Research On Operation Characteristics And Control Optimization Of The Ice-storage System Driven By Off-grid Wind Power

In recent years,grid-connected wind power has been affected by factors such as excess power generation by wind farms and poor grid load capacity.The rate of wind power rejection has increased significantly,and the scale of development of off-grid wind power system has been increasing.Off-grid wind power system must be equipped with energy storage device because the supply and demand are not synchronized.At present,the common energy storage devices are batteries,but the problems of its high battery cost,short life and pollution have hindered the further development of off-grid wind power system.In order to solve the problem of energy storage of off-grid wind power system,this paper proposed the off-grid ice-storage system driven by distributed wind energy to replace partial battety to store wind power with ice storage.In this paper,the following work is carried out to realize the efficient and stable conversion of wind-electric-cold:(1)By establishing wind-electric-cold energy transfer theoretical model and system performance evaluation index,the energy matching relationship and energy transfer law between system components were analyzed,and the key parameters affecting the overall efficiency of the system were the wind power conversion efficiency and the battery energy loss.(2)Under the steady and unsteady wind conditions,the comparative experiments of the system direct-drive and floating-charge modes were carried out to study energy transfer law of the system and found the corresponding relationship between the wind speed and the optimal operation mode of the system.When the steady average wind speed reaches 5m/s,the wind power output power reaches the upper limit of the operating power of the inverter compressor,and the battery achieves the balance of charging and discharging.The system coefficient of performance(COP)is similar and reaches a peak value of about 0.31 in the two operating modes.In the unsteady wind condition,the direct-drive mode cannot achieve stable operation and the floating-charge mode has strong dependence on the battery.The system COP is 0.063 and 0.17 respectively.(3)A variable step-size maximum power point tracking(MPPT)hill-climbing search(HCS)algorithm based on system impedance variation is proposed.The controller is fabricated by software compilation and circuit design.The results show that the optimized MPPT algorithm increased the system average wind-electric efficiency from 23% to 29%.(4)Based on the influence of multi-boundary conditions such as wind speed,wind speed stability and battery terminal voltage on the wind-electric-cold conversion of the system,a system operation mode-switch control is proposed,and the optimal operating mode of the system is selected according to the real-time wind condition.Under the condition that the average wind speed is 4.9m/s,the system compressor frequently stopped in the direct-drive mode and the starting power consumption was large causing the COP is only 0.074.Under the floating-charge mode,the battery terminal voltage clamping effect was obvious,and the charging and discharging loss was obvious causing the system COP is 0.187.The system adopting the mode-switching control minimizes the limitation of the battery to the energy transfer of the system under the premise of ensuring the stable operation of the compressor.The cumulative operation time of the direct-drive mode accounts for 46.9% of the total duration,and the system COP is 0.229 which is 209.46% higher than the direct-drive mode and 22.46% higher than the floating-charge mode,achieving stable and efficient conversion of wind-electric-cold.And the system only needs small capacity battery to achieve round-the-clock operation.