Anotherpower.com Forum
Renewable Energy Questions/Discussion => Solar (heating or electric) => Topic started by: rossw on March 06, 2012, 12:10:22 am
-
This is a subject that was thrown about in IRC a while ago.
There seems to be a lot of confusion, particularly to the newcommers, about what these things mean and how to measure them.
Couple of basic points to start.
Watts. Watts are the basic unit most panels are sold by. "100 watts" or "215 watts" or whatever.
From there on, it gets just more and more confusing...
So... lets look at a typical panel.
[attachimg=1]
Maximum power: 100 watts.
Open circuit voltage: 22V Short circuit current: 6.14A.
Anyone knows "Power = volts times amps" - so 22V * 6.14A = 135W. Huh??
The answer is, that the "Open circuit" voltage is with no load. As you apply load, the voltage will fall.
And the "Short circuit current" of 6.14A is the maximum current the panel can deliver into a dead short.
It can't provide any POWER in that circumstance of course, because a load resistance of 0 ohms means no work done!
The "maximum power point" is the sweet-spot where the voltage hasn't dropped too far, AND the current is as high as we can manage while delivering a reasonable voltage. The product of the two produces the "maximum usable power". And for this module, that's about 17.5 volts and 5.72 amps (100 watts).
If you were to take a large variable resistor and an amp meter in series and connect them across your solar module under perfect illumination (square on to full sun), you would expect to be able to vary the current from not much (depends on the maximum resistance), up to a little over 6 amps.
If you also connected a voltmeter across the panel while you did this, you would see the voltage change from almost zero (when you measure maximum amps), up to close to 22V.
Simply measuring open circuit volts is almost meaningless.
Measuring maximum (short circuit) current can tell you a number of things about your panels however. If you only get a few milliamps from a large panel like this (but you DO get full open circuit voltage) almost certainly means a cracked, broken or burned conductor in the panel, or a high-resistance connection.
-
This pertains to standard solar controllers.
For real life guesstimates, I use:
About 10% less than Isc for peak amps into the battery.
However, if the Imp is higher than that, I use Imp.
The actual peak charging amps into a battery is higher than Imp, even if the user is not there to see it when it happens.
For the panel shown, my real life guesstimate would be 5.72A.
The battery would be between 11.9V and 14.4V usually, under standard conditions.
The actual watts of output would be between 68W (11.9V x 5.72A), and 82W (14.4V x 5.72W).
Under good conditions, it is not uncommon to see more charging amps than Imp.
Even if the panel was making Isc, into a 11.9V (0% charge remaining), the output would only be 73W.
(11.9V x 6.14A = 73W)
G-
(PS- "Operatine"?) :D
-
This pertains to standard solar controllers.
For real life guesstimates, I use:
About 10% less than Isc for peak amps into the battery.
Hard to say. I agree the quoted figures are often "difficult to believe". However these ones I have, I *REGULARLY* see stated power. I will admit, I'm using MPPT charge controller, 6 modules in series for each array and nominally 100V DC from the arrays, to batteries at (nominally) 48V (around 50-53V charging)
Under good conditions, it is not uncommon to see more charging amps than Imp.
That would usually be at lower voltages, where current may be more than Imp, but volts are less than Vmp and power into batteries is below rated watts.
Even if the panel was making Isc, into a 11.9V (0% charge remaining), the output would only be 73W.
(11.9V x 6.14A = 73W)
Yes, which is why I've said time and again, MPPT makes a lot of sense, because you can't ever get anything like full panel output into batteries without it some sort of voltage stepdown.
(PS- "Operatine"?) :D
Chinglish at its finest :)
-
Just to prove the point: it's currently 13:40. Daylight savings is in force, so it's just after solar noon here.
My batteries are in a reasonable state of charge. Controller had dropped back to "Absorb" mode.
I isolated all except 2 arrays, so I could make those two deliver the most power they could, and took the following two photographs for you.
Here, two arrays, 6 of the above photographed modules each, in series.
(http://house.albury.net.au/07mar2012/100_5277.JPG)
And here is the charge controller display:
(http://house.albury.net.au/07mar2012/100_5278.JPG)
That was taken with zero setup, or waiting for any special event - just walked in, flicked 4 breakers off, snap, snap, flicked the breakers back on and came here to post it.
Edit: typo. Also not sure why the CC shows 14.2A when the analog meters show about 6.5A ea.
Analog meters are not particularly accurate, but should be within 5% I'd have guessed. I might go check with clamp meter..
-
Well, the analog meters may be showing off...
sorry for the shaky-cam pics. Trying to get stable readings with gusty wind making the charge controller keep backing off as battery volts change, is tricky!
I took a picture of the CC screen and hit HOLD on the clamp meter at the same time.
(http://house.albury.net.au/07mar2012/100_5284.JPG)
(http://house.albury.net.au/07mar2012/100_5285.JPG)
-
We are on the same page.
(when I said "This pertains to standard solar controllers", I meant PWM)
The vast majority of my emails related to solar are for arrays under 250W. For those people, the money for a MPPT controller would almost always be better off spent on more PV watts and a nice less expensive PWM controller, at least here in the US.
When it get up to 1KW, like yours, MPPT at todays pricing is a no brainer.
Do you have any idea why the meters are reading so much different?
Maybe try loosing, wiggling, retightening the shunt terminals? You knew that.
G-
-
I have a 720 watt array now, not yet setup, but at this point I only have a standard Xantrex C-60 charge controller. Its that or some home brew setup.. I think for now the charge controller is the easiest for me to get this system doing something. Maybe in the future I will get a MPPT, as I get more panels I will be able to move these to a better solar location. With a MPPT I can raise the voltage so I would have less losses in the longer lines needed because of higher voltages.
This is another good reason for, and what my main interest for a MPPT is, not eeking out a couple percent boost of output at this point
-
Do you have any idea why the meters are reading so much different?
Maybe try loosing, wiggling, retightening the shunt terminals? You knew that.
No, I'm puzzled. I *THOUGHT* they were pretty much on the money.
They have internal shunts, not much I can do wrong!
(http://house.albury.net.au/23may2010/100_4448.JPG)
-
I'm puzzled. I *THOUGHT* they were pretty much on the money.
Even more puzzled now.
(http://house.albury.net.au/07mar2012/100_5286.JPG)
(http://house.albury.net.au/07mar2012/100_5287.JPG)
Can only surmise that conditions had changed in the few seconds between the first and second photos.
It's quite possible - wind has been quite gusty, it's making the charge controllers job more difficult for sure.
-
... wind has been quite gusty, it's making the charge controllers job more difficult for sure.
Ok, I'm confused. I thought this was solar.
How does the wind affect the output?
Kevin
-
... wind has been quite gusty, it's making the charge controllers job more difficult for sure.
Ok, I'm confused. I thought this was solar.
How does the wind affect the output?
Because I, like many off-grid dwellers, have a hybrid system.
My PV goes through one charge controller onto the batteries.
My turbine goes through another controller also onto the batteries.
My propane genset goes through a third controller (actually my main inverter), to the same batteries.
If there is input from more than one source, they tend to interfere with each other.
Today at time my turbine was putting its full output into my batteries, which were already virtually fully charged, so the solar controller was backing off what it was trying to put in.
Make sense now?
-
Because I, like many off-grid dwellers, have a hybrid system.
Ah, OK. Got it now.
Thanks Ross.
Kevin
-
I have the same affliction as Ross, multiple controllers ( different manufacturers) + mini wind. (700W Solar (soon to be 900W)).
Since this was a growing system, I have a smaller controller for some panels, and a larger one for the bigger array).
Bad news is the absorption set points don't match up (actually float, etc don't match and the little guy is not adjustable)... so my little guy shuts down as my larger one rises to a higher set point.
Really not a big deal for me as my batteries are usually fully charged, and if not it will take more from the smaller set until the larger set takes over and basically shut down the smaller one.
Rover
-
I have the same affliction as Ross, multiple controllers ( different manufacturers) + mini wind. (700W Solar (soon to be 900W)).
Since this was a growing system, I have a smaller controller for some panels, and a larger one for the bigger array).
Bad news is the absorption set points don't match up (actually float, etc don't match and the little guy is not adjustable)... so my little guy shuts down as my larger one rises to a higher set point.
Really not a big deal for me as my batteries are usually fully charged, and if not it will take more from the smaller set until the larger set takes over and basically shut down the smaller one.
Rover
Rover,
You know it, but it is a good topic to touch on...
Not a problem usually, since the Smaller array shuts down when the Larger one is close to tapering down the amps.
When the smaller array shuts down, the larger array keeps going or increases the current.
The end result is about the same.
"A man with 2 watches never knows the correct time.
A man with 1 watch always knows the correct time"
(or something like that...)
The simplest things can mess up the calculations.
G-
-
The simplest things can mess up the calculations.
Or, in my case - the photographs :)
-
The simplest things can mess up the calculations.
Or, in my case - the photographs :)
Or in my case - the pencil on the paper.
I can not write as fast as a camera!
I think a lot of 'published' calculations are faulty due to when and how the original numbers were obtained and recorded.
G-
-
I used to write test and calibration procedures and I can tell you "how" it is done matters. Everything from connections to equipment settings should be laid out to ensure accuracy and repeatability.
Long ago in another life when the details mattered. Nobody wants the autopilot or glide slope receiver to fail on an airliner because a master oscillator is out a percent or two from design specification due to a shift in capacitance from a test probe. An unusually picky case but illustrates the point.
Basically "how you do it" matters.
I digress.
Tom
-
Hi Readers,
back to the topic of testing PV.
I have dabbled with PV for many years. Since early mono days using ARCO panels made in WA, Solarex poly cells and panels in the 1980's and even Green cells from the one and only. Have not had the funds to go the GaAs, or hybrid cell route yet.
Solar panel performance data is only as good as the incident solar radiation measuring equipment used to compare panel output with.
I made a solar radiation [pyranometer] sensor from a single silicon solar cell. I was given some space grade cells [75mm round], but any silicon solar cell will do, with suitable current output. I connected an Alegro Hall effect current sensor, directly across the cell, with the shortest leads. This measures cell current, that happens to be directly proportional to incident solar radiation. Temperature effects are minimal, as this effects the cell voltage, and in this instance this is close to "0V". I used a current sensor with a 0-5A range, and this worked well with my solar cell that produced 3A at 1050W/m^2 solar radiation. This was calibrated against my Davis weather station incident solar radiation sensor, and gives better than 1% correlation.
Visual human interpretation of how much solar is available is a guess at best. Probably have more luck winning Lotto.
It is surprising the effects of high cloud, [glare] and haze have on solar energy harvest, compared to cloud free conditions.
I have incident solar records going back years, logged at 10s intervals at my location. I have test records for many comparison solar systems including Amorphous panels, Poly-Crystalline, Mono-Crystalline [Std] and Sanyo [HIP] and mono-Crystalline [Sunpower buried conductor] types. I have found that most panels produce their rated power. Deficiencies occur usually with MPPT equipment connected to them. In certain weather conditions I have measured up to 20% difference in energy harvest from the same panels in the same orientation, but with different inverters.
It is very difficult to measure objectively and conclusively say one particular system is better than another, or a system is not performing properly, based on spot measurements.
I have spent a lot of time identifying the interactions of weather and MPPT equipment on solar panel energy yields. I am no expert yet.
I have to conclude that even from an informed position, removing the confusion is difficult.
The one thing that has happened over the years is that solar panels are far cheaper now than even 6 years ago. The low price makes the confusion dated. When I first looked at home solar, I had to really work the numbers to justify a system. The honeymoon period has passed with free systems, but systems are still about 1/10 the price I paid in 2006, for a 1.5kW installed. I will not go off grid, but continue with a hybrid system. I have made the grid my friend, and look forward to the checks they pay me.
At the end of the day, as long as the panel voltage is reasonably matched to the batteries [inverter], and the regulator can handle the maximum power, and the load demands are met, then you are most of the way there. The OFF grid systems I have seen that have the most problems are related to insufficient, battery capacity and recharging sources, to meet the demands of the loading. Typically battery loss of capacity, due to prolonged periods of low SOC. It really takes a typical lead acid battery many more hours than are left in a solar day to fully replace the energy that is drawn out. This is especially noticeable if the battery has been drawn down to the 50%SOC level. The best system I have monitored has 1300Ah @ 48Vnom battery, with 7kW of solar RE, and a 2.5kW windmill, with an auto start Genset, supplying a house with 10-14kWh daily loading. There are times during the year when the genset is called due to low battery SOC [<50%]. When there is plenty of solar, the battery is effectively charged by 10AM, and the bulk of the day is holding the battery at float voltage. You really can't beat a surplus of RE, in reducing the worry of maintaining an OFF-grid system.
Goood luck deConfusing.
Gordon.
-
Gordon;
Nicely put. Should help clear some confusion.
I had not even thought about the inability of the human eye to gage solar values. I "knew" it but neglected it as a testing factor. :o I like your sensing & evaluation rig.
Thanks.
Tom
-
Hi Tom,
Attached here is some test graphs of solar energy yesterday. The power is normalized to give kW per kW of incident solar, for each system. This is a feature I have added to my CC128 Professor program.
Not sure of how to show an image in a post.
[attachimg=1]
I think this works.
The white trace is the plot of the pyranometer. This is orientated horizontally. The other traces are as per the colour codes on the LHS check boxes.
The green trace is from a west orientated system, 5.24kWh/kW. The red is of an east/south orientated system, 5.36. The blue and orange is north orientated, 6.0kWh/kW.
The fattening of the output graphs, compared to the incident solar, is due to the fixed panel angles relative to the sun during the day.
The spikey component near the middle of the graphs is an effect of cloud.
This will add to the confusion.
Gordon.