solar cells

[DrPizza]

Anyone know of some good online sources for information using solar cells? The tech department at my school is experimenting with a single module. (supposed to be about 50 watts at peak performance.)

I couldn't find the manufacturer listed on the module, but connected to a large (fire truck size) 12 volt battery, I was reading a voltage of about 9 to 9.5 volts. Open circuit voltage is about 10 volts, on a somewhat cloudy day.

I thought (and this is my big question) - isn't the voltage supposed to be roughly constant & only the current vary significantly depending on light intensity? The markings on the module:
Vmax 16.7V
Imax 3.0A
Vopen circuit 21.6V
Ishort circuit 3.35A
(someone else copied these down and thought he saw "BP", wondering when British Petroleum started manufacturing solar modules. I'm suspecting it was possibly SP (SunPower)??)

There's a voltage regulator between the module and the battery - specs say to keep the battery from over-charging & from draining too far; exactly what I expected it to do & knew should be present. But, the manual for the voltage regulator says nothing about changing the voltage; in fact, input and output voltage are supposed to be within 1/2 or 1/4 volt of each other (some fraction of a volt.)

So, how the heck is it supposed to charge the battery if the solar module output is less than 12 volts? I'm confuzzled & have very little experience with solar power but have now found myself in the position of "okay, your turn to figure this all out" - a position I generally succeed in. But, I see a quick learning curve in the next couple of days.

Still though - with a commercial voltage regulator that actually has a small pretty small voltage drop across it, it seems that the solar module should have its output vary by only 1 or 2 volts at most; just the current varying by more. With dozens of voltage readings over the past couple weeks, they haven't seen a voltage above 9.5 volts. (and charging currents - the battery is quite low - of less than a half amp.) Otherwise, the solar module would be completely useless at anything less than 12-13 volts.

Should I suspect something is wrong with the solar module?

Very little clue here.

 

[CycloWizard]

The first thing that comes to mind is that, with such a large battery, it's placing a huge load on the module. Have you measured the actual charging current? It seems that the voltage could drop if the battery is drawing near the peak current, even with a voltage regulator. The regulator is likely designed to prevent an overvoltage at low loads rather than undervoltage at high loads. I would recommend trying it on a smaller battery that is nearly fully charged just to check if it's working properly.

 

[William Gaatjes]

I remember from memory if i am not mistaken that a solar cell under it's maximum power output functions as a voltage source. After you reach it's maximum power output and cross it, it start to functions as a current source. Meaning the voltage will drop when the current gets to high.
This all assuming optimum sunlight conditions. The maximum power is the bulge in the graph of the solar cell.

 

[DrPizza]

That's one of the things I considered. Hence, I measured the voltage with an open circuit (disconnected either the positive or negative from the battery) and got around 10 volts.

I slapped an ammeter rated to 5A into the charging circuit - afaik, it has yet to exceed 0.5 amps. Where as, shorting across the positive and negative leads from the module gave about 3.0 Amps.

 

[William Gaatjes]

Are you measuring direct at the solar cell or behind the regulator ?

I am guessing here, it makes me think that the solar panel needs a parabolic mirror or something to reach peak power. How much is the voltage directly at the solar cell ? That should give some clues. maybe it is indeed defect...

 

[herm0016]

bp does build solar panels and cells, i see them all the time on their wells, the flow control equipment and the flow meters are all run off of a solar panel and battery setup, along with things like methonol drips to keep the pipes from freezing in the winter.

 

[DrPizza]

Yep, double checked - it actually is BP. And, while not right at the solar cell, I measured the voltage with it disconnected from the voltage regulator. The line from the solar module to the regulator is 10 gauge copper; about 12 feet, so I doubt there's any significant drop there.

 

[canis]

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[canis]

..

 

[DrPizza]

I guess I could have used the word panel rather than module, sorry. Wild guess, 15" by 30"; one independent manufactured hunk of stuff that mounts on the roof, and has a rated output of 50 watts. One that independently is supposed to be able to charge 12 volt batteries. It doesn't.

 

[Cogman]

Solar cells preform much like a battery will. At low power, the voltage of a battery will drop, at high power the voltage stays constant and the current will vary depending on the attached circuit.

Unfortunately, solar cells are more picky about the sunlight and their voltage output. Luckly, you aren't totally screwed. You can make it perform more like you want it to, for a fairly low price . http://en.wikipedia.org/wiki/Boost_converter These are really pretty cheap (the problem of wrong voltage is a common one in robotics, with 1.5V batteries and 5.0v required input).

Get one of those (or build it yourself if you are feeling daring enough), and it should maintain a constant 12 V for you.

 

[DrPizza]

 

[DrPizza]

 

[DrPizza]

Yep, I figured I could purchase or build a booster. However, the current output peaks at less than 5 watts. Pretty useless.

 

[William Gaatjes]

I think linear technology is your friend...

http://parametric.linear.com/html/e...4][]=20||*&c[1035][]=18||540&c[1107][]=4||120


EDIT :

Forgot to ask, how old is it ? Is the solar panel been used for a long time ? I have heared that some solar panels degrade pretty fast...
Another option can be that 1 or both of the wires has been corroded at some specific location, that would possibly depending on the situation increase the ohmic resistance. But it would not explain the 10 volt output voltage measuring with a multimeter which has an high input resistance of at least 1MΩ.
Interesting...

 

[DrPizza]

The age of the panel (module) is approximately 8 years. From what I've found, it's common that panels are warranted for 90% rated power for 10 years, then 80% until year 20.

 

[William Gaatjes]

I guess this solar panel wants to retire young...

 

[Rubycon]

That's a pretty small panel for 50 watts.

It could also be damaged if you're not reading 20VDC or higher under full sun with only the meter connected.

 

[canis]

Is the second picture the back of the module? Is 30"x15" a guess or measured? Solar cells have reduced short circuit current when aged. The open circuit voltage does not significantly change.

 

[canis]

..

 

[DrPizza]

Is the second picture the back of the module? Is 30"x15" a guess or measured? Solar cells have reduced short circuit current when aged. The open circuit voltage does not significantly change.

Yes, it's the back. Yes, it's a "guess". However, my hand is there & I know the dimensions of my hand. If it's not 30"x15", it's damn close.

 

[canis]

A bp 50watt multicrystalline module available now measures 33"x21.1"x1.97". What is the last letter in the model number? I can't make it out from the picture.

 

[DrPizza]

Not a great picture, but I believe it was E
I struck out googling it earlier today.

 

[silverpig]

Voltages and currents can both vary a lot, and if you're only testing on a cloudy day, you'll get very different results. I did some testing with a single PV cell once under various light sources. Here are some of my results (dug the old ones up).

First, it should be noted that the power output is a product of open-circuit voltage, short-circuit current, and the fill factor.

Pwr = OCV * SCC * FF

Green diode laser:
OCV: 0.54V
SCC: 6.9A
FF: 0.38

Halogen Lamp:
OCV: 0.57V
SCC: 2.15A
FF: 0.8

Xenon-Arc Spotlight:
OCV: 0.57V
SCC: 3.6A
FF: 0.8

Sunlight:
OCV: 0.57V
SCC: 3.5A
FF: 0.8

It should be noted that the peak power I got out of one of these cells (4"x4" monocrystalline silicon) was 1.9W. After it heated up (60W lasers get things hot ), the thermal equilibrium power output was about 1.5W per cell.

It looks like your panel has 36 cells. 36*1.5 = 54W, although my cells were 4"x4" monocrystalline, and yours look like polycrystalline (less efficient). I can't quite tell the size, but yours do look like 4"x4" too. Seems plausible it could be a 50W panel, although that's under very ideal conditions.

One thing to consider is that while the cells generally last a really long time, the leads do corrode. Also, ensure that no part of the panel is shaded. Depending on how the thing is wired, when you shade a cell it not only doesn't produce power, but it acts as a diode put in backwards so it basically cancels out one other cell.

Try putting it out in the sun and connect it to the meter again. Then take your hand and shade one cell at a time to see if it affects the power levels. It should have a fairly dramatic effect unless BP went all expensive and put bypass diodes on every single cell.

 

[silverpig]

Ah, re-reading it seems like one of your chains is not producing any power.

It looks like each row of 9 cells is wired in series. Further, rows are then wired in series so that every cell is wired in a big long series circuit. See if you can find a wiring diagram on the panel somewhere to confirm this.

Thus, with each cell contributing about 0.5V, each row should produce 4.5V. 4 such rows should get you to ~19V (with some variance due to load etc).

Now, when you have a cell in series with others and you shade it, or corrode its leads somewhat, it not only stops producing power, but it also cancels another cell out as it acts like a diode (see my last post). It looks like you have some wiring issues or something.

Try the single-cell shading thingy I suggested and see if you can identify which cells are producing power and which cells aren't. Hopefully it should be fixable with a soldering iron.