I’m sure that this question will simply make public my remarkable lack of understanding about electricity and batteries and all other things in their neighborhood, but here goes.
Would it be theoretically possible to run powered devices such as cell phones, laptops, and possibly even vehicles on radio waves that simply exist in the air around us? I live in a city with a dozen or so radio stations that are bombarding my every move with their signals, and it seems to me a grand idea to be able to take their electromagnetic energy and aim it into my cell phone so that it always remains charged. Plus, it’d be a hell of a great way to actually make ad nauseum replay of crap like the Jonas dudes worthwhile.
Are these signals a completely different animal from whatever the shock-y stuff that comes out of my wall sockets is? Could it possibly be done? What are the limitations in devising such a technology?
There is energy in the radio waves in the air, and it can be used to power a device. Some very simple radios don’t have a power source other than the waves they’re picking up. But even with the simplest possible design, you still only get a whisper of sound out of the set-- Something like a cell phone or iPod would be hopeless. Any form of long-distance transmitted energy adequate to power such devices would also be adequate to cook the person wearing the device.
You’ll occasionally see mention of inductive power, which can be safely transmitted through the air, but for that to work, the distance between the antennas has to be smaller than the size of the antennas themselves. Any practical device using that technology would pretty much be mounted in a charger cradle like the ones we already have for portable devices, just without needing a couple of exposed metal contacts.
Not being wise to the ways of such calculations, perhaps someone can ballpark for me just how short of energy the radio waves are. That is, how long would it take to, say, trickle charge an empty cell battery using just radio waves. My Berry charges from dry in about 2 hours or so. How long would that same phone take with radio waves? Days? Months? Years?
I did some back-of-the-envelope calcs based on being 10km from a radio station with 50,000 watts of transmit power and a very efficient antenna array that keeps 80% of the power within 3 degrees of a horizontal plane. The further assumption is that your cellphone has a receiving area of 4 cm[sup]2[/sup] and is 100% efficient in using received radio energy.
Based on this, your cellphone will charge at about one-millionth its normal rate. If it takes 2 hours with a wall charger, you’re looking at something like 220 years.
RFID tag systems, like those used on security key card entry systems, E-Z pass toll booths, embedded ID chips for pets, etc. basically use this principle. There’s actually two types of tags, battery powered and beam powered. Beam powered is the most common, and they work like what we are talking about here. They use energy from the incoming radio beam to charge up a tiny capacitor. Once the capacitor is charged, the microchip in the device switches on and does its stuff (generally making sure that the incoming signal matches a request then it transmits out a radio signal indicating its number).
RFID chips are very low power, and the beam powered ones don’t have a very long range as a result, but it is an example of using radio waves to power something.
By the way, Nicola Tesla (the combination genius and psychotic whack job that created the AC motor and AC power system that we all use today) had an idea to simply transmit out large amounts of radio energy all over the place. Then consumers could basically get free energy out of the air. Unlike some of Tesla’s other ideas, this one isn’t really practical.
I don’t think it’s actually clear what Tesla’s broadcast power plan was supposed to be… By that time, he was pretty deep into the nutso end of the pool, and wouldn’t share technical details of anything, because the conspiracies were out to get him.
Every stoner I know is quick to remind me that “Tesla thought of this, dude, pass the bong.” In reality you would have no usable spectrum for radio, tv, wireless, etc as all of it would be used to power devices. Interference would be a nightmare. I guess you can have wireless power you want, but no tv, cell phone, cb, ham, satellite, etc. Not to mention possible health issues and the incredible waste of power this would be considering the inverse square law of radiation.
Beam powered RFID is like a radio version of a bar code. The energy the little laser gun that reads the bar code is quite a bit for what gets reflected back. Same with RFID. The power thats sent is a lot compared to what gets sent back.
There IS electromagnetic radiation all around us that’s available free for the taking - it’s called light, and things called photovoltaic (“solar”) cells can be used to charge batteries for things as small as a calculator to as large as a house.
Light and radio waves both follow the inverse square law, which is that the power level drops off as the square of the distance due to the fact that it’s radiating out in all directions. In other words, 2 feet away from the transmitter you’ve got 1/4th the power (not 1/2) that you had at 1 foot. 3 feet away you’ve got 1/9th (it’s always 1 over n squared, hence it’s called the inverse square law).
If you were in a dark room and generated your own light for “solar” power, then you’d have the same problem that radio energy has. You have to generate a lot of power, and most of it gets radiated away and not used. Solar power is only useful because we’ve got a really big light generator already installed (called the sun) and we don’t have to feed it energy ourselves.
Solar cells have a fairly wide frequency response. They create electricity from visible light as well as light that is slightly into the IR and UV parts of the spectrum.
Really? They have solar cells that work in the infrared? I never would have expected a work function that low in energy. Possibly my rudimentry understanding of photovoltaics is outdated or wrong.
Many types of photovoltaic cells respond up to about the 800 to 1200 nm wavelength range (visible light ends around 700 nm).
Also, one of the ways that they have been increasing the efficiency of solar cells in recent years is that they have been coming up with different ways to trap and use the IR portion of the spectrum. They have created new materials that respond farther into IR, and they have also used tricks like using different layers of materials where the visible gets absorbed by the upper layer and the longer wavelength IR passes through to a lower layer where it then gets absorbed. Another trick is to reflect the longer wavelengths so that they pass through the material multiple times, this increasing their absorption even though the material doesn’t respond well to those frequencies.
