Wednesday, November 26, 2014

Huge solar energy improvement - Nano-sized antenna arrays

Solar energy is not only the most popular energy source in the world, it is also the most abundant energy source of them all. Still, solar energy is still far behind fossil fuels in terms of primary energy consumption, mostly because of its lack of cost-effectiveness when compared with traditional energy sources (coal, oil, hydro and natural gas).

One of the latest promising technologies comes from the University of Connecticut engineering professor Brian Willis. The first tests have showed the excellent potential and this technology looks to have what it takes to vastly improve today's solar energy systems.

The basic principles of this technology that rely on incredibly small nano-sized antenna arrays that are in theory capable of capture more than 70 percent of the sun's electromagnetic radiation and simultaneously converting it into usable electric power thus greatly improving the efficiency of currently used solar cells.

With today's technology, even the most effective silicon panels are not able to collect more than 20 percent of available solar radiation, not to mention that the separate mechanisms are needed to convert the stored energy to usable electricity for the commercial power grid. This pretty much efficiency as well as quite an expensive development costs are the two major stumbling blocks to the widespread adoption of solar power as a practical replacement for traditional fossil fuels.

Theoretically promising is not anything for commercial production and scientists have lacked the technology required to construct and test because the fabrication process is extremely challenging. The nano-antennas because of their ability to both absorb and rectify solar energy from alternating current to direct current must be capable to operate at the speed of visible light and be built in such a way that their core pair of electrodes is a mere 1 or 2 nanometers apart, which is approximately 30,000 times smaller than the diameter of human hair.

Illustration of a working nanosized optical rectifying antenna or rectenna (source).
However, the potential solution for fabrication issue can perhaps be found in the selective area atomic layer deposition (ALD) that was developed by Willis. The Professor Willis developed the ALD process while teaching at the University of Delaware, and patented the technique in 2011.

As he explained it is through atomic layer deposition that scientists can finally fabricate a working nano-antenna device. In a nano-antenna, one of the two interior electrodes must have a sharp tip, similar to the point of a triangle. The trick here is getting the tip of that electrode within one or two nanometers of the opposite electrode, which is something similar to holding the point of a needle to the plane of a wall. Before the ALD came into the picture, the existing lithographic fabrication techniques were unable to create such a small space within a working electrical diode. Even when using sophisticated electronic equipment such as electron guns, the closest scientists could get was about 10 times the required separation. However, through ALD, Willis was able to precisely coat the tip of the nano-antenna with layers of individual copper atoms until a gap of about 1.5 nanometers is achieved. The process is self-limiting and stops at 1.5 nanometer separation.

The size of the gap is of vital importance because it creates an ultra-fast tunnel junction between the nano-antenna's two electrodes, which allows a maximum transfer of electricity. This gap also gives energized electrons on the nano-antenna just enough time to tunnel to the opposite electrode before their electrical current reverses and they try to go back. The triangular tip of the nano-antenna prevents electrons to reverse direction, thus capturing the energy and rectifying it to a unidirectional current.

These nano-antennas, because of their incredibly small and fast tunnel diodes, are capable of converting solar radiation in the infrared region through the extremely fast and short wavelengths of visible light, something that has never been accomplished before. The current solar panels build on silicon have a single band gap which, allows the panel to convert electromagnetic radiation efficiently at only one small portion of the solar spectrum unlike nano-antennas that are able to harvest light over the whole solar spectrum, creating maximum efficiency.

Prior to the advent of selective atomic layer deposition (ALD), it has not been possible to fabricate practical and reproducible nano-antenna arrays that can harness solar energy from the infrared through the visible and ALD is what makes the creation of these devices possible. The atomic layer deposition process is favored by science and industry because it is simple, easily reproducible, and scalable for mass production. The method being used to fabricate nano-antennas can also be used in thermoelectrics, infrared sensing and imaging, and chemical sensors.

Willis has already made the prototype device and as he says „now we're looking for ways to modify the nano-antenna so it tunes into frequencies better.“ The question whether these devices really function at this high level of efficiency is yet to be answered?’ Theoretically this is possible, but further tests in practice will tell us the rest of the story.

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