Can we exploit the Earth’s natural radiation? (Image: Shutterstock)
Heat is constantly flowing into space from the ground beneath our feet. Might we capture some of it to generate electricity?
EARTH glows. Not in a fiery, intense visible light as the sun does. But lie down on the ground and you can feel its warmth. Take an infrared photo from orbit and you will see the heat radiation it constantly pumps out into the frigid void of space.
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It is an immense flow of energy – and one that currently goes to waste. But need it? Not if a bright idea from a group of researchers at Harvard University works out. They think we can use some of this heat radiation as it passes out into space to generate electricity.
It is, on the face of it, an odd idea, using a heat flow away from us, rather than towards us, to make power. “To generate power by emitting, not by absorbing light, that’s weird… it’s highly counter-intuitive,” says team leader . But get it to work, and solar panels could be built that absorb light from below, as well as from above – and even continue delivering juice long after the sun has set. Too good to be true?
It’s not often that someone comes up with a new way to generate clean energy. Like the others, this one is all about thermodynamics. “Whenever you have energy flowing from hot to cold, you have a chance to harvest renewable energy,” says team member . Solar cells use the direct flow of light from the sun to Earth. Biofuels do the same less directly, exploiting solar energy stored in growing plant matter. Wind turbines are powered by energy flows between hotter and colder regions.
The total heat given off by Earth’s surface amounts to some 200 million gigawatts spread over a range of infrared wavelengths. That’s more than twice the power it receives in visible light from the sun (see diagram). Most is absorbed by gases and clouds in the atmosphere and re-radiated, some back toward the surface – a phenomenon familiar to us as the greenhouse effect. But the atmosphere is mostly transparent to infrared radiation at wavelengths in a small spectral window between 8 and 13 micrometres, resulting in a constant outward stream of heat.
To get an idea of how much of this power might be siphoned off, the Harvard researchers used measurements of Earth’s infrared emissions made by an instrument located in the little town of Lamont, Oklahoma. Their calculations showed that over a year a device capable of sucking up all the heat of the right wavelengths and converting it to electricity would generate about a tenth of the power of a typical solar cell, with an efficiency of 15 per cent.
That’s an appreciable, if small, amount. The real attraction, though, is not in how much, but when. Solar power is intermittent, and its production peaks do not generally coincide with peaks in electricity demand. Ground heat flow, on the other hand, is highest in the afternoon and early evening, roughly at times of peak demand. It is slightly greater in winter, when the air is colder and drier, and trickles out even at night.
“Power trickles from the ground all the time – even at night”
Garret Moddel of the University of Colorado at Boulder has been investigating ways to use Earth’s heat for many years, and welcomes the proposal. “This would be a great idea if it could be made to operate efficiently enough,” he says.
And that is the big question. The simplest way to exploit this heat would be to coat the up side of a thermoelectric slab – a material that converts heat to electricity – with paint that reflects sunlight and is a good infrared emitter. This would cool the slab below the surrounding temperature, and heat sucked up from the ground would generate current as it flowed through the material.
The problem is the thermoelectrics we know are very inefficient, generally converting just a few per cent of any passing heat into electrical energy. “This system would be totally impractical because it would produce very little electricity and cost a lot,” says Byrnes.
The Harvard vision instead builds on schemes devised in the 1960s for a “rectifying antenna”, or rectenna. This is essentially an antenna to collect or transmit a varying electromagnetic field, connected to a circuit containing a diode – a valve that lets current flow in only one direction (see diagram). The basic idea is that the diode would be warmed by the Earth while the antenna radiates away heat to keep the circuit cool. The heat flow sets up an AC voltage that alternates with each infrared wave cycle. The diode then converts this to a DC voltage, and a current flows (PNAS, vol 111, p 3927).FIG-mg29830601.jpg
In the 1960s the defence firm Raytheon developed microwave rectennas to . These reached conversion efficiencies as high as 70 per cent. But transferring that success to the infrared realm is no trivial matter. The wavelengths of interest are some 10,000 times smaller – so the antennas would need to be 10,000 times smaller as well.
That is now feasible with techniques developed in the past decade to print thousands of miniature radio antennas directly onto surfaces. “They really look the same as the antennas that you see on roofs, only a million times smaller,” says Byrnes. Anarray of tiny heat harvesters might be laminated onto a conventional solar panel, boosting its electricity output and extending production across the day.
That doesn’t mean it’s job done. Infrared rectennas now available would produce voltages too small to drive a significant current. What’s more, the voltages would flip so quickly with the fast infrared wave cycle that existing diodes could not keep up.
Ultra-high-speed diodes that exploit the way quantum mechanics allows electrons to “tunnel” across circuit gaps could supply a solution, the researchers suggest. Moddel isn’t so sure, and thinks trying to implement the idea will have to wait for next-generation diodes now only in the early stages of development by his group and others. And he points to an additional obstacle yet to be overcome: over any significant area, heat waves can arrive out of phase – in a jumble at slightly different times – and cancel each other out, limiting what any device could harvest.
Nevertheless, he thinks the Harvard idea is food for thought. “Their proposal will certainly give a needed push to the ongoing work in this area,” he says. That glow may be low, but there is enough incentive that it might be harnessed yet.
This article appeared in print under the headline “Earth electric”
