(1). Characteristics of photovoltaic cells
Solar cell characteristics can be fully displayed by I-U characteristics. Therefore, it is particularly important to understand the output voltage U and output current I and how they change. In order to determine these characteristics, a photovoltaic cell characteristic measurement platform as shown in Figure 1 is established.
Open circuit voltage Uoc refers to the voltage measurement value in the case of open circuit (R=RMAX). In this case, the current is 0. As the resistance decreases, the current increases while the voltage decreases. The maximum current is also called the short-circuit current Isc, which is the current measurement value in the case of a short circuit (R=0). In this case, the voltage is 0. Figure 2 shows a typical I-U characteristic curve of a photovoltaic cell. Each photovoltaic cell has an I-U characteristic curve. The introduction of Isc and Uoc is to help describe the characteristics of photovoltaic cells.
(2). Photovoltaic cell power curve
The power P produced by photovoltaic cells is the product of voltage and current under specific operating characteristics, namely
Therefore, when I or U is 0, P is 0. This situation occurs when the circuit is short-circuited (U=0 at this time) or open (I=0 at this time).
Drawing the power curve through the I-U characteristic curve, we can see how the power changes between the two extreme cases, as shown in Figure 3.
When U=Ump, the current is Imp at this time, and the photovoltaic cell output power at this point is the maximum (Pmp), which is called the maximum power point (MPP). It is important to ensure that the photovoltaic cell is operating at (close to) the maximum power point.
(3). Photovoltaic cell performance
Many factors affect the efficiency of photovoltaic cells, some of which are inherent in the manufacturing process, while others depend on operating conditions.
The efficiency of a photovoltaic cell is the ratio of the power incident on the cell to the power produced by the cell. Under ideal conditions, all incident energy can be converted into electrical energy, but in reality, this situation cannot be achieved. The pie chart in Figure 4 shows the typical losses of photovoltaic cells. The loss resulted in an overall conversion efficiency of about 17%, which is the conversion efficiency of photovoltaic cells. The average conversion efficiency of commercial silicon cells is about 14%-17%, while in a laboratory environment, a conversion efficiency of 24% can be obtained. The improvement of battery conversion efficiency is a current research hotspot.
The conversion efficiency of commercial battery cell manufacturers varies from 12% to more than 20%.
The fill factor FF reflects the size of the series resistance and shunt resistance in photovoltaic cells and circuits. The fill factor is the maximum power and the short-circuit current Isc. The ratio of the product of the open circuit voltage Uoc and the open circuit voltage Uoc is an operating index that characterizes the performance of the photovoltaic cell.
The decrease in the fill factor indicates that the battery may have a problem. The calculation formula of the fill factor is
The typical fill factor value range is 0.6~0.7.
A more important application of fill factor is to determine the performance of components under low light conditions. If the fill factor is high, it means that the I-U curve of the component is quite smooth, as shown in Figures 5 and 6.
It can be seen that if the fill factor is small (Figure 6), when the radiation is weak, the module I-U characteristic shows that the battery is not charged, because the voltage is not within the voltage range required for battery charging.
Therefore, fill factor may be important when choosing photovoltaic modules in areas where radiation is always low.
(4) Factors affecting the performance of photovoltaic cells
When the temperature of the photovoltaic cell rises, the open circuit voltage Uoc decreases and the short circuit current Isc slightly increases. The combined effect is power reduction, as shown in Figure 7.
According to experience, for crystalline silicon cells, the output power changes by 0.5% for every 1°C change in temperature. When the temperature is above 25°C, the output power decreases; when the temperature is below 25°C, the output power increases. It can be seen from Figure 7 that when the temperature rises, the voltage decreases and the current increases. The voltage change (percentage) is very similar to the power change percentage, the same is that for every 1°C change in temperature, the voltage change is about 0.5%.
If photovoltaic cells (in the form of modules) are installed horizontally on the roof, it will become difficult to dissipate heat through convection cooling. If a bracket is used to support the module, then sufficient air flow can be provided around the module. However, many customers and architects of photovoltaic grid-connected want to integrate photovoltaic modules with the roof. Therefore, the form of ventilation must be considered in the design. Reduce the adverse effects of high temperature.
The rated temperature of photovoltaic cells is 25°C. However, under normal operating temperature conditions, the battery temperature is generally higher than the ambient temperature, that is, 25°C higher than the battery temperature under standard experimental conditions (STC). The standard experimental conditions give conditions under which all batteries can be compared, and the nominal operating temperature (NOCT) better predicts the expected output power of the battery under rated operating conditions. It is worth noting that the battery can still operate at a temperature higher than NOCT, and is usually 25°C higher than the ambient temperature, depending on the battery technology, photovoltaic module design and installation technology.
When the radiation intensity changes, the change of the short-circuit current is approximately linear, while the open circuit voltage change is not very significant. It increases slightly with the increase of radiation, as shown in Figure 8. The figure assumes that the battery temperature is a constant that is not affected by the change of radiation. .