The volt-ampere characteristics of photovoltaic cells are shown in Figure 1, which shows the relationship between the output voltage and output current of photovoltaic cells under certain sunshine intensity and temperature, referred to as U-I characteristics. It can be seen from the U-I characteristics that the photovoltaic cell is neither a constant voltage source nor a constant current source, nor can it provide arbitrarily large power to the load, and it is a nonlinear power source. The output is approximately constant over most of the operating voltage range, with a large rate of current droop near the open circuit voltage.
Several important technical parameters of photovoltaic cell arrays are as follows:
(1) Short circuit current (Ics):
The maximum output current under a given sunshine intensity and temperature, that is, short-circuit the two ends of the PN junction of the photovoltaic cell through the outer wire to form a current flowing through the outer circuit, the magnitude of which is proportional to the light intensity.
(2) Open circuit voltage (Uoc): the maximum output voltage under a given sunlight intensity and temperature, that is, when the external resistance of the photovoltaic cell is infinite, the voltage measured on the resistance. There is a non-linear relationship between the open circuit voltage and the illuminance, and the illuminance is greater than 1000 lx, showing a saturation characteristic.
(3) Maximum power point current (Im): the current corresponding to the maximum power point under a given sunshine intensity and temperature.
(4) Maximum power point voltage (Um): The voltage corresponding to the maximum power point under a given sunlight intensity and temperature.
(5) Maximum power point power (Pm): the maximum power that the array may output under a given sunshine intensity and temperature.
Photovoltaic cells play the role of energy conversion, which converts solar energy into electricity. When the photovoltaic cell module is short-circuited, that is, when U=0, the current at this time is the short-circuit current Ics; when the circuit is open, I=0, and the voltage at this time is the open-circuit voltage Uoc; when the voltage across the photovoltaic cell rises from 0, for example, when the load resistance or the voltage of the component gradually increases from 0 (under short-circuit conditions), under the condition of constant light radiation, the output current of the photovoltaic cell is almost unchanged, and the output power continues to increase. When the battery voltage increases to a certain value, the output current begins to decrease, and the output power reaches a maximum value Pm, that is, the maximum power point. After that, as the battery voltage increases, the output current and power continue to decrease, and finally the output current decreases to 0, and the output voltage reaches the maximum value, that is, the open-circuit voltage Uoc. The voltage corresponding to the maximum power point is the maximum power point voltage Um (also known as the maximum power voltage), and the corresponding current is the maximum power point current Im (also known as the maximum power current).
The output of photovoltaic cells is determined by many factors, such as sunshine conditions, temperature, etc. In different environments, the output curves of photovoltaic cells are different, and the corresponding maximum power is also different. The stronger the sunshine, the greater the power that the photovoltaic cell can output, and the higher the voltage and current value corresponding to the maximum power; and the temperature is just the opposite. As the temperature increases, the open-circuit voltage decreases, and the photovoltaic cell can output less power. The voltage of each cell decreases by about 5mV for every 10°C increase, which is equivalent to a temperature coefficient of 0.4%/°C at the maximum power point. The following analyzes the influence of sunlight and temperature on its output power from the output curve of the photovoltaic panel.
Figure 2 is the characteristic curve of photovoltaic cells at different temperatures when the sunlight intensity is constant. It can be clearly seen that: under a fixed sunshine intensity, when the temperature increases, the photovoltaic panel opens and the voltage decreases, but its short-circuit current is almost unchanged. Overall, the rated output power of the photovoltaic panel decreases slightly when the temperature increases. It can be seen that the temperature of the working environment will also have a direct impact on the maximum output power of the photovoltaic panel.
Figure 3 is the characteristic curve of photovoltaic cells under different sunshine intensities when the temperature is constant. When the sunlight intensity changes, the open circuit voltage of the photovoltaic panel will not have much influence, but the maximum current value it can provide has a considerable change. When the sunshine intensity increases, the short-circuit current increases and the maximum power point increases. Therefore, the intensity of sunlight is an important factor affecting the output power of photovoltaic panels.
Peak power is used to measure the performance of photovoltaic cell modules, and its unit is peak watts (Wp). Under standard conditions (spectral irradiance 1000W/m2, spectrum AM1.5, cell temperature 25°C, the maximum power output by photovoltaic cell modules is called peak power).