The solar thermal power generation system developed by Northeastern University in Japan has the highest generation efficiency in the world

Tohoku University announced that its solar-thermo photo voltaic (PV-PVT) system has a power generation efficiency of 5.1%, reaching the highest level in the world. The principle of such a power generation system is to convert sunlight containing a wide range of wavelengths of light into heat radiation of a wavelength optimum for the solar cell and generate the power, which is expected to achieve high-efficiency photovoltaic power generation different from the concept of a multi-junction solar cell.

The light emitted by the sun (thermal radiation) has a broad wavelength distribution (spectrum). A single-junction solar cell can only convert light having a smaller wavelength bandgap than that of the used semiconductor material into electrical energy, and light of other wavelengths can not be converted into electrical energy and become lossy. Multi-junction solar cells, which are formed by overlapping a plurality of solar cells, can convert the solar spectrum into electrical energy without waste by enlarging the absorbable wavelength region. However, compared with single-junction solar cells, there is a problem of high production cost.

Solar-TPV heats solar selective materials and wavelength-selective emitters by concentrating sunlight, which is then used to generate heat using thermal radiation from a wavelength-selective emitter that matches the wavelength region of sensitivity. The sunlight is characterized by the fact that it can be converted into heat first and then converted into light of other wavelengths (thermal radiation) with the same total energy of the photons contained. In this way, high-efficiency power generation can be achieved even with inexpensive single-junction solar cells.

In this research, Northeastern University proposed a new scheme based on the concept of "thermal radiation spectrum control" and "thermal radiation unidirectional transportation" to improve the thermal radiation conversion and transportation efficiency. Based on this concept, Solar- TPV system as a whole design. The Solar-TPV system converts sunlight into thermal radiation, a wavelength conversion system that converts photons into one another, unlike conventional concentrating solar thermal power that converts sunlight into heat.

Therefore, the focus of improving efficiency is that the absorbed solar energy is delivered to the wavelength selective emitter without loss, matching the thermal radiation spectrum emitted by the wavelength selective emitter with the sensitivity wavelength region of the photoelectric conversion unit. In other words, it is required to have higher "heat radiation conversion and transportation efficiency" and "photoelectric conversion efficiency".

Both of these efficiencies can be enhanced by the optical design and geometry of the solar selective absorber and wavelength selective emitter. The selection of absorbing materials for the sunlight is required to have high absorption in the short wavelength region where the spectral intensity of sunlight is high and low emissivity (absorption) in the long wavelength region. A requirement for a wavelength selective emitter is to have high emissivity in the sensitivity wavelength region of the photoelectric conversion unit and low emissivity in other wavelength regions.

This time, the researchers designed a higher thermal radiation conversion efficiency and transport efficiency of the area ratio, inhibition of solar selective absorber material reflection and radiation losses, the successful design and production of thermal radiation transport efficiency is expected to reach 54% , Photoelectric conversion efficiency is expected to reach 28% of the solar selective absorption material and wavelength selective emitter. In a power generation test using a trial-manufactured solar selective absorbing material, a wavelength selective emitter, gallium-antimony photoelectric conversion unit, the power generation efficiency reached 5.1%.

Relevant research results have been published on "Applied Physics Express" on October 25, 2016 and have been selected as Spotlights essay.

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