Seductive light from the dark side

Researchers at the University of California, Los Angeles (UCLA), and Stanford University in the US have generated light “from the darkness of space itself” with a simple, inexpensive device they trialled at night on a Stanford rooftop.

Their report, published in the journal Joule, details how UCLA’s Aaswath Raman – the lead author – and Stanford’s Wei Li and Shanhui Fan used the low-tech thermoelectric device to harness the cold of space, without active heat input, and generate a low-power electrical flow that illuminated an LED.

“Remarkably, the device is able to generate electricity at night, when solar cells don’t work,” says Raman. “Beyond lighting, we believe this could be a broadly enabling approach to power generation suitable for remote locations, and anywhere where power generation at night is needed.”

The capacity of solar cells to efficiently produce renewable energy in daylight is long known and well developed, but there’s currently no similar, renewable approach to generating power at night. Solar cells can be linked to batteries to store energy produced in daylight hours for night-time use, but the addition drives up costs.

The Joule report emphasises the paucity of options for small-scale, renewable power generation that lacks storage. This is particularly challenging for the 1.3 billion people worldwide without access to electricity – a quarter of the developing world.

Demand for electricity among these mainly rural populations is surging, driven by particular interest in three applications: cell-phone charging, cooking, and lighting. 

The device developed by Raman’s team takes advantage of radiative sky cooling, in which a sky-facing surface passes its heat to the atmosphere as thermal radiation, losing some heat to space and reaching a cooler temperature than the surrounding air – a phenomenon that explains how frost forms on grass during above-freezing nights.

The same principle can be used to generate electricity, harnessing temperature differences to produce renewable electricity at night, when lighting demand peaks. 

Raman and colleagues tested their modular thermoelectric generator under a clear, early-winter sky. Radiative cooling maintained the cold side of the device several degrees below ambient temperature, while the surrounding air heated the warm side, with the ensuing temperature difference converted into usable electricity.

The thermoelectric module – a polystyrene enclosure covered in aluminised mylar to minimise thermal radiation and protected by an infrared-transparent wind cover – was positioned on a table one metre above the roof. 

It drew heat from the surrounding air and released it through a simple black emitter. When the device was connected to a voltage boost converter and a white LED, it passively powered the light. 

Power output was measured over six hours and as much as 25 milliwatts of energy per square metre was generated. A typical human hearing aid uses about one milliwatt of power; an inexpensive laser pointer outputs about five milliwatts of light power.

Since the device consists mainly of off-the-shelf components, Raman and his team believe that it can be easily scaled for practical use. Their report includes a theoretical model for a 20-fold improvement its performance, with an output of up to about half a watt per square metre.

Gains would come through improved engineering – such as by suppressing heat gain in the radiative cooling component to increase heat-exchange efficiency – and using the device in a hotter, drier climate.

“Our work highlights the many remaining opportunities for energy by taking advantage of the cold of outer space as a renewable energy resource,” says Raman. “We think this forms the basis of a complementary technology to solar. While the power output will always be substantially lower, it can operate at hours when solar cells cannot.”