The electrical characteristics of solar cell modules mainly refer to current-voltage characteristics, also known as I-V curves, as shown in Figure 2-22.
The I-V curve can be measured according to the circuit device. The I-V curve shows the relationship between the current Im delivered through the solar cell module and the voltage Vm under a specific solar irradiance.
If the solar cell module circuit is short-circuited, that is, V=0, the current at this time is called the short-circuit current Isc; if the circuit is open, that is, I=0, the voltage at this time is called the open-circuit voltage Voc. The output power of the solar cell module is equal to the product of the current flowing through the module and the voltage, that is, P=VI. When the voltage of the solar cell module rises, for example, by increasing the resistance of the load or the voltage of the module from zero (under short-circuit conditions) When increasing, the output power of the component also increases from 0; when the voltage reaches a certain value, the power can reach the maximum, then when the resistance continues to increase, the power will jump over the maximum point and gradually decrease to 0, that is, the voltage reaches Open circuit voltage Voc. The point where the output power of the component reaches the maximum is called the maximum power point. The voltage corresponding to this point is called the maximum power point voltage Vm (also called the maximum working voltage); the current corresponding to this point is called the maximum power point Current Im (also called the maximum operating current); the power at this point is called the maximum power point Pm. The fill factor of the solar cell module is the ratio of the maximum power point power to the product of the open circuit voltage and the short circuit current, expressed by FF: FF=Pm/(IscVoc). The series resistance and parallel resistance of the solar cell module will affect the fill factor, and the fill factor greater than 0.7 indicates that the quality of the module is good. The fill factor is an important parameter for judging the quality of solar cell modules.
Since the output power of the solar cell module depends on the solar irradiance, the distribution of the solar energy spectrum and the temperature of the solar cell, the measurement of the solar cell module is carried out under standard conditions (STC). The measurement conditions are defined as Standard No. 101 by the European Commission. The conditions are: spectral irradiance is 1000W/m², spectrum is AM1.5, and battery temperature is 25°C. Under this condition, the maximum power output by the solar cell module is called peak power, and when watts are used as the unit of calculation, it is called peak watts, which is represented by the symbol Wp. In many cases, the peak power of the module is usually measured with a solar simulator and compared with the standardized solar cells of international certification bodies.
It is very difficult to measure the peak power of solar cell modules outdoors, because the actual spectrum of sunlight received by the solar cell modules depends on the atmospheric conditions and the position of the sun; in addition, during the measurement process, the temperature and radiation of the solar cells The illuminance is also constantly changing. The error of outdoor measurement can easily reach 10% or more.
As the temperature of the solar cell increases, the open circuit voltage decreases. In the range of 20~100℃, the voltage of each cell decreases by 2mV for every 1℃ increase; while the photocurrent increases slightly with the increase of temperature, about every 1℃ increase. The photocurrent of each cell is increased by 1%, or 0.03mA/(℃·cm²). In general, the power of solar cells decreases when the temperature rises. The typical power temperature coefficient is -0.35%/℃. When the temperature increases by 1℃, the power is reduced by 0.35%. Figure 2-23 shows the effect of temperature on photovoltage and photocurrent.
What is introduced here is the effect of temperature on the performance of crystalline silicon solar cells. Amorphous silicon solar cells are different. According to a report by Uni-Solar, the company’s three-junction amorphous silicon solar cell module has a power temperature coefficient of only -0.21%.
The solar irradiance is directly proportional to the photocurrent of the solar cell module. In the range of irradiance 100~1000w/m², the photocurrent always increases linearly with the increase of irradiance. The irradiance has little effect on the photovoltage. Under the condition of a fixed temperature, when the irradiance changes within the range of 400~1000W/m², the open circuit voltage of the solar cell module remains basically constant. Because of this, the power of the solar cell is basically proportional to the irradiance.