A detailed explanation of the structure and working principle of the photovoltaic array

A detailed explanation of the structure and working principle of the photovoltaic array

Solar energy is a low-density planar energy source that needs to be collected by a large-area solar cell array. However, the output voltage of solar cell modules is not high, and a certain number of solar cell modules need to be connected in series and parallel to form a square array. A photovoltaic array contains two or more photovoltaic modules. The specific number of modules required and how to connect the modules are related to the required voltage (current) and the parameters of each module, which are determined by the system design.

  1. Structure

The structure of the flat-panel photovoltaic array depends on the needs of users. According to the voltage level, the voltage of the independent photovoltaic system is often designed to correspond to the nominal voltage of the battery or an integer multiple of them, and is consistent with the voltage level of the electrical appliance, such as 220V, 110V, 48V, 36V, 24V, 12V Wait. For AC photovoltaic power supply systems and grid-connected power generation systems, the voltage level of the square array is often 110V or 220V. For photovoltaic power station systems with higher voltage levels, multiple square arrays are often used in series and parallel, and they are combined into the same voltage level as the grid level, such as 600V, 10kV, etc., and then connected to the grid. For more information on battery voltage, please visit Tycorun Battery.

In addition to the need for brackets to gather many solar cell modules together, the solar phalanx also requires cables, blocking diodes and bypass diodes to electrically connect the solar cell modules, and requires a special branch junction box and a main junction box with built-in arresters. . Sometimes in order to prevent the hot spot effect caused by bird droppings contaminating the surface of the solar cell array, it is necessary to install a bird repellent on the top of the array.
The electrical connection diagram of the solar cell array is shown in Figure 1.

Figure 1 - Electrical connection diagram of solar cell array
Figure 1 – Electrical connection diagram of solar cell array

When assembling solar cell modules in series and parallel to form a square matrix, the principles that need to be paid attention to in series and parallel of solar cells should be referred to, and special attention should be paid to the following points:

(1) Components with the same operating current are required in series, and bypass diodes are connected in parallel for each component;
(2) Components with the same working voltage are required in parallel, and blocking diodes (anti-reverse charging diodes) are connected in series to each parallel line;
(3) Try to consider the principle of the shortest component interconnection wiring;

(4) It is necessary to strictly prevent individual solar cell modules with deteriorated performance from being mixed into the solar cell array.
Figure 2 shows the same square matrix composed of 64 solar cell modules with 4 parallels and 8 series, but there are two different electrical connections: vertical and horizontal and horizontal and vertical. As can be seen in Figure 2, when encountering partial shadows, the bus voltage connected in (a) drops, the output battery also drops significantly, and the system may not work properly; while the bus voltage connected in (b) can remain unchanged. Change, although a group of current is missing, but the system can work normally.

Figure 2 - Solar cell module square array
Figure 2 – Solar cell module square array
  1. Features

The basic characteristics of solar cell modules and square arrays are also used under IEC standard conditions: open circuit voltage UOC, short circuit current ISC, optimum working voltage Um, optimum working current Im, optimum output power Pm, fill factor FF and photoelectric The conversion efficiency η is expressed as:

Here, P0 is the solar radiation energy received on the potential area. According to IEC standard, under the condition of 25℃, AM1.5 spectrum, P0=100mW/cm2. Aa is the area of ​​the component and the square matrix, which usually refers to the actual area of ​​the component and the square frame, and the efficiency at this time is the efficiency of the component and the square matrix.

The phalanx works outdoors, and the output power and efficiency are greatly affected by temperature and solar irradiance. Good ventilation can reduce the operating temperature of the components and increase the output of the phalanx. Fig. 3 is the light volt-ampere characteristic curve of the solar cell square array. The nominal power of the square array is the maximum output power Pm under IEC standard conditions.

Figure 3-Light volt-ampere characteristic curve of the solar cell array
best working point
Figure 3-Light volt-ampere characteristic curve of the solar cell array
best working point

When the components are combined into a square matrix, there will be voltage loss and current loss, and thus there will be a loss of output power. The power loss factor of the square array can also be referred to as the combination factor ηa of the photovoltaic square array. When n components are combined into a square matrix, the combined loss factor can be expressed as:

In the formula, Pm——the actual output power of the square matrix;
Pmi – the output power of each of the n components.
The main sources of power loss of the square array are inconsistent component characteristics, series-parallel diodes, and wiring losses.

  1. Measurement

The measurement of photovoltaic arrays is not easy to perform under standard conditions, but is usually tested with a portable photovoltaic array tester under natural sunlight and then converted to IEC standard conditions. Special attention is to place the standard reference cell and the photovoltaic array under test on the same plane. Before measurement, cover the measured array with an opaque cover, and wait for the temperature to be the same as the ambient temperature and suddenly remove the cover. After measuring the light volt-ampere characteristic curve and basic parameters of the square array in the shortest possible time, and comparing with the standard reference battery, the output power of the square array under the IEC standard condition is calculated.

  1. Hot spot effect

The phenomenon that a phalanx has localized hot spots in sunlight is called the hot spot effect, and this kind of hot spot often occurs on a single cell. For example, a shaded solar cell module in a series branch will be used as a load to consume the energy generated by other illuminated solar cell modules, and the shaded solar cell module will heat up at this time, which is the hot spot effect. This effect can seriously damage solar cells, and part of the energy generated by illuminated solar cells may be consumed by shaded cells. In larger square arrays, the temperature of hot spots may be as high as about 200°C in severe cases. The hot-spot effect can melt solder joints, destroy packaging materials (eg, no bypass diode protection), or even disable the entire phalanx. The mechanism of the hot spot effect is shown in Figure 4. In Figure 4, 12 solar cells are connected in 3 parallel and 4 series,

Figure 4 - Schematic diagram of hot spot effect of solar cell modules
Figure 4 – Schematic diagram of hot spot effect of solar cell modules

Assuming that each cell has the same light I-U characteristic, the light I-U characteristic of every three cells in parallel is shown on the left side of the figure. The voltage and current of each node after the 4 groups of parallel batteries are connected in series are: U1, I, U2, I, U3, I, U4, I. When for some reason the left solar cell in the 2nd group is suddenly damaged and there is almost no current output, the total current 3I in the entire series circuit will flow through the two cells on the right. From the parallel characteristics of solar cells, it can be seen that when these two good cells are subjected to a photo-generated current that exceeds their operating point, the voltage corresponding to their operating point enters the reverse bias region U2. Sometimes the absolute value of U2 can be several times larger than the open circuit voltage of the good cell. Thus, in this parallel battery pack with one bad cell, the power experienced on the other two and the battery is U2I, and the power experienced in parallel battery packs 1, 3, 4 without the bad cell is UI, because U2 is Several times of U, so the two good batteries in parallel with the bad batteries began to heat up rapidly, and the entire square array appeared hot spots in the second group.

The output of other good battery packs in the phalanx is also affected before the irreversible hot spot effect occurs. Since the terminal voltage U4 of the phalanx is connected to the battery or the controller, all other good batteries also share the influence of the bad battery pack, resulting in a drop in the output power of the phalanx.

The root cause of the hot spot effect is the mixing of individual bad batteries, the virtual welding of electrode pads, the evolution of the battery from cracks to breakage, the deterioration of individual battery characteristics, and the partial shadowing of the battery.
In order to avoid the hot spot effect, in addition to eliminating the above situation, the main method is to add a bypass diode to increase the reliability of the square matrix. If all components are connected in parallel, bypass diodes are not required, that is, if the output voltage of the array is required to be 12V, and the output of each component is exactly 12V, there is no need to add bypass diodes to each component. If 24V is required Array (or higher), then there must be 2 (or more) components in series, and bypass diodes need to be added.

  1. Blocking diodes

Blocking diodes are used to control current flow in a PV system, and any stand-alone PV system must have a way to prevent reverse current flow from the battery to the array or have a way to protect failed cells. If the controller does not have this function, a blocking diode should be used. As shown in Figure 4-20, a blocking diode can be added to each parallel branch or the trunk between the array and the controller. However, when multiple branches are connected in parallel to form a large system, blocking diodes should be used on each branch to prevent the current from flowing from the strong current branch to the weak current branch due to branch fault or shading. In a small system, it is enough to use one diode on the main circuit, not both, because each blocking diode will cause a step-down of 0.4~0.7V, and its voltage loss is 6% of a 12V system, which is also an inconvenience small proportions.

Read more: How much do you know about the different types of solar power systems?