The electrical characteristics of solar cell modules mainly refer to the I-V output characteristics, as shown in Figure 1. The I-V characteristic of the solar cell is similar to the characteristic of the diode, generally called the I-V characteristic curve. The focus on the I-V curve is: short-circuit current (Isc), open-circuit voltage (Voc) and maximum power (Pmpp). The conversion efficiency of solar cells is affected by illuminance and ambient temperature.
The I-V characteristic curve shows the relationship between the current Im delivered by the solar cell module and the voltage Vm under a specific solar irradiance. Im is the maximum working current, that is, the current in the maximum output state; Vm is the maximum working voltage, that is, the voltage in the maximum output state. If the circuit of the solar cell module is short-circuited, that is, V=0, the current at this time is called the short-circuit current Ise; 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=V×I.
When the voltage of the solar cell module increases, for example, by increasing the resistance value of the load or the voltage of the module increases from zero (under short-circuit conditions), the output power of the module also increases from 0; when the voltage reaches a certain value, the power can reach the maximum. At this time, when the resistance continues to increase, the power will jump over the maximum point and gradually decrease to zero, that is, the voltage reaches the open circuit voltage Voc. The internal resistance of solar cells exhibits strong nonlinearity. The maximum point that the output power of the solar cell module reaches 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 working current); the power at this point is called the maximum power Pm.
As the temperature of the solar cell increases, the open circuit voltage decreases, and the voltage of each solar cell decreases by 5mV for every 1°C increase in temperature, which is equivalent to a typical temperature coefficient of -0.4%/°C at the maximum power point. In other words, if the temperature of the solar cell increases by 1°C, the maximum power is reduced by 0.4%. Therefore, in the summer when the sun is directly exposed to the sun, although the solar radiation is relatively large, if the ventilation is not good, the solar cell temperature rises too high, and the solar cell may not output a lot of power. The solar cell temperature change and I-V curve are shown in Figure 2.
The output of the solar cell decreases as the surface temperature of the solar cell rises, and the output changes with the seasonal temperature. Under the same intensity of sunlight, the output in winter is higher than in summer.
Since the output power of the solar cell module depends on the solar irradiance, the distribution of the solar spectrum and the temperature of the solar cell, the measurement of the solar cell module must be carried out under standard conditions (STC). The measurement conditions are defined by the European Commission as Standard No. 101. The test is set under the simulated light source conditions of the surface temperature of the solar cell module at 25°C, the solar radiation intensity at 1000W/m2 and the light distribution AM1.5, which is called the standard test state, as shown in Figure 3.
Under this condition, the maximum power output by the solar cell module is called the peak power, expressed as Wp (peakwatt). 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 Fengzhi power of solar cell modules outdoors, because the actual spectrum of sunlight received by solar cell modules depends on the atmospheric conditions and the position of the sun. In addition, during the measurement process, the temperature of the solar cell is constantly changing, and the error of outdoor measurement can easily exceed 10%.
If the solar cell module is shaded by other objects (such as bird droppings, tree shade, etc.) for a long time, the shaded solar cell module will heat up severely at this time, which is called the “heat island effect” in the industry. This effect can cause serious damage to solar cells. Part of the energy or all of the energy produced by solar cells with sunlight may be consumed by solar cells that are shaded. In order to prevent solar cells from being damaged due to the “heat island effect”, a bypass diode needs to be connected in parallel between the positive and negative poles of the solar cell module to prevent the energy generated by the solar cell module from being consumed by the shaded solar cell module. Its function is to provide a current path when the module is open or shaded, so as not to cause the entire string of solar cell modules to fail.
Pay attention to the polarity when connecting the bypass diode to the solar cell. The anode of the bypass diode is connected to the cathode of the solar cell module, and the cathode of the bypass diode is connected to the anode of the solar cell module. Usually the bypass diode is in a reverse biased state and basically does not consume power. However, the withstand voltage and allowable forward current of the bypass diode should be greater than the operating voltage and current of the component.
The connection box of the solar cell is a very important component, which protects the interface between the solar cell and the outside world and the wires and other system components connected inside each component. The connection box contains a junction box and one or two bypass diodes. The main technical parameters of solar cells are:
(1) Photoelectric conversion efficiency. Photoelectric conversion efficiency is an important indicator for evaluating the performance of solar cells. The conversion efficiency of solar cells refers to the ratio of solar cells converting received light energy into electrical energy
Η＝P0/E×100% (1-1)
In the formula: η is the conversion efficiency, %; P0 is the output power; E is the amount of solar energy irradiated by the solar cell module.
The conversion efficiency of solar cell modules is an important factor in determining whether solar cells have use value. The theoretical conversion efficiency limit of crystalline silicon solar cells is 29%, while the conversion efficiency of current solar cells is 17% to 19%. Therefore, there is still a lot of room for technical development of solar cells. At present, the conversion efficiency of the laboratory is η≈24%, and the conversion efficiency of the industrialization η≈15%.
(2) The voltage V of a single solar cell is 0.4~0.6V, which is determined by the physical properties of the material.
(3) The fill factor FF% is an important indicator for evaluating the load capacity of solar cells
FF=(Im×Vm)/(Ise×Voc) (1-2)
In the formula: Ise is the short-circuit current; Voc= is the open circuit voltage; Im is the best working current; Vm is the best working voltage. The power output capacity of a solar cell is closely related to its area. The larger the area, the greater the output power under the same light conditions. The pros and cons of solar cells are mainly measured by the two indicators of open-circuit voltage and short-circuit current.
(4) The ambient temperature and the temperature of the solar cell components directly affect the performance of the solar cell. When the temperature rises, the open circuit voltage of the solar cell decreases in a linear relationship. Solar cells of different materials have their own operating temperature range. For a certain type of solar cell, at different temperatures, the optimal load required to obtain the maximum output power is also different. For example: under standard conditions, AM1.5 light intensity, t=25℃, the output power of a certain type of solar cell is measured to be 100Wp, if the solar cell temperature rises to 45℃, the output power of the panel will be less than 100Wp.