1) Photoelectric characteristics of solar cells
When light hits the solar cell, currents IR and IJ flow through the load resistance R of the solar cell and inside the solar cell, respectively, where IJ is the forward current through the PN junction. When the illumination is constant, the photocurrent IP=IR+IJ is also constant. The flow of photocurrent inside and outside the solar cell can be represented by an equivalent circuit. The terminal voltage VJ of the solar cell should be equal to the voltage VR on the resistor, and the current IJ of the solar cell has an exponential relationship with the change of VJ:
where q is the electron charge, 1.6x 10-10, in C; T is the absolute temperature, in K; K is Boltzmann’s constant, 1.380x 10-23J/K or 0.86x 10-4V/K ; A is the effective area of the solar cell, the unit is mm².
The relationship between the current and voltage across the load resistor R is:
From formula (2), it can be concluded that the relationship between the current and the voltage on the load resistor is R=VJ/I, the voltage drop of the load resistor is equal to the junction voltage, and the electrical power obtained on the load resistor R is IxVJ. In order to obtain high conversion efficiency, solar cells must output as much power as possible under certain solar radiation.
2) Spectral characteristics of solar cells
The spectral characteristics of solar cells refer to the relationship between the output power of solar cells and incident light with the same energy but different wavelengths. In a solar cell, only those photons with energies greater than the “gap” width of the material can generate electron-hole pairs in the photovoltaic material when absorbed, while those photons with energies less than the “gap” width cannot be generated even if absorbed. Electron-hole pairs (they only heat the photovoltaic material), there is a cut-off wavelength for the absorption of light by the photovoltaic material. Theoretical analysis shows that for sunlight, the photovoltaic material that can get the best performance should have a “forbidden band” width of 1.5 electron volts. When the “forbidden band” width increases, the total solar energy absorbed by the photovoltaic material will increase. less and less.
Each solar cell has its own spectral response curve to sunlight, which indicates the sensitivity of the solar cell to different wavelengths of light (photoelectric conversion capability). From the data given in Table 1 , we can see the cut-off wavelength of photovoltaic materials and solar energy absorption efficiency. When sunlight hits a solar cell, the light of a certain wavelength and the spectral sensitivity of the solar cell at that wavelength determine the photocurrent value of that wavelength, and the total photocurrent value is the sum of the photocurrent values of each wavelength.
|Material||Band gap (ev)||Cutoff wavelength (μm)||Absorption efficiency of solar energy (%)|
3) I-V characteristics of solar cells
The electrical characteristics of solar cell modules mainly refer to the I-V output characteristics, also known as the I-V characteristic curve, as shown in Figure 1 . The I-V characteristics of solar cells are similar to those of diodes. The I-V curve focuses on: short circuit current (ISC), open circuit voltage (Voc) and maximum power (Pm). The conversion efficiency of solar cells is affected by illumination and ambient temperature.
The IV characteristic curve shows the relationship between the current Im of the solar cell module and the voltage Vm under a specific solar irradiance. Im is the maximum operating current, that is, the current in the maximum output state; Vm is the maximum operating voltage, that is, the maximum output state. voltage. If the external circuit of the solar cell module is short-circuited, that is, V=0, the current at this time is called the short-circuit current ISC; if the external circuit is open, that is, I=0, the voltage at this time is called the open-circuit voltage Voc. The output power of a solar cell module is equal to the product of the current and voltage flowing through the module, that is, P=VxI.
When the voltage of the solar cell module rises, 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 At this time, when the resistance value 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. When the output power of the component reaches the maximum 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 voltage). 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, approximately 5mV per solar cell for every 1°C increase in temperature, equivalent to a typical temperature coefficient of -0.4%/°C at the maximum power point. That is, if the solar cell temperature increases by 1°C, the maximum power decreases by 0.4%. Therefore, in the summer when the sun is directly exposed to the sun, although the amount of solar radiation is relatively large, if the ventilation is not good, the temperature of the solar cell may be too high, and it may not output a lot of power. The solar cell temperature change and I-V curve are shown in Figure 2.
Solar cell sunshine intensity-maximum output characteristics are shown in Figure 3. The short-circuit current of the solar cell is proportional to the sunshine intensity. The solar cell temperature-maximum output characteristics are shown in Figure 4. The output of the solar cell decreases as the surface temperature of the solar cell rises, and the output varies with the temperature of the season. At the same insolation intensity, the output in winter is higher than that in summer.
Since the output power of a 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 is carried out under standard conditions (STC), which is defined by the European Commission as Standard No. 101 , set the test under the condition that the surface temperature of the solar cell is 25℃ and the solar radiation intensity is 1000W/m², which is called the standard test state, as shown in Figure 5.
Under this condition, the maximum power output by the solar cell module is called peak power, which is expressed as Wp (peakwatt). In many cases, the peak power of the solar cell module is usually measured by a solar simulator and standardized by international certification agencies. Solar cells for comparison.
It is usually difficult to measure the peak power of a solar cell module outdoors because the actual spectrum of sunlight received by a solar cell module depends on the atmospheric conditions and the position of the sun; in addition, during the measurement process, the temperature of the solar cell is also constantly variable, so the error in outdoor measurements can easily reach 10% or more.
If the solar cell module is blocked by other objects (such as bird droppings, tree shade, etc.) for a long time, the blocked solar cell module will be seriously heated at this time, which is the “heat island effect”, which will cause serious damage to the solar cell. of destruction. Some or all of the energy produced by illuminated solar cells may be consumed by shaded solar modules. In order to prevent the solar cell 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 avoid the energy generated by the illuminated solar cell module being consumed by the shaded solar cell module. The function is to provide a current path when the module is open or shaded, so that the entire string of solar cell modules will not fail.
When using solar cells, pay attention to the polarity. 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 consumes no power. However, the withstand voltage value and allowable forward current value of the bypass diode should be greater than the operating voltage and current of the solar cell module.
The junction box of the solar cell is a very important component, which protects the wires and other system components that connect the solar cell to the outside world and the interior of each component. The junction box contains a junction box and 1 or 2 bypass diodes.
The main technical parameters of solar cells are:
(1) Photoelectric conversion efficiency. Photoelectric conversion efficiency is an important index to evaluate the performance of solar cells, and the conversion efficiency of solar cells refers to the ratio of solar cells to convert the received light energy into electrical energy;
In the formula, η is the photoelectric conversion efficiency; P0 is the output power; E is the solar energy irradiated by the solar panel.
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 cells is 29%, while the conversion efficiency of current solar cells is 17% to 19%. Therefore, solar cells There is still a lot of room for development in technology. At present, the laboratory conversion efficiency η≈24%, and the industrialized conversion efficiency η≈15%.
(2) The voltage V of the single solar cell is 0.4~0.6V, which is determined by the physical properties of the material.
(3) Fill factor FF is an important indicator for evaluating the load capacity of solar cells:
In the formula, ISC is the short-circuit current: Voc is the open-circuit voltage; Im is the optimum working current; Vm is the optimum working voltage.
The power output capability of a solar cell is closely related to its area. The larger the area, the greater the output power under the same lighting conditions. The advantages and disadvantages of solar cells are mainly measured by the two indicators of open circuit voltage and short circuit current.
(4) The influence of temperature on the performance of solar cells, the ambient temperature and the temperature of solar cell components directly affect the performance of solar cells, and when the temperature increases, the open-circuit voltage decreases, which is linear. Solar cells of different materials have their own operating temperature range. For a certain type of solar cell, the optimal load required to obtain the maximum output power is also different at different temperatures. For example, under standard conditions, AM1.5 light intensity, t=25°C, the output power of a certain type of solar cell is 100Wp. If the solar cell temperature rises to 45°C, the solar cell output power will not reach 100Wp.