| For the off-grid photovoltaic greenhouse system,the demand of the crops in the greenhouse determines that the load regulation method is different from other off-grid photovoltaic power generation systems.The primary function of the greenhouse load is to adjust the indoor temperature to promote the growth of crops,and the indoor temperature will change with the outdoor irradiance and temperature so that the load working state in the greenhouse will change with the external conditions.On the other hand,the electricity of the off-grid greenhouse all comes from photovoltaic power generation,and the output of photovoltaic modules will impact the working state of the load.Therefore,both irradiance fluctuations and load working state changes will cause the photovoltaic system to deviate from the maximum power point and reduce the solar energy utilization rate of the greenhouse system.Therefore,this paper will build models and conduct simulation research on components such as bifacial photovoltaic modules,batteries,DC control circuits,and inverter circuits.The performance evaluation of the greenhouse system was carried out according to the regulation strategy for the operation under the load.The main work contents are as follows:（1）Using the single-diode model of the photovoltaic module,according to the law of the change of the double-sided irradiance of the module,the backside irradiance gain is corrected to the five parameters of the module to obtain the double-sided power generation photovoltaic module under the actual working conditions and standard test conditions（STC）output characteristics.By analyzing the relative error,the error of the model to calculate the maximum output power mainly comes from the current,while the relative error of the measured condition and the optimal operating point voltage under STC is 0.83%and 3.26%,respectively.（2）According to the charging and discharging characteristics of lead-acid batteries,this paper designs a maximum power point tracking（MPPT）control strategy for photovoltaic systems that cooperates with battery state of charge（SOC）and stable bus voltage.By judging the bus voltage and battery SOC threshold,disturbance observation or integral separation PID voltage regulation control is used to realize the photovoltaic system MPPT;on the other hand,according to the battery SOC and bus voltage value,PI feedback is used to control the charging and discharging stages of the battery.The results show that when the disturbance observation method is used when the irradiance suddenly changes from 1000 W/m~2 to 600 W/m~2 and then increases to 800 W/m~2,the bus voltage is stable within 0.05s,and the MPPT overshoot is 5.231%.The peak time is 0.023s.When the irradiance variation is 300W/m~2,the PID voltage regulation control mode with integral separation is adopted.The output oscillation time of the system is shortened by 0.015 s compared with the time without integral separation control,and the voltage overshoot is reduced from 2.504%to 1.136%.（3）According to the load characteristics of the greenhouse,the relevant load mathematical models are established for linear and nonlinear loads,and corresponding control strategies are set to drive according to different load types.The main circuit uses a full-bridge inverter circuit.For the non-motor loads in the greenhouse,the current and voltage double closed-loop control of current feedforward is adopted;for the motor load,the DC brushless motor model is used,and PWM is used at the inverter output end.Rectifier circuit and double closed-loop control circuit suitable for brushless DC motor.The simulation results show that when the irradiance changes from1000 W/m2 to 600 W/m~2 to 800 W/m~2,the maximum voltage fluctuation of the DC bus voltage is 3.18%,the duration is 0.15 s,and the inverter output AC voltage amplitude is 311 V,the frequency is 50 Hz,the effective value is 220 V,the voltage fluctuation is0.18%,the AC voltage THD is 0.02%,and the power factor is 0.95;there is no apparent fluctuation in the output terminal and input terminal voltage of the rectifier,and the DC voltage fluctuation at the output terminal is 2.5%;the DC motor usually starts and can run at a stable speed.（4）A low-power constant voltage circuit with integral separation was built,and a comparison test was carried out with the circuit without integral separation under the condition of irradiance fluctuation.The results show that when the irradiance suddenly rises from 574W/m~2to 862W/m~2,the overshoot of the voltage regulator regulation voltage without integral separation is 14%,the peak time is 1.23 s,and the regulation time is 0.21 s;From 403 W/m~2 to 708 W/m~2,the overshoot of the integral and separate voltage regulation regulation is 7.5%,the peak time is 1.12 s,and the adjustment time is 0.125 s.By analyzing the photosynthetic characteristics of strawberry crops in greenhouse,two control modes of time domain regulation and crop growth demand were adopted.On the summer solstice,the crop growth demand strategy was used to control the load.During the period from 12:00 to 15:00,the photosynthetic rate（Pn）of strawberry was 4.1μmol·m~2/s,which was better than the time domain control of 3.6μmol·m~2/s;On the winter solstice,after adopting the crop demand regulation strategy,the indoor temperature was 1℃higher than the room temperature regulated in the time domain before 23:00,and increased by 3℃-6℃after 23:00.Finally,the system self-consumption rate SCR model is used to evaluate the time domain regulation and the regulation based on crop demand month by month.The results show that when the indoor temperature range needs to be regulated,the crop demand regulation strategy can improve the system SCR.When the ambient temperature in the greenhouse is adapted to the crop growth conditions,the time domain regulation can increase the system SCR. |
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