Forget solar panels, optical ‘rectenna’ converts light directly to electricity

From Georgia Tech, a news release that we missed when it first came out, but still well worth talking about.

First Optical Rectenna – Combined Rectifier and Antenna – Converts Light to DC Current

Using nanometer-scale components, researchers have demonstrated the first optical rectenna, a device that combines the functions of an antenna and a rectifier diode to convert light directly into DC current.

This schematic shows the components of the optical rectenna developed at the Georgia Institute of Technology. (Credit: Thomas Bougher, Georgia Tech)

This schematic shows the components of the optical rectenna developed at the Georgia Institute of Technology. (Credit: Thomas Bougher, Georgia Tech)

Based on multiwall carbon nanotubes and tiny rectifiers fabricated onto them, the optical rectennas could provide a new technology for photodetectors that would operate without the need for cooling, energy harvesters that would convert waste heat to electricity – and ultimately for a new way to efficiently capture solar energy.

In the new devices, developed by engineers at the Georgia Institute of Technology, the carbon nanotubes act as antennas to capture light from the sun or other sources. As the waves of light hit the nanotube antennas, they create an oscillating charge that moves through rectifier devices attached to them. The rectifiers switch on and off at record high petahertz speeds, creating a small direct current.

Billions of rectennas in an array can produce significant current, though the efficiency of the devices demonstrated so far remains below one percent. The researchers hope to boost that output through optimization techniques, and believe that a rectenna with commercial potential may be available within a year.

“We could ultimately make solar cells that are twice as efficient at a cost that is ten times lower, and that is to me an opportunity to change the world in a very big way” said Baratunde Cola, an associate professor in the George W. Woodruff School of Mechanical Engineering at Georgia Tech. “As a robust, high-temperature detector, these rectennas could be a completely disruptive technology if we can get to one percent efficiency. If we can get to higher efficiencies, we could apply it to energy conversion technologies and solar energy capture.”

The research, supported by the Defense Advanced Research Projects Agency (DARPA), the Space and Naval Warfare (SPAWAR) Systems Center and the Army Research Office (ARO), was reported September 28 in the journal Nature Nanotechnology.

Developed in the 1960s and 1970s, rectennas have operated at wavelengths as short as ten microns, but for more than 40 years researchers have been attempting to make devices at optical wavelengths. There were many challenges: making the antennas small enough to couple optical wavelengths, and fabricating a matching rectifier diode small enough and able to operate fast enough to capture the electromagnetic wave oscillations. But the potential of high efficiency and low cost kept scientists working on the technology.

“The physics and the scientific concepts have been out there,” said Cola. “Now was the perfect time to try some new things and make a device work, thanks to advances in fabrication technology.”

Using metallic multiwall carbon nanotubes and nanoscale fabrication techniques, Cola and collaborators Asha Sharma, Virendra Singh and Thomas Bougher constructed devices that utilize the wave nature of light rather than its particle nature. They also used a long series of tests – and more than a thousand devices – to verify measurements of both current and voltage to confirm the existence of rectenna functions that had been predicted theoretically. The devices operated at a range of temperatures from 5 to 77 degrees Celsius.

Fabricating the rectennas begins with growing forests of vertically-aligned carbon nanotubes on a conductive substrate. Using atomic layer chemical vapor deposition, the nanotubes are coated with an aluminum oxide material to insulate them. Finally, physical vapor deposition is used to deposit optically-transparent thin layers of calcium then aluminum metals atop the nanotube forest. The difference of work functions between the nanotubes and the calcium provides a potential of about two electron volts, enough to drive electrons out of the carbon nanotube antennas when they are excited by light.

In operation, oscillating waves of light pass through the transparent calcium-aluminum electrode and interact with the nanotubes. The metal-insulator-metal junctions at the nanotube tips serve as rectifiers switching on and off at femtosecond intervals, allowing electrons generated by the antenna to flow one way into the top electrode. Ultra-low capacitance, on the order of a few attofarads, enables the 10-nanometer diameter diode to operate at these exceptional frequencies.

“A rectenna is basically an antenna coupled to a diode, but when you move into the optical spectrum, that usually means a nanoscale antenna coupled to a metal-insulator-metal diode,” Cola explained. “The closer you can get the antenna to the diode, the more efficient it is. So the ideal structure uses the antenna as one of the metals in the diode – which is the structure we made.”

The rectennas fabricated by Cola’s group are grown on rigid substrates, but the goal is to grow them on a foil or other material that would produce flexible solar cells or photodetectors.

Cola sees the rectennas built so far as simple proof of principle. He has ideas for how to improve the efficiency by changing the materials, opening the carbon nanotubes to allow multiple conduction channels, and reducing resistance in the structures.

“We think we can reduce the resistance by several orders of magnitude just by improving the fabrication of our device structures,” he said. “Based on what others have done and what the theory is showing us, I believe that these devices could get to greater than 40 percent efficiency.”

This work was supported by the Defense Advanced Research Projects Agency (DARPA), the Space and Naval Warfare (SPAWAR) Systems Center, Pacific under YFA grant N66001-09-1-2091, and by the Army Research Office (ARO), through the Young Investigator Program (YIP), under agreement W911NF-13-1-0491. The statements in this release are those of the authors and do not necessarily reflect the official views of DARPA, SPAWAR or ARO. Georgia Tech has filed international patent applications related to this work under PCT/US2013/065918 in the United States (U.S.S.N. 14/434,118), Europe (No. 13847632.0), Japan (No. 2015-538110) and China (No. 201380060639.2)

CITATION: Asha Sharma, Virendra Singh, Thomas L. Bougher and Baratunde A. Cola, “A carbon nanotube optical rectenna,” (Nature Nanotechnology, 2015).


127 thoughts on “Forget solar panels, optical ‘rectenna’ converts light directly to electricity

  1. All this is very good, but really only in space.

    On earth it comes down to the number of watts/ sq meter.
    Insolation is is the upward boundary of what can be generated by any system. In the upper latitudes it simply won’t work.

  2. I don’t know when and if this technology will be reliable, but the use of the diode embedded into the antenna element looks as a good choice to me.
    I admit that the first time I read about the nanotech antenna arrays for light conversion, I had been pessimistic about coupling all those nano-aerials to achieve a useful electrons flux.
    This solution seems a good candidate to solve that issue.

    Let’s see where these guys go.

    Have a great day.


    • So what material are the diodes made of ? This story reads like they simply rectify the 150 THz AC waveform of the Hertzian waves that one calculates from Maxwell’s equations.

      Calling “heat” …. waste …. is repetitious. And it can’t be 100% converted to electricity or to anything else that is useful, like mechanical work.

      When they finally build their carbon nanotube tether rope, they can sail this thing up to a geo-synchronous orbit.

      I hope they have already figured out how to keep the rope geo-synchronous all the way from the ground up.


      • So what material are the diodes made of ?

        Al/Ca/Al, sounds like basically a point contact whisker diode.

        I hope they have already figured out how to keep the rope geo-synchronous all the way from the ground up.

        Easy peasy, just need the center of mass at geo-sync.

    • Have these researchers forgotten, or perhaps not realised what is already known about receiving electromagnetic “radio” waves, and that is that the antenna can have appreciable gain in itself, particularly with “Yagi Arrays”. Some researchers have already applied the Yagi principle to micro manufactured substrates, though not with carbon nanotube. The issue with the carbon nanotube, so far as I see it, is that the hexagons are all the same size. They need to be of an ever diminishing size, the further they get from the substrate, in a calculated progression, so as to get the wideband amplification effect they seek. This will be quite difficult to achieve, and may be prohibitively expensive, and perhaps even impossible to achieve due to inherent properties of carbon nanotube production where hexagons are bound to be of a certain exact dimensions.

      What may be more interesting, and productive in this field is Photonic Crystalography, and a good explanation of this research may be seen at …. “Photonic Crystal Research at the CMCE”, which is part of The Joannopoulos Research Group at M.I.T. Perhaps someone from that group will read these comments and enlighten us further – are these two techniques of Photonic Crystals and Carbon Nanotubes incompatible, or could they be combined to produce some desirous effect which could increase efficiency, and including the Yagi Effect in manufacture ?

      See this page for an explanation of Photonic Crystal Research

      See also this paper (PDF)
      Introduction to Photonic Crystals:
      Bloch’s Theorem, Band Diagrams, and Gaps
      Steven G. Johnson and J. D. Joannopoulos, MIT
      3rd February 2003

      See also this paper for relevance to Yagi principle (PDF)
      M. M. Abd-Elrazzak
      Electronics & Communication Department,
      Faculty of Engineering
      Almansoura University,
      Progress In Electromagnetics Research C, Vol. 19, 15–24, 2011

      It seems to me that we need to combine disparate techniques,
      but are researchers reluctant to step outside their own parish?

      • Not so fast Pilgrim !

        “””””…… and that is that the antenna ,can have appreciable gain in itself, ……””””””

        Nosirreebob Not on my watch it can’t !

        An Antenna’s “Gain” such as your Yagi, for example (or anyone else’s magic wand) refers to its peak sensitivity AS COMPARED TO A DIPOLE ANTENNA.

        It is entirely a consequence of a redistribution of the field. A dipole for example, has a circularly symmetrical toroidal shaped field, aligned along the axis of the dipole.

        You can count on the field intensity along the axis of the dipole as being essentially zero.

        But if you add other antenna elements, say parallel to the dipole, in some kind of array, you can either drive those elements too or drive some and not others; you can drive some with phase shifted currents.

        The result of such architectures is to change the directionality of the radiated field, and yes you can concentrate more of the “beam” into a smaller angle or direction space, so that the peak field intensity in some preferred direction is higher than a dipole gives, but now you have directions with lower or even no radiated field at all.

        The antenna cannot have any gain in power or energy or field, and still have the same field distribution pattern.

        You get nothing for free.


  3. Its clever physics but. Loads of mineral content to be extracted, not just inefficient but massively inefficient. Expensive production techniques. Its a long way from useful.

    Love the physics

  4. How is that different from solar panels? Solar works by exciting an electron so it separates from an atom, passes through a dielectric, and has to find the long way around to get the charge back. Instead of having one large plate of dielectric, this is just a forest of nanotubes, each with its own dielectric component. That’s all the difference I can see.

    • Not remotely similar. A solar cell relies on the photovoltaic effect, wherein the absorption of light causes an electron to be excited to a higher energy state. Through various bits of magic this electron excitation generates a voltage potential across a dielectric, which is then used to generate DC current.

      This system relies on the induction of current in a material by an alternating electromagnetic wave, exactly the same as a radio antenna, but at a much shorter wavelength – an effect known as transduction. The diode rectifies this AC current into DC.

      Theoretically it’s a much more efficient method of converting light to electricity.

    • Conventional solar cells use photons (a particle of light) to knock electrons off a silicon atom. The atom is then collected and used to create an electric differential. It only works with a few wavelengths of light (certain energies of photons) and that is why solar cells have a theoretical maximum of 21% efficiency.

      This new technology works by using light as a wave instead of a particle. It rectifies the wave in the same way as mains power is rectified in your computer power supply. It can work at as high a frequency as possible and this determines the theoretical maximum efficiency, which the article noted as being 40%.

      • a. Conventional solar cells already get near 40% efficiency. Google multi-junction solar cells.

        b. This article did not place an upper limit on this technology of 40%, but offered it as a target.

      • a. Conventional solar cells already get near 40% efficiency. Google multi-junction solar cells.

        quibbling, that’s likely because it’s 2 x ~20% efficient junctions stacked one on top of another.
        And Solar panel (well mono-crystalline ones) cells are just big diodes, photons create electron/hole pair as they travel through the doped Si, when they form across the diode junction, they can’t recombine.

      • Just to keep a proper perspective, microwave rectennas (if I remember correctly) are in the ballpark of 90% conversion efficiency. What will be of interest is the spectral bandwidth that this will accept.

    • MAYBE the ‘fossil fuel’s’ carbon (atoms) should be used DIRECTLY rather than later on WHEN a couple of Oxygen atoms come into play … …

  5. Very interesting. One problem will be getting the bandwidth to be able to use sunlight and does the light need to be polarized?

  6. If the nano-tubes are acting as antennae resonant at light frequencies, then I would expect a given be effective at a narrow bandwidth of the light falling on it. So unless that bandwidth can be made wider, it’s difficult to imagine a high efficiency of conversion from light to electricity to be achieved. The equivalent of a wide band antennae, would need to be achieved.

    • Most antennae work at a much wider range of frequencies than is practically useful. The receiving equipment then has to select those frequencies that are desired. Think of your TV set.

      In this application, a wider frequency range is actually useful.

      • Hi Hivemind,
        but antennas have gain respect the ideal isotropic radiator which is function not only of their directivy but that it is function of the bandwidth.
        The wider the band the lower the gain.
        So TedM issue is real, because the sunlight is not a narrow band source as the laser used by the one authors of this research.
        It could be very difficult to achieve a wide band antenna that efficiently collect the whole energy available on the irradiated surface in the full sunlight spectrum, because different wavelength antennas can’t occupies the same place.
        In my opinion, it’s a real hard to solve puzzle.
        Have a great day.


      • re: Hivemind October 20, 2016 at 4:57 am:

        “Most antennae work at a much wider range of frequencies”

        Suggest you endeavor to employ that ‘principle’ you have in mind and file a patent on it. Meanwhile, pick up a RigExpert model AA-30 (or better) and actually DO some experiments in the lab first …

      • Wide-band system? Use a beam-splitter to segment the incoming light into narrower wavelength bands. Next question.

      • @Michael J. Dunn October 20, 2016 at 3:44 pm
        “Wide-band system? Use a beam-splitter to segment the incoming light into narrower wavelength bands. Next question.”

        Hi Michael,
        not so easy, your splitter should be wavelength selective, that is it must canalize only the right wavelengths on the matched wavelength rectenna, otherwise the issue wasn’t solved.
        Not so easy indeed.

        Have a great day.


    • Awww, give ’em a break. This is the Mark 1 version. With 40 years of development, or maybe only 20 given moderner techniques, it may get to 20%, 40% or even better.
      Very clever thinking and execution, so far.

    • Well the only part of the solar spectrum that is of much use energy wise would be from 250 nm wavelength (1200 THz) out to 4.0 microns (750GHz).

      That’s only a 16:1 frequency ratio.

      Any Ham radio guy can probably gen up a suitable antenna design from the ARRL Handbook.


      How many Tom Swifts do we have here reading WUWT ??

      • Hi George,
        I’m an HAM radio too, and I assure you that the goal of a wide bandwidth antenna isn’t to get all the incoming energy, but just extract from the flux as much energy as it can for discriminate the information from the incoming signal (not so much indeed, at least for broadband antennas).
        “That’s only a 16:1 frequency ratio.”

        I don’t know your knowledge in the matter, but you are considering the “only” 16:1 ratio an easy to achieve goal for a efficient antenna, but I assure you that it isn’t so.
        The log periodic antenna or now the Vivaldi antenna are the widest on the market, but they are anything but efficient in capturing all the the incoming energy on that so wide range.

        I wish to those scientists who are working to this project to achieve all their goals, but I admit that I’m still a little pessimistic about a future for this device as a substitute for silicon based PV panels.

        Have a great day.


      • @Massimo
        Disregarding for the moment any fabrication issues, couldn’t one grow nanotubes with a range of suitable characteristics on the same substrate and thereby create a wide bandwidth antenna?

      • @D. J. Hawkins
        “Disregarding for the moment any fabrication issues, couldn’t one grow nanotubes with a range of suitable characteristics on the same substrate and thereby create a wide bandwidth antenna?”
        IMHO the problem is that you can’t have more than one efficient antenna for any wavelength in the same place, so you can’t get all the incoming energy on the receiving array surface.
        Just to say a resonating dipole @300nm has a very low efficiency @600nm.

        Have a great day.


      • @Massimo
        But the fabrication dimensions are on the order of 10 nm. Wouldn’t that allow an incoming photon to interact with multiple structures?

      • @D. J. Hawkins
        “But the fabrication dimensions are on the order of 10 nm. Wouldn’t that allow an incoming photon to interact with multiple structures?”
        Yes you are right, but that’s no more an antenna at all, it’s just an another application of Einstein’s photovoltaic effect.
        What that scientist are trying to do is an antenna array that in their intentions should convert light from its wave nature directly in electric energy.
        They claim that the rectenna should be better than the PV elements, I still have some doubts about that.
        Anyways, as we say here in Italy, if you are not optimistic is better you don’t start any task.
        So, I wish them all the best and good luck with that.

        Let’s see where they go.

        Have a great day.


  7. Interesting technology. But do we really need another source of energy that cannot provide base load needs? Show ne something that works on a calm night, please.

    That said…this might be good technology for space craft.

    • Dang ! I think you may have just put your finger on a matter of some importance there. Give them a couple of days to find a workaround.


  8. I see this as more useful in the immediate term for making highly accurate photo detectors. Eventually it may produce more efficient solar panels, which would be good for spacecraft, but if the past is any kind of guide, that is probably a long ways off. As in decades.

  9. A tiny antenna will have a tiny voltage. Any rectifier has some forward voltage drop. I’m surprised the antenna voltage exceeds the diode’s forward drop. Anyway, I wouldn’t bet money they can get much efficiency.

      • That ” drop ” as you call it, is the Output Voltage of the cell.

        In a way, that’s exactly what you are trying to get !


      • A solar cell is a diode, so the drop is built into them, as well internal resistance.

        there is no drop (ie no voltage across it), if there is no light.

  10. Sounds like something you might put where the sun don’t shine! But I’d agree with the comments above about the maturity of the technology for application in commercial solar power generation, and the abiding need for baseload power sources.

  11. Nanotubes!! Gooolllyyy!!! CARRRBOOONNNN Nanotubes no less!

    Do they work when the sun isn’t shining?… Oh. Never mind.

    • Paul Westhaver October 20, 2016 at 4:50 am
      “Do they work when the sun isn’t shining?… Oh. Never mind.”

      Perhaps they do,,,

      From the article

      ” energy harvesters that would convert waste heat to electricity – and ultimately for a new way to efficiently capture solar energy.”

      It would have been better if they had expounded more or that statement.


      • My take was that they were stating that they could build one of these devices that was tuned to the infra-red, rather than optical wavelengths.

      • Infra-red wavelengths are NOT waste “heat”; they are not ANY kind of heat.

        They are electro-magnetic radiation, which is an entirely different form of energy from garbage that is ” heat.”


      • @Mike the Morlock

        October 20, 2016 at 5:04 am: Like those warmists working on IR receivers for electric power a few years ago – haven’t heard from them, so perhaps they learnt the energy Physics lesson all CO2 cool-heats-warm idiots do not understand. At least this topic is nearer to correct, energetics-wise.

    • The same nanotechnology European greens lobbied to ban under guise of the precautionary principle. Two promising and fascinating applications in 3 days I’ve read about using nanotechnology. Both applications greens would adore.

    • The fatal flaws in solar power are 1. the linear linkage to the area of the collector — no matter what harvesting technology you use or how efficient it is — you need square miles of collectors to get close to current, let alone future, grid capacities, and 2. you must have a non solar system to cover the 4400 hours per year that the sun does not shine* and to make up the difference between summer sun and winter sun.

      All in all, those two factors guarantee that solar will never be economic, even if the cells were as efficient as the laws of physics allows and the they were free.

      * We call it night.

      • The sun is ALWAYS shining.

        Ask Kevin Trenberth .It shines down continuously from 186 million miles away, at about 432 w/m^2.


  12. Interesting technology

    I can see a use in IR detection. Of which they can engineer a device that is tailored for a given range of colors or one specific color. In order to use it as a replacement for cones and rods?

    However, the article says “As a robust, high-temperature detector, these rectennas could be a completely disruptive technology if we can get to one percent efficiency. If we can get to higher efficiencies, we could apply it to energy conversion technologies and solar energy capture.”
    Sounds like they need $$$ to begin.

    So, efficiency seem to be the problem. If they want to market the product in a year, they are not talking about insolation gathering.

    Down to it, solar panel technology is designed to convert “insolation” into high current DC. Presently it takes a full sun to produce enough current to charge a battery. Full sun does not begin to convert until well into the morning times some 1-1/2 hours after the first light of day for about 8 hours. Maybe they think that this “rectenna” can be used to convert moonlight into energy. Both technologies need to be much more efficient.

    • On the high temperature end of things, they sound *perfect* for capturing the energy put out by Randell Mills’ “Brilliant Light Power” devices …

  13. The problem with solar cells has litle to do with their efficiency (now around 20% – this new technology apparently looks to possibly double that). There is also the issue of environmental contaminents coating the rectennas, like they do solar panels, reducing the light that can get to the receptors. Energy from sunlight still suffers from its uncontrollability, lessened somewhat (but made more expensive) by the presence of batteries. With two companies that are developing molten salt nuclear reactors promising sub 2 cents per kWhr levelized cost for power, I see no technology that can compete against that emerging technology, especially uncontrollable power generators, whose power is inherently worth far, far less than controllable on-demand power. Solar roofs are disrupting our grid by their characteristics of uncontrollability – both in their house’s sudden need for power from the grid when clouds roll in, and then dumping unasked-for, often unwanted power onto the grid when the panels produce more than the home requires.
    The latter , in effect, introduces power input to the grid that is unreliable, and often requires that reliable power plants scale down their output , which increases, in an almost linear fashion (given cheap fuel)
    their cost per kWhr, paid for mostly by homes w/o solar roofs. This grid situation is absurdly stupid, made
    even more so by Fed and state govt subsidies to solar providers.

  14. “We could ultimately make solar cells that are twice as efficient at a cost that is ten times lower”

    True, if they improve efficiency by a factor of 50+ and reduce cost by a factor of…. thousands? Meanwhile, if they reduce R to increase efficiency, then the system Q will be increased, meaning the bandwidth will be reduced, but I have know idea where that will leave them. As a device for generating power, however, the Achilles heel will be the top substrate, which must carry the working current of a column of nano-tubes while remaining optically transparent. Perhaps a “wire” conductor could be added between each column of nano-tubes so that the film must only carry the current from one tube to the “wire”, and the “wire” could carry the summed current down to the SiOx junction. More complexity and cost.

    This is a very cool technology. But as a source of grid solar power it is dependent upon the improvement of a fair number of additional technologies, not the least of which is nano scale mass production.

    • Apparently NOT a math guy.

      They already cost ten times less so they need an improvement in cost of thousands.


    • Solar power conversion improvement can be very important to commercial space operations and human space exploration, so please don’t get fixated on terrestrial baseload power generation as a kneejerk consideration. (I notice it also doesn’t cure world hunger or banish marlaria.)

      • Thank you for your comment on possible space applications. Also, the very high efficiency solar cell technology depends on solar concentrators (typically 200 to 500 suns) to reduce the cost per watt for the solar cell. The use of solar concentrators then requires active cooling of the solar cell to maintain temperature within safe limits. This technology may also need to use concentrators to initially reduce cost. There are other considerations that may be driving the military funding of this technology (other than cost).

  15. First off, antennas operate best for a single frequency. Does this device also suffer from that limitation?

    Secondly, while it sounds very useful for sensors, I highly doubt that it will ever be possible to boost the efficiency and drop the cost enough for these beasties to be economical as power sources.

    • Perhaps they could build an array that had nano-tubes tuned for one frequency, gradually shifting tuned frequencies as you moved across the array. Then use a prism to break up the light and project it onto the array.

    • First off, antennas operate best for a single frequency. Does this device also suffer from that limitation?

      There are wide band antennas. My favourite is the discone which can have a 10:1 bandwidth. It is literally a disc and a cone.

      The solar thingie is described as follows:

      Based on multiwall carbon nanotubes and tiny rectifiers fabricated onto them, …

      That doesn’t sound conducive to any kind of wide band construction I’m familiar with.

      • A cone is actually an infinite number of antennas running from highest frequency at the small end to the lowest frequency at the big end.
        Each nano-tube is an antenna.

      • A ‘construction’ more akin to “fan dipoles” may be used to achieve multi-octave performance as well; I hasn’t seen that mentioned yet …

        Other ‘structures’ might be used as well, like multi-wavelength loops, with multiple rectifiers (diodes) at multiple points to take advantage of those points where voltage maximas occur as the incoming frequency or wavelength changes.

        If they fabricated enough nanotubes of varied length in either a monopole or even dipole form, then they would have intrinsic wide-banded-ness with perhaps several diodes each as well.

      • _Jim October 20, 2016 at 11:48 am

        … take advantage of those points where voltage maximas occur …

        Good point. I have always lived in a world where a good impedance match is next to Godliness. That usually has the effect of reducing voltages and currents.

        At microwave frequencies, it’s easy to inadvertently get some wicked high voltages. A sharp corner on a pcb trace will often do that quite nicely. They could take advantage of that.

  16. This might be a breakthrough in the conversion of light to electricity, however the efficiency does not solve the problem of the off-set of peak solar electric production from peak usage. This off-set is diurnal and seasonal. The solution to the production/usage off-set is a cheap and reliable storage system.

  17. I got a kick out of their statement that they believe a rectenna with commercial potential may be available in a year. Yea, and I believe a unicorn will fly out of my butt some day.

  18. I can’t believe that no one noticed the 77K number. That’s liquid N2, uses a lot of energy getting there.

  19. “We could ultimately make solar cells that are twice as efficient at a cost that is ten times lower, and that is to me an opportunity to change the world in a very big way”
    Better hurry up, we are already headed for 25 cents per watt LCOE with thin film panels using chemical vapor deposition within 1 to 1.5 years. That is down from 32 cents now.

  20. I love the “it will ultimately make solar cells twice as efficient and cost half as much” stuff. It almost never pans out. Decent silicon cells now produce 20 – 25% percent efficiency at about $3 per watt (not installed). So we have a technology that has demonstrated 1% efficiency but will eventually get to 50% efficiency for $2 per watt. Give me a break.

  21. This has also been done with planar graphene. The system theoretically can convert any light of any frequency. It uses two electrodes, one a sawtooth pattern and the other a simple stripe, spaced at optical wavelengths away from the sawtooth electrode. Because of the points on that electrode charge builds up on the points and quantum tunneling gets it across the space. And, graphene is transparent and a very good conductor of electricity, so the rectifying pattern can be implemented several layers deep. The substrate can be any none conducting material.

  22. Doesn’t matter. Solar cannot be a major power source for modern civilizations — way too diffuse & undependable.

    As Scotty said, “Capt’n, I canna change the laws of physics”.

  23. in a year?sounds like rectennas are in use now…though for some reason sounds as if the the research was abandoned years ago so has to be a good reason why…anyway I would not get to excited as distance attenuates anything associated with this type of energy instead of volts we are talking microvolts…it simply does not work

    • I’ve heard of radio and micro-wave rectennas. This is a first for optical rectennas.

      For the most part, it’s cheaper to just run a wire than it is to build a big enough transmitter and rectenna when it comes to transmitting power.

      • The *measured* thermal equivalent power (IOW, I used a bolometer or thermocouple HP uWave power meter) was about -17 dBm on my outside, multi-element “Log Periodic” TV antenna (A Jerrold model 933 at the time) some years back. The effective ‘peak’ RF voltage when all voltage phasors are ‘in phase’ briefly would, of course, be a much higher value …

    • re: “distance attenuates anything”

      You are probably speaking of the “inverse square law” wherein received energy is inversely proportional to the distance squared.

      This is *not* so much a matter of ‘attenuation’ (otherwise, where does the energy go? It does not get converted to ‘heat’ in free space.) but rather a ‘spreading’ of the energy in *two* dimensions (when the source is acting like a ‘point source’. Large ‘Line Arrays’ (as in acoustics) as sources, for instance, act differently.) …

  24. What ever happened to Dr. Alvin Marks(inventor of Polaroid film) and his Lumeloid photovoltic film (80% efficient)

  25. “That’s what gets me out of bed in the morning…”

    Son, if I was doing what you’re doing, I’d never go to bed at all!

    • Did I forget to say I love this? I love this! (And not because it’s “Green”, you ninny!)

      (Not that some nearly-free kilowatts would be a bad thing…)

  26. Rectenna efficiency potential
    Optical rectenna operation: where Maxwell meets Einstein
    2016 J. Phys. D: Appl. Phys. 49 265602 May

    Optical rectennas are antenna-coupled diode rectifiers that receive and convert opticalfrequency electromagnetic radiation into DC output. The analysis of rectennas is carried out either classically using Maxwell’s wave-like approach, or quantum-mechanically using Einstein’s particle-like approach for electromagnetic radiation. One of the characteristics of classical operation is that multiple photons transfer their energy to individual electrons, whereas in quantum operation each photon transfers its energy to each electron. We analyze the correspondence between the two approaches by comparing rectenna response first to monochromatic illumination obtained using photon-assisted tunnelling theory and classical theory. Applied to broadband rectenna operation, this correspondence provides clues to designing a rectenna solar cell that has the potential to exceed the 44% quantum-limited conversion efficiency. The comparison of operating regimes shows how optical rectenna operation differs from microwave rectenna operation.

    Summary If broadband rectennas operate in the quantum regime, their efficiency is limited to the maximum efficiency for quantum conversion [8] that also limits conventional solar cells, which is 44% for the solar spectrum. Conventional rectifiers are not subject to this limit because they usually operate in the classical regime. Therefore, shifting from the quantum to the classical regime of operation for optical rectennas provides the potential for substantially higher conversion efficiencies.
    We investigated rectenna operation using the quantum theory of photon-assisted tunnelling under different operating conditions. We found that there are different regimes of rectenna operation, from quantum to classical. Rectennas follow Einstein’s photon-based approach if the diode voltage (VS) is
    less than the photon energy divided by the electronic charge (ω/q), and follow Maxwell’s classical electromagnetic wave nature of light if VS ω/q. The correspondence between classical and quantum operation of the rectenna in the limit of large VS compared to ω/q allows classical operation at optical and IR frequencies for high incident intensity and for large source and diode impedances.

  27. Seems to me a very, very long way to go, and the longer the way, the more expensive it becomes. I hope the taxpayer won’t have to pay the taxi-bill…

  28. “though the efficiency of the devices demonstrated so far remains below one percent.”

    Translation: Greater efficiency can be had by burying thermocouples in a compost heap.

    • It’s a current that you can’t turn off, so it flows forever. Well of course you can’t turn it on in the first place or it wouldn’t be DC.


  29. More than 50% is not possible as for a normal antenna. The currents needed to make some power would radiate back.

    • Microwave rectennas are cited as attaining 85-90% conversion efficiency. Don’t be dogmatic.

    • Svend Ferdinandsen October 20, 2016 at 2:23 pm: “More than 50% is not possible as for a normal antenna. The currents needed to make some power would radiate back.

      Stipulate “normal.”

      You realize, of course, with proper engineering (choice of structure, configuration, topology, etc.) this is not the limiting factor to efficiency.

  30. The money shot: “I believe that these devices could get to greater than 40 percent efficiency.”

    I believe. Only two words, but charged with infinite potential to delude and distract.

  31. Gottlieb Daimler and Carl Benz first produced automobiles with engines of 1 horsepower. Where are we today? What is the point in casting aspersions (“delude and distract”) at those who are honestly working at the forefront of technology. Nowhere in that article were any Greenies touting it as The Path to an environmental utopia. Microwave rectennas are in the 85-90% efficiency range. There is no outstanding physical reason to think optical rectennas are incapable of matching this.

    • Thank you for having highlight that technology that I didn’t know before.
      It looks a good attempt to do something different in the field.
      But the claim that that system doesn’t need an inverter because it produces an AC current directly by the Stirling motor is clearly naive, if not deceptive.
      When in late 80s I worked on the FIAT (now FCA automobiles) TOTEM generators thought to work attached to the power grid, I learnt how much is important that not only the frequency must perfectly the same of the power grid, but the phase too.
      So I wonder how they dealt the phase issue, since the Stirling motor have to be synchronized to the grid sinusoid.
      The only way I know, it is continuously adjusting the motor speed to match the motor instantaneous rotational position with the sinusoidal instantaneous voltage of the power grid.
      That should not be an easily feasible task with a Stirling motor when the incoming heat is not under control, such as in the parabolic heater in subject.

      Have a great day.


      • Massimo,

        Why not ‘skip’ the middle man and a) heat water directly or b) bake foodstuff also directly with the sun?

        The intermediate step, converting to ‘moving’ electrons along a wire is a process that always wastes a fair amount of energy in a conversion process using the Carnot cycle ..

      • Massimo and Jim
        It’s a shame big business monopolies the patent system . “a government authority or licence conferring a right or title for a set period, especially the sole right to exclude others from making, using, or selling an invention.” This process not only stifles energy innervation but also hobbles science across the board. Turning science into a competition means everyone holds their playbook close to their chest. and that should be a crime against humanity. Here’s how big business works .

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