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I was reading about Nikola Tesla today (via the Oatmeal) and read about the Wardenclyffe Tower which (among other things) was intended to transmit electricity wirelessly. Forgive the naivety of the question, but if technology that could transmit electrical current wirelessly was invented over 100 years ago, why don't we use wireless electricity in our day to day lives today? In other words, why do we have to physically plug in our electrical devices (phones/computer, etc) if there exist such a thing as wireless electricity? If its an issue of efficiency/cost then I would imagine that some rich people still wouldn't mind paying extra, in light of the waste, for the added convenience.

Please explain in layman's terms (though a simple answer would suffice).

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    \$\begingroup\$ If the power company distributed wireless electricity, it would be much harder to meter the usage from each customer, or prevent non-customers from setting up an antenna and using the service without paying. \$\endgroup\$ – The Photon Aug 17 '12 at 17:25
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    \$\begingroup\$ Don't believe everything you read on the Oatmeal. \$\endgroup\$ – endolith Aug 17 '12 at 17:49
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    \$\begingroup\$ @ThePhoton: It would just be a non-excludable good, and could be paid for with taxes like any other. That doesn't say anything about whether it's feasible or not. I've yet to see a good explanation showing that Tesla's worldwide power idea would even work. Just because someone thought of something doesn't make it viable. \$\endgroup\$ – endolith Aug 17 '12 at 18:00
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    \$\begingroup\$ Nobody has been able to make it work cost effectively over any substantial distance. Much that Tesla did was great - but not everything. Even if t he worst case conspiracy theories were true, which they aren't, some people would do this if they could because it makes sense to do so technically if it can be done cost effectively. I had personal experience of short range application of people doing this 40 years ago (literally) over very short distances. It was "lost" when the professor doing it died and people lost interest, reintroduced by a student of the prof and formed the basis of ...\ \$\endgroup\$ – Russell McMahon Aug 17 '12 at 21:11
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    \$\begingroup\$ @endolith "Don't believe everything you read on the (Oatmeal) Internet." :-) \$\endgroup\$ – Anindo Ghosh Jun 19 '13 at 4:09
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I use wireless electricity everyday.

In my toothbrush:

toothbrush

And in my cell phone:

qi phone

The method used in my devices is called Inductive Charging. I talk a bit more about it in my answer to this question. This is the most common and most practical form of transmitting energy wirelessly at the moment. But as many of the comments have noted, this is considered near field transmission. And with an effective range of only a few millimeters, it is very near field.

The amount of energy transferred and the efficiency of the transfer can be increased quite a bit (although still considered to be near field) by adding a capacitor to each of the inductor coils and tuning the resultant RLC networks to have a high Q factor at the same (resonant) frequency. A team from MIT did research into using inductive resonance as a wireless power transfer system.

resonant induction recharging

The researchers have since formed a company called WiTricity to further develop the technology. While they still haven't brought a product to the commercial market, they have made some impressive demonstrations:

The term WiTricity was used for a project that took place at MIT, led by Marin Soljačić in 2007. The MIT researchers successfully demonstrated the ability to power a 60 watt light bulb wirelessly, using two 5-turn copper coils of 60 cm (24 in) diameter, that were 2 m (7 ft) away, at roughly 45% efficiency. The coils were designed to resonate together at 9.9 MHz (≈ wavelength 30 m) and were oriented along the same axis. One was connected inductively to a power source, and the other one to a bulb. The setup powered the bulb on, even when the direct line of sight was blocked using a wooden panel. Researchers were able to power a 60 watt light bulb at roughly 90% efficiency at a distance of 3 feet. The research project was spun off into a private company, also called WiTricity.

It's important to note that the distance between the transmitter and the receiver plays a crucial factor in determining how much energy can reliably be transferred. As can be seen in this paper based on the MIT project, the decay in voltage with respect to distance between the coils is exponential:

exponential decay

But there are many other methods such as microwave and laser that are capable of much greater distances. However, these methods are very directional and so are applicable over a much smaller area than Tesla's proposed Wardenclyffe Tower which would be omnidirectional. There are also many other factors to consider when implementing one of these methods:

Microwave:

Power transmission via radio waves can be made more directional, allowing longer distance power beaming, with shorter wavelengths of electromagnetic radiation, typically in the microwave range. A rectenna may be used to convert the microwave energy back into electricity. Rectenna conversion efficiencies exceeding 95% have been realized. Power beaming using microwaves has been proposed for the transmission of energy from orbiting solar power satellites to Earth and the beaming of power to spacecraft leaving orbit has been considered.
...
For earthbound applications a large area 10 km diameter receiving array allows large total power levels to be used while operating at the low power density suggested for human electromagnetic exposure safety. A human safe power density of 1 mW/cm2 distributed across a 10 km diameter area corresponds to 750 megawatts total power level. This is the power level found in many modern electric power plants.
...
Wireless high power transmission using microwaves is well proven. Experiments in the tens of kilowatts have been performed at Goldstone in California in 1975 and more recently (1997) at Grand Bassin on Reunion Island. These methods achieve distances on the order of a kilometer.

Laser

Advantages of laser based energy transfer compared with other wireless methods are:

  1. collimated monochromatic wavefront propagation allows narrow beam cross-section area for energy transmission over large ranges.
  2. compact size of solid state lasers-photovoltaics semiconductor diodes fit into small products.
  3. no radio-frequency interference to existing radio communication such as Wi-Fi and cell phones.
  4. control of access; only receivers illuminated by the laser receive power.

Its drawbacks are:

  1. Laser radiation is hazardous, even at low power levels it can blind people and animals, and at high power levels it can kill through localized spot heating
  2. Conversion to light, such as with a laser, is inefficient
  3. Conversion back into electricity is inefficient, with photovoltaic cells achieving 40%–50% efficiency. (Note that conversion efficiency is rather higher with monochromatic light than with insolation of solar panels).
  4. Atmospheric absorption, and absorption and scattering by clouds, fog, rain, etc., causes losses, which can be as high as 100% loss
  5. As with microwave beaming, this method requires a direct line of sight with the target.

And of course there is the "disturbed charge of ground and air" method used by Tesla. As far as the Tesla system goes, that got shut down because funding ran out and the stock market crashed. As for why it's not been tried since, it's primarily because such a system could not be strictly metered. Therefore, the power companies could not charge per usage and make lots of money. Without a way to monetize the technology, no investment into research and development will ever be made. That's the (conspiracy) theory, anyways. Although there are many other reasons why this method is either unfeasible or just outright wouldn't work.

I couldn't find an article with definitive numbers as to efficiency. But it's my guess that efficiency is the main reason you do not see this technology in more wide spread usage. However, it does exist, people like me (read: not rich) do have access to it, and it works quite well.

Edit:

I found a case study done by the Wireless Power Consortium, makers of qi charger for my phone, which states (emphasis mine):

In this section we compare the total power consumption in a 5-year period

Case Study:

Average system efficiency of wireless charger N sys-wireless = 0.50 (50%)

Average system efficiency of wired power adapter N sys-wired = 0.72 (72%) Assume that the average charging power is 2W.

So the wired part of their system has an efficiency of 72% and the wireless part has an efficiency of 50%. That is using an inductive method where the coils are a few millimeters apart. Compare that to the WiTricity from Joel which states an efficiency of 40% over 2 meters.

Factor in the additional costs associated with the extra circuity and components for a wireless system as compared to the cost of a length of copper wire and you can see why long distance wireless energy transfer is still considered impractical for mass market use.

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    \$\begingroup\$ Possibly relevant link to go with your answer: en.wikipedia.org/wiki/WiTricity, it's a third form of energy transfer that relies on the reactive near-field response (not inductive, yet not radiative). \$\endgroup\$ – Joel Cornett Aug 17 '12 at 20:23
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    \$\begingroup\$ @Droid: Here is the original research paper on WiTricity: sciencemag.org/content/317/5834/83.short Incidentally, the study cites 40% efficiency at a distance of about 2 meters. In wireless energy terms, that's pretty good. \$\endgroup\$ – Joel Cornett Aug 19 '12 at 22:23
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    \$\begingroup\$ @Droid Yes, your understanding is correct. I've added a case study to illustrate the differences in efficiency of an inductive system versus a wired system. \$\endgroup\$ – embedded.kyle Aug 20 '12 at 13:27
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    \$\begingroup\$ @RocketSurgeon that would be radiated EM, if you radiate in every direction chances are no more then .001% of your power will be picked back up. Beyond the fact that the best antenna in existence cannot do better then 50% pickup. \$\endgroup\$ – Kortuk Aug 20 '12 at 13:40
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    \$\begingroup\$ Small directly coupled devices like your toothbrush and cell phone are totally different cases than what Tesla was trying to do and the OP is asking about. \$\endgroup\$ – Olin Lathrop Mar 20 '13 at 20:06
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If you radiate power spherically (equal in all directions), power received at the other end will be proportional to the percentage of the sphere covered by the receiver. The further away you get, the less energy you capture for the same size antenna, proportional to 1/r^2. The rest of the energy is wasted into free space. This is a massively oversimplified model, of course. If you know where the receiver is you'd make the transmitter directional, use resonance etc., but you get the idea. Wireless energy does not magically find its way to your receiver with 100% efficiency. On top of that you have power conversion circuitry that isn't 100% efficient either.

If send & receive are millimeters apart and power levels are low, as in a toothbrush or phone dock, then efficiency is tolerable and the lost power doesn't cost much. A toothbrush only costs pennies a year to keep charged, so trading off extra energy cost against waterproofing the product for a bathroom environment is worth it. A pad under your electric car transmitting thousands of watts over a foot of ground clearance would waste tens of dollars a month in energy cost compared to plugging in. Trying to run a clothes dryer directly from an electric company tower on top of a hill miles away simply wouldn't work.

We may yet see wireless or ambient power become popular for tiny embedded devices, such as a low-power microcontroller monitoring something. If microcontroller power consumption gets low enough, it can run continuously from a tiny solar panel, a coil of wire as in an RFID badge, piezoelectric device or so on. Energy could be harvested from WiFi signals, heat, mechanical movement, or other ways that are not used today because the power levels are too low to be useful. Transmitting collected data over, say, Bluetooth LE takes much more energy than simply running the microcontroller, so the transmit bursts have to be short and infrequent, with some energy storage (capacitor) filling slowly in between. This is the realm of microwatts or maybe nanowatts, so forget about having your cellphone continuously charged as you walk around. Texas Instruments seems to be paying attention to energy harvesting if you want more information about it.

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    \$\begingroup\$ It's worth noting that AM radio sets that received their power wirelessly could for many years be purchased much more cheaply than those which used batteries or plugged into the wall. One would have to hook them up to a very big antenna, and listen to them with earphones in an otherwise quiet room, but they worked and were relatively cheap. The usefulness of such radios would be limited in many areas today because their tuners were not very selective, but the technology for radios to be powered wirelessly is hardly new. Microcontrollers might need even less electricity than an earphone. \$\endgroup\$ – supercat Aug 20 '12 at 22:06
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    \$\begingroup\$ As I understand it, those AM radio sets demodulated audio from the carrier and sent it directly to the earpiece as an analog waveform with enough energy to mechanically move the earpiece diaphragm. This is different from what we usually think of power today, as a regulated voltage supply that can output up to some level of current before falling out of regulation. You might be able to harvest AM radio energy into some storage and regulate it, then power a microcontroller from it, but let's not forget about all that extra circuitry in the middle. \$\endgroup\$ – Matt B. Aug 21 '12 at 20:23
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The reason we don't distribute power as Tesla had tried is because it doesn't work. It is basically a dumb idea because:

  1. The power available in any fixed volume decreases with the cube of the distance from the transmitter. Lets say for example that you could extract 100 kW from a cubic meter 10 meters from the transmitter. At 100 meters that would be 100 W. At 200 meters 12.5 Watts, which is barely enough to power a light.

  2. There is no way to measure individual use, so how do you charge people? You can't expect me to pay just because you put up a tower. I can claim I never used any of the power, and you can't prove otherwise.

  3. We don't really know what the health effects of long term exposure to significantly powerful electric fields are. Think about it. If a lightbulb is supposed to intercept the power from this field to light itself, how exactly is your body not supposed to intercept some power?

  4. How do you keep ordinary object that happen to have the right electrical properties from intercepting the power and heating up? You'd have to be very careful with using any material that isn't a good insulator. You'd have to keep its size, orientation, and impedance in mind carefully to avoid it grabbing power from the E field all around it. Think of all the metal objects you take for granted. Even a aluminum can of soda could be a problem.

  5. It is horribly inefficient, even if it did work. There will be many ordinary object as in #4 above. Let alone what happens to those objects when they intercept this power, but think of the huge waste of power from the producer's side. Every wet tree branch, the ground, and all sorts of things will take power from this E field.

Like I said, it's dumb idea, and was a dumb idea when Tesla tried it too, as some of his own equations should have told him.

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  • \$\begingroup\$ @DrFriedParts - #1 Then let's assume, quite reasonably that the power is distributed radially from the power station. Then you still have to suffer power density falling off as the square of the distance. Better than the cube, but much worse than a cable. \$\endgroup\$ – Rocketmagnet Mar 20 '13 at 23:44
  • \$\begingroup\$ @DrFriedParts - #2 Detect and localise disturbances to E-fields for thousands of closely spaced customers surrounding the transmitter? \$\endgroup\$ – Rocketmagnet Mar 20 '13 at 23:45
  • \$\begingroup\$ @DrFriedParts - #3 Oh really? There are studies where they have subjected large numbers of people to these much more powerful fields for several decades? No, of course not. You are probably thinking of much lower powered transmitters, like those used for phones and FM radio. \$\endgroup\$ – Rocketmagnet Mar 20 '13 at 23:48
  • \$\begingroup\$ @DrFried: #1, no isotropic or not has nothing to do with it. In any one direction, the power per area still falls of by the square of the distance and power per volume by the cube. The power level will be higher in a focused beam (at the expense of other directions), but the falloff is still the same. \$\endgroup\$ – Olin Lathrop Mar 20 '13 at 23:51
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    \$\begingroup\$ @DrFried: #3 Show me just one study at that power level. Perhaps we don't have proof it is dangerous, but we certainly don't have proof it's not. This is a case where you need to know its safe before you subject whole neighborhoods, as Tesla intended, to very powerful electric fields. \$\endgroup\$ – Olin Lathrop Mar 20 '13 at 23:57
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Counsel:

Let's see if I understand this correctly. If you have radiation or electromagnetic waves going from your system, the energy is wasted?

Tesla:

Absolutely wasted. From my circuit you can get either electromagnetic waves, 90 percent of electromagnetic waves if you like, and 10 percent in the current energy that passes through the earth. Or, you can reverse the process and get 10 percent of the energy in electromagnetic waves and 90 percent in energy of the current that passes through the earth.

It is just like this: I have invented a knife. The knife can cut with the sharp edge. I tell the man who applies my invention, you must cut with the sharp edge. I know perfectly well you can cut butter with the blunt edge, but my knife is not intended for this. You must not make the antenna give off 90 percent in electromagnetic and 10 percent in current waves, because the electromagnetic waves are lost by the time you are a few arcs around the planet, while the current travels to the uttermost distance of the globe and can be recovered.

This view, by the way, is now confirmed. Note, for instance, the mathematical treatise of Sommerfeld,[*] who shows that my theory is correct, that I was right in my explanations of the phenomena, and that the profession was completely misled. This is the reason why these followers of mine in high frequency currents have made a mistake. They wanted to make high frequency alternators of 200,000 cycles with the idea that they would produce electromagnetic waves, 90 percent in electromagnetic waves and the rest in current energy. I only used low alternations, and I produced 90 percent in current energy and only 10 percent in electromagnetic waves, which are wasted, and that is why I got my results. . . .

You see, the apparatus which I have devised was an apparatus enabling one to produce tremendous differences of potential and currents in an antenna circuit. These requirements must be fulfilled, whether you transmit by currents of conduction, or whether you transmit by electromagnetic waves. You want high potential currents, you want a great amount of vibratory energy; but you can graduate this vibratory energy. By proper design and choice of wave lengths, you can arrange it so that you get, for instance, 5 percent in these electromagnetic waves and 95 percent in the current that goes through the earth. That is what I am doing. Or you can get, as these radio men, 95 percent in the energy of electromagnetic waves and only 5 percent in the energy of the current. . . . The apparatus is suitable for one or the other method. I am not producing radiation with my system; I am suppressing electromagnetic waves. . . . In my system, you should free yourself of the idea that there is radiation, that the energy is radiated. It is not radiated; it is conserved. . . .

Tesla was not stupid!

:)

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    \$\begingroup\$ But he was also wrong. \$\endgroup\$ – Olin Lathrop Apr 6 '13 at 18:32
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I read somewhere he stopped it because of fears of the physical effects the system might have on us. In the end, I think if he said it would work it will work.... gotta go with the guy who invented electricity as we use it to this day....and the radio...and x-ray...too bad he isn't still around, the advancements he would make today!

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protected by Pops Oct 13 '14 at 19:48

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