Saturday, April 02, 2022

How close is wireless power technology?

[This is a transcript of the video embedded below. Some of the explanations may not make sense without the animations in the video.]

“Battery’s almost empty. Connect to a power source.” Why do we still not have wireless power? That’s what we’ll talk about today.

Wireless power would have many benefits. First, it’d get rid of all those different plugs. We’d also no longer have to crawl around on airport floors and fight over the only two available outlets. And if you don’t have to plug and unplug your things constantly, that’s one piece less that can break. It could power devices that can’t easily be reached, such as medical implants or robots used in contaminated areas. It’d also be much easier to make devices waterproof, though I’m afraid even with wireless power it’ll still be difficult to dry your hair underwater.

So why don’t we have wireless power already? The problem is fairly basic. Power is energy transferred per time, and energy travels locally. This means if you want to get energy from one place to another, it has to travel at least one path between these two places. If you have a cable, then the energy goes along that path, except for some unavoidable losses mostly due to heat. If you don’t have a cable, well, then you have to find some other way to get the energy where you want it to go. That leaves you with two options.

You either generously splatter energy into all directions and hope that some of it arrives at the intended destination. That’s like filling up your car tank not with a hose, but instead by parking it next to a gasoline sprinkler. Sure, it’ll work if you wait long enough, but it’s extremely wasteful.

The other thing you can do is to focus the energy at a particular target. This will be much more efficient, but now you have a focused beam that can deposit energy into anything that gets in the way, like walls, trees, or humans. That isn’t good. Maybe we could put some kind of shield around that energy so that nothing gets in the way. Like – a cable?

But, but, you may say, we already have wireless chargers. Indeed, wireless charging exists, but it currently works only over short distances. This is called “near-field” wireless power transfer. In these systems the power does not travel freely through space but can be extracted near a source, though the power you can extract that way drops quickly with the distance to the source.

There are a variety of ways this can be done with alternating currents, that is the stuff that comes out of your wall outlet. The simplest and also the oldest method is inductive power transfer.

Inductive power transfer uses two coils – one transmitter and one receiver – very close to each other, that’s usually millimeters to centimeters. If an alternating current goes through the transmitter coil it creates an oscillating magnetic field. The same field then also passes through the receiving coil, where it induces another alternating current. So you’ve moved some power from the one coil to the other.

This is currently the most common method of wireless power transfer. It’s used for example to recharge electric toothbrushes and shavers and things like that. With this method, the power transferred increases with frequency but decreases with distance between the coils. That’s because the larger the distance, the less of the magnetic field from the transmitter coil reaches the receiving coil. In practice this means that the two coils have to be right next to each other.

There are several variations of this method, for example magnetic resonant power transfer. In this case you add capacitors to both circuits. They then have a preferred resonance frequency at which the power transfer is more efficient. This method also works over somewhat larger distances than the inductive transfer, up to a few times the size of the coils. This method is often used for wireless phone chargers.

But let’s be honest this isn’t what we’re looking for. We want our phone to magically recharge while we’re queuing for coffee, right? And for this we need far-field wireless power transfer.

The most obvious way to get this done is by sending energy with electromagnetic waves that travel freely. It’s like radio, except you don’t want to transmit the sound signal, but the energy.

This too is an old technology. Already on 1964, the American engineer William Brown did a series of experiments in which he powered a small helicopter for hours using microwaves.

Microwaves, guess what, are also used in microwave ovens. Those ovens are shielded for a reason, the reason being that you don’t want this energy to bounce around in your kitchen. Before microwave ovens became consumer products, they were used in scientific laboratories and often didn’t have shields. And that had some interesting consequences, which I learned from Tom Scott’s video about unfreezing hamsters. Yes, hamsters. Here’s how James Lovelock recalls these first microwave ovens.

"In the course of the experiments, while we were building it, the thing was running open and the radiation was bouncing all around the room. And the light bulbs would light up without warning. The filaments just had the same wavelength as the radiation and it would absorb it and light up. And the pound notes were the funniest ones, they’d catch fire because the metal strip inside was just about the wavelength of the magnetron."

If you haven’t watched Tom’s video, you should, I swear you won’t regret it. But don’t forget to come back. 

So, yes, you can use electromagnetic radiation for wireless power transfer, but most of us don’t want things in our vicinity to catch fire every now and then. This means this technology can really only be used at small power. But at small power it is being used already.

Samsung, for example, has produced a TV remote that is powered by energy extracted from your home Wireless. Another example that you have probably all seen are RFID security tags. The abbreviation stands for radio frequency identification. They are powered by another device that serves as a reader. The power transferred in those examples is typically a micro Watt. This isn’t remotely enough to charge your phone, which needs some million times more.

At this point you may be wondering why not just extract energy from all that radiation which is in the air already? Indeed, there’s a group at Georgia Tech which has developed a receiver that grabs energy of the 5G network, again that’s not much, some microwatts, but at distances up to 180 meters from a network antenna.

If you want to transfer more power without setting things on fire, you have to find a way to focus power transfer smartly. This isn’t all that easy with electromagnetic waves. They tend to spread out into many directions, and that reduces the amount of energy that arrives at the target. You could use lasers, but again there’s the issue that things might get in the way. Also, lasers are themselves not exactly power efficient.

However, the intensity of the radiation drops with 1/R^2 in the far field only if you’re far away from all antennas, not if you are close by, or actually inside, an array of antennas. Then the intensity may drop far less.

The start-up Ossia makes use of this. Their system is called Cota. It’s an array of about 60 times 60 centimeter size with a few hundred tiny antennas. They emit radiation in the frequency range of several GHz, so that’s somewhat higher than the frequency used in a typical home wireless. The system can detect the position of your phone or other devices and then focus a signal to only that position, instead of wasting energy by emitting it everywhere.

The company claims that their system should be able to deliver 2 to 3 Watts at one meter distance, 1 Watt at 2 meters, and 10 to 50 miliwatt across 10 meters.

For comparison, a USB-3 port, which you most likely currently charge your phone with, delivers up to 4.5 Watts. So this isn’t so bad, but still really only good to power small devices like maybe alarm clocks and other things you don’t use. Also, the machine consumes between 40 and 60 Watts, which is roughly half the power consumption of a typical fridge, so not exactly energy efficient.

This technology was approved for sale and use in the USA already in 2019, and a few months ago it was also approved in the UK and the EU. The company says that Walmart is piloting the system in some distribution centers for inventory tracking and asset management, and Toyota is testing its viability to replace some wiring in cars, where it could power sensors and make it easier to replace them. These are all cases where you need really small amounts of power.

There are now a number of other companies with similar products, for example Energos whose system is called Watt-Up, and GuRu. The GuRu system operates at somewhat higher frequencies, at 24 GHz, which they claim makes it easier to miniaturizing the device.

I had a run-in with some guys who worked on wireless power about 15 years ago because they claimed in their product description that the electromagnetic energy “tunnels” from one place to another. If you’ve followed my blog for a really, really long time, you may remember this.

The reason I picked on this “tunneling” description is that I was afraid it may raise the impression you can somehow avoid the need for energy to go from one place to another with quantum something. This is not the case. Energy doesn’t just jump from one place to another, and quantum mechanics doesn’t change anything about this. For all I know that start-up no longer exists.

But referring to quantum something to attract customers hasn’t gone entirely out of fashion. There is a new wireless power company now, technovatar, which says on their website that they use neither electric nor magnetic fields but instead “transfer energy through energy quantization” which is “based on the creation of energy structures in space”. I have no idea what that means. They also claim their system does not use “any of existing methods of energy transfer”.  So I guess this means they sell a non-existing method of energy transfer.

Some of you may also remember that a few years ago there was a lot of buzz around wireless power transfer by ultrasound. The idea is that you convert electric energy into sound, transmit that sound, and then convert it back. You *can do that and it *does work, but of course this too doesn’t solve the problem that the energy has to somehow get from one place to another, and if you get in the way, some of that energy might be deposited into your body.

This company was called “u-Beam” and at the time and it made quite a few headlines. But in 2016, the entire engineering team left the company, and a former employee wrote a series of blog posts explaining that the technology did not work. He wrote: “While in theory [uBeam] may be possible in limited cases, the safety, efficiency, and economics of it mean it is not even remotely practical.” The company still exists, though it’s been renamed to SonicEnergy. It seems like wireless power by ultrasound isn’t going to become reality any time soon, maybe not ever.

There are however several technologies under development that might soon improve the efficiency of wireless power transfer and make the transfer more stable.

For example, in 2018 a group of researchers published a paper in PRL in which they proposed a way to improve the absorption of wireless power by a method of self-tuning called “Coherently enhanced wireless power transfer”. It’s basically a feedback loop that allows the system to adapt to reflections in the environment. A further step forward was made in 2020 by researchers from Stanford University. They developed a circuit that can adapt wireless power transfer to a moving source at times less than a millisecond. This drastically reduces loss and improves efficiency.

And near-field wireless power systems can be improved with metamaterials. Metamaterials are engineered to have desired responses to electromagnetic fields, sometimes also in the optical part of the spectrum. They can therefore much enhance the efficiency of the system. There’s a lot more to say about metamaterials. Let me know in the comments if you would be interested in a video on metamaterials specifically.

It looks like wireless power technology slowly gets going, but there’s already a new problem on the horizon. Wireless power might do away with the plugs, but the receiver has to be embedded inside the device. And unless everybody agrees on some standard receiver, which seems incredibly unlikely going by past experience, we’re going to see a lot of devices that won’t work in a room with another sender.

Personally I clearly think the way to go is tiny flying robots that deliver charge to your phone by crawling inside. Now I just need a catchy name and a website and soon I’ll be rich and famous. Until this happens, please don’t forget to like this video and subscribe.

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