The Sun can be seen going down and under beyond the horizon, but down under has made use of solar energy even at night. Solar scientists in Sydney have harnessed the power of the Sun to generate energy after the Sun has set. Jamie Hanson, a postgraduate student at the University of New South Wales (UNSW), fancifully calls this technology a “reverse solar panel” that produces electricity by emitting light rather than absorbing it.
Hanson is one of a team of researchers at the university’s School of Photovoltaic and Renewable Energy Engineering, developing novel methods to harness solar energy, even after the sun has set. Solar energy absorbed by the Earth during the day does not stay here forever; most of it is released into the atmosphere after sunset as infrared radiation, which is invisible to the naked eye but felt as heat.
Scientists at UNSW have struck gold in this infrared radiation, which they can convert into electricity using a thermoradiative diode. Although the night seems dark, through the lens of an infrared camera, it glows. In the process, the planet cools down, and temperatures drop. UNSW researchers didn’t invent the thermoradiative diode; instead, they built on the work from Harvard and Stanford universities in the USA. However, this cohort of scientists was the first to demonstrate electrical power from these devices.
The electrical power generated by body heat can barely power a Casio wristwatch, about 100,000 times less than that of a conventional solar panel. The power is generated from the difference in temperature between the heat source and the environment. At optimal efficiency, this tech produces electricity with a power density of only 1 watt per square meter. This happens because water vapor and other atmospheric gases also absorb energy from the Sun. This shrinks the temperature difference between the Earth’s surface and the night sky.
Despite mediocre results on Earth, Professor Ned Ekins-Daukes, who leads the team at UNSW, has high hopes for the tech to power satellites in space, where the absence of an atmosphere creates a cooler environment that steepens the temperature gradient. This method is touted to perform better than conventional solar panels, which don’t offer much benefit in the absence of direct sunlight.
“Particularly in lower orbit … you have 45 minutes of sunlight and then 45 minutes of darkness,” he says. “Obviously, your solar panel only works when the sun’s shining. So, the opportunity here is … (to) use other surfaces on the spacecraft, not to totally power it, but provide some auxiliary power,” he explains.
The thermoradiative diode will harness the heat absorbed by the satellite while in the presence of the Sun, and radiate it during periods of darkness into the harsh, cold space. Darkness satellites are powered by batteries charged by solar energy during periods of sunlight. Using this device as a complement to these technologies can help squeeze out more energy from the satellite’s radiated heat, which would otherwise go to waste.
In line with current trends, many smaller satellites are being sent into lower orbits, retaining the functions of a normal-sized satellite. Since thermoradiative diodes are lightweight and generate power from unused surfaces, they are an excellent fit for power generation in these satellites. The team plans to test the technology in space for the first time this year in a test flight.
Dr Geoffrey Landis, a scientist working on thermoradiative technologies at the NASA John Glenn Research Center, sees the technology as a positive development in generating power in low-orbit satellites. Still, the cost of producing these devices can be a significant hurdle in making them the default choice. Landis considers batteries to be relatively cheaper than thermoradiative diodes. Instead of using these diodes to power satellites, Landis sees their potential to power deep-space missions to the outer planets of the solar system or land rovers in perpetually shadowed regions of the moon.
These missions are powered by special thermoelectric generators that convert the heat produced by the radioactive decay of plutonium isotopes. These generators are heavy and cost a fortune, generally reserved for big, flagship missions, as plutonium is difficult and expensive to make. Plutonium will still be needed to generate heat for thermoradiative diodes, but the diodes are much simpler to build than thermoelectric generators. Smaller diodes can be connected to each other, resembling a solar panel array currently used to power satellites.
Thermoradiative diodes are made from the same semiconductive materials used in night-vision goggles; however, more work is needed to determine their endurance to the high temperatures generated by radioactive decay. Current thermoelectric systems in space that use these isotopes as heat sources operate at temperatures of around 540° Celsius (1,004 ° Fahrenheit) or 1,000° Celsius (1,832° Fahrenheit).
Semiconductive materials, such as these, have not been thought to be used in this manner, so their longevity is debated. Optimally, the materials used in these missions need to last at least a decade or two, or even longer. Materials that might be used to develop and test a thermoradiative cell, can withstand temperatures of up to 375° Celsius (707° Fahrenheit). Researchers hope that a working thermoradiative cell will become a reality in next five to ten years at this pace.
At UNSW, Ekins-Daukes’ team has received funding from the United States Air Force to perfect the diode, mainly to assist low-orbit satellites. His team is also working with materials used in solar panels, essentially piggybacking on the core technology and repurposing it.
Copyright @smorescience. All rights reserved. Do not copy, cite, publish, or distribute this content without permission.
