Hydraulic compressed air energy storage system based on gas storage salt cavern and optimization design method

Abstract

The application discloses a kind of based on gas storage salt cave's hydraulic pressure gas energy storage system and optimization design method, comprising: setting the initial parameter of energy storage system;Considering the heat exchange between salt cave wall surface and fluid, the improved thermodynamic model is calculated;With the optimization goal of maximizing indicated cycle efficiency, based on the improved thermodynamic model, the optimal salt cave height-diameter ratio is calculated.The application can optimize the height-diameter ratio of salt cave, thereby significantly improving the electric cycle efficiency, energy storage density and operating income of system.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of compressed gas energy storage combined with hydroelectric power generation, and in particular to a hydro-compressed air energy storage system based on a gas storage salt cavern and an optimization design method. BACKGROUND

[0002] Compared with traditional compressed air energy storage schemes, hydro-compressed air energy storage has the advantages of high energy transfer efficiency, simple system structure, and low cost. However, current hydro-compressed air energy storage technology faces many problems in practical application, which not only limits its efficiency and reliability, but also increases the complexity and cost of system operation.

[0003] The structural design of the energy storage container of the hydro-compressed air energy storage system will affect heat exchange during operation, thereby affecting energy conversion efficiency and system stability. The compression and expansion process of gas is generally divided into isothermal, polytropic and adiabatic processes. Under the same flow conditions, the pressure change range and power of the isothermal process are the smallest, while under the same final pressure conditions, the energy stored by the isothermal process is the largest. Proper structural design of the energy storage container helps to enhance the heat exchange capacity of compressed gas with the outside world and achieve near-isothermal operation of the cycle process. On the contrary, improper design of the energy storage container makes the entire cycle process close to adiabatic, and under the same input power conditions, the output energy will be greatly reduced.

[0004] In the hydro-compressed air energy storage system based on a gas storage salt cavern, the salt cavern serves as an energy storage cavity, and its geometric structure directly affects the thermodynamic performance, structural stability and operating efficiency of the system. Among them, the aspect ratio of the salt cavern is one of the key parameters that determine the performance of the system. Different aspect ratios will directly affect the proportion of the contact area between compressed gas and the wall and liquid, thereby affecting the thermodynamic loss of the energy storage system. A smaller aspect ratio has a larger gas-liquid contact area ratio, better heat exchange effect, smaller temperature rise during gas compression, and more energy dissipated to the liquid during the static phase than a salt cavern with the same volume and a larger aspect ratio.

[0005] Furthermore, the energy transferred to the salt cavern walls in the form of heat can be fed back to the compressed gas during the expansion process, while the energy transferred to the liquid is directly released to the outside of the system as the liquid flows. Existing computational models treat the temperature of the salt cavern walls as a fixed constant, ignoring the heat storage capacity of the salt rock layer. These models assume that the external environment of the energy storage container remains in a fixed, isothermal state. This assumption has certain limitations when applied to salt cavern energy storage systems. It ignores the heat transfer and storage capacity of the salt cavern walls, which may result in insufficient consideration of heat changes, thereby affecting the energy efficiency during compression and expansion. During compression, the heat exchange between the rising gas temperature and the salt cavern walls is not effectively simulated, which may lead to excessive temperature rise or heat loss. During expansion, the salt cavern walls fail to effectively replenish the heat released by the internal gas, which may cause the gas temperature to drop too low, affecting expansion efficiency and system stability. Summary of the Invention

[0006] The existing technology has problems such as a large range of water head variation, easy corrosion of metal equipment and pipelines, and limited system efficiency. The present invention discloses a hydraulic compressed gas energy storage system based on a gas storage salt cavern and an optimization design method. It fully considers the multi-physical field coupling problems such as the hydraulic system, thermal system, gas-liquid interface mutual mass transfer, and structural mechanics involved in the cyclic operation of the compressed gas energy storage system, and can optimize the salt cavern height-to-diameter ratio, thereby significantly improving the system's electrical cycle efficiency, energy storage density, and operating benefits.

[0007] In order to achieve the above technical objectives, the technical solution adopted by the present invention is:

[0008] In a first aspect, the present invention discloses an optimization design method for a hydraulically compressed gas energy storage system based on a gas storage salt cavern, characterized in that the method comprises the following steps:

[0009] S1, setting the initial parameters of the energy storage system based on the capacity and power requirements of the grid, as well as the local environment and geological conditions;

[0010] S2, considering the heat exchange between the salt cavern wall and the fluid, calculates an improved thermodynamic model. The thermodynamic model is used to describe the heat exchange between gas and liquid during gas compression and expansion. Its input data is the liquid flow rate, and the output data is the temperature change of the gas and the temperature change of the liquid;

[0011] In S3, the average flow rate of the hydraulic gas energy storage system is set as the liquid flow rate, and the maximum withstand pressure value and minimum suction pressure are used as constraints to limit the operating pressure range of the system. With maximizing the indicated cycle efficiency as the optimization goal, the optimal salt cavern height-to-diameter ratio is calculated based on the improved thermodynamic model.

[0012] Furthermore, in step S1, the initial parameters of the energy storage system include constraint parameters and input parameters; the constraint parameters include the maximum withstand pressure value and the minimum suction pressure; the input parameters include the initial temperature, effective volume, effective storage capacity, minimum working time and average flow rate;

[0013] Among them, the maximum withstand voltage The maximum pressure that a salt cavern can withstand is used to limit the maximum working pressure of the salt cavern energy storage system; the minimum suction pressure The minimum working pressure for the pump-turbine unit, taking into account the suction height of the pump-turbine unit and the underground depth of the salt cavern, is used to specify the minimum pressure at which the pump-turbine unit can operate normally; effective volume The maximum volume that can be used to store gas in a salt cavern; effective storage capacity is the volume of liquid in the salt cavern that can be used to exchange energy; minimum working time The minimum time for the process of pumping energy storage or expansion energy release; the average flow rate It is calculated by dividing the effective storage capacity by the minimum working time and is used to evaluate the impact of the salt cavern height-to-diameter ratio on the energy storage system.

[0014] Furthermore, in step S2, the improved thermodynamic model is divided into a thermodynamic system of gas working fluid and a thermodynamic system of liquid working fluid with the salt cavern wall and the gas-liquid interface as boundaries;

[0015] The thermodynamic model of the gas system is as follows:

[0016] ;

[0017] Where, is the internal energy of the gas in the salt cavern over time changes, is the time step, is the mass of gas in the salt cavern, is the specific heat capacity of the gas, is the average temperature of the gas at the current moment; is the heat exchange between gas and liquid, is the heat transfer coefficient at the gas-liquid interface, is the area of ​​the gas-liquid interface, is the average temperature of the liquid at the current moment; is the heat exchange between gas and salt cavern wall, is the heat transfer coefficient between gas and salt cavern wall, is the contact area between gas and salt cavern wall, is the wall temperature of the salt cavern in contact with the gas at the current moment; is the rate at which work is transferred across the boundary, is the volume of the gas. The gas volume at the current moment is the sum of the gas volume change rate and the gas volume at the previous moment. The gas volume change rate over time is the volume flow rate of liquid replacing gas: , is the flow rate of the pump-turbine unit; assuming that the behavior of the gas conforms to the ideal gas state equation, the following equation is used to solve the gas pressure at each time step: , is the gas mass, is the gas constant;

[0018] The thermodynamic model of the liquid system is as follows:

[0019] ;

[0020] Where, is the internal energy of the liquid in the salt cavern over time changes, is the mass of the liquid in the salt cavern, is the specific heat capacity of the liquid; It is the heat exchange between liquid and gas; is the heat exchange between the liquid and the salt cavern wall, is the heat transfer coefficient between the liquid and the salt cavern wall, is the contact area between the liquid and the salt cavern wall, is the wall temperature of the salt cavern in contact with the liquid at the current moment; is the energy transfer rate as the liquid flows into the control volume, is the mass flow rate of the liquid, The temperature at the depth of the underground salt cavern.

[0021] Furthermore, in the improved thermodynamic model, the temperature field distribution of the salt cavern wall is The unsteady one-dimensional heat transfer formula is used for calculation:

[0022] ;

[0023] In the formula is time, the distance between the working medium and the contact surface of the salt cavern The temperature at the place where the gas working medium directly contacts the salt cavern wall is , then the wall temperature of the salt cavern in contact with the gas is , the wall temperature of the salt cavern in contact with the liquid ; is the thermal diffusivity of the object.

[0024] Further, in step S3, the process of calculating the optimal salt cavern aspect ratio based on the improved thermodynamic model with the optimization goal of maximizing the indicated cycle efficiency includes the following steps:

[0025] S31, set the value range of the salt cavern aspect ratio, and set the minimum aspect ratio in the value range as the initial salt cavern aspect ratio;

[0026] S32, input the average flow of the hydraulic compressed energy storage system as the liquid flow, and calculate the gas volume change and liquid volume change in the salt cavern in the current time step;

[0027] S33, input the gas volume change and liquid volume change obtained in step S32 into the improved thermodynamic model respectively, obtain the change of the gas temperature, add the gas temperature change per time step to the gas temperature in the current time step to update the gas temperature in the next time step;

[0028] S34, input the gas volume change in the current time step obtained in step S32 and the gas temperature in the next time step obtained in step S33 into the ideal gas state equation to update the gas pressure, and judge whether the gas pressure reaches the stop pressure, wherein the stop pressure in the charging process is the maximum pressure value, and the stop pressure in the discharging process is the minimum suction pressure, if yes, go to step S35, otherwise, increase a time step and repeat steps S32 to S34;

[0029] S35, calculate the indicated cycle efficiency under the corresponding working condition by combining the process data including the gas pressure, the temperature changes of the gas and the liquid;

[0030] S36, gradually increase the salt cavern aspect ratio by a preset increment each time, take the aspect ratio after each increase as a new input parameter, repeat steps S31 to S35 until the indicated cycle efficiency of the new iteration is less than 1% higher than that of the previous iteration, and consider that the system performance has converged, and take the aspect ratio at this time as the optimal salt cavern aspect ratio.

[0031] Further, in step S3, the calculation formula of the indicated cycle efficiency is:

[0032] ;

[0033] In the formula, is the isothermal compression efficiency, is the isothermal expansion efficiency;

[0034] The isothermal compression efficiency is the ratio of the theoretical power consumption of the ideal isothermal compression process to the theoretical power consumption of the actual compression process .

[0035] ;

[0036] Where, is the initial temperature of the gas compression, is the initial pressure of gas compression, is the end pressure of gas compression, is the pressure of the gas at each time step;

[0037] The isothermal expansion efficiency is the theoretical output work of the actual expansion process The theoretical output work of the ideal isothermal expansion process Ratio:

[0038] ;

[0039] Where, is the initial temperature of the gas expansion, is the initial pressure of the gas expansion, is the pressure at which gas expansion ends.

[0040] Furthermore, the optimization design method further comprises the following steps:

[0041] Step A: Establish a computational fluid dynamics simulation model of the pump-turbine unit of the hydraulic compressed air energy storage system, and fit the hydraulic efficiency calculation formula of the pump-turbine unit to describe the relationship between the flow rate and hydraulic efficiency of the pump-turbine unit;

[0042] Step B: setting the initial speed range of the pump-turbine unit based on the determined optimal salt cavern aspect ratio and the determined operating pressure range of the gas in the salt cavern; using the minimum suction pressure as the initial pressure of the simulation operation, and the maximum withstand pressure value as the end pressure of the simulation operation; running from the initial pressure to the end pressure, and then returning from the end pressure to the initial pressure constitutes a complete cycle of the energy storage system's charge and discharge process;

[0043] Step C: Substituting the initial pump-turbine unit speed and working head into the hydraulic model for different working pressures of the gas in the salt cavern, and calculating the flow rate and corresponding hydraulic efficiency under the corresponding working conditions; by adjusting the speed of the pump-turbine unit, calculating the optimal speed range under the corresponding working conditions, so that the pump-turbine unit is in a high-efficiency operating state; wherein the high-efficiency range refers to the hydraulic efficiency of the pump or turbine in the operating mode reaching above 90%;

[0044] Step D, repeat step C, analyze the optimal speed range corresponding to different working pressures within the working pressure range, and comprehensively analyze the results to determine the optimal speed range that makes the pump-turbine unit operate in a high-efficiency state within the entire pressure range.

[0045] Furthermore, the pump-turbine unit adopts a variable speed pump-turbine unit, which adjusts the speed of the permanent magnet motor unit according to the pressure difference between the inlet and outlet ends, changes the speed triangle of the hydraulic machinery, and makes the water flow entering the impeller tend to be stable;

[0046] The hydraulic model is:

[0047] ;

[0048] Where, is the flow rate of the pump-turbine unit, is the water pressure head; is the operating characteristic constant of the pump-turbine unit, which is determined by the design parameters and operating speed of the pump-turbine unit;

[0049] The calculation formula of the hydraulic efficiency is:

[0050] ;

[0051] ;

[0052] Where, is the efficiency of the pump-turbine unit in pumping mode, It is the efficiency of the pump-turbine unit in the turbine working mode. The two are collectively referred to as hydraulic efficiency.

[0053] In a second aspect, the present invention discloses a hydraulically compressed gas energy storage system based on a gas storage salt cavern, the system comprising a gas system, a liquid system, a water pump turbine unit and a pipeline system;

[0054] The gas system contains compressible gas as the working fluid for energy storage, which is stored in an underground salt cavern and equipped with a gas replenishment device. The liquid system contains incompressible liquid as the working fluid for energy transmission, which is pressed into the salt cavern by a water pump. The compressible gas and incompressible brine are both stored in the underground salt cavern, which adopts an L-shaped connecting well structure with air intake at both ends and drainage in the middle.

[0055] The pump-turbine unit includes a variable-speed pump-turbine unit, a frequency converter, and a permanent magnet motor; the pipeline system includes an air supply pipeline and a liquid transmission pipeline, wherein the liquid transmission pipeline is the main transmission channel for liquid in the open water pool on the ground and the closed salt cavern underground;

[0056] The salt cavern height-to-diameter ratio is designed using the method described above.

[0057] Furthermore, the incompressible liquid is brine.

[0058] Compared with the prior art, the present invention has the following beneficial effects:

[0059] First, the hydraulic pressure gas energy storage system and optimization design method based on the gas storage salt cavern of the present invention fully consider the multi-physics field coupling problems of the hydraulic system, thermal system, gas-liquid interface mass transfer and salt cavern mechanical properties, comprehensively evaluate multiple parameters such as flow rate, pressure, temperature, heat exchange, etc., and can optimize the height-to-diameter ratio of the salt cavern to ensure the design scheme has optimal performance.

[0060] Second, the hydraulic pressure gas energy storage system and optimization design method based on gas storage salt caverns of the present invention propose an improved thermodynamic model to address the defect that the existing model ignores the heat transfer and storage capacity of the salt cavern wall, which may lead to insufficient consideration of heat changes and thus affect the energy efficiency during compression and expansion. By optimizing the solution process of the salt cavern wall temperature, the shortcomings of the existing model are overcome.

[0061] Third, the hydraulic compressed gas energy storage system based on gas storage salt caverns and the optimization design method of the present invention improve the energy conversion efficiency. The present invention adopts a variable speed water pump turbine unit to effectively cope with large changes in water head, reduce impact loss and eddy current loss, and ensure that the hydraulic machinery is always in an efficient operating state, thereby significantly improving the energy conversion efficiency of the system.

[0062] Fourth, the hydraulically compressed gas energy storage system and optimized design method based on gas storage salt caverns of the present invention reduce corrosion and gas loss problems, use compressed gas that does not contain oxygen molecules as the energy storage medium, avoid the corrosion problem caused by oxygen dissolving in salt brine in ordinary compressed air energy storage, and greatly extend the service life of the energy storage equipment; for example, compressed natural gas does not contain oxygen molecules, which reduces gas dissolution and loss, eliminates the need for frequent gas replenishment, and reduces operating costs. BRIEF DESCRIPTION OF THE DRAWINGS

[0063] Figure 1 This is a schematic structural diagram of the hydraulic compressed gas energy storage system based on gas storage salt caverns of the present invention;

[0064] Figure 2 A schematic diagram of the multi-physics field coupling calculation of the present invention;

[0065] Figure 3 This is a flow chart of the optimization design method of the hydraulic compressed gas energy storage system based on gas storage salt caverns of the present invention;

[0066] Explanation of the accompanying numbers: 1 is gas replenishment equipment, 2 is gas supply pipe, 3 is natural gas, 4 is brine, 5 is brine injection / discharge pipeline, 6 is water pump turbine unit, 7 is water tank, 8 is permanent magnet motor, 9 is frequency converter, 10 is power system, 11 is intelligent control terminal, 12 is large salt cavern, and 13 is small salt cavern. DETAILED DESCRIPTION

[0067] The embodiments of the present invention are described in further detail below with reference to the accompanying drawings.

[0068] The present invention discloses a hydraulically compressed gas energy storage system utilizing a gas storage salt cavern, comprising: a gas system, a liquid system, a water pump and water turbine unit, and a pipeline system; the gas system comprises compressible gas as a working medium for energy storage, which is stored in an underground salt cavern and is equipped with a gas supply device; the liquid system comprises an incompressible liquid, the main component of which is brine, which is used as a working medium for energy transmission and is pressed into the salt cavern by a water pump; both the compressible gas and the incompressible brine are stored in the underground salt cavern, and the salt cavern adopts an L-shaped connecting well structure with air intake at both ends and drainage in the middle, which is conducive to continuing water injection, brine extraction, and cavity expansion in the salt cavern during the interval between gas injection and extraction; the water pump and water turbine system comprises a water pump and water turbine unit with variable speed operation, a frequency converter, and a permanent magnet motor; the pipeline system comprises an air supply pipeline and a liquid transmission pipeline, wherein the liquid transmission pipeline is the main transmission channel for liquid in an open water pool on the ground and an underground closed salt cavern.

[0069] The compressible gas used in the present invention includes hydrogen, natural gas or other gases that can be stored in salt caverns. For ease of description, this embodiment only uses natural gas as an example for illustration.

[0070] The hydraulic pressure gas energy storage system is mainly divided into two working processes, including charging process and discharging process; the charging process occurs during the trough period of the power grid, converting the power grid energy into the pressure potential energy of natural gas and storing it in the salt cavern; the discharging process occurs during the peak period of the power grid, releasing the energy stored in the salt cavern.

[0071] The charging process of the hydraulic compressed air energy storage system is powered by the grid. A frequency converter adjusts the voltage and frequency, controlling the speed and power output of the permanent magnet motor. The motor generates mechanical energy through rotation, driving a water pump to pressurize brine from the open pool into the salt cavern through a pipe, completing the energy storage process by converting electrical energy into mechanical and potential energy. The discharge process of the hydraulic compressed air energy storage system is powered by the expansion of compressed natural gas, pushing water out of the salt cavern and driving the turbine. The turbine's mechanical energy is converted into electrical energy through a connected permanent magnet generator, completing the conversion of mechanical energy into electrical energy and completing energy feedback.

[0072] During the charging process of the hydraulic compressed air energy storage system, the pump-turbine unit operates in pump mode, with the pump flow rate regulated by the grid dispatch and the pump speed determined by the pressure difference between the inlet and outlet. During the discharging process of the hydraulic compressed air energy storage system, the pump-turbine unit operates in turbine mode, with the turbine flow rate regulated by the grid dispatch demand and the turbine speed determined by the pressure difference between the inlet and outlet.

[0073] During the charging process, the kinetic energy of the hydraulic machinery is converted into the pressure potential energy of natural gas, the internal energy of the compressed natural gas increases, and the pressure and temperature of the gas working fluid rise. Due to the temperature difference, the heat of the natural gas is transferred to the salt cavern walls and liquid, and stored in the salt cavern soil layer and liquid working fluid. During the discharge process, the pressure potential energy of the compressed natural gas is converted into the kinetic energy of the hydraulic machinery, and the internal energy of the compressed natural gas gradually decreases. The pressure and temperature of the gas working fluid gradually decrease as the flow continues. When the temperature of the salt cavern walls and liquid is higher than that of the compressed natural gas, the heat stored in the salt cavern walls and liquid is transferred to the compressed natural gas.

[0074] like Figure 1 As shown, this embodiment specifically provides a hydraulic pressure gas energy storage system based on energy storage salt caverns. First, natural gas 3 is injected into salt caverns 12 and 13 through gas supply pipe 2 by gas supply equipment 1 for storage.

[0075] During the compression energy storage phase, power system 10 provides electricity, and inverter 9 adjusts the frequency and speed of permanent magnet motor 8, driving pump-turbine unit 6 in forward rotation. This pumps brine 4 from reservoir 7 into salt caverns 12 and 13 through brine injection channel 5. As brine 4 is pumped into salt caverns 12 and 13, it further compresses the stored natural gas 3, storing it at a higher pressure, completing the conversion of electrical energy into gas pressure potential energy.

[0076] During the expansion and energy release phase of the system, the compressed natural gas 3 expands, and at the same time, the water pump turbine unit 6 switches to turbine mode, using the pressure potential energy stored in the salt cavern to drive the water pump turbine unit 6 to perform work, driving the permanent magnet motor 8 to generate electricity. After frequency modulation by the inverter 9, the electricity is connected to the power system 10 for transmission.

[0077] The intelligent control terminal 11 adjusts the flow rate and speed of the pump-turbine unit 6 according to the grid dispatching requirements.

[0078] The system utilizes salt caverns as compression chambers, compressed natural gas as the energy storage medium, and brine as the energy transfer medium. This technology can be developed for applications in salt caverns already used for gas storage, sharing the functions of gas storage and compressed gas energy storage within the same salt well, thereby achieving efficient recycling of salt caverns. Compared to hydraulic compressed gas energy storage systems based on freshwater working fluids, the use of brine working fluids, combined with the salt chemical industry's own salt mining and cavity creation processes, will not cause changes to underground salt caverns during long-term operation. Its technical development is low-difficulty and requires minimal investment. This system uses compressed natural gas as the energy storage medium. Natural gas is not easily soluble in water and does not contain oxygen molecules. This solves the corrosion problem caused by oxygen dissolving in brine in conventional compressed air energy storage and eliminates the need for frequent gas replenishment.

[0079] This paper takes maximizing the indicated cycle efficiency as the ultimate optimization goal and proposes an optimization design method for a hydraulically compressed gas energy storage system based on a gas storage salt cavern. The core optimization parameter of this method is the salt cavern height-to-diameter ratio, which provides guidance for the direction of salt cavern expansion and cavity creation. Figure 2 and Figure 3 , the optimization design method specifically includes the following steps:

[0080] Step S1: Set initial system parameters. Based on the grid's capacity and power requirements for the energy storage system, as well as the local environment and geological conditions, the initial parameters of the energy storage system are set. These parameters primarily include constraint parameters and input parameters. Constraint parameters include the maximum withstand voltage and minimum suction pressure; input parameters include initial temperature, effective volume, effective storage capacity, minimum operating time, and average flow rate.

[0081] The constraint parameters in step S1 include the maximum pressure resistance value and the minimum suction pressure, and the input parameters include the initial temperature, effective volume, effective storage capacity, minimum working time and average flow rate. The maximum pressure resistance value is the maximum pressure that the salt cavern can withstand. It is used to limit the maximum working pressure of the salt cavern energy storage system; the minimum suction pressure is the minimum working pressure considering the suction height of the pump turbine unit and the underground depth of the salt cavern. It indicates the minimum pressure at which the pump turbine unit can work normally; the effective volume is the maximum volume that can be used to store gas in the salt cavern. It is indicated that this parameter, as an input parameter, determines the total amount of gas that can be stored in the energy storage system; the effective storage capacity is the liquid volume in the salt cavern that can be used to exchange energy, which is expressed as Indicates that this parameter is the liquid that can be used for energy exchange in theory, which affects the charging and discharging capacity of the system; the minimum working time is the minimum time of the pumping energy storage or expansion energy release process, that is, the charging or discharging process is not less than this time. Indicates that this parameter, as an input parameter, determines the minimum time requirement during the charging or discharging process, ensuring that the energy storage system can complete charging or discharging within the set time, while avoiding excessive energy exchange in a short period of time that leads to reduced efficiency or system instability; the average flow rate is the effective storage capacity divided by the minimum working time, expressed as Indicates that this parameter, as an input parameter, is the limit condition of the energy storage system and the minimum working time of the liquid in a single process of the energy storage system. The average flow rate within can be used as a fixed working condition for the reference working condition for evaluating the optimization design method of the energy storage system. In the calculation of the effective storage capacity, the compression and expansion cycle is considered to be an isothermal process, so the effective storage capacity is for:

[0082] ;

[0083] In the formula The average flow rate is the limit condition and can be used as a fixed parameter to evaluate the impact of the salt cavern height-diameter ratio on the energy storage system. It can be calculated as:

[0084] ;

[0085] The average flow rate is actually the maximum average flow rate that the system can withstand within the minimum working time. When the actual flow rate is less than this value, or the cycle process is close to an isothermal process, the time of a single working process of the energy storage system can be further improved.

[0086] Step S2, Calculating a Thermodynamic Model: The thermodynamic model describes the heat exchange between gas and liquid during gas compression and expansion. Its input is the liquid flow rate, and its output is the temperature change of the gas and liquid. Liquid flow not only affects the volume of the gas but also removes or transfers heat, thereby changing the temperature of the gas and liquid. Using the law of conservation of energy, the thermodynamic model calculates the temperature change of the gas and liquid during compression and expansion, further determining the isothermal compression and expansion efficiencies of the gas.

[0087] The thermodynamic calculation model is divided into a thermal system of gas working fluid and a thermal system of liquid working fluid, with the salt cavern wall and the gas-liquid interface as the boundaries.

[0088] The thermodynamic model of the gas system is as follows:

[0089] ;

[0090] Where, is the internal energy of the gas in the salt cavern over time changes, is the time step, is the mass of gas in the salt cavern, is the specific heat capacity of the gas, is the average temperature of the gas at the current moment; is the heat exchange between gas and liquid, is the heat transfer coefficient at the gas-liquid interface, is the area of ​​the gas-liquid interface, is the average temperature of the liquid at the current moment; is the heat exchange between gas and salt cavern wall, is the heat transfer coefficient between gas and salt cavern wall, is the contact area between gas and salt cavern wall, is the wall temperature of the salt cavern in contact with the gas at the current moment; is the rate at which work is transferred across the boundary, is the volume of the gas. The gas volume at the current moment is the sum of the gas volume change rate and the gas volume at the previous moment. The gas volume change rate over time is the volume flow rate of liquid replacing gas: , is the flow rate of the pump-turbine unit; assuming that the behavior of the gas conforms to the ideal gas state equation, the following equation is used to solve the gas pressure at each time step: , is the gas mass, is the gas constant;

[0091] The thermodynamic model of the liquid system is as follows:

[0092] ;

[0093] Where, is the internal energy of the liquid in the salt cavern over time changes, is the mass of the liquid in the salt cavern, is the specific heat capacity of the liquid; It is the heat exchange between liquid and gas; is the heat exchange between the liquid and the salt cavern wall, is the heat transfer coefficient between the liquid and the salt cavern wall, is the contact area between the liquid and the salt cavern wall, is the wall temperature of the salt cavern in contact with the liquid at the current moment; is the energy transfer rate as the liquid flows into the control volume, is the mass flow rate of the liquid, The temperature at the depth of the underground salt cavern.

[0094] The present invention proposes an improved thermodynamic model, which overcomes the shortcomings of the existing model by optimizing the solution process of the salt cavern wall temperature. Specifically, the present invention introduces a non-steady-state one-dimensional heat transfer formula, takes into account the heat exchange between the salt cavern wall and the fluid, and accurately describes the heat diffusion process of the salt cavern soil during compression, as well as the compensation of the salt cavern soil for the heat of the internal gas during expansion. The aforementioned improved thermodynamic model effectively improves the accuracy of the heat exchange model and ensures that the salt cavern energy storage system can achieve a more stable and efficient thermodynamic process under different operating conditions. Among them, in the improved thermodynamic model, the temperature of the salt cavern wall is calculated using the non-steady-state one-dimensional heat transfer formula:

[0095] ;

[0096] In the formula is time, the distance between the working medium and the contact surface of the salt cavern The temperature at the place where the gas working medium directly contacts the salt cavern wall is , then the wall temperature of the salt cavern in contact with the gas is , the wall temperature of the salt cavern in contact with the liquid ; is the thermal diffusivity of the object. Here, the temperature field distribution of the salt cavern wall is calculated, and the temperature of the gas or liquid system in contact with the wall is calculated. Only the temperature at the first node of the temperature field at the current moment is extracted.

[0097] The gas volume at the current moment is the sum of the gas volume change rate and the gas volume at the previous moment, where the gas volume change rate over time is the volume flow rate of liquid replacing gas:

[0098] ;

[0099] The gas is assumed to behave according to the ideal gas equation of state, which is solved for the pressure of the gas at each time step:

[0100] ;

[0101] Where, is the gas mass, is the gas constant.

[0102] Isothermal compression efficiency Defined as the theoretical power consumption of an ideal isothermal compression process Theoretical power consumption compared to actual compression process Ratio:

[0103] ;

[0104] Where, is the initial temperature of the gas compression, is the initial pressure of gas compression, is the pressure at which gas compression ends.

[0105] Isothermal expansion efficiency is the theoretical output work of the actual expansion process The theoretical output work of the ideal isothermal expansion process Ratio:

[0106] ;

[0107] Where, is the initial temperature of the gas expansion, is the initial pressure of the gas expansion, is the pressure at which gas expansion ends.

[0108] Step S3, salt cavern height-to-diameter ratio optimization: The average flow rate of the hydraulic gas energy storage system is set as the liquid flow rate as the input condition; the maximum withstand pressure value and the minimum suction pressure are used as constraints to limit the system's operating pressure range; the main goal is to maximize the indicated cycle efficiency; and the salt cavern height-to-diameter ratio is the parameter to be optimized. The salt cavern height-to-diameter ratio optimization process is as follows:

[0109] S31, setting a value range of the salt cavern height-to-diameter ratio, and setting the minimum height-to-diameter ratio within the value range as the initial salt cavern height-to-diameter ratio;

[0110] S32, using the average flow rate of the hydraulic pressure gas energy storage system as the liquid flow rate input, and calculating the gas volume change and liquid volume change in the salt cavern at the current time step;

[0111] S33, using the gas volume change and liquid volume change obtained in step S32 as inputs, respectively, and substituting them into the improved thermodynamic model to obtain the change in gas temperature, and adding the gas temperature change per unit time step to the gas temperature of the current time step to update the gas temperature of the next time step;

[0112] S34: Substitute the gas volume change at the current time step obtained in step S32 and the gas temperature at the next time step obtained in step S33 as input into the ideal gas state equation, update the gas pressure, and determine whether the gas pressure has reached the stop pressure. The stop pressure during the charging process is the maximum withstand pressure value, and the stop pressure during the discharging process is the minimum suction pressure. If so, proceed to step S35; otherwise, add one time step and repeat steps S32 to S34.

[0113] S35, combining process data including gas pressure and temperature changes of gas and liquid, and calculating the indicated cycle efficiency under corresponding working conditions;

[0114] In step S36, the salt cavern aspect ratio is gradually increased by a preset increment. The aspect ratio after each increase is used as a new input parameter. Steps S31 to S35 are repeated until the indicated cycle efficiency of the new iteration is less than 1% higher than that of the previous iteration. The system performance is considered to have converged, and the aspect ratio at this time is used as the optimal salt cavern aspect ratio.

[0115] The aforementioned indicative cycle efficiency is the isothermal compression efficiency multiplied by the isothermal expansion efficiency, and the calculation formula is as follows:

[0116] .

[0117] As a preferred example, to address the problem of large head variations in salt cavern hydraulic pressure gas energy storage systems, the present invention utilizes a variable-speed pump-turbine unit. This pump-turbine unit automatically adjusts its speed based on changes in head. Adjusting the speed alters the velocity triangle of the hydraulic machinery, ensuring smooth and fluent water flow into the impeller, reducing blade impact losses and eddy current losses, and thus ensuring that the hydraulic machinery consistently operates at high efficiency. The introduction of a variable-speed pump-turbine unit not only improves the operating efficiency of the hydraulic machinery under large head variations but also enhances the energy conversion efficiency and stability of the energy storage system. The variable-speed pump-turbine unit is connected to a permanent magnet motor unit via a coupling, and the motor stator winding is connected to the power grid via a full-power converter. During variable-speed power generation, the variable-frequency AC power generated by the permanent magnet motor is rectified to DC power by the converter. This DC power is then inverted to AC power with the same frequency and phase as the grid based on grid signals. During variable-speed pumped hydropower storage, grid power is converted to DC by a converter, then inverted by a generator-side converter into adjustable AC power to drive the motor at variable speeds. The speed of the permanent magnet motor is controlled by the pressure differential between the inlet and outlet of the pump-turbine unit, ensuring efficient operation while matching the pump-turbine unit's speed under variable head conditions.

[0118] Based on the above-mentioned pump-turbine unit, a hydraulic model is constructed. The hydraulic model describes the dynamic change process of the liquid in the pipeline and hydraulic machinery in the energy storage system. The input data of the hydraulic model is the liquid flow rate, and the output data is the gas volume change and gas pressure change. In order to ensure the working efficiency of the variable speed pump-turbine unit, the flow rate of the pump-turbine unit is With water pressure head The hydraulic model is proportional to the square root of .

[0119] ;

[0120] Where, is the flow rate of the pump-turbine unit, is the water pressure head; It is the operating characteristic constant of the pump-turbine unit, which is determined by the design parameters and operating speed of the pump-turbine unit.

[0121] After determining the salt cavern's height-to-diameter ratio, dynamic simulation can be used to further optimize the pump-turbine unit's operating parameters. The pressure within the salt cavern is the pump-turbine unit's operating pressure, which varies with the input and output of liquid. This pressure directly impacts the hydraulic machinery's operating efficiency. By adjusting the pump-turbine unit's speed range, the system's flow and pressure can be dynamically adjusted to keep the unit operating within a high-efficiency range. The specific optimization process is as follows:

[0122] Step A: Establish a computational fluid dynamics simulation model of the pump-turbine unit and obtain a hydraulic efficiency calculation formula of the pump-turbine unit by fitting to describe the relationship between the flow rate and hydraulic efficiency of the pump-turbine unit. The calculation formula of the hydraulic efficiency is:

[0123] ;

[0124] ;

[0125] Where, is the efficiency of the pump-turbine unit in pumping mode; It is the efficiency of the pump-turbine unit in the turbine working mode. The two are collectively referred to as hydraulic efficiency.

[0126] In step B, the initial speed range of the pump-turbine unit is set based on the determined optimal salt cavern height-to-diameter ratio and the determined operating pressure range of the gas in the salt cavern; the minimum suction pressure is used as the initial pressure of the simulation operation, and the maximum withstand pressure value is used as the end pressure of the simulation operation. The operation from the initial pressure to the end pressure and then from the end pressure back to the initial pressure constitutes a complete cycle of the energy storage system's charge and discharge process.

[0127] In step C, for different working pressures of the gas in the salt cavern, the initial pump-turbine unit speed and working head are substituted into the hydraulic model to calculate the flow rate and corresponding hydraulic efficiency under the corresponding working conditions; by adjusting the speed of the pump-turbine unit, the optimal speed range under the corresponding working conditions is calculated to put the pump-turbine unit in a high-efficiency operating state; wherein, the high-efficiency range refers to the hydraulic efficiency of the pump or turbine in the operating mode reaching more than 90%.

[0128] Step D, repeat step C, analyze the optimal speed range corresponding to different working pressures within the working pressure range, and comprehensively analyze the results to determine the optimal speed range that makes the pump-turbine unit operate in a high-efficiency state within the entire pressure range.

[0129] The optimization design method of the present invention comprehensively considers the multi-physics coupling issues involved in the operation of hydraulic compressed gas energy storage systems, including the hydraulic system, thermal system, gas-liquid interface mass transfer, and structural mechanics during system modeling. These considerations include the heat transfer characteristics of gas and liquid working fluids during charging and discharging, the transient processes of the unsteady hydraulic system, the heat and mass transfer processes at the gas-liquid interface, and the maximum stress that the salt cavern can withstand. This optimization design method ensures that the maximum operating stress of the salt cavern remains within a safe range under various grid dispatch scenarios, maximizing the operational benefits of the energy storage system.

[0130] The following is an example of system optimization design in practical application.

[0131] In the rock salt mining area, it is planned to develop 450,000 m3 at a depth of 850 m underground. 3 The salt cavern is used as the compression chamber of the hydraulic pressure gas energy storage system. The upper limit of the pressure that the salt cavern can withstand is 20 Considering the suction height of the pump turbine unit, the lower limit of pressure is 9.5 The grid's scheduling requirements for the energy storage system are that the single working process of energy storage or release is no less than 2 hours, and the intermediate energy storage static time is no less than 5 hours. By executing the system optimization design method involved in the present invention, it is determined that the maximum average flow of the designed energy storage system can reach 26m 3 / s, and then different aspect ratio schemes were compared. The aspect ratio was optimized from the original design of 1:1 to 5:1, with power density increased by 11%, energy storage density increased by 4%, and indicated cycle efficiency increased by 2%.

[0132] Although the preferred embodiments of the present application have been described, those skilled in the art may make additional changes and modifications to these embodiments once they have learned the basic creative concept. Therefore, the appended claims are intended to be interpreted as including the preferred embodiments and all changes and modifications that fall within the scope of the present application.

[0133] Obviously, those skilled in the art may make various changes and modifications to this application without departing from the spirit and scope of this application. Thus, if these modifications and variations of this application fall within the scope of the claims of this application and their equivalents, this application is intended to include these modifications and variations.

Claims

1. An optimization design method for a hydraulically compressed gas energy storage system based on a gas storage salt cavern, characterized in that: The method The following steps are involved: S1, setting the initial parameters of the energy storage system based on the capacity and power requirements of the grid, as well as the local environment and geological conditions; S2, considering the heat exchange between the salt cavern wall and the fluid, calculates an improved thermodynamic model. The thermodynamic model is used to describe the heat exchange between gas and liquid during gas compression and expansion. Its input data is the liquid flow rate, and the output data is the temperature change of the gas and the temperature change of the liquid; S3, setting the average flow rate of the hydraulic compressed gas energy storage system as the liquid flow rate, and using the maximum pressure resistance value and the minimum suction pressure as constraints to limit the operating pressure range of the system; With the maximization of indicated cycle efficiency as the optimization goal, the optimal salt cavern height-to-diameter ratio was calculated based on the improved thermodynamic model. In step S2, the improved thermodynamic model is divided into a thermodynamic system of gas working fluid and a thermodynamic system of liquid working fluid with the salt cavern wall and the gas-liquid interface as boundaries; The thermodynamic model of the gas system is as follows: Where, is the change of internal energy of gas in salt cave with time t, dt is the time step, m G is the mass of gas in the salt cavern, c G is the specific heat capacity of the gas, T G is the average temperature of the gas at the current moment; -h G，L A G，L (T G -T L ) is the heat exchange between gas and liquid, h G，L is the heat transfer coefficient of the gas-liquid interface, A G，L is the area of ​​the gas-liquid interface, T L is the average temperature of the liquid at the current moment; -h Wall，G A Wall,G (T G -T Wall，G ) is the heat exchange between gas and salt cavern wall, h Wall，G is the heat transfer coefficient between gas and salt cavern wall, A Wall，G is the contact area between gas and salt cavern wall, T Wall，G is the wall temperature of the salt cavern in contact with the gas at the current moment; is the rate of work transfer through the boundary, V G is the volume of the gas. The gas volume at the current moment is the sum of the gas volume change rate and the gas volume at the previous moment. The gas volume change rate over time is the volume flow rate of liquid replacing gas: Q(t) is the flow rate of the pump-turbine unit. Assuming that the gas behaves in accordance with the ideal gas state equation, the following equation is used to solve the gas pressure at each time step: m G is the gas mass, R G is the gas constant; The thermodynamic model of the liquid system is as follows: Where, is the change of internal energy of the liquid in the salt cave with time t, m L is the mass of the liquid in the salt cavern, c L is the specific heat capacity of the liquid; h G，L A G，L (T G -T L ) is the heat exchange between liquid and gas; -h Wall,L A Wall，L (T L -T Wall,L ) is the heat exchange between the liquid and the salt cavern wall, h Wall，L is the heat transfer coefficient between liquid and salt cavern wall, A Wall,L is the contact area between the liquid and the salt cavern wall, T Wall,L is the wall temperature of the salt cavern in contact with the liquid at the current moment; is the energy transfer rate as the liquid flows into the control volume, is the mass flow rate of the liquid, T cavern is the temperature at the depth of the underground salt cave; In step S3, the process of calculating the optimal salt cavern height-to-diameter ratio based on the improved thermodynamic model with maximizing the indicated cycle efficiency as the optimization goal includes the following steps: S31, setting a value range of the salt cavern height-to-diameter ratio, and setting the minimum height-to-diameter ratio within the value range as the initial salt cavern height-to-diameter ratio; S32, using the average flow rate of the hydraulic pressure gas energy storage system as the liquid flow rate input, and calculating the gas volume change and liquid volume change in the salt cavern at the current time step; S33, using the gas volume change and liquid volume change obtained in step S32 as inputs, respectively, and substituting them into the improved thermodynamic model to obtain the change in gas temperature, and adding the gas temperature change per unit time step to the gas temperature of the current time step to update the gas temperature of the next time step; S34: Substitute the gas volume change at the current time step obtained in step S32 and the gas temperature at the next time step obtained in step S33 as input into the ideal gas state equation, update the gas pressure, and determine whether the gas pressure has reached the stop pressure. The stop pressure during the charging process is the maximum withstand pressure value, and the stop pressure during the discharging process is the minimum suction pressure. If so, proceed to step S35; otherwise, add one time step and repeat steps S32 to S34. S35, combining process data including gas pressure and temperature changes of gas and liquid, and calculating the indicated cycle efficiency under corresponding working conditions; In step S36, the salt cavern aspect ratio is gradually increased by a preset increment. The aspect ratio after each increase is used as a new input parameter. Steps S31 to S35 are repeated until the indicated cycle efficiency of the new iteration is less than 1% higher than that of the previous iteration. The system performance is considered to have converged, and the aspect ratio at this time is used as the optimal salt cavern aspect ratio.

2. The optimization design method of the hydraulic compressed gas energy storage system based on gas storage salt caverns according to claim 1 is characterized in that: In step S1, the initial parameters of the energy storage system include constraint parameters and input parameters; the constraint parameters include the maximum withstand pressure value and the minimum suction pressure; the input parameters include the initial temperature, effective volume, effective storage capacity, minimum working time and average flow rate; Among them, the maximum withstand voltage value p max The maximum pressure that the salt cavern can withstand is used to limit the maximum working pressure of the salt cavern energy storage system; the minimum suction pressure p min The minimum working pressure is used to determine the minimum pressure at which the pump-turbine unit can operate normally, taking into account the suction height of the pump-turbine unit and the underground depth of the salt cavern. The effective volume V0 is the maximum volume that can be used to store gas in the salt cavern. The effective storage capacity V val is the liquid volume in the salt cavern that can be used to exchange energy; the minimum working time t0 is the minimum time of the pumping energy storage or expansion energy release process; the average flow rate Q ave It is calculated by dividing the effective storage capacity by the minimum working time and is used to evaluate the impact of the salt cavern height-to-diameter ratio on the energy storage system.

3. The optimization design method of the hydraulic compressed gas energy storage system based on gas storage salt caverns according to claim 1 is characterized in that: In the improved thermodynamic model, the temperature field distribution T Wall (x, t) is calculated using the unsteady one-dimensional heat transfer formula: Where T Wall (x, t) is the temperature at the point x from the contact surface between the working medium and the salt cavern at time t; let the position where the gas working medium directly contacts the salt cavern wall be x0, then the wall temperature T on the side where the gas working medium contacts the salt cavern is Wall，G =T Wall，G (x0, t), the wall temperature T on the side of the salt cavern in contact with the liquid Wall，L =T Wall，L (x0, t); α is the thermal diffusivity of the object.

4. The optimization design method of the hydraulic compressed gas energy storage system based on gas storage salt caverns according to claim 1 is characterized in that: In step S3, the calculation formula of the indicated cycle efficiency is: or ind =the c or e ; Where η c isothermal compression efficiency, η e is the isothermal expansion efficiency; The isothermal compression efficiency η c is the theoretical power consumption W of the ideal isothermal compression process c，t Theoretical power consumption W of the actual compression process c Ratio: Where T0 is the initial temperature of gas compression, p1 is the initial pressure of gas compression, p2 is the final pressure of gas compression, and p G is the pressure of the gas at each time step; The isothermal expansion efficiency η e is the theoretical output work W of the actual expansion process e The theoretical output work W of the ideal isothermal expansion process is e,t Ratio: Where T3 is the initial temperature of gas expansion, p3 is the initial pressure of gas expansion, and p4 is the end pressure of gas expansion.

5. The optimization design method of the hydraulic pressure gas energy storage system based on gas storage salt caverns according to claim 1 is characterized in that: The optimization design method further comprises the following steps: Step A: Establish a computational fluid dynamics simulation model of the pump-turbine unit of the hydraulic compressed air energy storage system, and obtain a hydraulic efficiency calculation formula of the pump-turbine unit based on the model to describe the relationship between the flow rate and hydraulic efficiency of the pump-turbine unit; Step B: setting the initial speed range of the pump-turbine unit based on the determined optimal salt cavern aspect ratio and the determined operating pressure range of the gas in the salt cavern; using the minimum suction pressure as the initial pressure of the simulation operation, and the maximum withstand pressure value as the end pressure of the simulation operation; running from the initial pressure to the end pressure, and then returning from the end pressure to the initial pressure constitutes a complete cycle of the energy storage system's charge and discharge process; Step C: Substituting the initial pump-turbine unit speed and working head into the hydraulic model for different working pressures of the gas in the salt cavern, and calculating the flow rate and corresponding hydraulic efficiency under the corresponding working conditions; by adjusting the speed of the pump-turbine unit, calculating the optimal speed range under the corresponding working conditions, so that the pump-turbine unit is in a high-efficiency operating state; wherein the high-efficiency range refers to the hydraulic efficiency of the pump or turbine in the operating mode reaching above 90%; Step D, repeat step C, analyze the optimal speed range corresponding to different working pressures within the working pressure range, and comprehensively analyze the results to determine the optimal speed range that makes the pump-turbine unit operate in a high-efficiency state within the entire pressure range.

6. The optimization design method of the hydraulic compressed gas energy storage system based on gas storage salt caverns according to claim 5 is characterized in that: The pump-turbine unit adopts a variable speed pump-turbine unit, which adjusts the speed of the permanent magnet motor unit according to the pressure difference between the inlet and outlet ends, changes the speed triangle of the hydraulic machinery, and makes the water flow entering the impeller tend to be stable; The hydraulic model is: Where Q(t) is the flow rate of the pump-turbine unit, H(t) is the water pressure head; k1 is the operating characteristic constant of the pump-turbine unit, which is determined by the design parameters and operating speed of the pump-turbine unit; The calculation formula of the hydraulic efficiency is: or pump =3.76959+0.18899Q(t)-0.000112Q(t) 2 ; η turbine =87.96+2.24857Q(t)-0.27143Q(t) 2 ; Where η pump is the efficiency of the pump-turbine unit in the pump working mode, η turbine It is the efficiency of the pump-turbine unit in the turbine working mode. The two are collectively referred to as hydraulic efficiency.

7. A hydraulically compressed gas energy storage system based on gas storage salt caverns, characterized in that: The system includes a gas system, a liquid system, a pump-turbine unit and a piping system; The gas system contains compressible gas as the working fluid for energy storage, which is stored in an underground salt cavern and equipped with a gas replenishment device. The liquid system contains incompressible liquid as the working fluid for energy transmission, which is pressed into the salt cavern by a water pump. The compressible gas and incompressible brine are both stored in the underground salt cavern, which adopts an L-shaped connecting well structure with air intake at both ends and drainage in the middle. The pump-turbine unit includes a variable-speed pump-turbine unit, a frequency converter, and a permanent magnet motor; the pipeline system includes an air supply pipeline and a liquid transmission pipeline, wherein the liquid transmission pipeline is the main transmission channel for liquid in the open water pool on the ground and the closed salt cavern underground; The salt cavern height-to-diameter ratio is designed by the method described in any one of claims 1-6.

8. The hydraulic pressure gas energy storage system based on gas storage salt caverns according to claim 7 is characterized in that: The incompressible liquid is brine.

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