When the sun goes down, solar panels are played out. But that could be about to change.

Solar panels are on the rise; more and more roofs are covered with it. But those panels have a major drawback; they are only able to generate energy during the day. But there is hope; researchers have shown that it is also possible to generate solar energy at night. Not on the basis of sunlight, but on the basis of solar heat radiated by the earth.

Heat

“All warm bodies give off infrared light,” researcher Ned Ekins explains Scientias.nl from. “If you point a thermal camera at someone, you can see that person glow because his or her body is warmer than the surrounding space. In a similar way, the Earth radiates all the energy it receives from the sun back into space in the infrared.” And Ekins and colleagues have now used that energy to generate electricity.

Diode

They use a so-called thermoradiative diode, consisting of materials that you also find in night vision goggles. “The thermoradiative diode is a semiconductor device that can transmit part of that radiant energy. Just as a solar cell intercepts the energy flow from the sun to the earth, the thermoradiative diode intercepts the energy flow from the earth as it radiates out to space.” That energy flow does come indirectly from the sun, emphasizes Ekins; sunlight has previously reached the earth and heated it up, and that heat is then sent back into space by the earth in the form of infrared radiation.

And with the diode, part of that infrared light can be absorbed and then converted into electricity. “Just as a solar cell can generate electricity by absorbing sunlight given off by a very warm sun, the thermoradiative diode generates electricity by emitting infrared light to a colder environment,” explains researcher Phoebe Pearce. “In both cases, the temperature difference is what allows us to generate electricity.”

Temperature difference

“So the device generates electricity by emitting light,” Ekins added. “It is important to note that we can always generate power if there is a temperature difference. That temperature difference exists between the sun (very hot) and the earth (warm), but also between the earth (warm) and space (very cold). In the case of solar cells, we have the hot sun and a cool solar cell. But the same effect can be used – with the help of other materials – to generate electricity when a source gives off heat to a cold environment.”

Small yield

However, the amount of electricity generated using the thermoradiative diode does not come close to the yield of traditional solar panels. In fact, the amount of energy that the researchers can generate at this stage with the thermoradiative diode is no less than 100,000 times smaller than that of a solar panel. But Ekins does think that the yield can be increased considerably. “Right now we can generate a very small amount of electricity with our thermoradiative diode,” says Ekins. “One of the challenges of our research was even detecting the generated electricity. But the theory dictates that it should be possible to eventually provide about one-tenth of the energy generated by solar cells.”

Combination

With that limited yield, the thermoradiative diodes are far from being able to compete with the traditional solar panels. However, they could be used in addition to the solar poles. “Ultimately, the technologies could be combined, with the thermoradiative element behind the solar cell. And for a household, a thermoradiative device should then be able to generate enough energy in the dark to power devices that also run at night – such as a freezer, the router for WiFi, etc..”

body heat

But there are more possibilities. For example, our body heat can also become a source of energy in a similar way. “Wherever there is a temperature difference, we can generate thermoradiative current,” says Ekins. “If you take a thermal image of me, I will glow brightly in infrared light because I am about 37 degrees Celsius and my environment is 20 degrees Celsius.” And that temperature difference can be used to, for example, provide a smartwatch with energy. “If we scale our current thermoradiative diode to the size of a watch, the diode can generate enough energy based on body heat to power such a watch. And who knows, in the future we may also be able to develop semiconductors that can be integrated into clothing. In that case, it would be possible to generate energy from the infrared radiation emitted by our clothing.”

There’s work to be done

It’s future music for now. After all, some work still needs to be done to transform the thermoradiative diode that researchers are now using as part of their research project proof of concept have developed, to move towards much more efficient devices that can harness the nighttime solar energy on a much larger scale. Still, Ekins and colleagues are cautiously optimistic that this technology will make a difference in the future. “The thermoradiative diode can help by providing energy when the sun has set,” says Ekins.

In the future, however, a key role will be reserved for the business community. “We think we can increase the yield by a factor of 10,000. Once the technology reaches the stage where companies can make money by selling the product, we expect the industry to pick up on the idea and start helping to scale up and reduce costs.” And then it can go fast. “If the industry sees value in it, we can move forward extremely quickly.”