Media release

From: The University of New South Wales

"UNSW researchers bring “night-time solar” technology a new step closer to reality, by using the infrared light spectrum to tap into Earth’s radiant heat and generate power." 

– UNSW lab researchers have used a thermoradiative diode–a semiconductor sensor found in existing technologies like night-vision goggles–to capture photons leaving Earth along the infrared spectrum, and converted them into electricity. 

– Technically, the diode works as “the inverse of a solar cell”: A solar cell generates electricity because it receives more photons than it emits. The thermoradiative diode generates electricity because it emits more photons than it receives. The current flows in the opposite direction as in a solar cell, and the voltage is also inverted compared to a solar cell. The power is the product of voltage and current, so the power stays positive. 

– As ‘incident solar radiation’ hits the Earth during the day, some of this energy is absorbed by the atmosphere and the surface, thereby warming the planet. This same energy also radiates away from Earth and back into the cold of space, at night. The only way for heat to escape the Earth is via light (photons), and this “night time light” travels on the infrared spectrum. 

– The breakthrough is an exciting confirmation of a previously theoretical process: it is the first step in making specialised devices to one day capture this energy at larger scale. The first silicon solar cell produced only 2% power; decades later it’s now at ~26.6%. 

Competition is heating up in the international field of solar power research to develop “night solar” technologies. Now, researchers at the University of New South Wales in Sydney have made a major breakthrough on the path to the seemingly impossible: demonstrating solar power generated at night, for the first time anywhere in the world.

Though the field itself is only ten years old, the advances being made in research are moving quickly. “Night solar” might sound like the stuff of pure science fiction, but for researchers in the field, how to capture the energy of Earth's radiant heat that is released into space at night isn’t just pie in the sky. In a new study published in ACS Photonics, researchers at UNSW’s School of Photovoltaic and Renewable Energy Engineering (SPREE) have proven that a semiconductor device exploiting the infrared spectrum of light and converting it into electrical power is possible, in a lab environment.

Sunlight provides us with more than enough energy to power all electricity on earth if it were able to be harnessed effectively. Solar panels can tap this flow of radiant energy from the sun and generate useful quantities of electricity. At night, that huge amount of energy supplied by the sun is re-radiated back out into space through our atmosphere, but at much longer wavelengths in the infrared spectrum, an effect which stops the planet from heating to an immense degree. The only way to release energy back into space from Earth is through the emission of light; it’s this escaping radiant energy into the cold of space that lets us live our lives in the perfect temperature comfort to sustain life. If the flow of this radiant heat could be tapped by a power cell device and converted into electricity (as a solar cell does), there’s a large and unused spectrum of potential power to be exploited. This could mean being able to achieve the ultimate dream of renewable energy: power generation uninterrupted by the setting of the sun, and proving the feasibility of this technology is the first step, an extremely exciting breakthrough for the UNSW team.

They did this with a “thermoradiative diode”; a new type of power-generation device which works by using the same semiconductor materials that are found in infrared photodetectors (used in night-vision goggles), to make a “night” solar cell. This process is known as “thermoradiative power”: the generation of electrical power from the thermal emission of radiant heat into a cold environment. The diode device can generate electricity from anything that glows brightly when viewed with a night-vision camera, working essentially as an inverse solar cell.

The team’s lead, N.J. Ekins-Daukes, puts the process into context this way: “In the late 18th and early 19th century it was discovered that the efficiency of steam engines depended on the temperature difference across the engine, and the field of thermodynamics was born. The same principles apply to solar power: the sun provides the hot source and a relatively cool solar panel on the Earth’s surface provides a cold absorber. This allows electricity to be produced with an intensity of a few hundred watts of electrical power per square meter of solar panel. However, when we think about the infrared emission from the Earth into outer space, it is now the Earth that is the comparatively warm body, with the vast void of space being extremely cold. By the same principles of thermodynamics, it is possible to generate electricity from this temperature difference too: the emission of infrared light into space.”

Then new UNSW results build on previous work from the group where co-author Andreas Pusch developed a mathematical model that helped guide their laboratory experiments. This built on work by Norwegian researcher Rune Strandberg who first explored the theoretical possibility of such a device. Researchers at Stanford University led by Shanhui Fan are concurrently looking into other ways to capture the radiant heat energy of the sun at night, though using a different method, a development that illustrates the keen interest in the emerging technologies.

While for the UNSW team, the amount of energy produced is small (roughly equivalent to 1/100,000th of a solar powered cell), it is the proof of concept that is big. Ekins-Daukes explained why the results of the work are so exciting: “We usually think of the emission of light as something that consumes power, but in the mid-infrared, where we are all glowing with radiant energy, we have shown that it is possible to extract electrical power. In theory it is possible for this technology to produce about 1/10th of the power of a solar cell but we need some new infrared materials to achieve that level of performance. The situation is similar to the demonstration of a silicon solar cell, it debuted at 2% efficiency in 1954 and was extremely expensive.

Almost 70 years later, silicon solar cell technology is now in the hands of industrial manufacturers who can produce solar panels on a huge scale, at efficiencies close to the theoretical limit that in turn, can provide some of the cheapest electricity on the planet. Returning to the thermoradiative diode, we do not yet have the miracle material that will make the thermoradiative diode an everyday reality, but we made a proof of principle and are eager to see how much we can improve on this result in the coming years”.

Co-author Michael Nielsen said, “Even if the commercialization of these technologies is still a way down the road, being at the very beginning of an evolving idea is such an exciting place to be as a researcher. By leveraging our knowledge of how to design and optimize solar cells, and borrowing materials from the existing mid-infrared photodetector community, we hope for rapid-progress towards delivering the dream of solar power at night!”

The team are now excited to move to the next research phase in creating and refining their own devices to harness the power of the night, and welcome potential industry partners. Beyond night sky power generation, this technology could find use anywhere radiant heat is being emitted; a quick survey with a thermal camera shows all the possible sources of waste radiant heat: our bodies, buildings, vehicles and even satellites in space, since the only way they can cool is, like the Earth, by emitting infrared light.

For media interviews and supplementary images, please contact Elmo Keep.