The solar panel standard is an important reference basis for its design and production. The physical and electrical properties of solar panels will be introduced.
The physical and electrical performance requirements of solar panels include the power size, size, load-bearing, installation, etc. of solar panels, and they need to meet standards such as IEC61215 and IEC61730 or UL 1703. Unreasonable or poorly considered design will cause unnecessary power loss due to mismatch effects, hot spot effects and other factors in the produced solar panels, thus failing to meet the design requirements. The solar panel is a product in which solar cells are connected in series and parallel according to the requirements of working voltage and output power, as shown in Figure 1, and then the solar cells are packaged with special materials. Therefore, the series-parallel connection of solar cells will have a significant impact on the electrical performance of solar panels. The following is a brief analysis of the impact of solar cells in series and parallel on the electrical performance.
(1) Circuit analysis of solar cells in series. The total voltage of the two batteries in series is equal to the sum of the voltages of the two batteries. The currents of the two batteries are equal, which means that the current mismatch between the two batteries makes the total current equal to the current of the smallest battery, such as
As shown in Figure 2.
There is no current mismatch after two batteries with equal short-circuit currents are connected in series. The total current is equal to the current of the two single cells, and the total voltage is equal to the sum of the voltages of the two single cells, as shown in Figure3.
In the current source formed by two single cells, since the current is generated by light and the battery pack is short-circuited, the forward current flowing through the single battery is zero, and the voltage of the battery pack is also zero.
If two batteries with unequal short-circuit currents are connected in series, there is a current mismatch. The total current is equal to the current of the battery with the smallest current among the two single cells, and the total voltage is equal to the sum of the voltages of the two single cells.
When the currents generated by the two batteries in series are not equal (for example, part of the irradiation light of battery 2 is blocked), then the short-circuit current of battery 2 is the maximum current flowing through the external circuit, and the extra current of battery 1 , Which is mathematically equal to Isc1-Isc2 will flow through battery 1 and generate a forward bias voltage applied to battery 1, which in turn generates a reverse bias voltage on battery 2. Since the total circuit is short-circuited, the total The voltage is zero, as shown in Figure 4.
(2) Circuit analysis of parallel solar cells. When two single batteries are connected in parallel, the total current is equal to the sum of the currents of the two single batteries, and the total voltage is equal to the voltage of the two single batteries, as shown in Figure 5 and Figure 6.
(3) Summary: The effects of series and parallel connection of solar panels on the electrical performance of solar panels are as follows:
1) The effect of series and parallel connection of the same parameters on the electrical performance of solar panels:
(a) When connecting in series, U0=U1 +U2 +…=nU1(U2, U3,…);
(b) When connected in parallel, I0=I1 +I2 +…nI1(I2, I3,…)
2) The effect of series and parallel connection of different parameters on the electrical performance of solar panels:
(a) When connected in series, the open circuit voltage of U is equal to the sum of the open circuit voltage of each sub-battery:
I short-circuit current is between the maximum and minimum short-circuit current;
U best is equal to the sum of the voltages of the remaining (n-1) batteries except for the battery with the smallest short-circuit current;
The best must be less than the minimum short-circuit current.
(b) When connected in parallel, the U open circuit voltage is between the maximum and minimum open circuit voltages of each sub-battery;
I best = working current of the sub-battery-working current of the battery with the worst performance.
(c) When in series and parallel, the voltage and current will be less than the theoretical value, so the calculated solar panels will decrease in power after the actual production is completed. The only way to solve this problem is to screen the cells and try to use cells with the same performance. On the same solar panel, this can significantly reduce the power attenuation of the solar panel. The number of series connection of single solar cells should be determined according to the nominal working voltage, and the number of parallel connection of solar cells should be determined according to the nominal output power. In order to save packaging materials, the solar cells should be arranged reasonably so that the total area is as small as possible. The peak output power of common solar cells and solar panels are 5W, 10W, 20W, 40W, 160W, 240W, 300W, etc. The overall size of the solar panel is related to the number of solar panels; the peak voltage is related to the total unit number of solar panels; the power is matched with the size of the solar panel to determine the power of a single cell. The photovoltaic modules of JA Solar are shown in Figure 7.
(4) The circuit equation of the solar panel. If the batteries in the solar panel have the same electrical performance, and in the same light and
At temperature, then the circuit equation of the solar panel is
Where: M is the number of batteries in series: n is the number of batteries in parallel: IT is the total current of the battery: IL is the electricity of a single battery
Flow; UT is the total voltage: N is the ideal factor of a single battery: k and T are physical constants.
The I-U curve of photovoltaic modules is shown in Figure 8.
The condition of the battery interconnection circuit in the solar cell module has a significant impact on the actual performance and working life of the module. When solar cells are connected in series and parallel, the operating characteristics of a single cell, such as current mismatch (or detuning), make the output power of the component less than the sum of the maximum output power of each cell. This power loss caused by mismatch is most obvious when the cells are connected in series. The mismatch effect of the module is caused by the cells in the module do not have the same electrical performance or have different electrical properties.
It includes current mismatch and voltage mismatch. Since the output power of the module is determined by the most
It is determined by the battery with small output power, so the mismatch of the module is a serious problem, such as the irradiated light of a battery in the module
When the battery is blocked and other batteries are still working, the power generated by the working battery is consumed on the non-working battery instead of the external load, which will cause local power consumption and local overheating. Cause irreparable damage to the components. The component mismatch caused by shading is also called the hot spot effect, and the hot spot is the specific manifestation of the mismatch effect.
Some literature pointed out that for monocrystalline silicon solar cells, the mismatch loss caused by monomer differences during the manufacturing process is about 0.2% to 1.5%. For amorphous silicon, the mismatch problem has not been studied enough, but there is a significant attenuation phenomenon in amorphous silicon. After a group of components are connected in series and parallel, individual differences will become very large in actual use, and the mismatch can be definitely The phenomenon is more serious and can even affect the normal use. The power loss of photovoltaic modules is shown in Figure 9, and the influence of a mismatched output cell on the series battery pack is shown in Figure 10.
In the case of a short circuit, the poorer output characteristics are reverse-biased and consume a lot of power. The current output of the series battery pack depends on the worst battery. When the current of one battery in a series of batteries is significantly lower than the current of the other batteries, the hot spot effect will occur. For example, when one battery in a series of batteries is blocked by the light, the current on the battery will be generated. Significantly smaller than other batteries. At this time, the total current is limited to a minimum. The positive bias generated on the battery that generates a large current will be added to the shielded battery, and the shielded battery will become a consuming load. The power generated by other batteries is released in the form of heat, which can cause damage to the components.
(5) Methods to reduce the hot spot effect. Use the parallel method to reduce the hot spot effect, as shown in Figure 11.
By increasing the number of series modules and parallel battery strings for each component or branch, the tolerance of the components to battery mismatch, battery rupture, and partial shadows can be improved. This is the series-parallel method.
The hot spot effect can also be avoided by connecting a bypass diode in parallel. The parallel diode and the solar cell have opposite electrode directions. Under normal operation, each solar cell will be at a forward bias voltage, and the bypass diode will be in a reverse direction. Under bias voltage, so the bypass diode is in an open state and does not work. However, when one battery in a string is reverse-biased due to current mismatch, the bypass diode will be forward-biased and conduct, so the current generated by the good battery will flow through the external circuit instead of being positive on the good battery. Bias voltage, so the maximum reverse bias voltage applied to a bad battery will reduce the voltage drop of the diode, thus limiting the current and preventing the hot spot effect.
In the case of a short circuit, there are currents that match each other, so the voltage applied to the solar cell and the bypass diode is zero, as shown in Figure 12.
In the case of current mismatch, the current flowing on a good battery is equivalent to adding a forward bias voltage, and at the same time adding a reverse bias voltage to the bad battery, but at this time, because there is a forward-biased bypass diode, Make the current flow through the bypass diode to avoid the power consumption on the bad battery, as shown in Figure 13.
In the case of an open circuit and the currents between the batteries match each other, the short-circuit current is forward biased to each solar cell and reverse biased to each bypass diode. At this time, the bypass diode does not work, as shown in Figure 14. Shown.
In the case of open circuit and current mismatch, although the voltage of the solar cell whose part of the irradiated light is blocked is reduced, the bypass diode is still reverse biased, so it does not work. If the irradiating light is completely blocked, the blocked battery at this time is equivalent to no photo-generated current, so the bypass diode still does not work, as shown in Figure 15.
If there is no bypass diode, when the battery is shaded and the current mismatch is caused, the bad battery will bear a larger voltage drop (depending on the number of batteries in series); if there is a bypass diode, it will be reverse biased. When the voltage is greater than the threshold voltage of the diode, the bypass diode is turned on, so that the voltage drop on the bad battery is only the threshold voltage of the diode, as shown in Figure 16.
Without the bypass diode, when a battery is blocked, the total current is equal to the current of the bad battery, and the total voltage is equal to the sum of the voltages of all batteries connected in series. At this time, the bad battery consumes a lot of power generated by the good battery. , As shown17 shown.
In the case of a bypass diode, when the battery is blocked, the bypass diode is turned on when the reverse voltage is equal to the threshold voltage (this voltage is equivalent to the open circuit voltage), so that the total current will not be reduced to the current of the bad battery Size, thereby reducing the power loss on the bad battery, as shown in Figure18. In practice, the cost of adding a bypass diode to each battery is relatively high, so in practice, a string of batteries always share a bypass diode, as shown in Figure19.
The bypass diodes in photovoltaic modules usually use ordinary rectifiers or Schottky diodes. Ordinary rectifier diodes are cheap, but when the hot spot effect of photovoltaic modules occurs, the forward voltage drop is relatively large (about 1V), which will generate a lot of heat. If the heat dissipation measures are not done well, it may burn the junction box and Photovoltaic modules can even cause fires. Compared with ordinary rectifier diodes, the forward voltage drop of Schottky diodes is smaller (about 0.3V), but its withstand voltage is low and the reverse leakage current is also larger. In order to solve the problems that may be caused by bypass diodes, MOS transistors are proposed to bypass photovoltaic modules. The on-resistance of the MOS tube is usually only a few milliohms, so the on-voltage drop of the MOS tube is small, and it comes with a heat sink, which is an ideal photovoltaic module bypass component.