- The relationship between voltage, current and temperature
Figure 1 shows the effect of temperature changes on photovoltaic cell or module voltage. When the temperature decreases (cooler), the voltage of the device increases, and when the temperature increases (warmer), the voltage of the device decreases. The temperature here is the surface temperature of the photovoltaic cell or module measured under Standard Test Conditions (STC). (This voltage-temperature relationship is characteristic of crystalline silicon cells only and not applicable to all photovoltaic cell types.) Designers and installers should consider cooling needs when installing modules. When the temperature of the battery increases, its power also decreases in proportion to the voltage, because as we have observed, the current increases with temperature very little and is not enough to compensate for the power loss caused by the voltage drop.
- Electrical and thermal properties
Electrical and thermal properties of photovoltaic devices are available from the manufacturer. These parameters can also be found on the device nameplate or in the manufacturer’s catalog. Section 690.7 of the 2008 National Electrical Code defines a correction factor for the maximum voltage of crystalline silicon components at ambient temperature. If there is no manufacturer’s data, you can refer to Table 1, which is an example of manufacturer’s parameters.
- Temperature coefficient
Manufacturers report the effect of temperature on a device as a temperature coefficient, V/°C, or %/°C. The temperature coefficient is the rate of change in device voltage, current, or power due to changes in battery temperature. A rise or fall in temperature can cause changes. A negative (-) temperature coefficient indicates that the measured value decreases with increasing temperature and increases with decreasing temperature; conversely, a positive (+) temperature coefficient indicates that the measured value increases with increasing temperature. large and decreases with decreasing temperature.
Voltage temperature coefficients and power temperature coefficients are generally negative, and current temperature coefficients are generally positive. Unless otherwise noted, the standard reference temperature is 25°C (770°F), and this example applies to crystalline silicon cells. This method can also be used for other types of batteries, but the temperature coefficient may be different and the trend of change may not be the same. Because temperature changes have little effect on battery current, the current temperature coefficient may not be listed.
Cell temperature is a function of ambient temperature, solar radiation intensity, wind speed, cell packaging, and how the components are mounted. The manufacturer will identify the temperature coefficient of the component. The temperature coefficients of the voltage, current and power of the battery cells can be converted into the temperature coefficients of the components. The module voltage temperature coefficient (Cv-module) can be calculated from the temperature coefficient of the cell (Cv-cell) by the following formula:
In the formula: N is the number of series monomers. The component current temperature coefficient (C1-component) is
In the formula: Np is the number of parallel strings; Ac is the area of one string (cm2).
Calculate the temperature coefficient of a PV module. The assembly contains 54 battery cells, which are connected by two strings in parallel
Example: In the specifications of photovoltaic products, the temperature variation coefficient is also expressed as %/℃. This percentage is the unit temperature variation coefficient. Convert the percentage to a decimal and multiply by the reference (nominal) value to get the unit change. The unit temperature coefficient of variation applies to voltage, current, and power, but temperature has little effect on current, so it may not be listed in the specification. In this example, when converting the coefficient %/°C into unit change/°C, the specification parameters of the components are used: Voc (component)=21V@STC 25°C, cell voltage temperature coefficient (Cv-cell)=-0.369 %/℃, the temperature coefficient of the power of the cell (Cp-single) = -0.480%/°C, the maximum power is 87W, and now the unit change of the component is calculated by the temperature coefficient of the cell voltage:
In this way, the coefficient of the monomer is converted to the coefficient of the assembly by the unit temperature variation coefficient.