Photovoltaic cell is a non-mechanical device that uses the photovoltaic effect of semiconductor materials to directly convert sunlight energy into electrical energy. In people’s opinion, photovoltaic cells should be the “master” who is not afraid of the sun. Under the scorching sun, it should be a time when photovoltaic cells can do a lot. Is it really so?
Photovoltaic effect and photoelectric effect
The use of solar energy by humans has a long history. The conversion and utilization of solar energy can be divided into three main methods: photoelectric conversion, photothermal conversion, and photochemical conversion. Photovoltaic power generation is the use of solar photovoltaic conversion.
In 1839, French scientist Edmund Becquerel discovered that light can produce potential differences in different parts of semiconductor materials. This phenomenon was later called the photovoltaic effect, or photovoltaic effect for short.
In 1905, Einstein successfully explained the photoelectric effect with the light quantum hypothesis, so he won the Nobel Prize in Physics in 1921. The photoelectric effect is a phenomenon in which electrons are emitted from the conductive material when light is irradiated on it. Unlike the photoelectric effect that occurs on a single conductive plate, the photovoltaic effect occurs on the boundary of two semiconductor plates and accumulates along the boundary to form an electric field. When a wire is used to connect two boards, current will flow.
The original solar cell was designed and manufactured according to the photovoltaic effect. When the surface of the cell is exposed to light, current will flow through the external circuit.
Photovoltaic power generation, heat is inevitable
How do photovoltaic cells convert sunlight into electricity? Sunlight is electromagnetic radiation with a wide range of wavelengths. When it is irradiated on a photovoltaic cell, the radiation may be reflected, absorbed or directly passed through. Only the absorbed radiation can be converted into electrical energy.
For silicon semiconductors, it takes about 1.11 electron volts to “bump” electrons from its atoms at room temperature. This means that only photons with absorbed energy higher than that energy can excite electrons and generate current. If the energy of a photon is 1.7 electron volts, and the energy required to “hit” the next electron from a silicon atom is 1.11 electron volts, then the remaining part of the energy (0.59 electron volts) will be lost as heat . Of course, there are other heat production factors that will affect the efficiency of power generation. The consequence is that these factors work together to heat the solar cell module to a temperature higher than the ambient temperature.
The sun is like fire, a “double-edged sword”
People originally thought that in the scorching summer, silicon-based photovoltaic cells would be extremely “excited” and generate more electricity. Who knows that silicon-based photovoltaic cells also like a cool environment. Of course, photons are still needed, after all, they are “raw materials” for power generation!
So, how does the temperature of the solar panel affect the electrical output performance? In fact, the solar panel temperature is high, the energy produced under the same light conditions will be reduced.
As the temperature increases, although the short-circuit current (the current when the positive and negative electrodes of the photovoltaic cell are short-circuited) is basically unchanged or slightly increases, the open circuit voltage (the voltage when the positive and negative electrodes of the photovoltaic cell are open) will decrease a lot, almost Linear relationship. The consequence of this is that the conversion efficiency of photovoltaic cells decreases and the output power decreases.
The standard test temperature for photovoltaic cells is 25°C. If the temperature of the solar panel reaches above 60°C, the reduction in output power cannot be ignored. Generally speaking, for every 1°C increase in silicon-based photovoltaic cell modules, the short-circuit current will increase by 0.04% and the open circuit voltage will decrease by 0.4%.
However, despite the fact that under the same light conditions, the increase in temperature will reduce the conversion efficiency, but in the hot summer, the electricity harvested by the sun will still be more than in other seasons.
How to cool down photovoltaic cells
Photovoltaic cells, like other electronic devices, have higher working efficiency at lower temperatures. Because photovoltaic power generation uses light rather than heat, photovoltaic cells are more suitable for sunny and cool working environments.
How can we cool down photovoltaic cells in the hot summer? How about adding a parasol? No! The reason is very simple. Without light, photovoltaic power generation will become empty. How about some “sunscreen”? Nor! The use of physical sunscreen is tantamount to reducing the absorption of light; while the use of chemical sunscreen does not help lower the temperature.
For rooftop solar panels, natural ventilation cooling is an economical and practical method. For example, a certain gap is left between the roof surface and the panel during installation to allow airflow to cool the panel. However, it is necessary to avoid leaves and other debris entering the gap to prevent the temperature from becoming too high due to poor airflow.
Someone has studied the influence of different cooling methods on the efficiency of solar power generation. In addition to natural circulation cooling, forced circulation cooling and solar photovoltaic thermal cooling are also included in the experimental research, which undoubtedly has important guiding significance for reducing the temperature of photovoltaic cells and improving power generation efficiency.
As a messenger of clean energy, photovoltaic cells have entered our lives and brought us a breeze of low carbon and environmental protection.