# RF Energy harvester

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1. Aug 14, 2015

### jaymin915

I am using RF energy harvester circuit to charge mobile in my project.The rectification part is done by Villard Voltage multiplier circuit .I need to used a supercapacitor to store the harvested energy .As the battery rating of the mobile phone is 3.7V and 2000mAh.So can i use 0.33 Farad 5 Volt supercapaciotor to store energy?

2. Aug 14, 2015

### Staff: Mentor

Welcome to the PF.

Please post your working calculations. What RF frequency are you trying to harvest? What is your antenna structure? What is your RF front end circuitry like?

3. Aug 14, 2015

### jaymin915

I am working with frequency 900Mhz and I am using a chip antenna by Prestta p5223304 the front end of the circuit contains impedance matching circuit followed by 10 stage viilard voltage multiplier.Schematic of the design is attached........

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4. Aug 15, 2015

### meBigGuy

To charge a mobile you need to supply 5V to the phone's charger circuit (USB connector). The phones internal charge controller then deals with all the complex issues of actually charging a li-ion battery.

If your capacitor drops below about 4.5V, the phones charger circuit will probably shut off.

A supercapacitor's voltage drops steadily as you draw current from it. (unlike a battery) so you need to build a boost regulator that will convert the super-capacitor to 5V for the phone. So put a 5V boost circuit in front of the USB.

Also, phones have battery charger protocols. They won't just charge by applying 5V to the +5USB pin to ground. Many will be happy if you simply short DP and DN, but not all (for example iphone will not charge). This document explains a lot of this in figure 3 (Note that the part is designed to be used inside the phone, not as a charger -- sort of backwards -- It just happens to have a nice table of what chargers present to some phones to make them charge): http://datasheets.maximintegrated.com/en/ds/MAX14578AE-MAX14578E.pdf

The USB battery charging protocol can be quite complex. Those proprietary methods illustrated above are outside the USB spec.

Last edited: Aug 15, 2015
5. Aug 15, 2015

### sophiecentaur

What is the site at which you plan to harvest the energy? It would be worth while considering the actual power flux density of RF signals at this site because you can only expect as much power out of your system as actually is passing through your receiving site in the effective aperture of your receiving antenna. Imo, you are probably talking in terms of a few mV (into 75Ω). That, at least is around what a UHF TV requires from its (standard design) antenna so it's the sort of signal level you will get near the fringe of a TV reception area. Signal strength will improve as you get nearer, of course, but the transmitting antenna is often arranged so that the main beam direction is towards the edge of the reception area so you cannot rely on a simple Inverse Square Law to calculate the likely received signal at a nearby receiving site.
There is a massive difference between the signal level needed for receiving TV pictures (where you use amplification - plenty of it) and the signal level that can give the sort of power level needed for battery charging (in a reasonable length of time).
Your "chip antenna" will not do the job for you. You would need an antenna of significant dimensions - like a large multiple Yagi array - to provide a useful energy gathering aperture.

6. Aug 15, 2015

### rbelli1

What is the average power that you need?

7. Aug 15, 2015

### jaymin915

Average power will be the power required to charge the mobile phone.......So with 3.7 volts and 2000m Ah current is needed........

8. Aug 15, 2015

### rbelli1

Do you have any dBm measurements of the environment you are operating in?

BoB

9. Aug 16, 2015

### meBigGuy

Did you even read my post regarding how to charge a phone?

If the phones battery is rated 2000maH, that won't tell you much about what is required to fully charge it.
The only sure way to know what will be required to charge the phone is to measure the USB 5V current over time for a full charge duration.
Plus, You need a boost converter to boost your supercap to 5V +- 5%. That will have some efficiency number, like 70 or 80%.

To summarize some issues that require that you measure the charge cycle:
1. 2000maH is only a battery rating based on a discharge rate of 0.1C (200ma). Has little to do with what is really obtained during operation.
2. 3.7V is the average voltage. It actually goes as high as 4.2V when charging
2. You have no idea of the efficiency of the battery charger in the phone. It could be anywhere from 50 to 90%. (notice phones get warm when charging)
4. The lithium battery itself is pretty efficient at converting charge (otherwise the batteries would heat up), but I don't know how to tell you how much energy is actually required to fully charge a typical 2000maH battery.
5. The phone is operating while charging, so you have no idea how much power it will consume. Also, that will depend on what is happening while charging (any calls, etc?)

10. Aug 16, 2015

### sophiecentaur

Charging or discharging are the same ball park figures. There may be many orders of magnitude between those figures and what is actually available.
Important thing is to show any product that is available on the market to do this job. Of all strange (non working) devices you can buy on the Internet , I have never spotted a charger of this sort. Why?

11. Aug 16, 2015

### sophiecentaur

Speaking in terms of 'available power density', start with 1kW/msq, which is how much you can get from the Sun. PV cell arrays of reasonable sizes can provide a useful amount of power for battery charging. Even so, the so-called 'trickle charger' panels that you can buy for your car are very often totally inadequate for anything more than making you feel better about whether your car or boat will start after you've left it for a month or so. A dodgy battery can leak faster than one of these chargers can keep up. They are, of course, very good for all sorts of applications - road signs, weather stations etc. Even so, they need a receiving area of something like 1/4msq.
The total flux of power in the RF bands (except when you are actually sitting on top of a transmitting station) is a tiny fraction of what the Sun provides. If it were otherwise, we would feel warm on a winter's night. On top of that. restricting the bandwidth of any 'harvesting' device means selecting only a few percent of what's buzzing around us.
Let's face it, the nearest approach to what's being proposed here is the good old Crystal Set, which used the power of MF Radio signals, received off a long wire, to power a sensitive headphone. That's a matter of less than 1mW, probably. This proposal is an Engineering Project and will only work if the Rules for Engineering Projects are followed. It doesn't even start in that direction and no amount of technology can summon up Energy when it doesn't exist.

12. Aug 16, 2015

### mr166

This whole thread is a valuable illustration of how little science the general public knows. The media is a huge contributor to this lack of knowledge also. I must have read 10 articles in Physics.org that featured cell phones being charged by devices capturing various ambient RF signals. These articles were all green power hoaxes.

13. Aug 16, 2015

### sophiecentaur

The general public doesn't even grasp the facts about its money and its taxes. That's why so many money scams work. No surprise that they just don't 'get' Energy.

14. Aug 31, 2015

### Mr.Robot

http://patentimages.storage.googleapis.com/pages/US4685047-1.png
take a look at this circuit.

15. Aug 31, 2015

### sophiecentaur

And where is this power available - out in the aether? (You don't actually say how much)
I liked the patent in the above post. Patented but not marketed. Could you consider why?

16. Aug 31, 2015

### meBigGuy

The enemy of understanding is classification.

http://www.mouser.com/new/powercast/powercast-Px110-powerharvester/ [Broken]

Just needs -6dBm in the 900 MHz band.

http://www.ti.com/solution/energy_harvesting
http://ieeexplore.ieee.org/xpl/logi...re.ieee.org/xpls/abs_all.jsp?arnumber=4802822

https://sensor.cs.washington.edu/pubs/wisnet_ambient_rf_energy.pdf

Try googling "rf energy harvesting system from cell towers in 900mhz band"
"rf energy harvesting system and circuits for charging of mobile devices"
"ambient rf energy harvesting"

https://workspace.imperial.ac.uk/opticalandsemidev/Public/Publications/2013_07_TMTT.pdf

Last edited by a moderator: May 7, 2017
17. Aug 31, 2015

### sophiecentaur

Marketed, perhaps, then. But actually sold ? Do you, or does anyone you know (or any PF reader) , own and operate one, meBigGuy? If not, why not?

I have not just been 'classifying' I have been doing some sums (yet again) to reassure me that I am not way out in my estimation.
-6dBm is 112mV into 50Ω (I believe). Where are you going to get that sort of signal? Urban Median reception levels are around 77dBμV/m at Band V and to get -6dBm at your receiver input would need around 1V/m, if your receive antenna had 10dB gain. The figures just don't seem to match up. Ref1 and Ref2 were what I used. I remain skeptical about the practicability of such systems, mainly on the grounds that the service areas and transmitter power levels have been designed with a view to Active Amplification of received signals in the receivers. If the received power levels are so enormous then someone has not been planning correctly (imo). Why use expensive Klystron Amplifiers when low power transmitters would, it seems, serve an area the size of London (Main Station at Crystal Palace), quite well enough?

I could suggest that Energy Theft could be a more accurate description of a process that significantly affects the received signal for people behind you. I read (don't ask for a ref) of a guy who was using power from a local mf transmitter to light his farmhouse and I believe he was successfully prosecuted. But was he 'harvesting' or just plain 'nicking'? (very near field - no pun- operation and it only worked for one installation)

18. Aug 31, 2015

### Jeff Rosenbury

Still, with a better antenna and a very close transmitter ...

But that would be stealing, at least from a moral perspective. Laws vary by jurisdiction.

19. Aug 31, 2015

### sophiecentaur

Agreed. However, isn't the point of this harvesting idea to have a device that can be carried or used 'wirelessly' in any old spot in the house? If you need a wire to connect to a large external antenna then you have negated any advantage. The 'free'ness of the system has to take into the account, the capital cost. Running a personal electronic device is well below the noise level of your electricity bill.
You could imagine loud complaints from the large proportion of the population without high received signal levels if this took off. Also, you could expect complaints and litigation from people in the shadow of their neighbours who are using a 'harvester'.
Time will tell, I guess. But decades of time have already given a pretty good indication. I am not sure that there is any new technology that could alter the real situation.
There is a parallel thread about energy harvesting shoes. We are spoiled for choice.

20. Aug 31, 2015

### Mr.Robot

Exactly..
Agreed..
I was thinking the same thing,to harvest it and later use it for our purposes.
Ever corner of our house have RF waves.
We will store it keeping in mind for which purpose we will use it.
(Correct me if i am wrong!)

21. Aug 31, 2015

### Averagesupernova

That depends on who is doing the harvesting.

22. Sep 1, 2015

### sophiecentaur

Do you have any response to the (albeit mostly negative) comments on the thread. Do you see the basic principle behind the objections to your proposed scheme?
One should always be prepared to argue one's case. That's what forums are for.

23. Sep 8, 2015

### axemaster

In general RF power harvesters don't work. Most wifi transmitters emit only a few watts at most. That power is spread out in a sphere around the transmitter, so at most you'll only capture <1% of the signal.

Just from that you should see that charging a phone is completely non-viable (at least in less than a few days or more likely weeks).

Now consider the inefficiencies of the receiver:

1. The antenna will have a loss.
2. Voltage multipliers are very inefficient at low voltages and high numbers of stages.
3. The battery will have power loss when charging.

In other words, don't expect to get any meaningful power out of a wifi harvester. Hell, consider yourself successful if you can even detect a voltage from that multiplier.

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