The performance of photovoltaic arrays is affected by many factors: here are some of the most influential factors (irradiance, temperature and shading).
① Irradiance
The amount of solar radiation (sunlight) received by a photovoltaic cell largely determines the output power. The output of the photovoltaic array can be estimated using the performance data provided by the manufacturer’s data sheet. All photovoltaic arrays have a rated peak output power, that is, if an array is a 1.5kWp array, it means that the installed photovoltaic modules can provide 1.5kW peak power. The peak output power is determined by the manufacturer using standard test conditions. With this information and local solar radiation data, the output power of the photovoltaic array can be estimated.
For example
In fine weather, the 2kWp photovoltaic array receives 6 PSH of solar radiation: 6PSH is equal to 6000Wh/m2 of energy received per day. The output power is calculated as follows:
Peak output power x solar peak hours (PSH) = expected output electric energy
2kW x6PSH = 12kWh
The photovoltaic array produced 12kWh of electricity in that day (not included in the power loss factor). The monthly solar radiation data is changing, so the annual output power of the photovoltaic array can be estimated from the monthly radiation data. However, this method cannot be applied in practice because photovoltaic systems rarely work under STC.
| month | Daily average solar peak hours (inclination 31°) | month | Daily average solar peak hours (inclination 31°) |
| January | 5.38 | July | 4.04 |
| February | 5.11 | August | 4.68 |
| March | 4.84 | September | 5.33 |
| April | 4.42 | October | 5.51 |
| May | 3.87 | November | 5.44 |
| June | 3.90 | December | 5.57 |
| Annual average | 4.84 | 4.84 | 4.84 |
2kWp photovoltaic array can output an average of 2kW x 4.84 PSH/day=9.68kWh/day. The annual average output power is 9.68kWh/day x 365 days/year = 3533. 2kWh/year.
②Temperature
The solar radiation received by the module not only generates electricity, but also heats the module. Under suitable climatic conditions, it is not common for the temperature of photovoltaic modules to reach 70°C on a sunny day. As the temperature rises, the open circuit voltage drops rapidly, while the short-circuit current rises slowly. The output power is the voltage multiplied by the current, so it will also decrease. When designing a system, engineers usually use the following approximate formulas (according to local design codes and guidelines):
Battery temperature = ambient temperature + 25C
Because high temperature in turn affects the output power, the output power calculation of the photovoltaic array must consider the temperature effect, that is, reduce the output power of the array based on the operating temperature conditions. Similarly, low temperature conditions can increase the voltage, thereby increasing the output power. At the same time, it is necessary to accurately calculate the maximum voltage limit of the system to ensure that the voltage does not exceed the rated range of the inverter.
The installation method of the photovoltaic array directly affects the working temperature of the array itself. When the array is installed close to the roof surface, there is a lack of air flow on the back of the module to reduce the temperature of the module. This has two negative effects. The roof itself has to dissipate heat (especially tin roofs), and heat is collected under the modules, so it is usually necessary to ventilate the roof surface and the back of the modules. If the installation method does not include these two parts of heat dissipation, the output power of the array must be reduced to reflect this negative effect.
③Block
Solar cells need sunlight to generate electricity. If shading causes the photovoltaic cell to not be exposed to sunlight, it will not output power (even a small piece of shading on the battery will also cause a large amount of output power loss). The batteries in the module are usually connected in series, so when one or several batteries are blocked, the module’s output current will decrease. If the component is part of an array, the array output current will also decrease. This can also happen if the block battery is damaged and cannot generate electricity.
Array occlusion can cause irreversible damage. When a battery is blocked, the output power of the battery decreases, and most of the current is generated by other batteries (unblocked), and is forced to flow through the blocked battery, resulting in heat generation and hot spot effect. It often leads to damage (cracking) of the battery and damage to the glass package.
Blocking is difficult to prevent. However, diodes can be used to bypass the effects of temporary shading (ie, leaves falling on the array). When a battery is blocked or damaged, the diode provides another path for the current, thereby completely skipping the damaged or blocked battery, reducing the impact on the output power of the array. This type of diode is called a bypass diode, and the manufacturer installs one, two, or three bypass diodes on each component.
