| In view of the food refrigeration in the operation cost and energy consumption problems,using energy saving and environmental protection solar energy instead of traditional fossil energy as the direct driving power source of cold storage can reduce the pressure on the power grid.A direct-driven refrigeration compressor by photovoltaic usually operates at a precarious working state in the long term since the off-grid photovoltaic cold storage is affected by fluctuations of solar irradiance intensity.Hence,it is essential to control the compressor with efficient and reasonable ways.In order to ensure the well performance of photovoltaic cooling of the direct-driven mode,the performance characteristics of off-grid photovoltaic cold storage system were studied in this paper.At the same time,exploring a cooperative control strategy of photovoltaic maximum power point tracking(MPPT)control combined with constant voltage per frequency(V/f)control is beneficial to the stable operation of solar photovoltaic cold storage system.Based on this,the main work of this paper are as follows:(1)In order to study the influence of solar irradiation fluctuation on the operation stability of compressor under actual working conditions,the optimal working point impedance model of the photovoltaic array was established,which was related to temperature,irradiance,and the optimal impedance under standard test conditions.When the instantaneous irradiance of the compressor is greater than 800 W/m~2,it can also better track the optimal power of the photovoltaic array and at the same time broaden the available threshold of instantaneous irradiance.(2)A 5.4kW distributed photovoltaic direct-driven cold storage experiment system was built,and the compressor's rated power was 4.4 kW.The impedance converter was added between the photovoltaic array and the load,and the control method of photovoltaic maximum power point tracking(MPPT)combined with constant voltage per frequency control(V/f)was adopted.By controlling the on-off time of the switching circuit,the dynamic matching of the load impedance and the optimal impedance of the photovoltaic array is realized,and the utilization rate of the compressor was effectively improved.Compared to the conventional MPPT control,the utilization rate of compressor of PV system under the MPPT with the V/f control has been improved by 10.51%.(3)The control strategy of stable output of photovoltaic direct-driven compressor system under the condition of irradiance fluctuation was studied,and the correctness of the optimal working point impedance model of the photovoltaic array was verified by experiments.The error rates of SSE and RMSE under MPPT+V/f control were 2.58and 7.46%respectively,which were significantly lower than those of root mean square error and sum of squared errors under the MPPT control.At the same time,the performance characteristics and performance indexes of the refrigeration system under MPPT+V/f control and MPPT control are analyzed.The W_a and the PR of PV system under the MPPT+V/f control reached 13.6%and 95.42%respectively,which were10.51%and 14.36%higher than those controlled by MPPT.(4)The energy storage operation stability of the ice system with photovoltaic direct-driven compressor was analyzed.The experimental results showed that at night or without the sun,the technology of ice-making and energy storage can maintain the cold storage in the temperature range of-5?5?,and meet the demand of cooling capacity of fruits and vegetables.At the same time,it can also alleviate the adverse impact of solar intermittency on the cold storage system,and the PR under cloudy conditions reaches 88.53%,which reasonably solves the problem of off-grid photovoltaic cold storage cooling instability.The realization of the above research method is conducive to the stable operation of the refrigeration system in the photovoltaic drive mode,and provides a reasonable scientific optimization method and experimental basis for the coupling utilization and ratio of energy. |
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