High-efficiency heat exchange type energy storage pile and bearing capacity calculation method thereof

Abstract

The application discloses 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; 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 is greatly strengthened; the 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 is effectively solved; the bearing capacity calculation method considering a temperature field, thermal stress, thermal degradation of soil mechanical parameters, change of pile-soil interface contact pressure and long-term cycle attenuation effect is first proposed.

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 piles, 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 piles 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.

[0007] 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.

[0008] 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 pipe P changes with the depth to meet the requirement of uniform heat exchange along the depth, and the expression of the pitch of the spiral pipe h is as follows: P

[0009]

[0010] wherein, P 0 is the initial pitch at the top of the pile and has a value range of 150 to 300 mm; β p is the attenuation coefficient of the pitch and has a value of 0.001 to 0.005 m -1 , and needs to be calibrated according to the ground temperature gradient and the in-situ thermal property of rock and soil;

[0011] 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.

[0012] 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, the inside is filled with paraffin-based composite phase change material, and is used for storing heat during intermittent operation of the heat pump to improve the energy storage density of the system. ​

[0013] Preferably, the high thermal conductivity filler adopts steel fibers or carbon nanotubes, wherein the length of the steel fiber is 10-20 mm and the diameter is 0.5-1 mm, and the carbon nanotube has a tube diameter of 10-50 nm and a length of 1-5 μm.

[0014] Preferably, the high-efficiency heat exchange type energy storage pile further comprises an intelligent control system; the intelligent control system comprises 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 at an interval of 2-3 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 to realize dynamic adjustment of the fluid flow and flow direction.

[0015] A bearing capacity calculation method of a high-efficiency heat exchange type energy storage pile, comprising the following steps:

[0016] S1, collecting calculation parameters:

[0017] The pile body design parameters, soil parameters, heat cycle working condition parameters and load parameters are collected; the pile body design parameters include the pile diameter D , the pile length L , the equivalent cross-sectional area of the pile body A p , the geometric size of the groove, the density of the pile body p c , the equivalent thermal conductivity of the pile body l c , the specific heat capacity of the pile body B c , the thermal expansion coefficient of the pile body α c , the elastic modulus of the pile body E c and the axial compressive strength at normal temperature f c0 ; wherein the equivalent cross-sectional area of the pile body A p has deducted the area occupied by the internal reserved hole, the equivalent thermal conductivity of the pile body l c is obtained by weighted average calculation of the thermal conductivities of the modified concrete, the fins and the heat conducting sheets; the soil parameters include the thermal conductivities of the soil layers l s , the density of the soil p s , the specific heat capacity of the soil B s , the cohesion of the soil at normal temperature C s0、 Internal friction angle of soil at normal temperature f s0 Shear modulus of soil at normal temperature G 0 Characteristic value of pile tip resistance of soil at normal temperature q p0 The heat cycle working condition parameters are according to the historical operation data of the intelligent control system in at least one complete heat load cycle, and the measured temperature distribution along the depth direction of the pile body is extracted 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 to the pile top by the upper structure N ;

[0018] S2, calculation of additional thermal stress of pile body:

[0019] 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:

[0020]

[0021] 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, p = p s , B = B s , l = l s ; T Actual temperature, ℃ t Time, s

[0022] The parameters collected in step S1 are input into the finite element analysis software, a model is established, and the temperature change amount r , z , t ) of any point of the pile body is calculated AT ( r , z , t ) ; wherein, AT = T - T 0 ,​T 0 For reference temperature and take T 0 = 20℃;

[0023] Considering that the vertical bearing capacity of the pile is mainly controlled by axial stress, the additional axial thermal stress of the pile... s th The expression is shown in the following formula:

[0024]

[0025] Among them, the axial additional thermal stress of the pile body s th A positive value indicates tensile stress, and a negative value indicates compressive stress.

[0026] S3, Correction for thermal degradation effect:

[0027] Soil cohesion considering temperature changes C s ( T ), soil internal friction angle f s ( T and soil shear modulus G 0 ( T The influence of ) was investigated based on indoor direct shear tests and shear modulus tests at different temperatures. C s ( T ), f s ( T )and G 0 ( T The expression for ) is shown in the following formula:

[0028]

[0029] in, T The actual temperature is in °C. T 0 For reference temperature and take T 0 = 20℃; β c , β φ , β G These are the temperature influence coefficients for cohesion, angle of internal friction, and shear modulus, respectively. They need to be calibrated based on in-situ soil samples, and their typical range is [value missing]. 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;

[0030] S4, calculation of pile side friction and pile tip resistance under thermal-mechanical coupling:

[0031] The pile side friction is provided by the cohesion and internal friction angle of the pile-soil interface, and the thermal degradation effect of the soil and the change of contact pressure caused by thermal expansion and contraction of the pile-soil interface need to be considered. The depth h , the pile side friction characteristic value T f s ( T , h ) is expressed as follows:

[0032]

[0033] 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 influence of temperature change AT on the contact pressure of the pile-soil interface, which is obtained by fitting in more than 1000 thermal cycle shear tests of the energy storage pile, and the fitting expression is as follows:

[0034]

[0035] wherein, β k1 is the temperature influence coefficient of the thermal correction coefficient k sf of the pile-soil interface, which is calibrated by indoor tests, and the typical value range is β k1 ∈[-0.02,-0.01]℃ -1 ;

[0036] The pile side friction characteristic value is integrated in the depth direction to obtain the pile side friction R s ( T ) under thermal-mechanical coupling, as shown in the following expression:

[0037]

[0038] ​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 compressibility. The pile tip resistance R p ( T ) satisfies the following expression:

[0039]

[0040] wherein, q p0 is the characteristic value of the pile tip resistance of the soil at normal temperature;

[0041] S5, limit state checking of bearing capacity:

[0042] Considering the degradation of the pile-soil interface 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:

[0043]

[0044] 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, based on the number of thermal cycles N cycle calibration, the expression is:

[0045]

[0046] 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];

[0047] S6, limit state checking of axial compressive strength of the pile body:

[0048] The energy storage pile mainly bears vertical load, and it is checked 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:

[0049]

[0050] wherein, s max is the maximum axial compressive stress of the pile body, kPa; f c ​T ) is the temperature-corrected axial compressive strength of the pile body concrete, satisfying the following expression:

[0051]

[0052] wherein, β f is the temperature influence coefficient of the design value of the axial compressive strength of the pile body concrete, calibrated by indoor test, and the typical value range is β f ∈ [-0.02, -0.01]℃ -1 .

[0053] 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 comprising an energy storage pile body, a heat exchange pipeline system and a phase change energy storage system, the bearing capacity calculation method comprising 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, limit state checking of bearing capacity, limit state checking of axial compressive strength of the pile body, realizing the following breakthrough improvements:

[0054] 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, greatly strengthening the heat exchange capacity of the pile body and the surrounding soil;

[0055] 2) Solve the imbalance between supply and demand through energy storage: through the encapsulated phase change energy storage module, the surplus heat during intermittent operation of the heat pump is dynamically stored, which can smooth the load fluctuation and effectively solve the imbalance between supply and demand caused by the intermittency of renewable energy;

[0056] 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, ensuring that the heat exchange capacity per unit length of the shallow, medium and deep parts of the pile body tends to be uniform, avoiding the risk of concrete cracking caused by non-uniform thermal expansion;

[0057] 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, pile-soil interface contact pressure change and long-term cyclic attenuation effect is systematically proposed; the soil parameters are dynamically modified by introducing the temperature influence coefficient, and the thermal correction coefficient and long-term attenuation coefficient are used to quantify the cumulative damage effect of thermal cycle, so that the bearing capacity calculation result is closer to the real working state of the energy pile, which overcomes the serious shortage of traditional methods considering only normal temperature working condition, and provides accurate theoretical basis for long-term safe design and evaluation of energy piles;

[0058] 5) Intelligent dynamic regulation: the intelligent control system equipped can monitor the key parameters such as pile body and soil temperature, fluid flow, etc. in real time, and dynamically adjust the flow and flow direction through the PLC controller, which not only can optimize the heat exchange performance and avoid extreme temperature damage, but also can provide real and continuous input parameters for the bearing capacity calculation model, to ensure that the system always runs in an efficient and safe state. BRIEF DESCRIPTION OF DRAWINGS

[0059] Fig. 1 is a structural schematic diagram of the high-efficiency heat exchange type energy pile according to the present application;

[0060] Fig. 2 is a flow chart of the bearing capacity calculation method of the high-efficiency heat exchange type energy pile according to the present application;

[0061] 1- energy pile body, 11- modified concrete, 12- groove, 2- heat exchange pipeline system, 21- main heat exchange pipe, 22- outer wall reinforcing 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

[0062] The specific embodiments of the present application will be 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.

[0063] The first aspect of the present application discloses a high-efficiency heat exchange type energy pile as shown in Fig. 1 The high-efficiency heat exchange type energy pile according to the present application comprises an energy pile body 1, a heat exchange pipeline system 2, a phase change energy storage system 3 and an intelligent control system 4.

[0064] The energy storage pile body 1 is poured by using modified concrete 11 with high thermal conductivity, 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 a specific implementation, the high thermal conductivity filler uses steel fibers or carbon nanotubes, wherein the length of the steel fiber is 10 to 20 mm, the diameter is 0.5 to 1 mm, the carbon nanotube has a tube diameter of 10 to 50 nm and a length of 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; the groove 12 is paved with a high-thermal-conductivity flexible heat-conducting sheet and is in contact with the soil around the pile, thereby enhancing the heat transfer efficiency and guiding the flow of underground water to form active convection heat transfer.

[0065] The heat exchange pipeline system 2 includes a main heat exchange pipe 21 and an outer wall reinforced structure 22; the main heat exchange pipe 21 is arranged in the reserved hole in the pile body, and is made of a single or multiple high-strength copper spiral pipes, the pitch P of the spiral pipe h changes with the depth P to meet the requirement of uniform heat exchange along the depth, and the expression of the pitch of the spiral pipe

[0066] is as follows: (1)

[0067] 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 a specific implementation, the initial pitch P 0 at the top of the pile is 200 mm, the attenuation coefficient of the pitch β p is 0.003 m -1 , the pile length L is 30 m, and then P ( h= 10m)=194mm, P ( h= 20m)=188mm.

[0068] The outer wall reinforced structure 22 is an integrally formed high-thermal-conductivity annular copper fin arranged on the outer wall of the main heat exchange pipe 21, 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 groove 12 on the outer side of the pile body, forming a continuous heat transfer channel and strengthening the heat diffusion efficiency.

[0069] The phase change energy storage system 3 is arranged above the energy storage pile body, adopts an encapsulated phase change material module, the module shell is made of stainless steel material, the inside is filled with paraffin-based composite phase change material, and is used for storing heat during intermittent operation of the heat pump and improving system energy storage density.

[0070] 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 regulating valve 43, a bidirectional circulating pump 44 and a PLC controller; the temperature sensor 41 is arranged at every 2 to 3 m along the depth, and the temperature of the pile body and the soil is monitored in real time; the flow sensor 42 is used for monitoring the flow at the inlet end of the main heat exchange pipe 21; the PLC controller controls the electric proportional flow regulating valve 43 and the bidirectional circulating pump 44 according to the monitoring data, and is used for realizing dynamic adjustment of fluid flow and flow direction.

[0071] The second aspect of the present application discloses a high-efficiency heat exchange type energy storage pile bearing capacity calculation method as shown in the formula (1). Fig. 2 The bearing capacity calculation method of the high-efficiency heat exchange type energy storage pile includes the following steps:

[0072] S1, collection of calculation parameters: collecting pile body design parameters, soil parameters, heat cycle working condition parameters and load parameters; the pile body design parameters include pile diameter D , pile length L , pile body equivalent cross-sectional area A p , geometric size of the groove, density of the pile body p c , equivalent thermal conductivity of the pile body l c , specific heat capacity of the pile body B c , thermal expansion coefficient of the pile body α c , elastic modulus of the pile body E c and normal temperature axial compressive strength f c0 ; wherein the pile body equivalent cross-sectional area A p has deducted the area occupied by the internal reserved hole, the equivalent thermal conductivity of the pile body l c is obtained by volume weighted average calculation of the thermal conductivities of the modified concrete, the fin and the heat conducting sheet; the soil parameters include the thermal conductivities of the soil layers l s , the density of the soil p s , the specific heat capacity of the soil B s , the cohesion of the soil at normal temperature C s0 、Normal temperature internal friction angle of soil f s0 Normal temperature shear modulus of soil G 0 Normal temperature pile tip resistance characteristic value of soil q p0 The thermal cycle working condition parameters are according to historical operation data of the intelligent control system in at least one complete thermal load cycle, and the measured temperature distribution along the depth direction of the pile body is extracted h T 1 ( h, t ) inlet and outlet temperatures of the heat exchange fluid T 2 Heat charging or discharging mode switching period t 3 ( h )The load parameters include the vertical static load transmitted to the pile top by the upper structure N .

[0073] S2, calculation of 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:

[0074] (2)

[0075] 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, p = p s , B = B s , l = l s ; T is the actual temperature, ℃; t is the time, s;

[0076] The parameters collected in step S1 are input into the finite element analysis software, a model is established, and the temperature change amount r , z , t ) of any point of the pile body is calculated AT ( r , z , t ); wherein, AT = T - T 0 , T 0 is the reference temperature and is takenT 0 = 20℃;

[0077] Considering that the vertical bearing capacity of the pile body is mainly controlled by the axial stress, the axial additional thermal stress s th of the pile body is expressed as follows:

[0078] (3)

[0079] wherein the numerical value of the axial additional thermal stress s th of the pile body is a positive value indicating tensile stress, and a negative value indicating compressive stress; in the specific implementation, for a typical point at a depth z = 15 m, the elastic modulus E c of the pile body is 35000 MPa, the temperature change 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:

[0080] (4)

[0081] that is, the axial additional thermal stress s th of the pile body at the typical point is 3.36 MPa, and the expansion causes compressive stress due to AT > 0.

[0082] S3, correction of thermal degradation effect:

[0083] Considering 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 ) when the temperature changes, according to the indoor direct shear test and shear modulus test at different temperatures, C s ( T ), f s ( T ), and G0 T The expression of the formula is as follows:

[0084] (5)

[0085] wherein, T is the actual temperature, ℃; T 0 is the reference temperature and takes T 0 = 20℃; 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]℃ -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.

[0086] S4, calculation of pile side friction and pile end resistance under thermal coupling:

[0087] The pile side friction is provided by the cohesion and 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 thermal expansion and contraction of the pile-soil interface need to be considered; the characteristic value of the pile side friction at the depth h and the temperature T is f s T , h The expression of the formula is as follows:

[0088] (6)

[0089] 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 influence of temperature change AT on the contact pressure of the pile-soil interface, which is obtained by fitting in more than 1000 times of thermal cycle shear test of the energy storage pile body, and the fitting expression is as follows:

[0090] ​​(7)

[0091] wherein, β k1 is the thermal correction coefficient of pile-soil interface k sf is the temperature influence coefficient of pile-soil interface, calibrated by indoor test, and the typical value range is β k1 ∈[-0.02,-0.01]℃ -1 ;

[0092] In the specific implementation, for a typical point at the depth z =15m, the lateral pressure coefficient of soil body K is 0.5, the temperature change AT is 8℃,

[0093] the thermal correction coefficient of pile-soil interface k sf is the temperature influence coefficient of pile-soil interface β k1 is-0.015, then the calculation of the thermal correction coefficient of pile-soil interface k sf is as follows:

[0094] (8)

[0095] The soil cohesion force considering temperature change C s ( T ) is 13.58kPa, the internal friction angle of soil body f s ( T ) is 31.5°, then the calculation of the characteristic value of pile side friction force f s ( T , h ) is as follows:

[0096] (9)

[0097] that is, the characteristic value of pile side friction force f s ( T , h ) at the typical point is 20.2 kPa;

[0098] The integral of the characteristic value of pile side friction force in the depth direction obtains the pile side friction force under thermal force coupling R s ( T ), as shown in the following expression:

[0099] (10)

[0100] The pile tip resistance is affected by the mechanical parameter decay caused by thermal degradation of the pile tip soil and the thermal-induced compressibility. The pile tip resistance R p ( T ) satisfies the following expression:

[0101] (11)

[0102] wherein, q p0 is the characteristic value of the pile tip resistance of the soil at normal temperature.

[0103] S5, limit state checking of bearing capacity:

[0104] 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-mechanical coupling R ( T ) is as follows:

[0105] (12)

[0106] 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, based on the number of thermal cycles N cycle calibration, the expression is:

[0107] (13)

[0108] 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];

[0109] 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 long-term decay coefficient s calculated is 0.86; the pile tip resistance R p ( T ) is 2595 kN, and 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, the equivalent cross-sectional area of the pile body A p is 0.48 m 2 , the total vertical bearing capacity of the pile body under thermal coupling R ( T ) is calculated according to formula (12) as shown in the following formula:

[0110] (14)

[0111] , i.e. the bearing capacity limit state calculation is satisfied.

[0112] S6, pile axial compressive strength limit state calculation:

[0113] 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 calculated, which is expressed as follows:

[0114] (15)

[0115] wherein, AT max is the maximum axial compressive stress of the pile body, kPa; f c ( T ) is the temperature-corrected axial compressive strength of the pile body concrete, which satisfies the following expression:

[0116] (16)

[0117] wherein, β f is the temperature influence coefficient of the axial compressive strength design value of the pile body concrete, which 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 is -0.015℃ -1 , the normal temperature axial compressive strength f c0 is 25 MPa, the temperature-corrected axial compressive strength of the pile body concrete is calculated as 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 calculated according to formula (15) as shown in the following formula:

[0118] (17)

[0119] That is, the axial compressive strength limit state checking requirements of the pile body are met.

[0120] The above is one or more embodiments of the application, which is described more specifically and in detail, but cannot be understood as a limitation on the scope of the patent of the application. It should be noted that for ordinary skilled persons in the art, without departing from the concept of the application, a number of modifications and improvements can be made, which are within the scope of protection of the application. Therefore, the protection scope of the patent of the application should be subject to the appended claims.

Claims

1. A high-efficiency heat exchange type energy storage pile, characterized in that, This includes energy storage piles, heat exchange pipeline systems, intelligent control systems, and phase change energy storage systems. The energy storage pile is constructed using high thermal conductivity modified concrete. The modified concrete uses ordinary Portland cement as the matrix and adds 5% to 15% by mass of high thermal conductivity filler, so that the overall thermal conductivity of the modified concrete is not less than 3.0 W / (m·K). Four to six axial grooves are evenly opened along the circumference on the outer side of the energy storage pile. The grooves are 30 to 50 mm deep and 40 to 60 mm wide. High thermal conductivity flexible heat-conducting sheets are laid in the grooves and keep in contact with the surrounding soil. The heat exchange piping system includes a main heat exchange tube and an outer wall reinforcement structure; the main heat exchange tube is installed in a pre-reserved channel inside the pile body, and adopts a single or multiple high-strength copper spiral tube with a pitch of P With depth h Variations to meet the requirement of uniform heat transfer along depth, pitch P The expression is shown in the following formula: in, P 0 represents the initial pitch at the top of the pile, with a value ranging from 150 to 300 mm; β p The pitch attenuation coefficient ranges from 0.001 to 0.005 m. -1 It needs to be calibrated on-site based on the geothermal gradient and the thermal properties of the soil and rock. The outer wall reinforcement structure is an integrally formed high thermal conductivity annular copper fin set on the outer wall of the main heat exchange tube. The fin thickness is 2 to 5 mm and the thermal conductivity is not less than 380 W / (m·k). The fin is connected to the groove on the outside of the pile body to form a continuous heat transfer channel and enhance the heat diffusion efficiency. The intelligent control system includes temperature sensors arranged in the pile body and surrounding soil, a flow sensor installed at the inlet of the main heat exchange pipe, an electric proportional flow regulating valve, a bidirectional circulation pump, and a PLC controller. The temperature sensors are arranged every 2 to 3 meters along the depth to monitor the temperature of the pile body and soil in real time. The flow sensor is used to monitor the flow rate at the inlet of the main heat exchange pipe. The PLC controller controls the electric proportional flow regulating valve and the bidirectional circulation pump according to the monitoring data to achieve dynamic adjustment of fluid flow rate and direction. The phase change energy storage system is located above the energy storage pile and adopts an encapsulated phase change material module. The outer shell of the module is made of stainless steel and the inside is 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 filler uses steel fibers or carbon nanotubes, wherein the steel fibers are 10 to 20 mm in length and 0.5 to 1 mm in diameter, and the carbon nanotubes are 10 to 50 nm in diameter and 1 to 5 μm in length.

3. A method for calculating the bearing capacity of a high-efficiency heat exchange type energy storage pile, characterized in that, The method applied to the high-efficiency heat exchange type energy storage pile as described in claim 1 includes the following steps: S1. Acquisition of calculation parameters: Acquire pile design parameters, soil parameters, thermal cycle parameters, and load parameters; the pile design parameters include pile diameter. D Pile length L equivalent cross-sectional area of ​​pile A p The geometric dimensions of the groove and the density of the pile body ρ c Equivalent thermal conductivity of pile body λ c Specific heat capacity of pile body B c The coefficient of thermal expansion of the pile body α c Elastic modulus of pile E c and room temperature axial compressive strength f c0 Among them, the equivalent cross-sectional area of ​​the pile body A p The area occupied by the internal reserved channels has been deducted, and the equivalent thermal conductivity of the pile body is... λ c The thermal conductivity was calculated by weighted average of the thermal conductivity of the modified concrete, fins, and heat-conducting fins; the soil parameters include the thermal conductivity of each soil layer. λ s Soil density ρ s Specific heat capacity of soil B s Soil cohesion at room temperature C s0 、 soil internal friction angle at room temperature φ s0 Soil shear modulus at room temperature Gs 0 Characteristic value of pile end resistance at normal temperature in soil q p0 The thermal cycle operating parameters are extracted along the pile depth based on historical operating data of the intelligent control system over at least one complete thermal load cycle. h Measured temperature distribution in the direction T 1 ( h,t ), heat exchange fluid inlet and outlet temperatures T 2 Heat charging or heat dissipation mode switching cycle t 3 ( h The load parameters include the vertical static load transferred from the superstructure to the pile top. N ; S2. Calculation of additional thermal stress in the pile: taking the central axis of the pile as... z Axis, radial direction r To establish a heat conduction control model in cylindrical coordinates, as shown in the following equation: When calculating the temperature field of the pile body, ρ=ρ c , B=B c , λ= λ c When calculating the soil temperature field, ρ=ρ s , B=B s , λ=λ s ; T The actual temperature is in °C. t For time, s; The parameters collected in step S1 are input into the finite element analysis software to build a model and calculate the values ​​at any point on the pile. r , z , t Temperature change ΔT ( r , z , t );in, ΔT=T - T 0 , T 0 For reference temperature and take T 0 = 20℃; Additional thermal stress in the axial direction of the pile σ th The expression is shown in the following formula: Among them, the axial additional thermal stress of the pile body σ th A positive value indicates tensile stress, and a negative value indicates compressive stress. S3. Correction for thermal degradation effect: Considering soil cohesion under temperature changes. C s ( T ), soil internal friction angle φ s ( T and soil shear modulus G 0 ( T The influence of ) was investigated based on indoor direct shear tests and shear modulus tests at different temperatures. C s ( T ), φ s ( T )and G 0 ( T The expression for ) is shown in the following formula: in, T The actual temperature is in °C. T 0 For reference temperature and take T 0 = 20℃; β c , β φ , β G These are the temperature influence coefficients for cohesion, angle of internal friction, and shear modulus, respectively. They need to be calibrated based on in-situ soil samples, and their typical range is [value missing]. β c ∈[-0.005,-0.002]℃ -1 , β φ ∈[-0.003,-0.001]℃ -1 , β G ∈[-0.004,-0.002]℃ -1 A negative temperature influence coefficient indicates that an increase in temperature has a deteriorating effect on the mechanical properties of the soil. S4. Calculation of pile side friction and pile end resistance under thermo-coupling: depth h Location, temperature T Characteristic value of pile side friction f s ( T , h The expression for ) is shown in the following formula: in, K This is the soil lateral pressure coefficient; k sf This is the thermal correction factor for the pile-soil interface, reflecting temperature changes. ΔT The influence on the contact pressure at the pile-soil interface was obtained by fitting the energy storage pile body through more than 1000 thermal cycle shear tests. The fitting expression is shown in the following formula: in, β k1 Thermal correction factor for pile-soil interface k sf The temperature influence coefficient, calibrated through indoor tests, typically ranges from [value missing]. β k1 ∈[-0.02,-0.01]℃ -1 ; Integrating the characteristic value of the pile side friction along the depth direction yields the pile side friction under thermo-coupling. R s ( T The expression is as follows: The pile end resistance is affected by the attenuation of mechanical parameters and thermal compressibility caused by the thermal degradation of the soil at the pile end. R p ( T It satisfies the following expression: in, q p0 The characteristic value of the pile end resistance of the soil at normal temperature; S5. Ultimate limit state verification of bearing capacity: Total vertical bearing capacity of the pile under thermal coupling. R ( T The expression for ) is shown in the following formula: in, N The vertical static load transferred from the superstructure to the pile top is expressed in kN. k r The safety factor is determined according to the "Code for Design of Building Foundations"; η This is the long-term decay coefficient, based on the number of thermal cycles. N cycle The calibration expression is: in, β η The attenuation coefficient, which is the long-term attenuation coefficient, is calibrated through indoor tests, and its typical value range is [value range missing]. β η ∈[0.01,0.03]; S6. Ultimate state verification of axial compressive strength of pile body: Verify whether the axial compressive strength of pile concrete under the combination of static load and thermal load meets the requirements, as shown in the following expression: in, σ max The maximum axial compressive stress in the pile body is expressed in kPa. f c ( T The axial compressive strength of the pile concrete after temperature correction is given by the following expression: in, β f The temperature influence coefficient for the design value of the axial compressive strength of the pile concrete is determined through indoor tests, and its typical range is [value missing]. β f ∈[-0.02,-0.01]℃ -1 .

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