Solar energy is a type of energy produced by the nuclear fusion of hydrogen in the sun. Only about one in 2.2 billion of the energy emitted by the sun can reach the Earth's atmosphere. At the upper limit of the Earth's atmosphere, it is about 1367W per square meter. When it reaches the photovoltaic module, it is converted into direct current. According to the current efficiency of 18.3% for a 300W monocrystalline module, it is about 183W. Where does the remaining 1184W of energy go?
1. Absorbed and reflected by the atmosphere
The Earth's atmosphere, spanning thousands of kilometers, is divided into the troposphere, stratosphere, mesosphere, thermosphere, and exosphere. Approximately 30% of the sun's energy is reflected into space, and about 19% is absorbed by clouds and the atmosphere, becoming wind, thunder, rain, and lightning. About 51% of this energy reaches the Earth's surface. Since most of the Earth's surface is covered by oceans, only about 10% of the energy radiated across the globe actually reaches the land surface. Even so, utilizing this remaining energy would be equivalent to 35,000 times the current global energy consumption.
2. The battery module only absorbs the energy from the visible light portion.
The solar spectrum: Sunlight is a mixture of continuously varying wavelengths of light, including various wavelengths such as infrared, red, orange, yellow, green, blue, indigo, violet, and ultraviolet. Red, orange, yellow, green, indigo, blue, and violet are visible light, visible to the human eye. The longer wavelengths are red light, and wavelengths even longer than red light are infrared light. The shorter wavelengths are violet light, and wavelengths even longer than violet light are ultraviolet light. Although the solar spectrum has a wide wavelength range, from a few angstroms to tens of meters, the distribution of radiant energy according to wavelength is uneven. The region with the highest radiant energy is in the visible light region, accounting for approximately 48%, the ultraviolet spectral region accounts for about 8%, and the infrared spectral region accounts for about 44%. Within the entire visible spectrum, the highest energy is at a wavelength of 0.475 μm. Solar cells can only absorb a portion of this energy and convert it into electrical energy. Energy conversion is not possible in the ultraviolet spectral region, and the excessively long wavelengths in the infrared spectral region can only be converted into heat.
In the solar spectrum, different wavelengths of light possess different energies and contain different numbers of photons. Therefore, the number of photons produced by a solar cell when exposed to light varies. Generally, silicon solar cells do not respond to ultraviolet light with wavelengths shorter than approximately 0.35 μm and infrared light with wavelengths longer than approximately 1.15 μm, with the peak response ranging from 0.8 to 0.9 μm. This response peak is determined by the solar cell manufacturing process and material resistivity; at lower resistivity, the peak spectral response is around 0.9 μm. Within the spectral response range of a solar cell, the longer wavelength region is typically referred to as the long-wavelength spectral response or red light response, while the shorter wavelength region is referred to as the short-wavelength spectral response or blue light response. Essentially, the long-wavelength spectral response primarily depends on the minority carrier lifetime and diffusion length in the substrate, while the short-wavelength spectral response primarily depends on the minority carrier lifetime in the diffusion layer and the recombination rate at the front surface.
Currently, there are two main methods to improve battery efficiency. One is to research new battery materials and broaden the range of their response spectra. For example, cascaded solar cells integrate sub-cells made of semiconductor materials with different spectral responses, making full use of various wavelengths of the solar spectrum. Multi-junction cell technology can further improve utilization. The second method is to improve cell manufacturing processes, such as diamond wire cutting, surface passivation technology, and laser processing technology, to increase solar energy utilization.
3. Component encapsulation loss
After encapsulation into modules, approximately 2 percentage points of total area efficiency are lost because the module area is larger than the total cell area. Secondly, 0.5 percentage points are lost due to light absorption by the photovoltaic glass and 0.5 percentage points due to light absorption by the EVA film. Thirdly, 1 percentage point is lost due to resistance in the interconnect/busbar strips. The total loss is approximately 4 percentage points. With the continuous development of module technology, multi-busbar modules, double-glass frameless modules, and MWT back-contact busbarless modules are now available, which can reduce module encapsulation losses to below 1%.
