Harvesting energy from WiFi and GSM networks?

By Confusion on Sunday 10 January 2010 21:21 - Comments (20)
Categories: Science, Technology, Views: 12.330

Source: Oh Gizmo, Engadget

At CES 2010, a company called RCA has presented a device called 'Airnergy', which is supposedly able to harvest energy from WiFi (and GSM) networks, for instance to charge your cellphone. Now this raises immediate suspicions: if that is possible, why aren't mobile devices powered by these networks in the first place? However, given the feeling that we are surrounded by a vast multitude of such networks, it still sounds remotely plausible. The only way to determine whether this makes sense, is to actually crunch the numbers:

My phone has a 0.9 Ah battery, at 3.7V. This means the battery holds 12kJ. With an uptime of 200 hours, this means a mobile phone consumes 17 mW on average.

A wireless accesspoint emits EM radiation in the order of of 100mW. If a device harvesting this power has an active area of 10x10cm and is, on average, located 1 meter from the accesspoint, it will pick up 0.01/(4*pi) * 100 = 0.08 mW, assuming the radiation is the same in all directions.

If the device in question can harvest 0.08 mW, it takes17/0.08 ~ 209 accesspoints and 200 hours to accumulate 12 kJ. My only conclusion can be that this product is completely bogus. Even at a conference, with perhaps 100 nearby cellphones as additional radiation sources, it wouldn't be useful.

This analysis grossly overestimates the power that could be harvested, as it assumes a 100% conversion of EM radiation into electricity and situates the accesspoints within one meter of the device. Is it a hoax? Is it fraud? Is it idiocy? Or is my calculation wrong?

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Comments


By Tweakers user golfdiesel, Sunday 10 January 2010 22:11

you can't calculate the amount of radiated energy by the capacity of the battery and the uptime of the phone.
The phone uses power for more things then only the transmitter.

By Tweakers user Hazurr, Sunday 10 January 2010 22:16

Also, radiated energy is inverse proportional to it's reception, a cellphone doesn't need to emit a strong signal if it's close to a GSM antenna anyway.

The same can be true with WiFi devices, especially battery powered ones.

By Tweakers user EvilWhiteDragon, Sunday 10 January 2010 22:18

Also, radiated energy is inverse proportional to it's reception, a cellphone doesn't need to emit a strong signal if it's close to a GSM antenna anyway.

The same can be true with WiFi devices, especially battery powered ones.
True, but don't forget that you can use the power of the GSM antenna in that case.

By Tweakers user Confusion, Sunday 10 January 2010 22:27

you can't calculate the amount of radiated energy by the capacity of the battery and the uptime of the phone. The phone uses power for more things then only the transmitter.
Also, radiated energy is inverse proportional to it's reception, a cellphone doesn't need to emit a strong signal if it's close to a GSM antenna anyway.
Updated the post; that bit should have read what it reads now: I mostly care about the minimum power consumption of the mobile phone and didn't intend to calculate the amount of power that could be harvested from cellphones, as it is obviously much lower than that of WiFi accesspoints.
The same can be true with WiFi devices, especially battery powered ones.
True, but that is just another reason that I've overestimated the amount of energy the device could possibly harvest.

[Comment edited on Sunday 10 January 2010 22:33]


By Tweakers user xantus, Sunday 10 January 2010 22:54

I can't see what radiated energy has to do with it at all. The phone itself doesn't have to radiate anything in order to collect energy. But none the less your calculations are correct.

surface of the accesspoint is 12,56 m^2
surface of the phone 0.01 m^2
the phone will pick up 0,01/12,56 *0,1 = 7.96*10^-5 W = 79.6 uW

The battery of the phone contains 3600*0.9*3.7=11988J
It will take 11988/79.6uW=150603015s to completely charge to phone. Or
150603015/(200*60*60)=209.1 accesspoints all radiating 100mW and located 1m from the phone.

The problems with these things is that if you want this to useful at all it at least needs to charge as much as the phone drains from the battery, as you don't want to wait 200 hours to charge you phone.

So:
Your phone drains on average 12000/(2000*60*60) = 1.6mW
1.6mW/79.6uW = 20x
This means you need an accesspoint of 2W spaced 2m apart everywhere you go just to keep you phone charged.

In this form wireless energy is just too impractical to be used.

Also you can't use the other phones as additional power sources as they are most likely also charged by the same accesspoints and so any energy you get from them you are getting indirectly from the accesspoint.

[Comment edited on Sunday 10 January 2010 23:00]


By Tweakers user redfox314, Sunday 10 January 2010 23:00

Every once and a while this airy story shows up somewhere. Despite everyone's wishful thinking there is still no such thing as a free lunch.


By Tweakers user YopY, Monday 11 January 2010 09:38

Wishful thinking. Sure, they might be able to harvest a small portion of the energy, but even with a 100% conversion rate (which is very, very unrealistic), the effect on a cell phone's battery life would be marginal.

Phone builders should invest in lowering the power consumption of their phones, instead of such largely theoretic claims. I mean, did they actually build something like this?

This can safely be chucked into the same corner as boiling eggs (or brains) with cellphones (see also Brainiac).

By Tweakers user xantus, Monday 11 January 2010 11:10

Phone builders should invest in lowering the power consumption of their phones, instead of such largely theoretic claims. I mean, did they actually build something like this?
It is not only a theoretical claim, a google search would have lead you to the following movie:
http://www.youtube.com/watch?v=IMMbihbeIls

As you can see they have actually build the thing. And they claim it is able to fully charge a phone in ~90 minutes. It will be available on the market Q2 of 2010 for around $39.

By Tweakers user SPee, Monday 11 January 2010 11:37

At the article posted on tweakers.net last year, they mentioned that it was a (very) low energy they could harvest. So they would require some transistors and some amount of time to have enough energy to be used.
Offcourse, this could be improved in time.

There was also some mention that the network providers would not be very happy with this.

By Tweakers user Confusion, Monday 11 January 2010 11:50

@Xantus
As you can see they have actually build the thing.
They have built something. However, if my calculation is correct (and I see no reason why it isn't), then it is completely bogus. Not unlike many other bogus devices that have been sold throughout the ages.

[Comment edited on Monday 11 January 2010 11:51]


By Sage, Monday 11 January 2010 13:35

There was a story this past Summer that Nokia could generate 5 mW from the RF waves in the air but it was later retracted. It was in EE Times, I think.

I think that I agree with the authors analysis.

There is a battery in the "Airnergy" thing -- lets see it work without the battery.

Sage

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