Scientists from Hanyang University in Seoul presented the development capable of turning to reality one of the most ambitious engineering projects of the humankind – outer-space solar power plants. Positioned at the geosynchronous orbit at about 36 thou km, such power plants could capture solar energy round-the-clock and transmit it to Earth with the help of microwaves avoiding atmospheric losses.
Peter Glaser, an American engineer, is the author of the idea. Back in 1968 he proposed to capture solar light in outer space and transmit it to Earth by way of microwave radiation. The principle is simple: a giant solar panel is positioned at the orbit, which converts light into electricity. This electricity feeds the microwave transmitter sending a ray to Earth, to the pick-up antenna – rectenna. It receives the signal, coverts it back to current and feeds it into the grid.
However, for many decades, natural technological restrictions hindered the practical implementation of this idea. Even today, the wireless transmission sub-system remains the most expensive and most complex element of the entire design. For example, according to the project of Chinese Academy of Space Technologies (CAST) for one 1 GW power plant about 128 thou of microwave generators – magnetrons 12.5 KW each are needed, and each of them has the efficiency factor only 54%. The cost estimate for just this part of the installation is almost USD 9.2 bn, and its mass exceeds four thou tons.
The main reason for such high costs was in structural constraints of traditional magnetrons with thermionic cathodes. In such magnetrons the electrons are knocked off the cathode surface due to heating, which requires complex heating and power feeding systems. With time, the cathode loses its properties, its surface is contaminated and degraded – this process is known as “cathode poisoning”. It results in reduction of capacity and service life, hence, the generator has to be replaced long before the resource of the remaining system is depleted.
The Korean researchers proposed to replace such cathodes to field emitters — “cold cathodes”, where electrons are released not be exposure to high temperature, but under the effect of strong electric field. This effect is called auto-electronic emission (field cold emission). The refusal from heating makes the system more simple, more reliable and much lighter, which is especially important in the situation, when every kilo at the orbit costs thousands of dollars.
On top of that, the engineers redesigned the very architecture of the system. They made the shape of resonator voids asymmetrical, applying the so-called “rising sun” scheme. In this configuration one part of the voids is slightly broader than the other, and due to this, electric oscillations in the resonator are unevenly distributed. This helps to naturally divide the frequencies of the oscillations and support the stable operating mode of the generation removing spurious oscillations, which previously required installing additional stabilizing elements.
The new design confirmed its efficient performance during a series of digital experiments using the computer electrodynamics methods and simulation of electron flows. Under 23.5 KV and 0.3 tesla magnetic field, the new magnetron demonstrated 85% efficiency factor and output power of above 100 KW at 2.45 GHz frequency. To compare: commercial analogues under the same conditions usually show the output power of only 10-15 KW and 60% efficiency factor. Hence, the new system turned out approximately eight times more powerful and 25% more efficient than the existing analogues given the same size and operating mode.
According to the calculations by the researchers, using new sources will allow for decreasing the mass and the cost of the wireless transmission sub-system by approximately one-third. In terms of the entire project, it means 30% reduction of the total cost of the orbital station – from USD 28 bn to USD 19.6 bn.
However, the benefits are not limited to outer space. The technologies of powerful wireless energy transmission open up new opportunities on Earth as well – from remote charge of e-vehicles to feeding remote infrastructure facilities.
