Structure diagram of a tungsten wire used to make a "sunlight trap" under a microscope

At present, people can use solar cells to directly convert sunlight into electricity. They can also use sunlight to heat water and then use steam to drive turbines. These are mature technologies. But recently, the British "New Scientist" magazine website pointed out in the report that, in theory, we still have a third method, that is, directly use the heat of the sun to generate electricity. Moreover, American scientists have done this with tungsten-made equipment, with a photoelectric conversion efficiency of 37%, which is superior to the best silicon-based solar cells.

Solar heat can be directly converted into electricity

In order to generate electricity from sunlight, one can use photovoltaic cells to directly convert sunlight into electricity, or use the heat of sunlight to heat the water, and then let the generated water vapor drive the turbine to generate electricity. At present, these two fields have some mature related technologies. But perhaps, in theory, we still have a third way to go, that is to use heat directly to generate electricity, without the need for steam or turbines. In this case, almost all incident energy can be converted into electricity. However, standard solar cells are very sensitive to sunlight at certain frequencies and are not sensitive to sunlight at other frequencies. Solar energy is converted into electricity. Moreover, unlike the heated water regenerative method, the third method does not require a cumbersome mechanical process. Once such a system is set up, it can run well without too much manpower and management.

The fly in the ointment is that this device that converts sunlight into heat and then converts it into electricity does not allow the water to be warmer than boiling water under direct sunlight. The reason is that when the temperature of the water is much higher than the temperature of the boiling water, the rate of heat absorption is as fast as the rate of heat dissipation. This is a bit maddening, because this kind of direct conversion needs to reach 700 degrees Celsius to become very efficient, and it is impossible to use a special and expensive concentrating spherical mirror to collect incident light.

But now, the Massachusetts Institute of Technology scientist Peter Bomore and colleagues in the "Nano Research Express" magazine pointed out that they have developed a sun-capture device that can collect incident sunlight without using a concentrating spherical mirror.

New sun trap made of tungsten wire

The "sunlight trap" proposed by Bomor is a thin layer of tungsten (which is a heat-resistant metal) that has been treated in a complicated manner. The surface facing the sun is covered with extremely small pits; the other side is etched into a photonic crystal structure and faces a special type of solar cell made of indium gallium arsenide. The photonic crystal structure allows this surface to be selected. The ground emits infrared radiation that is maximally acceptable to the solar cell. Moreover, both the front and the back can be fabricated by photolithography for manufacturing computer chips.

It is these cavities that are only 3/4 micrometers in diameter and 3 micrometers deep and are arranged in a 4/5 micrometer wide grid to capture light. When the devices are aligned such that the holes are directed at the sun, most of the incident light will pass through the holes into the bottom where they are absorbed by the tungsten. As the laws of thermodynamics point out, these rays are immediately released and radiated. However, when the heat radiation from the inside of a cavity escapes into the outside world, it is likely to encounter the wall of the cavity. As a result, the entire absorption and re-radiation process occurs again, with the result that the tungsten in the cavity becomes very hot.

To turn heat into electricity, the photonic crystal directs heat into the solar cell. A photonic crystal is a regular geometric pattern etched on the surface of a tungsten wire. As long as these patterns are well tuned, it can enhance the infrared at a specific frequency and suppress the infrared rays at certain frequencies, releasing as much as possible to be most effectively arsenic. Infrared rays captured by indium gallium solar cells to enhance their absorption efficiency.

According to Bomor's calculations, the photoelectric conversion efficiency of the new system can be as high as 37%. The standard conversion efficiency of standard silicon-based solar cells that do not use mirrors to collect incident light is only 28%, and the maximum conversion efficiency of standard silicon-based solar cells using mirrors is 31%. Therefore, the latest research has on photoelectric conversion efficiency. Significant improvement. Of course, the next step is to make it as realistic as possible, and Bomor is very confident in the accuracy of the calculation results.

People generally use tungsten wire to make the filament of an incandescent lamp. However, this effect of tungsten filaments is becoming "out of gas" because they convert much of the electrical energy that passes through them into heat rather than light. Ironically, it is this shortcoming that not only revitalizes the tungsten wire, but also helps solve the energy shortage problem.

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