Solar Antennas

This is an old idea I once had, in 2006 when I started college

Radio waves. You can build a radio that is powered entirely by the radio waves it picks up. These are called crystal radios. They’re not very loud, but nevertheless powered by radio. Now if you go up in frequency, the more energy there is. If you go way way way up in frequency – you’ll get to light waves. So why not build an antenna tuned to light? Back then, I worked it out using a magic value (not this kind) I recalled from getting my radio merit badge in boy scouts and came to approximately 225nm in size for an antenna tuned to blue light – and that’s where I stopped. It’s time to look at the idea again.

Turns out, I wasn’t the only one to have that idea – it’s called rectenna, from the words rectifying antenna. Light waves are very high in frequency – 400THz at the lowest – and that’s not a usable form of electric energy. So this would need to be straightened out – rectified – into direct current. And thus, in custom of making enormous names, such antennas made to operate at visible light waves were named optical rectennas. (Nowhere near cat’s whisker MWNT-Al₂O₃Ca diode though). There are others, for lower frequencies like infrared (say, waste heat) or microwave, but they somehow don’t have an explicit name. In any case, the rectenna word is used for any such antenna to convert the electromagnetic spectrum into electricity.

I’ve also seen ‘nantenna’ used for it. I personally thought of ‘nano-antenna arrays’, which can be abbreviated to NAA that you can then use in a conversation like so:
Other Person: “So, can you tell me what you’re workin’ on?”
You: “Naa”
Other Person: *eyes you suspiciously*
There’s also the title of this post as a name, which I also just made up.

 

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Theory

The current solar panels, photovoltaics, operate via having a photon smashing into them, which excite an electron up out of the valence band, leap across the band gap and into the conduction band where they join a marching band and proceed along the street into the item of your choice that requires electricity. This is the true pedant’s answer.

That band gap limits the amount of power you can collect from the sun. The amount of energy you can get is the amount it took to jump that gap. Current solar panels are around 20% efficient [1] at capturing the energy. That is, if there’s 1000 watts of light hitting the panel, you’ll get 200 watts of it. There’s also a bunch of really fancy (I.E highly expensive) panels that can reach to 47% [2], and there’s a theoretical limit of 55% [3]. There’s much more to be said about band gaps, but that is a story for another time.

Antennas on the other hand, operate in a different manner. They respond (resonate) to the wave form of the light hitting it, and begin to vibrate at the same frequency of the light and start to generate an alternating current [3]. Without a band gap preventing more energy for collection, the efficiency could be much higher.

Since most of this rectenna operation is still theory, there are arguments about how efficient they would be. The efficiency of these optical rectennas however could be anywhere from ~0. 00001% (it’s a proof of concept) to 85.4% (thermodynamic limit) to 93.3% (Landsberg limit) to 100% (ultimate maximum power!), depending on who you agree with [4][5][3][6]. A rectenna and diode combination was measured to have 90.59% efficiency in the microwave region for what it’s worth [7, 8]. This suggests that a value around the latter two values are possible.

A minimum goal to reach at any case I’d say would be 80%. With that, you could quadruple the amount of power for the same area that current solar panels cover. Or use a quarter of the area for the same amount of power. The estimated area needed to power the United States with solar panels is roughly 2000 square miles [9]. This could be cut down to ~500 sq. mi. For comparison, New York City covers 468 square miles – which, strangely enough, is the same value as the magic number I used in the first paragraph.

 

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Antennas

Various antenna types, as shown and randomly assigned a name either by me or by the literature:

The Dipole [3]

The Spiral [12]

The Bowtie [10] (Others call this the butterfly)

The Waffle [11] (side note: this Novack guy at INL here is one of the reasons that got me to look to Idaho for a graduate degree)

The Carpet [4] (of nanotubes {with a plastic cover because guests are coming over [you monster]})

The Feathers [3]

Others that I haven’t seen (that I’m making up as I go): The Menorah, The strangely Bent Paperclip, The Koch snowflake (nevermind), The Concentric Circles, The “Let Me Out!” Claw Marks, The Core and the uhm… Spikes and Pits Deathtrap Special.

 

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Rectifying

In this section, papers become more annoying to read.

There’s a few methods of rectifying the energy once collected. The issue is that of speed (among several other things). The higher the frequency, the faster it switches back and forth, so you need diodes that can respond just as quickly. One such diode is the metal-insulator-metal (MIM). Their fast times could reach to ~100THz [3 ~pg 91] or ~150 THz [3 ~pg 314]. If they’re small enough, they could reach to about 650THz [3]. There’s also various sub categories of MIM diodes. MOSFETs might also be used, but searching for them always leads to imaging papers, not rectifying papers.

A graphene geometric diode [10, 3] was made and operated at 28 THz, giving an efficiency of 12%. Making graphene however, is a constant issue. If you see “exfoliated graphene”, it’s made by hand with scotch tape. Carbon nanotubes are also looked into [4] and they could work up to PHz waves, which would be excellent, but like graphene, they somehow never appear outside of research.

Ballistic transistors are something I’ve also heard of, but haven’t seen much of it mentioned – it might also be useful. Mostly, all I can find of it is news briefs from 2006 when it was announced. There it mentions it might go to 3 THz but even the authors weren’t sure how high it could go – though they’re more interested in making faster computers with them instead of rectifying (which I’m fine with; a THz processor could really speed up my polyplots).

It’s possible they also go by a different name and I haven’t seen them connected; that sort of thing can happen. Incidentally, if this was chemistry, you’d be killed for even suggesting there’s another name/method of thought (“you can’t think that way!” is the nature of most chemists I’ve dealt with).

Ultimately what you’d like is a diode/transistor that works on petahertz scale thus giving you access to the whole of visible light; or, if need be, ones that work within a valid range for a certain wavelength; then build up several of those for various different wavelengths.

 

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There’s obviously far more to this whole idea than I could stuff into a thousand words, and hopefully I covered the main parts of it (however briefly). As such, I want to see these to commercialization, and not just in papers.

 

 

 

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References

[1]: V. Aggarwal, “Solar Panel Efficiency: What Panels Are Most Efficient?”, Solar News, 2019 [link][archive]

[2]: “Best Research-Cell Efficiencies”, NREL, 2019 [link][archive]

[3]: G. Moddel, “Rectenna Solar Cells”, Springer, New York, 2013 [doi]

[4]: A. Sharma, V. Singh, T. Bougher and B. Cola, “A carbon nanotube optical rectenna”, Nature Nanotechnology, 2015, 10 [doi]

[5]: R. Corkish, M. Green and T. Puzzer, “Solar energy collection by antennas”, Solar Energy, 2002, 73 [doi]

[6]: R. Bailey, “A Proposed New Concept for a Solar-Energy Converter”, Journal of Engineering for Power, 1972, 94 [doi]

[7]: J. Zhang and Y. Huang, “Rectennas for Wireless Energy Harvesting” [link][archive]

[8]: W. C. Brown, “Electronic and Mechanical Improvement of the Receiving Terminal of a Free-Space Microwave Power Transmission System”, Raytheon Company, NASA Report CR-135194, 1977 [link][archive]

[9] Jorgustin, K. “Amazing Map: Total Solar Panels To Power The United States”, Modern Survival Blog, 2019 [link][archive]

[10] M. Gadalla, M. Abdel-Rahman and A. Shamim, “Design, Optimization and Fabrication of a 28.3 THz Nano-Rectenna for Infrared Detection and Rectification”, Scientific Reports, 2014, 4 [doi]

[11] D. Kotter, S. Novack, W. Slafer and P. Pinhero, “Theory and Manufacturing Processes of Solar Nanoantenna Electromagnetic Collectors”, Journal of Solar Energy Engineering, 2010, 132 [doi]

[12] F. Gonzalez, B. Ilic, J. Alda and G. Boreman, “Antenna-coupled infrared detectors for imaging applications”, IEEE Journal of Selected Topics in Quantum Electronics, 2005, 11 [doi]

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