Generating light from darkness
New device generates electricity from the cold night sky
Scientists in California have developed a device that can power a light bulb using energy that flows from the ground into space at night. This technology could fill in the gap left by solar panels that only work during the day.
One problem with solar panels is they only produce electricity when the sun shines, so they're not providing power during the long hours of night when electricity is needed to keep the lights on. Batteries can be added to store the solar energy for night time use, but that increases cost.
This is particularly an issue in remote off-grid areas or for people in the developing world who can't afford the extra expense of batteries. This new, extremely low cost approach works like a solar panel in reverse, by literally capturing energy from darkness.
Taking advantage of losing heat to the night sky
Researchers at the University of California Los Angeles and Stanford University took advantage of a principle called radiative cooling, which happens at night as the Earth radiates heat into space that it had absorbed during the day. This process can actually cause the ground to become colder than the air, which is how frost can form on grass over night when the air temperature is still above freezing. That temperature difference between an object exposed to the night sky and the air that is used to produce electricity.
The technology to demonstrate this concept was remarkably simple, costing about $30 US. The thermoelectric module consisted of a Styrofoam-like box about the size of a shoebox. It was covered with aluminum material, with a metal disk painted black on the outside facing upwards and an aluminum block on the inside. The key component, though, was a commercial thermoelectric generator that coupled the disk and the block.
Thermoelectric generators or thermocouples are solid state devices that generate electricity when heat passes from one side to the other. They are used in a variety of applications, from small temperature sensors to devices that produce power from waste heat flowing through smokestacks, to generating electricity from body heat, such as the body heat-powered flashlight invented by Canadian teenager Ann Makosinski in 2013.
The technology has also been used on deep space probes where sunlight is too dim to provide sufficient power. They're a critical component in radioisotope thermoelectric generators (RTGs). RTGs use a small piece of radioactive material to generate heat that the thermoelectric material convert into electrical power. These devices have powered robots such as the Curiosity rover on Mars, the Cassini mission to Saturn, and the twin Voyagers to operate for decades exploring the outer reaches of the solar system.
Enough electricity to illuminate an LED bulb
It doesn't matter what the source of heat is and it doesn't take much of a temperature difference to produce a current of electricity.
The foam box contraption was placed on the roof in Stanford, Calif,. on a clear December night for over six hours, using a temperature difference of only a few degrees between the air and the black piece of metal. That produced a peak of 0.8 milliwatts of power, which is not a lot — it's a tiny fraction of the electricity a solar panel of equivalent size can produce in the day. But it was enough to illuminate an LED bulb and prove the concept that light can be produced from darkness.
The scientists say that with improvements and scaling it up, a larger device could provide enough clean electricity for lighting or charging electronics at night in a completely passive way.
Thermoelectrics will not meet all of our nighttime energy needs, but they could provide low-cost clean energy sources in developing countries and remote areas.
This project demonstrates that when it comes to clean energy, there is no shortage of it on Earth. Energy is all around us in many forms, even at night. We just need innovative ways to capture it.