Energy can be harvested any time heat flows spontaneously from a hotter to a cooler body. Solar cells, for example, avail of the large temperature difference between the sun (~6000K) and the earth (~300K) for efficient energy collection. On the other hand, the earth’s thermal radiation (~300K) into cold, deep space (~2.7K) has rarely been considered as a potential energy source.
This might be because in some ways it compares poorly with solar energy. Compared to a theoretical Carnot efficiency limit of about 95% for solar cells, earth’s IR radiation into outer space (mediated by the atmosphere) is constrained to 15%. But unlike solar power, which peaks for only a few hours a day, this energy is available day and night, and collectors can be imagined to run non-competitively alongside photovoltaics.
The hurdle is really the design: to make a working device that is cheap, practical, and can cover large areas. Additionally, the device has to live on earth, with most of its components in thermal equilibrium with the ambient atmosphere. The group’s proposal, based on the creative use of a rectifying antenna, is the subject of our most recent Proceedings of the National Academy of Sciences (PNAS) publication.
The design employs an emitter rather than an absorber, as in the case with photovoltaics, to create the effective temperature differential required for energy flow and collection. In the simplest realization of the device, the emitter, a radiatively cooled antenna, is joined in a circuit with an ambient-temperature diode. Thermal energy flows from the “hot” diode to the “cold” antenna, resulting in DC current. The paper discusses the physics behind these devices as well as material and engineering considerations relevant for their practical use. A network of such circuit elements may eventually be able to approach the Carnot efficiency limit*.
The project is led by post doc Steven Byrnes with contributions from graduate student Romain Blanchard. This and other emissive energy harvester designs continue to be tested and explored.
- Harvesting renewable energy from Earth’s mid-infrared emissions – Steven J. Byrnes, Romain Blanchard, and Federico Capasso, PNAS
- Infrared: A new renewable energy source? – Harvard SEAS Press
- Physicists May Have Found New Way To Turn Earth’s Radiation Into Energy – Huffington Post
- Earth Electric – New Scientist (full text)
*Carnot efficiency limit must be calculated in this case to account for an effective temperature for the antenna which is between that of the earth and sky. The result for such a device is about 1-2%.