Talk:Optical rectenna

Carnot efficiency

In wireless energy transfer an experiment is cited with shows that the efficiency for a microwave-rectenna exceeds 95%. Microwaves have wavelength (much) longer than infrared (10.6 µm). Therefore their "temperature" is much lower than 300 K. How does this fact relates to the Carnot efficiency?

Is the intensity of the microwave beam the reason for the hight efficiency? —Preceding unsigned comment added by Stefan.K. (talk • contribs) 17:39, 22 October 2009 (UTC)

See Brightness temperature. A microwave beam can easily have a brightness temperature of 10000 kelvin. That just means that the microwaves are exactly as intense as the microwaves that you would receive sitting in a 10000 kelvin blackbody cavity. --Steve (talk) 22:03, 16 July 2012 (UTC)

Infrared

Ideally, nantennas would be used to absorb light at wavelengths between 0.4-1.6 μm because these wavelengths have higher energy than infrared (shorter wavelength)

Much of that range is infrared, right? 68.239.116.212 (talk) 02:17, 18 February 2010 (UTC)

Carnot stuff should be removed

I have been reading through discussions about this debate on carnot efficiency, and frankly, while it does apply in actuality to solar topics, its assertions in this article are incorrect, and it is too difficult to characterize. The photons do have a temperature, but it is not room temperature. It is a result of their entropy, reradiation, and everything they came into contact with when they entered the earth's atmosphere. The temperatures I have seen given are almost always wrong. For example, the most thorough predictions of solar efficiency with regards to the carnot efficiency put the maximum efficiency of a typical solar cell at 30%. Solar cells have already exceeded this! This means even a thorough examination of photon temperature cannot give a reliable result. If solar cells have already exceeded their carnot limits, then treating solar cell topics like heat engines is simply not useful.

Tyhou (talk) 19:17, 19 October 2010 (UTC)

They must mean narrowband, a subset of Maxwellian population and thusly not in equilibrium with the whole solar spectrum... Cheaters. -lysdexia 21:33, 18 May 2011 (UTC) —Preceding unsigned comment added by 99.64.168.136 (talk)

It seems these claims have been removed. You are not correct about people saying the carnot-related "maximum efficiency of a typical solar cell [is] 30%" unless those people are truly idiots. (They are probably talking about the Shockley-Queisser limit.) I have seen dozens of solar cell textbooks, papers, reviews, etc., and everyone who discusses the Carnot efficiency gives a reasonably correct value, 85% (more or less depending on whether the light is concentrated etc.).

The photons coming from the sun have a brightness temperature of around 6000K (more or less depending on frequency-band, concentration, latitude, etc.), so the carnot efficiency is very high. --Steve (talk) 22:03, 16 July 2012 (UTC)

Cool a room, thermodynamics

In the section "future research and goals" the statement "cooling a room by using it's heat radiation" seems to violate the second law of thermodynamics. It should be clarified or removed. —Preceding unsigned comment added by 129.10.73.181 (talk) 22:39, 24 May 2011 (UTC)

The way it is stated now is "Novack claims the last of these will work by both absorbing the infrared heat available in the room and producing electricity which could be used to further cool the room." I presume that this is a correct statement of fact, i.e. that Novack did in fact make those claims. Novack is wrong, because of the second law of thermodynamics, for sure. I guess readers have to figure that out for themselves. I edited it to at least raise the question parenthetically, but it would nice to make a stronger statement if there was a reliable source. --Steve (talk) 22:03, 16 July 2012 (UTC)

I don't see why this should violate the second law of thermodynamics. Assuming the device is exhausting the heat outside or into a larger space (don't all refrigeration systems do this?) it seems possible. Take a battery operated refrigerator (possible), put the battery inside of the fridge (why not?) and you have a good analogy. I'm a layman and new to wikipedia, so please forgive any mistakes either in they physics of my point above or in my commenting style here. 198.0.251.1 (talk) 23:42, 6 December 2013 (UTC)

I don't think that's a good analogy. In your analogy, you start with a fully-charged battery, and you gradually discharge it (i.e., you run down the battery). That battery discharge process produces entropy -- more than enough entropy to compensate for the entropy reduction of the refrigerator. The overall system entropy increases!

In the alleged rectenna system, there is no battery running down, there is no process that is increasing the entropy. The refrigerator is lowering entropy, and there's nothing else compensating that reduction. So it would violate the 2nd law. --Steve (talk) 20:11, 7 December 2013 (UTC)

How does it violate the second law? The reason we need air conditioning is due to solar heating. ( Or lots of people at a party ) The energy used to cool the room is claimed to be absorbed and re-emitted solar energy. ( Or people energy ). It doesn't seem to violate the second law anymore than a solar powered air conditioner would. Am I missing something? SpinozaQ (talk) 17:55, 10 February 2014 (UTC)

We are talking about the second law, not the first law, i.e. whether or not entropy is being destroyed. Energy conservation is not the issue here.

Novack is describing a classic example of a perpetual motion machine of the second kind.

The signature of a perpetual motion machine of the second kind is that there is only one heat reservoir involved, which is being spontaneously cooled without involving a transfer of heat to a cooler reservoir. This conversion of heat into useful work, without any side effect, is impossible, according to the second law of thermodynamics. (Perpetual motion machine)

The prototype machine of the second kind is Gamgee's ammonia motor. It propels a ship that simply withdraws heat energy from the ocean to power itself. Obviously this doesn't violate the first law, because the ocean contains plenty of heat energy. The ship would simply extract some of it, leaving a cold wake behind. [1]

Now someone might say: "Gamgee's ammonia motor does not violate the second law of thermodynamics any more than a solar-powered ship does, because the heat of the ocean originally came from the sun! After all, if there was no sun, the ocean would have much less heat! The motor is using "absorbed and re-released solar energy"".

That is analogous to the argument you're making, if I understand it correctly.

Well, Gamgee's ammonia motor really is an impossible perpetual motion machine. And solar-powered ships really are possible. So there clearly must be a mistake in saying that they're similar. What is the mistake?

When heat flows from something hotter to something colder, entropy is created, which means there is an opportunity to extract energy to do useful work. For example, in a car engine, heat flows from hot exploding gasoline to the colder outside air. Solar power works the same way. Heat is flowing (mostly in the form of visible light) from the very hot sun to the much colder Earth. When this energy is absorbed, it is an entropy-creation process, which means there is an opportunity to extract energy to do useful work. Green plants synthesize sugar, solar panels create electricity, etc.

So, when sunlight shines on the ocean, there is an opportunity to extract energy to do useful work. But it is a transient opportunity. Once the sunlight heats the ocean, the entropy has now already been created. The opportunity is past. It's too late to extract useful work out of this process. Once entropy is created, it cannot ever be destroyed.

If the sun blinks out right now, algae stops being able to store new energy. It is still surrounded by an ocean that was warmed by the sun, but that doesn't do the algae any good!

In other words, there is a world of difference between solar energy and "absorbed and re-released solar energy". The solar energy carries low entropy, while the absorbed-and-re-released-solar-energy carries high entropy.

Anyway, if I have a room at 30°C, and everything in the room is at 30°C, it doesn't matter how it originally got to be that temperature. Sunlight, people, heater, it doesn't matter. Once it reaches that temperature we can remove the sunlight and people and heater, and then we can keep it that same 30°C temperature forever by sealing the whole room and surrounding it with thermal insulation. Then, if you believe Novack's description, a nantenna in this room can continually absorb the thermal radiation from objects in that room, use the energy to spin a motor forever, and the motor's friction would re-heat the room. This is a perfect example of a perpetual motion machine of the second kind. --Steve (talk) 15:00, 11 February 2014 (UTC)

I think your missing that the sun or a heater or people would continually be adding more heat to the system. The description that the nantenna could continue to produce electricty depends on an external heat source adding back the amount of energy absorbed from the infrared. In the closed inputless system, the temperature would go down as the nantenna absorbs infrared, reducing the infrared radiation thereby reducing the electricity produced until the system reaches a new equilibrium. --Erik — Preceding unsigned comment added by 151.151.109.21 (talk) 18:24, 31 March 2014 (UTC)

If I had a perpetual motion machine of the second kind, what could I do with it?

• I could collect energy from the heat of a room, and use that energy to run an electric heater, so the energy would get returned back to the room as heat. The room would stay the same temperature, and I could do this forever.
• I could collect energy from the heat of a room and use it to charge up batteries. The room would get colder and colder, approaching absolute zero, until eventually the perpetual motion machine would shut down.
• I could collect energy from the heat of a room and use it to charge up batteries, while another heat source in the room (e.g. people running around) added enough heat to the room to counteract the heat loss and keep the room the same temperature.

All three of these tasks are actually impossible because they violate the second law of thermodynamics, and there's actually no such thing as a perpetual motion machine of the second kind. (Please note that these tasks do not violate the first law. You don't need to convince me that energy is conserved. I know that energy is conserved.)

You can recognize a perpetual motion machine of the second kind from the fact that it is destroying entropy, instead of (or in addition to) creating it. In the first example, the electric heater is creating entropy, while the perpetual motion machine is destroying entropy. In the third example, the people running around are creating entropy, while the perpetual motion machine is destroying entropy.

In the second law of thermodynamics, there is a huge difference between "creating entropy over here while destroying it over there" versus "not creating entropy in the first place". The first one is forbidden, the second one is allowed. :-D --Steve (talk) 13:32, 1 April 2014 (UTC)

He is describing taking heat out of a room (a heat reservoir) and cooling it. At some point there will be no heat. He is not describing taking heat out and heating. I assume because the efficiency is not 100% that he also is not describing a perpetual motion machine...so...For cooling: if the only heat reservoir is the heat in the room, eventually there will be no heat left. However, if there are bodies, or some other heat source in the room to maintain the heat reservoir, the room will continually cool. This concept does not violate the second law. For heating: because the efficiency is <100% you will never be able to increase the heat of the room greater than what it already is. This is why it is explained that as it absorbs heat, it cools the room. This concept does not violate the second law. Example. Say I successfully absorb 85% of the heat, there will be another loss as that is converted into a heating mechanism (the rectifying device will have loss as well as the 'widget' that creates the heat from the energy absorbed). Hence you will be putting out a T < 85% < reservoir. This does not violate the second law -- signed by me — Preceding unsigned comment added by 67.240.83.224 (talk) 02:46, 10 March 2015 (UTC)

You seem not to understand the definition of a "perpetual motion machine of the second kind" (PMM2K) .... Don't be fooled by the word "perpetual": Gamgee's ammonia motor etc. are PMM2Ks (and they violate the 2nd law of thermodynamics) even if they can run for only one second, and extract just 1 joule of energy from the entire ocean, before shutting down.

If you have an engine that sucks heat out of a room and performs work (like Gamgee's ammonia motor above), it is a PPM2K, and it does violate the second law of thermodynamics, regardless of whether its efficiency is 100% or 85% or even 0.000000001%. The second law of thermodynamics says that exactly 0 energy is the maximum amount that can possibly be converted from heat to electricity / mechanical motion / etc. --Steve (talk) 20:15, 10 March 2015 (UTC)

I dont know if you understand. The second law does not state "exactly 0 energy is the maximum amount that can possibly be converted from heat to electricity / mechanical motion / etc." (by your words this would then say that a steam turbine or nuclear reactor violate the second law). The second laws says that there will be an increase in entropy from all systems involved as heat is converted into useable energy (this is what thermodynamics is, the study of heat to do work). Also a perpetual motion machine is (described from wiki) "A perpetual motion machine is a hypothetical machine that can do work indefinitely without an energy source.". The energy source is heat, or a oscillating wave. We already turn oscillating waves into electricity in the form of a solar cell. The efficiency described above is more or less a red herring. It just shows at a high level that as you lose your reservoir heat, you are increasing your entropy. Again, this does not violate the second law and by no means is a perpetual motion machine. 67.240.83.224 (talk) 16:19, 11 March 2015 (UTC)

Sorry I was not more explicit. Heat flows spontaneously from hotter to colder areas, and when it does, you can siphon off some fraction of the power according to Carnot's law. In a solar cell, heat is spontaneously flowing from the hot sun to the cold earth. In a nuclear reactor, heat is spontaneously flowing from a hot reactor core to cold river water. In a car engine, heat is spontaneously flowing from hot exploding gasoline (petrol) to (comparatively) cold air. When there is a flow like that, you can run a heat engine and generate electricity, as long as you satisfy the Carnot limit. But "Exactly 0 energy can be converted from heat to electricity / mechanical motion / etc." in the absence of a flow of this type.

If Novack said that the room is hot, and the nantenna is also connected to a much colder river and dumping heat into the river, then I would not say that he is proposing a PPM2K. But he's not saying that. The nantenna is alleged to generate electricity in a closed, sealed room with a single temperature. How efficient can a heat engine be in this circumstance? There is a simple formula that gives the answer: Carnot's theorem (thermodynamics). In for this situation, the formula says that the maximum efficiency is exactly zero.

I re-read your first comment. You said something which suggests that you are confusing the first and the second law of thermodynamics. You said: "you will never be able to increase the heat of the room greater than what it already is. This is why it is explained that as it absorbs heat, it cools the room. This concept does not violate the second law." That quote makes no sense to me, but if you just changed one word in it, then it would be a completely sensible and correct statement: If you just replaced "second law" with "first law". The first law of thermodynamics says "energy is conserved; energy can be neither created nor destroyed". Yes, I agree 100%, Novack's nantennna does not violate conservation of energy. Every PPM2K by definition does not violate conservation of energy, or else you would call it a PPM1K instead. The second law is entirely different than the first law. The situation you describe in that quote above is a clear-cut, textbook example of what a violation of the second law (but not the first law) would look like. (Sorry if I'm misunderstanding something.) --Steve (talk) 00:06, 12 March 2015 (UTC)

No I was not confusing them. You make a good point though, I am assuming a second heat sink. I just assumed Novack was also...hmmm...now I am really thinking about it...I guess best thing to do would be experiment...I appreciate your candor. 67.240.83.224 (talk) 01:18, 12 March 2015 (UTC)

This 1968 experiment by J._B._Gunn proves that no power is generated when the nantenna is at the same temperature as the room it's in. (There is power generated when the temperature is either higher or lower.) OK, maybe "proves" is too strong a word, the experiment differs in some superficial ways from what Novack is proposing. Gunn's experiment based on radio-frequency electromagnetic waves in a circuit, rather than visible-frequency electromagnetic waves in free space. But anyway, it is strongly suggestive. :-D --Steve (talk) 14:21, 12 March 2015 (UTC)

Cheaper?

The article says "Nantennas are cheaper than photovoltaics." This statement should probably be removed, because right now, nantennas cannot be purchased at any price. A complete nantenna would include a component capable of rectifying extremely-high-frequency alternating current. Such a component has not been invented yet; assuming that it will someday be invented, it may prove to be quite expensive. GPS Pilot (talk) 00:49, 9 July 2011 (UTC)

This comes from (perhaps purposeful) confusion between "nantennas" meaning nanoscale antennas, and "nantennas" meaning complete energy-harvesting systems which have nanoscale antennas and rectifiers and everything else. The former is proven to be cheap, the latter is nonexistent like you say. I will edit the wording. --Steve (talk) 22:03, 16 July 2012 (UTC)

COI note

I forgot to mention in the edit summary here, but one of those citations is my own paper. Full disclosure! :-D --Steve (talk) 18:18, 18 March 2014 (UTC)

Implementation

A recent flurry of reports around a Nature paper and press release. So far it's just proof of concept, but they've already made the world's fastest (40 PHz) diodes with a simple fabrication technique. Patent application WO 2014063149  relates. Very neat stuff! LeadSongDog come howl! 15:58, 1 October 2015 (UTC)

While it's great work, beware that the first sentence of the press release is misleading. For example, here is a rectenna detecting red light in 1978.

Also, it's certainly not 40 PHz, that must be a typo.

Hmmm, we should really rename this page to "optical rectenna". It's the more common term, and also more specific. ("Nantenna" can be either a nano-antenna or a nano-rectenna, i.e. antenna plus diode. Similarly, it can be any optical rectenna, or it can be the specific project of Dr. Novack.) --Steve (talk) 17:53, 1 October 2015 (UTC)

Granted the claim is poorly worded in the press release. The Nature Nanotechnology article is better, but even that is a bit much. However, this analysis on the MOM device found it best suited to 10 μm wavelengths, not red light and certainly not the full optical spectrum. I'm not so sure that 40 PHz is a typo: a 13 nm lambda_min is reasonable for these geometries and consistent with diode switching times in the attoseconds. The journal article asserts that the junction capacitance is on the order of ~2 aF and that the junction across the end of the 10 nm diameter MWCNT is at 2 eV energy gap. I don't think they're suggesting use of extreme UV illumination, though there might be some space-based astronomy applications. Just that longer wavelengths should be no problem. Anyhow, at this stage it's all primary, we'll have to wait for some secondary analysis to be published. LeadSongDog come howl! 19:20, 1 October 2015 (UTC)

Where did you see 40 PHz? The only wavelengths I see in the article is 1064 nm (282 THz), 532 nm (564 THz), and a solar simulation (NIR and visible and a bit of UV all mixed together). The paper says switching speed is on the order of femtoseconds, I don't see any mention of attoseconds.... Am I missing something? --Steve (talk) 20:54, 1 October 2015 (UTC)

From the patent: "The arrays can be used as rectenna at frequencies up to about 40 petahertz because of their intrinsically low capacitance."
I suggest reading the "Original document", since the "Claims" and "Description" on the WIPO site have mangled the text beyond comprehension. At this point I'm not sure where I saw the 2 as mentioned, I'll look som more. LeadSongDog come howl! 13:56, 2 October 2015 (UTC)

Now I can't find the attosecond mention, perhaps I imagined it, or inferred it from the 40 PHz. To rectify this 25as wave, the rectifier must function in at most 12.5as, significantly less for efficient operation. LeadSongDog come howl! 18:17, 2 October 2015 (UTC)

It needs rewriting

This article is in desperate need of a rewrite. There is factually incorrect stuff (the Schottky diode operation and efficieny comments are absurd). It also lack of any reliable reference on the rectification mechanisms (which are *not* as a diode/antenna operating at microwave, but are based on Brownian ratchets) and experimental results. I'll add those sections in later on. There no point in having the Carnot's efficiency section in any serious rewrite, if nobody complains I'll remove it. --c

I agree with "it needs rewriting" and "there is factually incorrect stuff". I'm not sure exactly what you're referring to in "schottky diode operation and efficiency comments". The article says that Schottky diodes don't work well in an optical rectenna because they're too slow (I think that's correct), and that they're too slow because of parasitic capacitance (I think that's partly true, although transit time / carrier lifetime is a bigger issue). What's the absurd part? (The comment that MIM capacitors do not have parasitic capacitance is the absurd comment from my point of view.)

I don't understand your implication that an optical rectenna's mechanism is fundamentally different than a microwave rectenna, but is rather based on Brownian ratchets. Specifically: If you're going to say that an optical rectenna is based on a Brownian ratchet, I won't object, but I would add that a microwave rectenna is also based on a Brownian ratchet. I'm not sure where the fundamental difference is here. (Except that the stereotypical microwave rectenna is working with narrowband energy inputs, whereas the stereotypical Brownian ratchet is working with more incoherent energy inputs. But that's not an important difference, and anyway there is such a thing as a broadband microwave rectenna, and a narrowband optical rectenna.) I'm not saying that there is no difference between the optical and microwave rectenna. You typically use different formulas for them. But that's mainly because visible rectennas usually operate with voltages much lower than the photon energy, while microwave rectennas usually operate with voltages much higher than the photon energy. I wouldn't elevate that to a fundamentally different rectification mechanism, just a different typical operating regime.

The article doesn't say much about Carnot efficiency, except that it has been claimed (by R. Bailey) that the Carnot limit doesn't apply to rectennas, but really it does. Well it could be more concise, but I do think that this is a popular misconception (it was widely aired in the popular press I believe), and that therefore it is worthwhile to explicitly refute it. --Steve (talk) 17:35, 14 July 2016 (UTC)

Delete Proof of principle

I am horrified at the section "Proof of principle". The measurement discussed was of infrared emissivity, which is to say, how well the wafers absorb light (in the infrared). Absorbing light is trivial, and does not need to be demonstrated; a lump of charcoal will do it. "Proof of principle" would require demonstrating generation of electricity, which this did not do.

I'm deleting the section. Geoffrey.landis (talk) 20:37, 23 July 2020 (UTC)