Nov
15
Energy Gathering Antennas Reach Further Into the Light Spectrum
November 15, 2011 | Leave a Comment
A solar panel composed of nano-antennas instead of semiconductors, made by adapting classic metallic antennas to absorb light waves at optical frequencies, has been announced by Tel Aviv University’s Department of Physical Electronics from its innovative new Renewable Energy Center.
We have a prior work to consider that harvests the infrared from nearly four years ago by Steve Novak at the Idaho National Lab. Then this year saw a group in Spain focus infrared light so that much more light could strike a collector.
Now scientists in Israel, professors Koby Scheuer, Yael Hanin and Amir Boag have achieved a much higher conversion rate from light into useable energy combined with a lower material cost, which could mean a cost-effective way to harvest a larger part of the solar radiation.
The professor’s technology was recently presented at Photonics West in San Francisco and will be published in the conference proceedings.
Professor Scheuer explained that both radio and optical waves are electromagnetic energy. When these waves are harvested, electrons are generated that can be converted into electric current. Traditionally, detectors based on semiconducting materials like silicon are used to interface with light, while radio waves are captured by antenna.
For optimal absorption, the antenna dimensions must correspond to the light’s very short wavelength — a challenge in optical frequencies that’s plagued engineers in the past, but now the Israeli team is able to fabricate antennas less than a micron in length.
To test the efficacy of the antennas, Prof. Scheuer and his colleagues measured their ability to absorb and remit energy. “In order to function, an antenna must form a circuit, receiving and transmitting,” said Prof. Scheuer, who points to the example of a cell phone, whose small, hidden antenna both receives and transmits radio waves in order to complete a call or send a message.
By illuminating the antennas, the researchers were able to measure the antennas’ ability to re-emit radiation efficiently, and determine how much power is lost in the circuit — a simple matter of measuring the wattage going in and coming back out. Initial tests indicate that 95 percent of the wattage going into the antenna comes out, meaning that only five percent is lost or wasted.
These are efficiencies undreamed of by the photovoltaic folks.
According to Prof. Scheuer, these “old school” nano sized antennas also have greater potential for solar energy because they can collect wavelengths across a much broader spectrum of light. The solar spectrum is very broad, he explained, with UV or infrared rays ranging from ten microns to less than two hundred nanometers. No semiconductor can handle this broad a spectrum, and they absorb only a fraction of the available energy. A group of antennas, however, can be manufactured in different lengths with the same materials and process, exploiting the entire available spectrum of light.
The team’s plan is a finished nanoantenna solar panel will be a large sheet of plastic which, with the use of a nano-imprinting lithography machine, will be imprinted with varying lengths and shapes of metallic antennas.
The researchers have already constructed a model of a possible solar panel. The next step, said Professor Scheuer, is to focus on the conversion process — how electromagnetic energy becomes electric current, and how the process can be improved.
That also is the most likely engineering issue faced by the earlier research. The folks at the Idaho National Lab found the energy harvested coming in at frequencies never before considered. What progress has been made isn’t known yet.
Everyone’s goal is not only to improve the efficiency of solar panels, but also to make the technology a viable option in terms of cost. Silicon is a relatively inexpensive semiconductor, but in order to obtain sufficient power from antennas, you need a very large panel — which makes them expensive.
Professor Scheuer pints out the obvious to most of us by noting energy sources need to be evaluated not only by what they can contribute environmentally, but also the return on every dollar invested. “Our antenna is based on metal — aluminum and gold — in very small quantities. It has the potential to be more efficient and less expensive,” he said.
It seems to this writer that nanoantennas are the leading candidate to make solar to electric power a fully economically viable path to harvesting energy. It comes with a great sense of surprise that the Israeli team has a working model losing only 5% of the incoming energy.
But after all this, antennas deliver current. So far the current they make isn’t very useful as the frequencies are so very, very high.
Now though, the incentive is getting clearer for investing the effort to harvest the antenna’s output and make free solar energy an affordable capital cost. Novak has figured out that his design could work into the night gathering the latent infrared energy. Just how the multiple sized antennas printed on plastic might perform will be an intensely interesting test.
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