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Topic: How to limit solar panel voltage? (Read 2379 times) previous topic - next topic

jremington

#15
Feb 05, 2018, 07:11 pm Last Edit: Feb 05, 2018, 07:23 pm by jremington
Quote
The cap will demand a HUGE amount of current from the panel which will drop it's voltage down too low to charge the cap at all.
You are confused, and need to do some reading and/or experimentation. PV panels are best modeled as current sources, not voltage sources.

If charging a supercap that is initially uncharged, the panel will supply the short circuit current, charging the supercap at the maximum possible rate. The supercap will eventually charge up to the PV open circuit voltage.

Circuit model for a PV cell:

venquessa

#16
Feb 05, 2018, 08:21 pm Last Edit: Feb 05, 2018, 08:29 pm by venquessa
You are confused, and need to do some reading and/or experimentation. PV panels are best modeled as current sources, not voltage sources.

If charging a supercap that is initially uncharged, the panel will supply the short circuit current, charging the supercap at the maximum possible rate. The supercap will eventually charge up to the PV open circuit voltage.

Circuit model for a PV cell:

Yes, but current without voltage is fairly useless.  150Amps @ 0V = 0 W, 0 J/s, 0 Charge/s, 0 Energy

You can't see them purely as a current source if you want any "power" out of them.  You have to select the voltage at with they produce the most power, which varies with conditions and light level.

Granted we are talking theoretically here and even a few joules flowing will, as you describe begin to raise the voltage of the cap.  However it still stands that the panel at a low voltage, regardless of the current less Joules of energy will flow out of the panel and thus it's being used in efficiently.

If what you were saying was purely and practically true I could connect my 50W panel directly to a flat SLA battery @ 9V and expect it to charge it at around 2.5A (24.5W).  Which it might, but if I PWM or buck the voltage down from it's maximum power point of 17V it will charge the battery with 50W and the battery will charge faster.

jremington

#17
Feb 05, 2018, 08:25 pm Last Edit: Feb 05, 2018, 08:26 pm by jremington
Quote
You can't see them purely as a current source if you want any "power" out of them.
Correct, but the panel will still charge the capacitor at the maximum possible rate. When the capacitor is charged, you can then draw power as needed. That is how these tiny, low power sensors are designed and intended to work.

venquessa

Correct, but the panel will still charge the capacitor at the maximum possible rate. When the capacitor is charged, you can then draw power as needed. That is how these tiny, low power sensors are designed and intended to work.
https://www.physicsforums.com/threads/charging-a-capacitor-with-solar-panel.713123/


TomGeorge

#19
Feb 05, 2018, 09:41 pm Last Edit: Feb 05, 2018, 09:47 pm by TomGeorge
Hi,

Quote
Yes, but current without voltage is fairly useless.  150Amps @ 0V = 0 W, 0 J/s, 0 Charge/s, 0 Energy
That is only for an instant, current flow charges the capacitor.
The instant you have just one electron difference in charge across a capacitor you have a potential difference.

A voltage develops across the capacitor as current charges the capacitor up.

The bigger the capacitor the longer it takes a charging source current to charge it to the desired voltage.

The charge current stops because the capacitors charged voltage equals the maximum voltage possible out of the PV.

Have you factored in the fact that any PV spec is a laboratory figure, in real life they need perfect conditions to acquire those specs.
Have you tried your project out in the sun and experimented if during the hours of no sun and low light you can keep the capacitor voltage high enough to run the controller and sensors.

Are you going to use a boost converter?
If you are doing any AtoD conversion, power supply stability may be an issue.
Do any of your sensors require a regulated supply for stable and accurate operation?

Tom.... :)
Everything runs on smoke, let the smoke out, it stops running....

venquessa

Hi,
That is only for an instant, current flow charges the capacitor.
The instant you have just one electron difference in charge across a capacitor you have a potential difference.

A voltage develops across the capacitor as current charges the capacitor up.
If the capacitor is at 1V and the MPP of the panel is 16V, at 1V it will provide very close to it's short circuit current.  Lets say that is 2A.  So it will provide 2W.  At it's MPP it might only provide 1.5A, but at 16V that's 24W.

If you put 2W into the capacitor you get 2W / the current voltage across it Amps, 2A.

If you put the 24W through a buck converter down to the 1V of the capacitor you will get 24A.  Assuming you can control the current draw on the panel to keep the voltage at 16V.

Minus inefficiencies and the devils of the real world, yes, but we are not talking about a 1200% factor in those details.

This is exactly why you don't just hook a panel to a capacitor or rechargeable battery.  It's just inefficient.

For simplicity, it might provide the OP with enough, but if it does he could achieve the same thing with half the panel area with even a fixed power point buck converter.

TomGeorge

Hi,
What we have to remember here is that the PV power is less than a couple of watts, any MPPT circuit will consume energy itself.

Tom... :)
Everything runs on smoke, let the smoke out, it stops running....

venquessa

#22
Feb 05, 2018, 11:07 pm Last Edit: Feb 05, 2018, 11:13 pm by venquessa
You might be surprised.  A PWM solution can apparently be done on as little as 5mA.

https://www.youtube.com/watch?v=NU8xf5vAG9s

EDIT: Actually I have a 5V panel.  It was claimed to be 200mA, but I highly doubt it.   I might have to give it a go at some point.   I know it didn't run an Nano sitting on a window sill in good sunlight.

Wawa

#23
Feb 05, 2018, 11:26 pm Last Edit: Feb 06, 2018, 09:14 pm by Wawa
It seems you're taking things out of proportions here.

OP has a 30mA panel that charges a 1.5F supercap.
Even a dead cap will fully charge with 10mA from the solar panel in less than 30 minutes.

Switching controllers might use more idle current here, and that could take longer for the cap to charge.
Leo..

jremington

#24
Feb 06, 2018, 12:14 am Last Edit: Feb 06, 2018, 12:15 am by jremington
Quote
It seems you're taking things out of proportions here.
Agreed.

The point is to power a sensor for a short time using a small solar panel, using simple circuitry, not to get the "maximum available power" out of the panel.

For this purpose, supercaps are pretty much ideal and much better than rechargeable batteries, because supercaps come close to lossless energy storage. Additional electronics will simply add unacceptable losses.

strange_v

The last thing which I want to implement is MPPT (according to KISS principle).
My sensor won't see direct sun light, so I expect about 5mA from panel. It is enough to charge supercap and I'll measure how match it take to charge it from 0V.

UPD:
8:35 solar panel was connected to the sensor with discharged capacitor
9:08 got first package of data (vcc 1.94V)
9:42 vcc ~4.7V

TomGeorge

Hi,
How are you collecting your data?

Tom.... :)
Everything runs on smoke, let the smoke out, it stops running....

strange_v

I have a REST API on Node JS and a client on Angular.
NodeMCU receives data from remote sensors and post them to the API.

I haven't found any flexible and ready to use solutions.

TomGeorge

#28
Feb 07, 2018, 10:03 pm Last Edit: Feb 07, 2018, 10:04 pm by TomGeorge
I have a REST API on Node JS and a client on Angular.
NodeMCU receives data from remote sensors and post them to the API.

I haven't found any flexible and ready to use solutions.
I'm sorry but that means little to me.
All I can "gather" is you are using a wifi/inet link.
But if REST API s a logging device fine, I understand.. :o


Tom... :)
Everything runs on smoke, let the smoke out, it stops running....

strange_v

Sorry for that.
Yes, I use NodeMCU with NRF24L01+ like a getaway between the sensors network and internet.
Also I have a small home server which collects all data from the sensors. All software for collecting and displaying information are custom.

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