Newbie In Need Of Solar Panel Advice

[cougarparts4u]

I'm needing a little help in putting together a simple solar panel project.

After much research and trying to figure this out on my own, I'm back to square one. Too many variables in solar panel current...and I'm not an electrical engineer nor much experienced in calculating volts, watts and amps.

What I'm trying to do is match a 12v solar panel with these specs: "4.5 watt, voltage voc 18, current ISC 250ma"---up to a 12v fan, such as a double bearing 120mm computer/server type fan.

My concern is that the fan may "over-rev" on a sunny day and burn out the bearings prematurely. I'm trying to avoid using a battery/circuitry type arrangement if possible, I would much prefer, for obvious reasons, to keep this project simple by using a direct wired fan that can take anything the solar panel could possibly "dish out" current wise...even if the Sun and Moon and Venus are all three shining brightly in a crystal clear sky on June 21st at high noon.

I'm putting this "project" together to do a "special situation" attic venting. I could use a typical 120v fan...but after my next door neighbor's attic caught on fire due to a defective gable vent fan...the idea of possibly slinging sparks all over my attic disturbs me somewhat (even if the odds are a million to one), and I've chosen to go a low voltage solar route instead.

Which brings up another question: What happens if a small 12v fan locks up and the solar panel keeps on pumping power into it? Could it overheat and catch fire? Or is the current too small to create such a situation?

Any suggestions that anyone might have to offer would be greatly appreciated!

Thanks,

MikeD

[Nando]

MIKED:

Your small FAN will not have enough power to really move the air in your attic.

I presume that you want to remove the HOT air in the attic, so one needs to think how fast the air is exchanged.

Air velocity ( extraction rate doubling in volume)will require 8 times the energy like if you have a FAN moving 100 CFM ( Cubic Feet per Minute) and you want to move or extract 200 CFM, the new fan needs to have 8 times the energy of the 100 CFM.

The attic fan fires are very strange and rare, though you report one, this means that the setup was done in a way to save a lot of money and not to properly do it to protect the house and at the same time cool the attic.

I recommend you do the necessary steps to do a good job with a much larger capacity fan, which as well, will require greater energy usage.

Nando

[cougarparts4u]

Hi Nando,

I guess I should have explained my project in a little greater detail. I'm not actually trying to vent an attic, just a 10' long area under the peak of some rafters in an enclosed area. I need to get some air flowing up through the rafters surrounding that area. It would take me most of this text block (plus some photos) to expain this "special situation", and my "off the wall" design solution to the problem, but suffice it to say that a small 12v computer/server type fan of perhaps 30 to 40 or even 50, preferably, cfm would provide all the ventilation required. Basically, I'm just trying to ID a 12v fan that would operate and handle anything that the solar panel could throw at it without being damaged by "over-revving". I may be trying to accomplish something that can't be done without a battery and/or special circuitry to control the current to the fan. I simply don't know enough about solar panel current flow to even take an educated guess at the fan specs required. An electrical engineer I ain't.

MikeD

[henjulfox]

Miked,

I'm not an electrical engineer either. That said - for testing purposes I have used a 24 volt fan run off of 12 volts no problem. If you look around on E-bay, 24 volt fans are more common than you might think.

For the record, I have a little 24 volt fan hooked up to my inverter running 24/7. My battery bank gets as high as 28.8 volts when fully charged and my fan has been going strong for close to a year now.

Good luck,

-Henry

[Don Cackleberrycreations]

There are solar powered attic vents on the market  they have inproved greatly in the last few years. for none powerd venting look into whirly bird turines they do a good job . (hope to use an industrial 36" diameter model for a small wind genny myself)

[cougarparts4u]

Thanks Henry,

You're steering me in the right direction. Sounds like a tough little fan...might be exactly what I need to try.

[cougarparts4u]

Hi Don,

I considered the SP attic vents with some rigged up duct work...unfortunately they busted my "cheap-skate" budget for this project, plus, I don't want to cut a hole in the roof AND the "whirly birds" won't do what I need to do. However, I do appreciate your input.

MikeD

[wdyasq]

You can probably use almost any small computer fan.  Match the panel output close to the fan draw.  A zenner diode could limit voltage if you are worried about it.  You could also run a 12V fan on 9 volts - or 7V or 5V as do the folks worried about noise on computers.

Ron

[Nando]

Go Ahead and use a 12 volts FAN for either 200 or 250 milliamps with the 4.5 watts, 12 volts that may go as high as 18 will not damage the fan and there is no fire danger.

The fan will load the panel to about 11 to 13 volts at maximum solar insolation, also any increase in current will represent a lowering of the panel voltage and any increase of air loading will draw higher current lowering the panel voltage -- in this case you have a self regulating system.

You may need to pay attention it the panel watts are greater than 3 times the fan power watts.

Remember than increasing the RPM will load the FAN to the square of the volume of air drawn and doubling the RPM of the FAN, the energy needed is 8 times higher

Nando

[ghurd]

Nando is 100% right.

If the panel volts get too high, the amps will drop. A lot and fast.

If the fan tries to overspeed, it will need high volts and amps.

The panel would have to make high volts AND amps to overspeed the fan.  A panel can't do both at once.

Match a '12V' (18~21V) panel at peak power, to a 12V fan amp draw of maybe 75%.

Like a 1 amp panel and a 750ma fan. 1/2A panel and 375ma fan. Etc.

All 12V 200ma fans are different. Some move a lot of air at low pressure, others move less air at a higher pressure.  One brand may use 5x the power to move the same air at the same pressure.  The best fan for the job depends on the natural flow in the attic.  If you know what I mean.

Most 12V fans will start at 5~6V, giving a jump on the solar day.

I have had good luck with 9V fans and 12V panels, both at the same max amp rating.

[ghurd]

Nando is 100% right.

If the panel volts get too high, the amps will drop. A lot and fast.

If the fan tries to overspeed, it will need high volts and amps.

The panel would have to make high volts AND amps to overspeed the fan.  A panel can't do both at once.

Match a '12V' (18~21V) panel at peak power, to a 12V fan amp draw of maybe 75%.

Like a 1 amp panel and a 750ma fan. 1/2A panel and 375ma fan. Etc.

All 12V 200ma fans are different. Some move a lot of air at low pressure, others move less air at a higher pressure.  One brand may use 5x the power to move the same air at the same pressure.  The best fan for the job depends on the natural flow in the attic.  If you know what I mean.

Most 12V fans will start at 5~6V, giving a jump on the solar day.

I have had good luck with 9V fans and 12V panels, both at the same max amp rating.

G-

[fungus]

You will not need any circuitry to limit the speed as that is done internally. You may have to limit the voltage. Think of computer power supplies which can supply to 10A of current and the fan is safe on them.