Editorial Feature

Nanoantennas to Harvest the Energy of the Sun

A nanoantenna is a very tiny antenna used to study nanoscale interactions. It is basically an electromagnetic collector that can absorb specific wavelengths proportional to its size. The first nanoantenna was built by Robert L. Bailey in 1973.

Nanoantennas have the ability to transform thermal energy from the sun to electrical energy. They target the mid-infrared wavelengths, just outside the range of what is visible to the eye, which conventional photovoltaic cells cannot do. The sun radiates a lot of infrared energy, some of which is soaked up by the earth and later released as radiation for hours after sunset. Nanoantennas can take in energy from both sunlight and the earth's radiation, with higher efficiency than conventional solar cells.

In 2008, researchers at Idaho National Laboratory, along with partners at Microcontinuum Inc. and Patrick Pinhero of the University of Missouri, developed a novel way to collect energy from the sun with a technology that could cost very less, be imprinted on flexible materials, and still draw energy after sunset. This approach garnered two 2007 Nano50 awards and used a special manufacturing process to stamp tiny square spirals of conducting metal onto a sheet of plastic. Each interlocking spiral "nanoantenna" was as wide as 1/25 the diameter of a human hair.

The miniscule circuits absorb energy just like the antenna on a television or cell phone. All antennas work by resonance, the same self-reinforcing physical phenomenon that allows a high note to shatter glass. Radio and television antennas must be large because of the wavelength of energy they need to pick up. In theory, smaller antennas absorb electromagnetic radiation closer to the visible range. However, finding an efficient way to stamp out arrays of atomic scale spirals took several years.

An Economical Alternative

Commercial solar panels usually transform only less than 20% of the energy available to them into electricity. Their cells are made of silicon and doped with exotic elements to boost their efficiency. Hence they are expensive and unaffordable by many.

Research shows that individual nanoantennas can absorb close to 80% of the available energy and convert them to electrical energy. The circuits themselves can be made of a number of different conducting metals, and the nanoantennas can be printed on thin, flexible materials such as polyethylene, a plastic commonly used in bags and plastic wrap. Thus, nanoantennas are more efficient than photovoltaic cells and are a sustainable alternative.

Fine-Tuning Fine Structures

The real challenge in making the solar nanoantenna panels is to be able to predict their properties and perfect their design before printing them. While it is relatively easier to work out the physics of a single resonating antenna, complex interactions start to happen when multiple antennas are combined. When hit with the right frequency of infrared light, the antennas also produce high-energy electromagnetic fields that can have unexpected effects on the materials.

Latest Research

Recently, researchers a collaboration of three labs in Mexico, demonstrated an innovative nanodevice that uses evolutive dipole nanoantennas (EDNs) and generates 3 times the thermoelectric voltage generated by a classic dipole nanoantenna (CDN), thus optimizing the thermoelectric efficiency nanoantennas for solar energy harvesting.

The bimetallic nanoantennas used nickel and platinum and were made with the help of e-beam lithography. The design optimization was done using simulations to measure the distance between these elements. I-V curves analyzed using a solar simulator showed that the EDNs were 1.3 times more efficient than conventional nanoantennas. The researchers hope that the EDN nanoantennas can be applied in harvesting of waste heat energy and will be useful in applications that require high thermoelectric efficiency such as in the aerospace industry.

A Charged Future

While manufacturing and deploying the nanoantennas seem easy, a crucial challenge would be to find a way to store or transmit the electricity produced by them. Although infrared rays create an alternating current in the nanoantenna, the frequency of the current switches back and forth ten thousand billion times in a second. That's too fast for electrical appliances, which operate on currents that oscillate only 60 times a second.

So we need ways to transform the high-frequency alternating current (AC) to direct current (DC) that can be stored in batteries. One potential candidate is high-speed rectifiers, special diodes that would sit at the center of each spiral antenna and convert the electricity from AC to DC.

With more focused research and development, the future might see nanoantenna collectors charging portable battery packs, coating the roofs of homes and, perhaps, even integrated into polyester fabric. Double-sided panels could absorb a broad spectrum of energy from the sun during the day, while the other side could take in the narrow frequency of energy produced from the earth's radiated heat.

Sources and Further Reading

  • Idaho National Laboratory.
  • Javier Mendez-Lozoya, Ramón Díaz de León-Zapata. Thermoelectric efficiency optimization of nanoantennas for solar energy harvesting. Journal of Nanophotonics, 2019; 13 (02): 1 DOI: 10.1117/1.JNP.13.026005
  • B. Mora Ventura, et al. Analysis of Metallic Nanoantennas for Solar Energy Conversion. Proc. of SPIE Vol. 9562 95620P-4.
  • Nanoantennas – detecting the very small. https://www.physicscentral.com/explore/action/nanoantennas.cfm
  • Sadashivappa G and Sharvari N. P.  Nanoantenna – A Review. International Journal of Renewable Energy Technology Research. Vol. 4, No. 1, January 2015, pp. 1 - 9, ISSN: 2325 - 3924 (Online) Available online at http://ijretr.org.

This article was updated on 23rd May, 2019.

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