Energy control strategy for compressed air and super capacitor hybrid energy storage system

Fund Project: Funded by the Central University's basic research business expenses (E13B00020) Microsmall Scale CAES power level is generally 100 kilowatts, storage pressure can reach more than 30MPa. MSCAES energy storage compression technology uses liquid pump to pump liquid to compress gas. High pressure gas expansion causes liquid to drive hydraulic motor to drive motor to generate electricity. It can realize reversible process of compression and release, greatly improve compression and release efficiency, and system thermodynamics can compress and release the whole. The efficiency is close to 70%. MSCAES is a physical storage energy source with zero emissions, no pollution and long service life. Supercapacitors have a sustainable constant discharge capability with a power density of approximately 0.8 kW/kg. Supercapacitors have a long service life and high efficiency, reaching over 95%. Due to the small cell voltage, the application needs to be combined by a large number of monomers, which increases the failure rate of the system. At the same time, the self-discharge rate of supercapacitors is close to 5% per day, so it is impossible to maintain long-term standby energy storage.

Different energy storage technologies have unique advantages, but there are also some quantitative density, high power density, reasonable cost and long life cycle optimization combined with B7. Design a hybrid energy storage system consisting of compressed air energy storage and super capacitor energy storage. It can achieve high-capacity storage and continuous energy conversion, as well as fast power response and energy replenishment. In order to improve the efficiency of energy conversion of compressed gas, the system adopts quasi-constant temperature compression release process control, and uses the maximum efficiency point tracking algorithm MEPT to realize coordinated control of speed and pressure to ensure high efficiency of liquid pump/hydraulic motor. Under the control of the MEPT algorithm that maintains the quasi-constant temperature change process, the energy accumulation and release of the compressed gas is reflected in the form of a similar intermittent pulse. The supercapacitor auxiliary system adopts adaptive power regulation control, and the response speed is fast, which can supplement the energy pulse gap of the compressed gas working.

1 Energy Management Control Strategy Design For an energy storage application project, the parameter level of the energy storage system is the primary design point. The parameter level of the hybrid energy storage system is described by its own characteristic parameters, including energy level, power level, volume size and cost. Through the above specific characteristic parameters, the energy storage system capacity level of an application scenario can be determined. On the other hand, the way the energy storage system works is determined by the application needs. The actual working mode of the energy storage system is the strategy and method of energy management and distribution. Different energy management strategies can meet different application requirements. Therefore, the parameter level of the hybrid energy storage system is coupled with the energy management method. Describes the design key to an energy storage system that needs to be measured.

Relationship between parameters and management of hybrid energy storage systems Energy management and control of energy storage systems include energy layer management, power layer management, and device level management. This hierarchical management is mainly distinguished by the inertia time characteristics of the system. It is a schematic diagram of this hierarchical management.

The management of energy is the top-level goal of the energy storage system control strategy. The purpose is to design a strategy to control the long-term evolution of the system. The control response time is long, generally greater than the second level, which mainly determines the control mode of the system and directly limits the range of power level control.

The management of power is the intermediate design layer goal of energy storage system control, and the purpose is to design a control method that satisfies the mid-term changes of the system.

The control response time is in the order of milliseconds, which determines the power characteristics of the system and limits the level of parameters used in the hardware. The underlying design goal of the energy storage system is device layer management. This layer design directly determines the dynamic performance of the execution process. The reaction time is in the order of microseconds, which can realize voltage and current regulation and power conversion. Table 1 shows the energy management level of the energy storage system.

Table 1 Energy Management and Control Hierarchy Table Management Hierarchy Design Target Control Output Response Aging Energy Management Management Strategy Working Mode Larger than Second Power Management Processing Method Power Size Milliseconds to Seconds Device Management Execution Process Modulation Modes Microseconds to Milliseconds Level 2 Mixed Storage The functional block diagram of the system adaptive power regulation control supercapacitor and liquid gas circulating compressed air hybrid energy storage system is shown in the figure. In the energy storage state, the excess power of the DC grid is first stored to the super capacitor according to the distribution, and the excess energy drives the motor to work, driving the hydraulic pump to compress and store the air by pumping the liquid. In the release state, the load requires additional energy injection in addition to the DC network. At this time, the compressed gas drives the hydraulic motor to rotate, which drives the generator to generate electricity, and the converter converts the electric energy into a form required by the load. As a power supplement, the super capacitor can maximize the smoothing of the instantaneous fluctuation of the DC network voltage by controlling the charging and discharging of the super capacitor.

In the SCHPCCAES hybrid energy storage system, the energy performance of the energy storage system is an important indicator, and the performance of the compressed air energy storage and super capacitor storage energy is achieved. When the liquid-gas cycle is compressed, a compression and pre-compression cycle can be regarded as equivalent to a process of mixed liquid-gas compression. In order to simplify the analysis of the power control strategy, the PHESS working process is used instead of the HPCCAES working process to study the hybrid energy storage power regulation. The external power supply and load constitute an energy fluctuation system. The energy difference between the source and the load will cause the network voltage to fluctuate. By controlling the energy storage system to absorb and supplement the source-charge energy difference, the fluctuation of the power grid can be stabilized. The control object of source-source energy difference is represented by the input-output current difference between the grid and the load. To maintain the same, it is necessary to control the difference current i t2 between the grid and the load. In the hybrid energy storage system, 2 is output by the super-capacitor converter. Current ich and motor converter current are combined. Automatic frequency power regulation control with variable power output is adopted to coordinate control of liquid gas compression (release) and supercapacitor charge and discharge to maintain the bus voltage stability and ensure load operation characteristics. The i control frequency is lower, and the transient performance of the SCPHESS system is mainly realized by controlling ich. The voltage and current double loop control PWM is used to control the super capacitor converter. The automatic frequency adjustment control realizes the switching control according to the voltage state of the super capacitor. Consider the simplest hybrid energy storage alternate operation, and consider the external energy power level to be smaller than the super capacitor energy storage power and compressed air energy storage power. 1 When the compression condition is used, the valve is closed first, and the external energy is first absorbed by the super capacitor. When the super capacitor voltage USc rises to Umax, the valve opens, the capacitor discharges, and the external power source cooperates to compress the gas storage energy; when the capacitor voltage drops to the liquid gas The compression stops, the valve closes, and the super capacitor recharges. 2 When the energy release condition is applied, the valve is first opened, and the compression energy is directly supplied to the load while charging the capacitor. When the capacitor voltage A. reaches the maximum voltage setting, the liquid gas storage stops and the valve is closed; the super capacitor is separately discharged. Load, capacitor voltage A. When the voltage is set to the minimum voltage, the valve is opened and the liquid gas storage is released again. Generally, it is necessary to close the valve for a certain period of time. After the fluid system is stable, the valve can be opened again, and the delay time is T; Flowchart for automatic frequency power regulation control.

The compressed air supercapacitor hybrid energy storage device converts the energy WcV and the PHESS action cycle into a Tonn proportional relationship. As the air pressure of the main storage tank decreases, the power delivered is gradually reduced. The Pcv is the power curve and shows a downward trend with time.

The capacitor is charged. The energy obtained by the super capacitor is Ws. In the time Tff n when the PHESS system does not change energy, it is only powered by the super capacitor. The output energy of the super capacitor is WR. It is considered that the super capacitor does not accumulate excess energy during each charging and discharging process. The following relationship 3 is satisfied. The energy allocation management strategy of the hybrid energy storage system adopts the energy management and allocation strategy controlled by the rules to realize the control of SCPHESS. Energy management and control are based on the instantaneous power difference between the power supply and the load. The basic rules for establishing the basic rules are the safe working area of ​​the supercapacitor energy storage system and the conversion efficiency of the liquid gas compression energy storage system. The rule of the supercapacitor system is the maximum and minimum withstand voltage of the capacitor UScmm. The rule of the compressed air energy storage system is the maximum power Pphmax and maximum efficiency nphmax of the liquid pump/hydraulic motor. It is a hybrid energy storage system. Energy management and control strategies are indicated.

Set the hybrid energy storage system to apply the load difference between the power supply and the load. The power supply is Pgnd and the load power is Pkd. Consider the power difference between the source and the load according to the storage and release conditions of the energy storage system (remaining power) Pex=Pgnd-Py, four working rules can be established: rule 1, compressed energy storage, and the remaining power is greater than the maximum power of liquid-gas compression.

The power provided by the power supply is excessive, the source-load power difference Pex>, the liquid-gas storage system is controlled to compress and store energy, and the stored energy Pph is the maximum compression power COM. If the remaining power is greater than the liquid-gas compression maximum power Pex>Pphmax, it can be divided into two. The energy management mode is carried out: 1 liquid gas compressed air energy storage system works at maximum power, Pph=Pphmax; compression efficiency h is the efficiency corresponding to the highest speed nhspeed, nph=nhspeed, at this time, the super capacitor energy storage system is charged; When the supercapacitor power PSc is the power Pscch at the time of charging, Psc = PScch until the highest withstand voltage is reached, and it can only be filled once. 2 Liquid-gas compressed air energy storage system works at maximum efficiency, Pph=Pphhspeed,%h=lax, supercapacitor discharges compressed air energy storage system at this stage, PSc=PScdis, when the capacitor discharges to the lowest discharge voltage, starts again Charging, liquid gas compression can work at maximum efficiency.

V load rule 2, compressing energy storage, the remaining power is less than the maximum power of liquid gas compression.

The remaining power is still greater than zero, but less than the maximum power of the liquid-gas compression system PeX > Pphmax. It can also be divided into two energy management modes: 1 liquid-gas compressed air energy storage system works at maximum power, input power is equal to residual power, Pph= Pex, regardless of conversion efficiency, supercapacitor does not work; 2 liquid-gas compressed air energy storage system works at the corresponding maximum efficiency point speed, if the corresponding power output at the maximum efficiency point under a certain speed and pressure is less than the remaining power, then super The capacitor is charged to absorb excess residual power.

Rule 3, release power generation, the underpower is greater than the maximum power of liquid gas compression.

The power provided by the power supply cannot fully support the load, and the power generation is lacking. The PexO. liquid-gas circulating compressed air energy storage system releases the power generation of the Beijing Jiaotong University, and the hydraulic motor power is the release power PphREL, Pph=PphEL. If the unpaid power is greater than The maximum power of liquid-gas compression, IP>Pph, can be divided into two energy management modes: 1 liquid motor works at high speed to maintain maximum power, Pph = Pph, nph=%Speed, at this time, it is necessary to output higher power to maintain Load supply, liquid motor conversion efficiency is not the highest efficiency at the moment of pressure and speed. Super capacitor discharge, PSc = PScdis, and can only be discharged once; 2 liquid gas compression works at maximum efficiency, liquid motor works at a certain speed corresponding to the power, not the maximum power, but guarantees the maximum conversion efficiency at this moment. Because all the maximum efficiency points correspond to less than the underpower, there is no power to charge the supercapacitor. The supercapacitor does not work at this stage.

Rule 4, release power generation, the under-committed power is less than the maximum power of liquid-gas compression.

The energy storage system is in the state of power generation release, and the underpayment power is less than the maximum power of liquid gas compression energy storage IPexI 4 System Simulation and Test Table 2 lists the main parameters of the operation simulation of the supercapacitor compressed air hybrid energy storage system.

Table 2 SC-HPCCAES hybrid energy storage system hybrid energy storage system design simulation parameter table external power supply rated power peak +50% rated voltage resistance load rated power range 0 super capacitor converter system rated power rated voltage (high voltage) peak + permanent magnet synchronization Motor converter system motor power rated speed rated torque peak + liquid pump / hydraulic motor displacement pressure (maximum) most local power high pressure tank rated capacity maximum pressure low pressure tank rated capacity maximum pressure assumed external power supply is ideal constant pressure The source, the load is a variable power load, and the maximum power value of the load change can be greater than the rated power of the compressed air energy storage system. The load energy storage system works under the condition that the load suddenly decreases by more than 15 kW and the external residual power reaches 24 kW and 20 kW. Characteristics, in which Pex, Ppump, Psc, and Usc are respectively external residual power, liquid pump power, supercapacitor power, and super capacitor voltage when the liquid pump compresses energy storage. The compressed energy storage system adopts the waveform diagram when the maximum efficiency point is controlled, mainly for the higher efficiency of transferring the external residual energy. The power corresponding to the maximum efficiency point will exceed the maximum power of the compression system by 15 kW, and the super capacitor needs relatively more storage power. Need to store more external energy. The 325V is shown at 45s on the time axis. The supercapacitor is fully charged and fills up faster than the maximum power point control.

Characteristic curve of external residual power, liquid pump power, supercapacitor power and supercapacitor voltage when compressing the energy storage for the liquid pump. The liquid-gas compression energy storage does not need to absorb the external residual power as soon as possible, and the super-capacitor system provides energy reception. This working mode ensures that the compression conversion efficiency of the liquid pump is large, the super capacitor is in a state of charging and discharging, and the capacitor voltage rises slowly.

Pex Adaptive power regulation control has been developed. Taking the available voltage range of the supercapacitor as the basis for judging the working relationship between the compressed air main energy storage system and the super capacitor auxiliary energy storage system, the working state of the SCHPCCAES system application is fully understood, and the energy management strategy of the rule basic control is designed. Different working conditions, application of maximum efficiency point tracking control and maximum power point tracking control ensure the working efficiency and power requirements of the liquid pump/hydraulic motor, and then realize the energy distribution and management of the main energy storage system and the auxiliary energy storage system. Simulation and experiment verify the correctness and feasibility of the design.

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