Solar panel direct to LED's (no battery) question

[scooterhead9996]

Hi all,
Long time lurker, first time poster. I started with building halogen bike lights - MR16's with SLA batteries, now progressed to 3 watt LED's with AA's all in PVC plumbing fittings, and a couple of 3 watters in the house off a battery. I'm in complete awe of the lights built by members around here, the ones in homemade aluminium look awesome !
This leads to my question, a magazine i got here in Australia talks about running 5 x 3 watt led's in series directly from a 10 watt solar panel with no battery, so basically only working when the sun is shining. That appeals to me for one room in our house that could do with more light during the day. The premise seems to be that the panel will run around 16.5 volts and the led's at 5 x 3.2 volts. To my mind that means they won't run at full power but i'm ok with that, my question is, is it really that simple ? And would a 10 watt panel really produce enough to light all the leds in full sun ? A 10watt panel seems to push 500-600ma , is that enough spread over that many leds ? I'm quite happy to admit to not fully understanding the relationship of current and forward voltage on a led :green: Using drivers on the lights i've built has helped me avoid that theory !
thanks and any thoughts welcome.
Steve.

 

[RobertK]

Hi Steve.

I'll assume that you'd use a Cree XR-E, just for assumed characteristics.

You can operate an XR-E at an amperage greater than 700ma if it is over 5000k, such as cool white. If you drove the LEDs at the 500-600ma you say it generates, you're right at the 2.4W mark; each LED would put out ~150lm. You could probably string 4 LEDs in series without much (if any) trouble.

(From what I understand, spare voltage isn't the issue spare amperage is.)

So as far as I know, and I'm sure someone else can speak more definitively than I, you could comfortably run 4 LEDs during the day.

 

[Dave_H]

As long as the panel cannot source enough current to overdrive the
LEDs you might get away with direct drive. You probably won't get
full current through 5 LEDs as I suspect the panel output voltage
will drop below 16.5v at full load even in bright light. It also depends
on the level of sunlight; there will be a set of I-V curves varying
with illumination. Best to check these if you have them.

The current through the LEDs will be the intersection of the I-V
curve for the LEDs with the I-V curve for the solar panel at the
particular light level.

If your panel position is fixed (solar tracking is nice but more
complex/expensive), the 10W being peak power, if not mistaken
it will vary over the day sinusoidally, proportional to the angle
of incidence. Therefore available light from the LEDs will follow.

A linear driver with 5 (or 4) LEDs may not work too well due
to voltage overhead. I'm not too familiar with boost-buck drivers
whether a good idea and how they'd react in a case like this.

Dave

 

[scooterhead9996]

Thanks for the replies, I think I'll give it a try, a boat shop over here has a special on 10 watt panels at the moment, I was looking to get 2 anyway and a bigger battery to upgrade my very small solar power setup at home which is a very small panel of a couple of watts and a 7.2a/h SLA driving 2 x 3watt crees and 2 x 1 watt crees. I also was going to get some more led's for more bike lights so might just string them together and give it a try , I really like the idea of no battery and just creating a bit of extra light from a free source to hopefully reduce the amount of time we have the lights on in the kitchen, you need lights on during the day to do anything in there. Having said that, we have always had a lot of blackouts where we live but i reckon it's been easily once every week since Xmas so maybe some batteries might just be the go.
Steve

 

[hank]

If that doesn't work out, assuming it's your own house, you can use brute force:
http://www.google.com/search?q=solatube

 

[scooterhead9996]

Thanks Hank, unfortunately above the Kitchen is an upstairs bedroom, so can't just bore a couple of nice big holes through for a solatube ! They look so good in the diagram, just up and through the roof, but not at my place
The other problem even if i did hide it away is that the kitchen is on the southern side of the house and has a 45 degree roof running east west between it and the sun ! I'd need one really long tube to catch direct sun, i guess it would still let some light in though but I don't think the home boss would give approval for the drilling !!
Steve

 

[Illum]

A DC-DC LED driver like Luxdrives Buck/Boost pucks are decent selections, they have sufficient voltage input overhead to take on 12V solar arrays, [5-30VDC input]. Might want to toss in a filter cap on the input side to cut down on ripple.

 

[Dave_H]

A DC-DC LED driver like Luxdrives Buck/Boost pucks are decent selections, they have sufficient voltage input overhead to take on 12V solar arrays, [5-30VDC input]. Might want to toss in a filter cap on the input side to cut down on ripple.

True, this would give flexibility on the number of LEDs in series
but at a money and efficiency cost. Just wondering though how
these things behave when they become current-starved i.e. the
source cannot supply enough current to drive the load at its
constant-current output; does it gracefully degrade, cut off,
go nuts or...? Anyone tried this?

Dave

 

[Ken_McE]

I'm thinking you'll need to run a wire from the panels to the lights. If you're running a line anyhow you could just run some fiber optic lines. They'd have a mirrored dish around each collector and a diffuser around each emiter. Main advantage is that it would have no maintenance, couldn't wear out.

 

[LukeA]

I'll agree with some posters above, the first thing I thought when I read the OP was "skylight".

A 10W solar panel takes in 50W+ of visible solar radiation, converts it into 10W of electric power, sends that power to some LEDs which convert about 4W (best case) of that power back into visible light. Kind of a losing proposition compared to just rerouting the light.

 

[scooterhead9996]

Just for anyone reading through this thread, i ended up with 2 x 10 watt panels with a charge controller into a big Four wheel drive car battery, runs my 2 kitchen 3 watters for hours and hours, even started charging my AA's and AAA's by building a cord for my battery charger that used a plug pack down to 12 volts Thanks all posters !

 

[rodfran]

You could also run a d.c. fan for summer cooling.

 

[kethd]

Some general-theory thoughts for anyone stumbling on this thread and curious about trying this kind of thing:

The idea of directly connecting LED lighting to solar panels, with no batteries, is crazy-intriguing...

There aren't many situations where you only want LEDs to light up when the sun is bright. Even if you only want light when there is sun, any physical way (skylights, mirrors, etc) would potentially get more light, cheaper, etc.

But let's consider just the technical aspects. This could be a great marriage, with properly matched parts. LEDs are odd loads, preferring a constant-current source. Solar cells are odd sources, tending to be constant current (proportional to the sunlight) when they are loaded.

To make this work, start with over-rated (over-adequate current) LEDs (like a nice BridgeLux BXRA-C0360 400 Lumen 350ma 13V 4W from Digi-Key for $5) and give it an adequate heatsink. Get an under-rated (inadequate current) set of solar cells, put them in series to put out too much open-circuit voltage (say 15-20V in this case). Make sure that the short circuit current in full sunlight is well within what the LED can handle - in this case, say about 300mA at the most. Less would be fine for first experiments.

In theory, you should be able to just wire these directly together and everything will work fine; the LEDs will light up in fair proportion to the sunlight. (Careful testers would start with some high series resistance first, and maybe a 1N400x rectifier to protect the LED from reverse voltage wiring mistakes.)

If you get over-rated solar cells with just barely adequate output voltage, the system will also work, but differently -- the LED light level will be more constant (independent of the sunlight level), but less stable with LED temperature etc. It would be safer to put in some series resistance or more fancy current limiting. Not such an interesting project.

The Wikipedia article on solar cells is not very helpful in understanding this area. The web page at http://chuck-wright.com/projects/pv-measure.html has a very relevant discussion with an Appendix A: The I-V Characteristic of PV cells that has good graphs for explaining the basics.