30 to 50 mm, and the width is 40 to 60 mm; high thermal conductivity flexible heat conduction sheets are laid in the grooves and keep contact with the soil around the pile; The heat exchange piping system includes a main heat exchange pipe and an outer wall reinforcement structure; the main heat exchange pipe is set in the reserved channel inside the pile body, and adopts a single or multiple high-strength copper spiral pipes, the pitch of which is P With depth h Changes to meet the need for uniform heat transfer along the depth, the pitch P The expression is as follows: wherein, P 0 is the initial pitch at the top of the pile, ranging from 150 to 300 mm; β p is the attenuation coefficient of the pitch, ranging from 0.001 to 0.005 m -1 , which needs to be calibrated according to the field temperature gradient and the thermal properties of rock and soil. The outer wall reinforcing structure is an integrally formed high thermal conductivity annular copper fin arranged on the outer wall of the main heat exchange pipe, the fin thickness is 2 to 5 mm, and the thermal conductivity is not less than 380 W / (m·k); the fin is connected with the grooves on the outer side of the pile body, forming a continuous heat transfer channel and strengthening the heat diffusion efficiency; The phase change energy storage system is arranged above the energy storage pile body, adopts an encapsulated phase change material module, and the module shell is made of stainless steel and filled with paraffin-based composite phase change material.

2. The high-efficiency heat exchange type energy storage pile according to claim 1, characterized in that, The high thermal conductivity fillers adopt steel fibers or carbon nanotubes, wherein the length of the steel fibers is 10 to 20 mm, the diameter is 0.5 to 1 mm, the carbon nanotube tube diameter is 10 to 50 nm, and the length is 1-5 μm.

3. The high-efficiency heat exchange type energy storage pile according to claim 1, characterized in that, The intelligent control system includes temperature sensors arranged in the pile body and the surrounding soil, a flow sensor arranged at the inlet end of the main heat exchange pipe, an electric proportional flow regulating valve, a bidirectional circulating pump and a PLC controller; the temperature sensors are arranged every 2 to 3 m along the depth to monitor the temperature of the pile body and the soil in real time; the flow sensor is used to monitor the flow at the inlet end of the main heat exchange pipe; The PLC controller controls the electric proportional flow regulating valve and the bidirectional circulating pump according to the monitoring data, so as to realize dynamic adjustment of fluid flow and flow direction.

4. A method for calculating the bearing capacity of a high-efficiency heat exchange type energy storage pile, characterized in that, The high-efficiency heat exchange type energy storage pile of claim 1 comprises the following steps: S1, collecting parameters: collecting pile design parameters, soil parameters, thermal cycle working condition parameters and load parameters; the pile design parameters include pile diameter D , pile length L , equivalent cross-sectional area of pile body A p , geometric size of groove, density of pile body ρ c , equivalent thermal conductivity of pile body λ c , specific heat capacity of pile body B c , thermal expansion coefficient of pile body α c , elastic modulus of pile body E c and axial compressive strength at room temperature f c0 ; wherein the equivalent cross-sectional area of pile body A p has deducted the area occupied by the internal reserved hole, the equivalent thermal conductivity of pile body λ c is obtained by weighted average calculation of the thermal conductivity coefficients of modified concrete, fins and heat-conducting sheets; the soil parameters include thermal conductivity coefficients of each layer of soil ρ s , density of soil ρ s , specific heat capacity of soil B s , cohesion of soil at room temperature C s0 、 , internal friction angle of soil at room temperature φ s0 , shear modulus of soil at room temperature G 0 , characteristic value of pile tip resistance of soil at room temperature q p0 ; the thermal cycle working condition parameters are obtained according to the historical operation data of the intelligent control system in at least one complete thermal load cycle, and the measured temperature distribution h along the depth direction of the pile body T 1 ( h,t ), inlet and outlet temperatures of heat exchange fluid T 2 , heat charging or discharging mode switching period t 3 ( h ); the load parameters include vertical static load transferred to the pile top by the upper structure N ; S2, calculation of additional thermal stress of pile body: taking the central axis of the pile body as z the axial direction and the radial direction as r the axial direction, a heat conduction control model under the column coordinate system is established, as shown in the following formula: wherein, when calculating the temperature field of the pile, ρ=ρ c , B=B c , λ=λ c ; when calculating the temperature field of the soil, ρ=ρ s , B=B s , λ=λ s ; T is the actual temperature, °C; t is the time, s; The parameters collected in step S1 are input into finite element analysis software, a model is established, and the temperature variation of the pile body at any point (x, y, z) is calculated r , z , t ); wherein, ΔT r , z , t ; the reference temperature is taken as ΔT=T T 0 , T 0 20℃. T 0 = 20℃.​​ Additional axial thermal stress of pile body σ th The expression is as follows: wherein the axial additional thermal stress of the pile body σ th is positive for tensile stress and negative for compressive stress. S3, Correction of thermal degradation effect: influence of temperature change on soil cohesion C s ( T ), internal friction angle of soil φ s ( T ), and shear modulus of soil G 0 ( T ) are expressed as follows: C s ( T ), φ s ( T ), and G 0 ( T ). where, T is the actual temperature, °C; T 0 is the reference temperature and taken as T 0 = 20 °C; β c , β φ , β G are the temperature influence coefficients of cohesion, internal friction angle and shear modulus, respectively, which need to be calibrated according to the in-situ soil sample, and the typical value range is β c ∈ [-0.005, -0.002] °C -1 , β φ ∈ [-0.003, -0.001] °C -1 , β G ∈ [-0.004, -0.002] °C -1 ; the temperature influence coefficient is negative, which indicates that the temperature rise has a deteriorating effect on the mechanical properties of the soil. S4, Calculation of the shaft friction and the toe resistance of the pile under thermal coupling: depth h at which the pile is located, temperature T of the shaft friction of the pile f s ( T , h ) is expressed as follows: wherein, K Kt is the lateral pressure coefficient of soil; k sf Kt is the lateral pressure coefficient of soil; ΔT the influence of the contact pressure of the pile-soil interface, which is obtained by fitting the energy storage pile in more than 1000 times of thermal cycle shear test, and the fitting expression is shown in the following formula: wherein, β k1 is the thermal correction coefficient of pile-soil interface k sf is the temperature influence coefficient, calibrated by indoor test, and the typical value range is β k1 ∈ [-0.02, -0.01]℃ -1 ; Integrating the characteristic value of the pile side friction in the depth direction, the pile side friction under thermal-mechanical coupling is obtained R s ( T ), as shown in the following expression: The pile tip resistance is affected by the mechanical parameter decay caused by thermal degradation of the soil at the pile tip and the thermal-induced compressibility, and the pile tip resistance R p ( T ) satisfies the following expression: wherein, q p0 is the characteristic value of the pile tip resistance of the soil at normal temperature. S5, bearing capacity limit state checking: total vertical bearing capacity of pile under thermal-mechanical coupling R ( T ) is shown as follows: wherein, N P is the vertical static load transmitted to the pile top by the superstructure, kN; k r γ is the safety factor, which is taken according to the Code for Design of Building Foundation; η η is the long-term attenuation coefficient, which is based on the number of thermal cycles N cycle calibration, the expression is: wherein, β η is the attenuation coefficient of long-term attenuation coefficient, calibrated by indoor test, and the typical value range is β η ∈[0.01, 0.03]; S6, pile axial compressive strength limit state checking: checking whether the axial compressive strength of the pile body concrete under the combination of static load and thermal load meets the requirements, as shown in the following expression: wherein, σ max Pmax is the maximum axial compressive stress of the pile body, kPa; f c Pmax is the maximum axial compressive stress of the pile body, kPa; T is the axial compressive strength of the pile body after temperature correction, satisfying the following expression: wherein, β f temperature influence coefficient of design value of axial compressive strength of pile body concrete, calibrated by indoor test, typical value range is β f ∈ [-0.02, -0.01]℃ -1 .

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