measuring power of solar panels

**duffy** · 5th March 2009 06:21 PM

How about a precision resistor? Once every ten seconds or so you switch the output of the panel from the charger (and everything else) onto a precision power resistor for about 10mS or so.

This would be enough to get a A/D read of the voltage and calculate power using Ohm's law with a known, fixed, temperature-invariant resistance many times higher than the sum of the contact resistances in the circuit.

**awright** · 5th March 2009 07:12 PM

"PS, I want to measure the Max point very accurately.. so need something thats very precise."

Out of curiosity, umer27, why do you want to measure the max point very accurately? It varies a little or a lot (depending upon weather) from moment to moment with variations in insolation and the location of the sun. Since the loss of energy due to orientation is a roughly cosine function, minor errors in pointing have negligible influence on energy production.

The max power point is not a property of the panel alone, but is influenced by atmospheric conditions, how high the sun is in the sky, pointing errors, shading, etc. What is your goal in determining max power point very accurately?

The overwhelming majority of solar installations are fixed, rather than tracking, and mostly do fine. I think you can find in the literature the statistical loss of energy production over a year due to not tracking, and I don't think it is very large. I'd guess that if you are just charging a small battery for some remote outdoor application your money would better be spent on a slightly larger panel that would make up for the losses in not tracking and you would gain a lot in reliability.

This is not necessarily the case for a utility level solar farm in which it pays to get every watt out of the system. This is why utility power plants are always optimizing operating conditions to get the last 0.01% efficiency out of the plant, but those economics don't apply to a small battery charging application in which you will be throwing away excess energy anyway to avoid overcharging your battery.

duffy, I may be misinterpreting what umer27 is seeking, but I don't think your suggestion will give him what he wants. I think he wants to determine the MPP, which, for a given insolation, is influenced by the load resistance. Switching to a precision resistor can allow you to measure the instantaneous power production with that specific load and insolation, but can't help you determine the MPP. Of course, many MPP controllers don't actually determine MPP dynamically, anyway. I think they determine the MPP only occasionally and remember the voltage to load the panel to.

awright

**duffy** · 5th March 2009 08:33 PM

Originally Posted by awright

I don't think your suggestion will give him what he wants. I think he wants to determine the MPP, which, for a given insolation, is influenced by the load resistance. Switching to a precision resistor can allow you to measure the instantaneous power production with that specific load and insolation, but can't help you determine the MPP.

"Insolation" - ? Do you mean "installation" or "insulation"?

Yes, power is influenced by load, that's why you want a fixed load to determine power. A fixed load will help you determine the maximum power point better than a variable load, which is just dumb.

All he really needs to do is physically align the assembly correctly, then establish the day/night cycle. The software may require a trivial amount of curve-fitting, but clouds and whatever make no difference to a clock and calendar tracker.

**mneary** · 5th March 2009 11:09 PM

"Insolation" - ? Do you mean "installation" or "insulation"?

He means insolation. Look it up.

**awright** · 5th March 2009 11:11 PM

ummm, duffy, you might want to be a little more diplomatic about using terms like, "dumb."

I meant insolation, the amount of solar energy per unit area.

The normal method of locating the MPP (Maximum Power Point) is to vary the load on a panel while simultaneously measuring the power output. It can get complex for large panels, but for small panels it is usually done simply using a variable resistor with suitable power rating and resistance range. There is some unique combination of voltage and current out of the panel for any given set of conditions (including orientation and insolation) that results in the output power being maximized. That is the MPP.

An MPPT (Maximum Power Point Tracking) controller is a load manager that loads the panel to its MPP for the specific conditions existing at any moment (or at least at some time in the recent past). It draws whatever current pulls the voltage out of the panel down to the MPP, thereby maximizing energy production from the panel at all times. It then converts that energy to the voltage and current appropriate for the load.

MPPT in this context does not refer to panel orientation to maximize power and umer27's initial post in this thread clearly referred to MPPT by load adjustment, not by orientation.

awright

**umer27** · 6th March 2009 05:11 AM

Thanks Guys , I think I got it..

pls see the attached files..

Its just a theory , but...
Im going to short the panel outputs .. not for very long , only for a very small amount of time..
By, approx calculations, the MPP is at 90 % of the Isc and at 70-75 % of the Voc .. remember this..

what I needed to do was to vary the load on the panel, and I would get the IV curve..

what a resistor essentially does, is to limit the flow of electrons.. Im going to use PWM @ 3Khz to do just that .. lets say I take the PWM to 100% for about 50ms .. keep it there.. I should get Isc... right.. then I take the PWm to 0 % ... calculate.. Voc..

I think if a vary the PWM little by little.. i.e take it from 0 to 100 % or vice versa .. and every 5 % or so, take a reading.. I should be able to generate the curve.. Im going to bulid a small model to test this theory .. Any ideas are welcome..

We also have a sun simulator at the factory, It uses a neon lamp , flashing around 1000 Hz , and has a reference cell next to it (whose, IV curve is known), with this data , they calculate the MPP, Im going to find the MPP with the solar simulator.. then with my setup.. if the results are similar.. I win !

**umer27** · 6th March 2009 05:13 AM

here is the schematic ..

**umer27** · 6th March 2009 05:15 AM

@awright...

kindly read the "very very good solar tracker post"

cheers

Umer

**awright** · 6th March 2009 06:24 AM

AHA! Now I see the nature of your project. I based my comments on the impression that you were working with a small remote powered application of some type.

I also looked at the Lorentz system and, additionally searched "solar tracker gain." Found several interesting discussions of the tradeoffs between fixed and tracking systems. The maximum benefit of tracking is surprisingly large to me at some latitudes and climates. I didn't study the analysis in detail but they did point out that the benefits are strongly dependent upon the latitude of the installation.

Good luck on your project. It is beyond my level of interest or knowledge at this point.

awright

**umer27** · 6th March 2009 06:45 AM

well from an instrumentation point of view, you could tell me , whats the fastest and best method for measuring the current and voltage..

For current, I plan to use a wirewound resistor with a value of 0.1 Ohms (see the differential voltage) across it, with my ADC.

For Voltage, Im simply going to feed it to my ADC, btw In using the on-board ADC of the ATmega8 , its an AVR microcontroller..

**duffy** · 6th March 2009 01:10 PM

Originally Posted by awright

ummm, duffy, you might want to be a little more diplomatic about using terms like, "dumb."

...it is usually done simply using a variable resistor with suitable power rating and resistance range. There is some unique combination of voltage and current out of the panel for any given set of conditions...

Get a reference voltage off a fixed resistor and get the loading off a lookup table. Swapping power resistors around in a production unit is [insert diplomatic term here].

Thank you for "insolation".

**umer27** · 6th March 2009 01:13 PM

you talking to me .. what about my idea ?

PS ROFLMAO !

**duffy** · 6th March 2009 01:42 PM

Sounds like you're on the right track. I wouldn't bother with an instrumentation amp, just an opamp, maybe an LM6132, which is fast and can do near-ground sensing. You would set the gain based on 5V/max volts across resistor, which is your maximum solar-cell current times the .1Ω resistor.

**Mr RB** · 17th April 2009 08:25 AM

Hi Umer27, I read some of your other thread too.

I designed a commercial high-efficiency switchmode solar MPPT a few years back, and if you really want to deliver maximum power into a 12v battery the ONLY factor you need to optimise is the CURRENT delivered into the battery itself. So you need to think of it as a "maximum current point tracker". This in effect gives you the greatest power into the battery itself, no need to complicate things trying to measure or control "power".

Of course if you want to measure voltage too, then your AVR can do a multiply calc and show "power" on the display to entertain the user.

Re the mechanics, in your other post you mentioned trying to make it as efficient as possible, I would suggest doing the sensing optically to determine solar position using 2 light sensors and one lens. Then close-loop the error output through the AVR to control your actuator. Your AVR can maximise efficiency by running the motor and leadscrew infrequently and leaving the entire actuator system in a zero power state for 90+% of the time, leadscrews are good for that because they are resistant to being backdriven by the wind.

For cloud conditions etc the AVR can guess solar posiiton based on previous days and its 24hour clock, an average of where the sun was at that time on previous days.

I think on the PIClist years ago someone had a mechanical tracker system where the PIC actually was able to realign its own 24hour clock based on solar zenith (noon) averaging, so it kept flawless time all year round and used that time to control the mechanical position of the solar panels. One of the benefits was it only needed one light sensor (noon detector) and dirt problems etc of the sensor were much more easily addressed than dual sensors (like when a bird craps on ONE of the 2 sensors).

**smanches** · 17th April 2009 12:47 PM

Originally Posted by Mr RB

if you really want to deliver maximum power into a 12v battery the ONLY factor you need to optimise is the CURRENT delivered into the battery itself. So you need to think of it as a "maximum current point tracker". This in effect gives you the greatest power into the battery itself, no need to complicate things trying to measure or control "power".

Just to clarify, this is the current after the switcher, not before it, right? So is the MPP on the IV curve both before and after the switcher directly proportional? EDIT: Answered my own question. IV curve IS a power curve, so of course they would both be directly proportional.

Before the switcher, both voltage and current fluctuate with sun, so you would have to track both to find the MPP. After the switcher, the voltage is usually being held at a steady level, so you only have to track the current to the load.

That makes it a lot easier.

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