Characteristics of photovoltaic cells and photovoltaic panels

Characteristics of photovoltaic cells and photovoltaic panels

The energy conversion device for photovoltaic power generation is a photovoltaic cell, which is an active element for power generation. It uses the photovoltaic effect to directly convert light into electrical energy, so that the object under the action of the light generates an electromotive force in a certain direction. When sunlight (or other light) shines on the solar cell, the cell absorbs the light energy and produces a photoelectron-hole pair. Due to the large area of the PN junction, under the action of the built-in electric field of the battery, the photogenerated electrons and holes are separated, and the accumulation of opposite signs occurs at both ends of the battery, generating an electromotive force. If electrodes are drawn on both sides of the built-in electric field and a load is connected, the load will have “photo-generated current” flowing through it, thereby obtaining power output. In this way, the light energy of the sun directly becomes electric energy that can be put into use.

Photovoltaic panels, also known as solar cell arrays, are composed of many photovoltaic cells connected in series and parallel, plus supporting substrates that increase mechanical strength, and surface strengthened glass coverage. The array is formed by connecting the solar cells in a certain way according to the required voltage and current. Because the photovoltaic cell will be affected by factors such as the intensity of sunlight, temperature, and materials, its output will vary. Therefore, in order for the photovoltaic panel to exert its maximum output, its instantaneous output power must be controlled so that it can output the maximum power under different environmental conditions. A panel composed of many solar cells is always less efficient than a single cell, and the entire panel may be blocked due to external reasons, while the rest of the panel is still in the sun. The blocking two-pole method is usually used to prevent a certain parallel branch from being blocked and causing breakage with the solar panel, damaging all the solar cells connected in series in the parallel branch.

Anti-reverse charge diode

The anti-reverse charge diode, also known as the blocking diode, is connected in series between the energy storage battery and the photovoltaic array. Its function is to prevent the photovoltaic cell array from generating electricity during rainy days and at night, or when the voltage generated by the photovoltaic cell during the day is lower than the DC bus voltage of its power supply, the battery pack reversely transmits power to the photovoltaic array, which consumes the energy of the battery and causes the photovoltaic panel to heat up. It is connected in series in the photovoltaic cell square array circuit, acting as a one-way conduction. It is required to be able to withstand a large enough current, and the forward voltage drop should be small, and the reverse saturation current should be small. Generally, suitable rectifier diodes can be used.

Anti-hot spot effect

Under certain conditions, the shaded photovoltaic cell module in a series branch will be used as a load, consuming the energy generated by other photovoltaic cell modules that are illuminated. The shaded photovoltaic cell module will generate heat at this time, which is the hot spot effect. This effect can seriously damage photovoltaic cells. Some or all of the energy produced by photovoltaic cells with sunlight may be consumed by photovoltaic cells that are shaded. In order to prevent the hot spot effect of photovoltaic cells, a bypass diode needs to be connected in parallel between the positive and negative poles of the photovoltaic cell module to prevent the energy generated by the light module from being consumed by the shaded module. The bypass diode usually does not work, withstands reverse bias, and there is no energy consumption during normal operation.

Isolation diode

When the photovoltaic array is connected in parallel by several series, diodes should be connected in series in each series, and then connected in parallel, as shown in Figure 1, in order to prevent a certain series of arrays from being blocked or failing to consume energy and affect the energy output of other normal arrays. This diode becomes an isolation diode. Diode selection usually uses rectifier diodes, and its capacity selection should leave a certain margin, its current capacity should be able to reach twice the expected maximum operating current, and the withstand voltage capacity should be able to reach twice the reverse maximum working voltage.

Figure 1 – Series and parallel connection of photovoltaic modules and the application of diodes