EMISSIVE ENERGY HARVESTER (EEH)
Is Earth’s infrared radiation a potential energy source?
by George Dvorsky / 6.3.14
Our Earth, which is heated by the sun, is much warmer than the frozen space that surrounds it. This creates a heat imbalance that could be transformed into direct-current (DC) power. But making this happen is considerably easier said than done. “It’s not at all obvious, at first, how you would generate DC power by emitting infrared light in free space toward the cold,” noted principal investigator Federico Capasso in a statement. “To generate power by emitting, not by absorbing light, that’s weird. It makes sense physically once you think about it, but it’s highly counterintuitive. We’re talking about the use of physics at the nanoscale for a completely new application.” Capasso, along with his colleagues Steven Byrnes and Romain Blanchard, have proposed two designs for a device they call an emissive energy harvester (EEH). Once developed, they would convert infrared radiation (IR) into usable power. And somewhat paradoxically, they would run in reverse.
The first type of EEH is comparable to a solar thermal generator. It would consist of a “hot” plate at the same temperature as the Earth and air, with a “cold” plate on top of it. Facing up towards the sky, the cold plate would feature a material that cools by very efficiently radiating heat. The resulting heat difference between the plates could generate a few watts per square meter — day or night. And in fact, the researchers are basing these measurements on a case study conducted at a location in Larmont, Oklahoma. The researchers admit that keeping the cold plate cooler than the ambient temperature will be a challenge, but it illustrates an important principle — that difference in temperature can generate work. “Solar panels for heating and cooking are already used in much of the world,” he says. “You could easily couple that to the (infrared) harvester.”
Earth’s light differences (Image: NASA/JPL)
As for the second device, it would rely on temperature differences between nanoscale electronic components (namely diodes and an antenna) rather than a temperature that can be felt with touch. “If you have two components at the same temperature, obviously you can’t extract any work, but if you have two different temperatures you can,” says Capasso. “But it’s tricky; at the level of the electron behaviors, the explanation is much less intuitive.” Unfortunately, the technology to build these devices doesn’t exist — at least not yet. But the groundwork certainly makes it look promising. “Now that we understand the constraints and specifications, we are in a good position to work on engineering a solution,” adds Byrnes. In fact, he envisions a sheet printed with thousands of tiny IR-harvesting rectennas that could be laminated on a solar panel or integrated into a solar water heater.
How the Earth might appear if your eyes were sensitive to infrared light (Image: Goddard Space Flight Center/NASA)
100 MILLION GIGAWATTS
Earth’s Infrared Radiation: New Renewable Energy Frontier?
by Prachi Patel / 4 Mar 2014
The Earth continuously emits 100 million gigawatts of infrared heat into outer space. That’s enough to power all of humanity many thousands of times over. Capturing even a fraction of that would mean an end to our energy woes. Harvard University researchers are now proposing a way to harvest this untapped source of renewable energy. They have come up with two designs for a device they call an “emissive energy harvester” that would convert IR radiation into usable power. Today’s technology isn’t sufficient for an efficient, affordable harvester, the researchers say. But they’ve laid out a few different paths towards such devices in the journal Proceedings of the National Academy of Sciences.
It seems counterintuitive, but the devices generate power by emitting infrared radiation. “The [device] is emitting much more radiation than it receives,” says Steven Byrnes, a postdoctoral researcher working in applied physicist Frederico Capasso’s laboratory at Harvard. “This is the imbalance that we can take advantage of to create DC power.”
The first design, which they admit is not the most promising, is a heat engine running between the Earth’s surface and a cold plate. The heat flowing from the ambient surface air to the cold plate, which radiates it out into the atmosphere, would be used to do mechanical work. The concept is simple, but cooling the plate efficiently to a low enough temperature is tough, Byrnes says. As a case study the researchers looked at how much power such a device would generate in Lamont, Oklahoma, where a facility has been measuring IR radiation intensity. They found that they would get an average of 2.7 Watts from the IR radiation emitted by a square meter of Oklahoma over 24 hours, which is pretty low for large-scale power generation.
So the researchers turn to rectifying antennas, or rectennas, devices that absorb electromagnetic radiation and convert it into direct current electricity. A rectenna is an antenna coupled with a diode. Radiation induces an AC voltage across the antenna, which the diode rectifies to DC. The researchers argue that rectennas can be run in reverse, generating DC power while emitting radiation, rather than absorbing it. In their design, a nanoscale antenna very efficiently emits Earth’s infrared radiation into the sky, cooling the electrons only in that part of the circuit. Because the diode is at a higher temperature than the antenna, current only flows from the diode to the antenna. And because the antenna acts as a resistor, this results in a voltage.
Rectennas are traditionally used to generate power from microwaves, but can be used for higher frequency radiation, all the way up to visible light. Infrared frequency rectennas are a developing technology and the proof-of-principle devices demonstrated so far would generate very little power. But technological advances could improve their efficiency, Byrnes says. Applying solar-cooking techniques such as reflectors to heat up the rectennas could also increase efficiency. For example, in the Lamont, Oklahoma case study, raising the temperature of a rectenna-based harvester from 20° C to 100° C using solar-cooking techniques would increase the power density of a rectenna from 1.2 W/m2 to 20 W/m2. “Solar panels for heating and cooking are already used in much of the world,” he says. “You could easily couple that to the (infrared) harvester.”
The researchers say that IR antennas should be easy to make on large areas at a reasonable cost. The critical challenge will be making diodes that would work well at the low voltages that would be expected in the harvester. The researchers suggest a few options to get around this problem. One is to use specially designed low-voltage diodes such as tunnel diodes and ballistic diodes.
Needless to say, this vision of IR energy harvesters for renewable power rests on engineers overcoming several technical challenges. But Byrnes says that this is a new energy frontier to tackle. He imagines one day a sheet printed with thousands of tiny infrared-harvesting rectennas that could be laminated on a solar panel or integrated into a solar water heater.
Three diode-resistor generator circuits with different temperature inputs. A circuit at thermal equilibrium (A) generates no current; (B) is a conventional rectifier circuit. The Harvard team proposes a twist—shown in (C). (Image: Federico Capasso/PNAS.)
Infrared: A new renewable energy source?
by Caroline Perry / March 3, 2014
When the sun sets on a remote desert outpost and solar panels shut down, what energy source will provide power through the night? A battery, perhaps, or an old diesel generator? Perhaps something strange and new. Physicists at the Harvard School of Engineering and Applied Sciences (SEAS) envision a device that would harvest energy from Earth’s infrared emissions into outer space.
Heated by the sun, our planet is warm compared to the frigid vacuum beyond. Thanks to recent technological advances, the researchers say, that heat imbalance could soon be transformed into direct-current (DC) power, taking advantage of a vast and untapped energy source. Their analysis of the thermodynamics, practical concerns, and technological requirements will be published this week in the Proceedings of the National Academy of Sciences.
Federico Capasso is a world-renowned expert in semiconductor physics, photonics, and solid-state electronics. He co-invented the infrared quantum-cascade laser in 1994, pioneered the field of bandgap engineering, and demonstrated an elusive quantum electrodynamical phenomenon called the repulsive Casimir force—work for which he has received the SPIE Gold Medal, the European Physical Society Prize for Quantum Electronics and Optics, and the Jan Czochralski Award for lifetime achievement. His research team seems to specialize in rigorously questioning dated assumptions about optics and electronics.
“The mid-IR has been, by and large, a neglected part of the spectrum,” says Capasso. “Even for spectroscopy, until the quantum cascade laser came about, the mid-IR was considered a very difficult area to work with. People simply had blinders on.” Now, Capasso and his research team are proposing something akin to a photovoltaic solar panel, but instead of capturing incoming visible light, the device would generate electric power by releasing infrared light. “Sunlight has energy, so photovoltaics make sense; you’re just collecting the energy. But it’s not really that simple, and capturing energy from emitting infrared light is even less intuitive,” says lead author Steven J. Byrnes (AB ’07), a postdoctoral fellow at SEAS. “It’s not obvious how much power you could generate this way, or whether it’s worthwhile to pursue, until you sit down and do the calculation.” As it turns out, the power is modest but real. As Byrnes points out, “The device could be coupled with a solar cell, for example, to get extra power at night, without extra installation cost.”
Two proposed devices — one macro, one nano
To show the range of possibilities, Capasso’s group suggests two different kinds of emissive energy harvesters: one that is analogous to a solar thermal power generator, and one that is analogous to a photovoltaic cell. Both would run in reverse.
The first type of device would consist of a “hot” plate at the temperature of the Earth and air, with a “cold” plate on top of it. The cold plate, facing upward, would be made of a highly emissive material that cools by very efficiently radiating heat to the sky. Based on measurements of infrared emissions in Lamont, Oklahoma (as a case study), the researchers calculate that the heat difference between the plates could generate a few watts per square meter, day and night. Keeping the “cold” plate cooler than the ambient temperature would be difficult, but this device illustrates the general principle: differences in temperature generate work. “This approach is fairly intuitive because we are combining the familiar principles of heat engines and radiative cooling,” says Byrnes.
The second proposed device relies on temperature differences between nanoscale electronic components—diodes and antennas—rather than a temperature that you could feel with your hand. “If you have two components at the same temperature, obviously you can’t extract any work, but if you have two different temperatures you can,” says Capasso. “But it’s tricky; at the level of the electron behaviors, the explanation is much less intuitive.”
“The key is in these beautiful circuit diagrams,” he adds. “We found they had been considered before for another application—in 1968 by J.B. Gunn, the inventor of the Gunn diode used in police radars—and been completely buried in the literature and forgotten. But to try to explain them qualitatively took a lot of effort.” Simply put, components in an electrical circuit can spontaneously push current in either direction; this is called electrical noise. Gunn’s diagrams show that if a valve-like electrical component called a diode is at a higher temperature than a resistor, it will push current in a single direction, producing a positive voltage. Capasso’s group suggests that the role of the resistor could be played by a microscopic antenna that very efficiently emits the Earth’s infrared radiation toward the sky, cooling the electrons in only that part of the circuit. The result, says Byrnes, is that “you get an electric current directly from the radiation process, without the intermediate step of cooling a macroscopic object.” According to the paper, a single flat device could be coated in many of these tiny circuits, pointed at the sky, and used to generate power.
(A) Thermal EEH device, (B) an infrared rectenna EEH. Image Credit: PNAS
Technological challenges—and promise
The optoelectronic approach, while novel, could be feasible in light of recent technological developments—advances in plasmonics, small-scale electronics, new materials like graphene, and nanofabrication. The Harvard team says a strength of their research is that it clarifies the remaining challenges. “People have been working on infrared diodes for at least 50 years without much progress, but recent advances such as nanofabrication are essential to making them better, more scalable, and more reproducible,” says Byrnes. However, even with the best modern infrared diodes, there is a problem. “The more power that’s flowing through a single circuit, the easier it is to get the components to do what you want. If you’re harvesting energy from infrared emissions, the voltage will be relatively low,” explains Byrnes. “That means it’s very difficult to create an infrared diode that will work well.” Engineers and physicists, including Byrnes, are already considering new types of diodes that can handle lower voltages, such as tunnel diodes and ballistic diodes. Another approach would be to increase the impedance of the circuit components, thereby raising the voltage to a more practical level. The solution might require a little of both, Byrnes predicts.
(A) In a conventional circuit, incoming radiation creates an AC voltage; (B) Reverse circuit creating DC power. Image Credit: PNAS
Speed presents another challenge. “Only a select class of diodes can switch on and off 30 trillion times a second, which is what we need for infrared signals,” says Byrnes. “We need to deal with the speed requirements at the same time we deal with the voltage and impedance requirements. Now that we understand the constraints and specifications,” Byrnes adds, “we are in a good position to work on engineering a solution.”
“As an aside, this could be potential bad news as far as the search for extraterrestrial intelligence is concerned; wide-scale energy capture measures like these could make an alien planet appear invisible to us.”
Could we find alien civilizations using infrared light?
by George Dvorsky / 6/03/13
Instead of listening for radio signals, a group of astronomers is proposing that we search for extraterrestrials by detecting their planet’s exaggerated heat signatures — signatures that could be detected in the infrared. But to do so, we’d have to build the largest telescope this world has ever seen. Writing in Astronomy Magazine, a team of astronomers, engineers, and physicists from the University of Hawaii, the University of Freiburg, and elsewhere is making the case for infrared SETI. The basic idea is that a sufficiently advanced extraterrestrial civilization will produce more power on its planet than it receives from its parent star. This delta in energy could indicate the presence of an alien civilization. “The energy footprint of life and civilization appears as infrared heat radiation,” says Jeff Kuhn, the project’s lead scientist. “A convenient way to describe the strength of this signal is in terms of total stellar power that is incident on the host planet.” Thus, given a large enough telescope — and one that’s designed specifically for infrared detection — astronomers could scan the heavens for planets within a 60 light-year radius. And indeed, there is already a proposal on the table for one such telescope: The Colossus.
Telescope via Innovative Optics (Image: NASA/JPL)
Space Daily describes the telescope:
The quest for direct infrared detection of extraterrestrial civilizations, along with many other research possibilities, has led the team to the funding and building of a giant telescope. Currently planned large infrared telescopes, the Giant Magellan Telescope, the Thirty Meter Telescope, and the European Extremely Large Telescope, would not be large enough. Instead, a telescope (dubbed Colossus) with a primary mirror about 250 feet (77 meters) in diameter could find hundreds of Earth-sized or larger planets in habitable zones, and perhaps dozens of extraterrestrial civilizations, by using a sensitive coronagraph — and the technology to build such an instrument exists. The international team thus seeks funding to build a 77-m telescope, which would be constructed from revolutionary thin-mirror slumping and polishing technologies developed by the Innovative Optics team. The telescope would consist of approximately sixty 8-m mirror segments, and would operate at a high-altitude site.
In addition, the telescope could be used to study stellar surfaces, black holes, and quasars.