Two years ago, an icy December evening on a California roof, a small light shone brightly with some help from the freezing night air. It wasn't very bright. But that was enough to demonstrate the possibility of generating renewable energy after the sun goes down.
The work of Stanford University engineers Wei Li and Shanhui Fan, University of California materials scientist Aaswath Raman of Los Angeles, put together a device that produces a voltage by channeling today's remaining heat to cool air.
"Our work highlights the many remaining opportunities for energy by taking advantage of the cold in outer space as a renewable energy resource," says Raman.
 "We believe this forms the basis for a complementary solar energy technology. Although the power output will always be substantially lower, it can operate at hours when solar cells do not can."
For all the benefits of solar energy, it's just not a 24-7 power source. Sure, we can store it in a giant battery or use it to pump water into a reservoir for later, but until we have more economical solutions, night turns into a quiet time for renewable solar energy.
Most of us return home from work when the sun goes down, and that is when energy requires momentum to meet our heating, cooking, entertainment and lighting needs.
Unfortunately, we often turn to fossil fuels to make up for the shortage. For those living off-line, it may require restrictive options and go without a few luxuries.
Shanhui Fan understands the need for a renewable power source at night well. He has worked on a number of similar devices, including a recent technology magazine that turned solar cells on its head by squeezing electricity from the glow of heat that radiated from the planet's solar-heated surface.
While the smart object relied on the optical properties of a hot object, this alternative device uses the good old thermoelectric effect.
By using a material called a thermocouple, engineers can convert a temperature change to a voltage difference. This requires something relatively tasty on the one hand and a place for the heat energy to escape to on the other.
The theory is the simple part – the real challenge is to arrange the right materials in such a way that they will generate a voltage from our cooling environment that makes it worthwhile.
To keep costs down, the team used simple, shelving items that pretty much any of us could easily get our hands on.
They put together a cheap thermoelectric generator and connected it to a black aluminum disc to throw heat into the night air while facing the sky. The generator was placed inside an isopor cabinet sealed with a window that was transparent to infrared light, and connected to a single tiny LED.
For six hours one evening the box was allowed to cool on a roof top in Stanford as the temperature fell just below freezing. As the heat flowed from the ground to the sky, the small generator produced just enough power to make the light flicker to life.
At its best, the unit generated around 0.8 milliwatts of power, equivalent to 25 milliwatts of power per square meter.
It may be enough to keep a hearing aid working. String several together, and you might just keep the cat entertained with a simple laser pointer. So we're not talking huge amounts of power.
But as far as prototypes go, it's a fantastic starting point. The team suggests that with the right adjustments and conditions, 500 milliwatts per square meter is not out of date.
"Beyond lighting, we believe this can be a broad enabling approach to power generation suitable for remote locations, and wherever power generation at night is needed," says Raman.
As we search for big, bright ideas to drive the renewable energy revolution, it is important to make sure that we do not let the smaller, simpler solutions that these slip away at night.
This research was published in Joule .