I would appreciate any advice here. I thought a Solar powered fan would be ideal for venting a polytunnel which rapidly heats up when the sun comes out if I haven’t opened both ends up in time.
I bought a cheap kit off Amazon but the result was pathetically underpowered. I then bought a 120 watt 12v car radiator fan and this is about the right power. To run it I connected a 12v 200watt solar panel directly assuming in my nativity that the voltage would be stable.
However after a week of running beautifully the sun came out full on and the motor overheated. Checking the panel output it varies up to 21volts in bright sun so clearly this is the problem.
So my question is, what sort of controller do I need, or do I need a 200watt fan. It’s only a polytunnel so cost is an issue.

 

I have one of these on my greenhouse. In opens and closes the shutter automatically depending on temperature.



I used to have a computer box fan controlled by an ordinary house thermostat. The automatic opener works so well that I stopped using the fan.

 

Hi. Thanks for the advice. I have one of those but on still days it’s not enough. I could install more I suppose but the fan pulls air through the polytunnel and helps with humidity so I would like to continue with that if I can.

 

I would charge a 12 V battery with the solar panel.
Then use the battery to power the fan.

 

I would charge a 12 V battery with the solar panel.
Then use the battery to power the fan.

Then you need some control mechanism to switch the fan on.
And unless it's rather a big battery, something to prevent a 200W panel from overcharging it.
We also don't know quite how sunny it is where the TS lives.
I'd stick with the 120W fan if it works well enough: it will then work when it is partially overcast.
The simplest answer (but it needs a big heatsink) would be a TL431 & power transistor based shunt regulator. A series regulator would need an equally big heatsink.

 

https://www.amazon.com/Converter-Vo...oof-Transformer/dp/B0C66635R1/ref=sr_1_2_sspa
Automotive electrics typically see voltages as high as 14V, and possibly as high as 15V. The link is meant to convert incoming voltage down to 12V. I don't know if it won't like having less than 24V when sun angle is low or sky is overcast. But it's not an expensive experiment.

 

I purchased a 19W 12V solar panel and connected it to a buck converter and dropped voltage out down to 9.2VDC. That was connected to a dog bark deterrent device that was 9V battery powered. When running on batteries, those darn dogs would bark incessantly and I'd be replacing batteries every 10 days. Hence, the solar panel. The whole thing proved successful. During night the dogs could bark without interruption. I figured at night is when dogs should be allowed to bark if there's a reason. But during the days the constant barking was causing problems.

I never measured the voltage during low sun angle or overcast days, I just know the buck converter worked perfectly. The dogs are gone now, so - - - the equipment now sits fallow in the shop.

 

When running on batteries, those darn dogs would bark incessantly

that's the problem with battery-powered dogs.

 

that's the problem with battery-powered dogs.

Oh be quiet! ")

 

You'll need a battery (preferably deep-cycle but car is ok) and a 15Amp solar controller, then it should all be just fine.

 

https://www.amazon.com/Converter-Vo...oof-Transformer/dp/B0C66635R1/ref=sr_1_2_sspa
Automotive electrics typically see voltages as high as 14V, and possibly as high as 15V. The link is meant to convert incoming voltage down to 12V. I don't know if it won't like having less than 24V when sun angle is low or sky is overcast. But it's not an expensive experiment.

I wondered about one of those, but I also wondered what would happen at low light when the load collapses the supply.

 

How about a relay to turn on a 50 watt automotive light bulb in parallel with the fan when the voltage exceeds 14 volts? This would drop the voltage so the circuit would need hysteresis so it only turns off again when the voltage is below say 10 volts. If the idea appeals I could sketch a circuit

 

Just wondering if a filament lamp in series might do the trick. With a low solar output the filament resistance would be low, increasing as the voltage increases. It needs to drop about 8V @ 10A at full power, so about 0.8Ω. Filament resistance varies with the square root of applied voltage, so you need a lamp that would have a filament resistance of about 1.05Ω at 14V, so perhaps a 12V 150W lamp Might be close.

 

Use a cheap ($10) Amazon solar charge controller and an appropriately sized 12V battery with your fan motor connected to the controller's load terminal. The controller will turn on the fan during programmable time periods.

 

I am guessing that the TS does not have a convenient way to run an extension cord with mains power out to this garden area. For a single 12 to 15 inch fan a number 16 cord or even a number 18 wire cord will work. And then just a simple thermostat to switch the fan on when it gets hot. And much cheaper than a 12 volt battery. The challenge might possibly be a lack of an outside outlet on the house. Those are simple to install and wire.