Devices Proposed To Generate Electricity by Emitting Earth's IR Energy To Outer Space

Researchers at Harvard School of Engineering and Sciences (SEAS) have come up with a very innovative way to harvest the infrared energy the earth radiates into outer space. For this discovery, the researchers turned to Thermodynamics- The good ol' friendof scientists and engineers alike. These scientists envision a device which will use the earth's IR emissions caused by heat imbalance between the earth and outer space. However, this device will generate DC power not by absorption, but by emitting energy in the IR spectrum. This concept has been aptly described by the scientists themselves as a 'highly counter-intuitive' one.

Harvard physicists Federico Capasso (left) and Steven J. Byrnes (right)
One of the big brains behind this idea is Federico Capasso, a world-known scientist in the field of semiconductor physics, photonics, and solid-state electronics. From a long time, he has been working on projects which question age old assumptions regarding optics and electronics. Capasso's team has proposed two devices: one on a macro scale and another one on a nano scale. Both these devices would represent the reverse-analogue to the photovoltaic cell.

The macro device would consist of a 'hot plate' at the temperature of earth and earth's atmosphere and a 'cold plate' made of a highly emissive material, facing upward. This 'cold plate' will cool rapidly thanks to the emissive material and the temperature difference between the two plates will be enough to generate energy of few watts per square meter. The measurements were carried out on the basis of infrared emissions in Lamont, Oklahoma as a case study. Nonetheless, "Temperature difference causes energy to flow"-the basic principle of thermodynamics is illustrated in this device. One of the problems preventing the practical implementation of such a device is the difficulty in maintaining the 'cold plate' cooler than the ambient temperature.

The nanoscale device relies on temperature difference between electronic components such as that between an antenna and a diode. Capasso et al rely on the electron behaviour in presence of a temperature gradient between electronic devices in a circuit. In the circuit diagrams below, figure A shows no net current flow when the devices are at same temperature. In figure B, a conventional rectifier circuit shows how temperature difference causes electrical noise in the circuit. What the Harvard researchers have proposed is depicted in figure C: Making the diode temperature higher so that current is pushed only in one direction, producing a potential difference. The resistors will be replaced by antennas that will efficiently radiate earth's IR radiation towards the sky, thus cooling the antenna and initialising the process. A single flat device could be coated in many of these tiny circuits, pointed at the sky, and used to generate power, says the paper which will be published this week by the researchers.

Interestingly enough, J.B.Gunn, the inventor of the Gunn diode used in police radars, had considered this concept way back in 1968 but the idea was confined to literature only.

A few problems persist though. The inability of IR diodes to operate at extremely low power levels is one of the major issues. Team member Steven J. Byrnes said that they are considering diodes such as tunnel diode and ballistic diode, which can handle lower voltages. Also, raising the overall impedence of the circuit could help in realising usable voltage levels from the circuit. Hence, a combination of both the techniques would be helpful, Byrnes added. Speed represents another constraint. Along with low voltage levels, these diodes will be required to switch on and off about 30 Trillion times a second, which is needed for IR signals. Very few types of diodes are currently capable of such high speeds.

However, Byrnes believes that since they are aware of the constraints and limitations as well as of the promises delivered by the current technology, they will soon engineer an efficient solution to their problems. Also, these devices could be implemented along with existing photovoltaic devices without extra installation cost, he pointed out.

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