The short answer: a bilge blower fan on 6v.
The first question to ask yourself if you’re considering an off-grid greenhouse, is, should I choose an inflatable?
It’s more work stretching the plastic perfectly tight over a non-inflating greenhouse, but, then you’re done. An inflatable is stronger, and warmer, but, is it worth it?
If you have a robust solar system and can hardwire your inflatable greenhouse into it, great. Otherwise, say if there’s a possibility of having to carry batteries from a charging station to the greenhouse, you may want to choose more work up-front vs. more ongoing work maintaining power to the GH.
We have an adequate solar array, not a generous one, and it is set up too far from the GH to directly wire it or the batteries stationed there into the controller. Therefore, we assumed from the beginning that we’d be carrying batteries. How often was another story.
Choosing an inflated GH off-grid, the first hurdle is the inflator fan. AC fans are readily available, but DC fans are not, and the issue is not readily answered by Google either. That’s why I’m writing this.
I’ll spare you the harrowing hair-pulling details in this quick overview of our journey to get our off-grid GH inflated:
1) Can the squirrel cage blower be detached from the AC motor it came with and be retrofit to a heater fan out of a car? Yes. It depletes a 12v battery in a few hours. Not sustainable.
2) Go see an electric motor specialist. Can a DC motor of appropriate specs be obtained that will run the squirrel cage at the right rate? In theory. It’s $349, and wait, no, it’s out of production.
Feeling very much trapped inside the box, 3) Call Inventor Dad. In 48 hours, he found the right thing. A bilge blower fan from a marine supply. It’s cheap ($25ish), it’s made to run on 12v, it’s the right size, and compact into the bargain. Yay!!!! This one is from Binnacle.com.
Our troubles are not over…
I hooked it all up, plugged it in, it started blowing like it was born to, filling the envelope entirely in about 7 seconds, and then it kept blowing, and blowing. Oh crap! The plastic started to strain and at about 12 seconds I lunged to yank the leads off the battery before it blew. Far too powerful.
4) Try a 6v battery. Perfect. It runs for two days on a charged 6v at exactly the right pressure. Are we done?
Not quite. The 70lb 6v batts that we have are, to put it mildly, no effing fun to carry back and forth from the cabin where our solar panels are mounted to the greenhouse. Put a panel by the greenhouse? A possibility, but there’s nowhere to mount ON the greenhouse, so it would require its own stand.
One last attempt. 5) Aha, I think, a dimmer switch. An AC dimmer switch does not work in a 12v line. DC dimmer switches exist, and are super cheap on eBay. I thought this would be the final answer. 12v batts are no prob to carry, and the dimmer would cut it down to 6v. The dimmer blew up on the first day. Turns out you really can’t load them with a motor.
If this sounds bad and you’re wondering how much hair-pulling I left out, just imagine 100s of trips over months at all hours, in all weather, carrying batteries, and add in periods of despair (while carrying batteries) between each breakthrough.
Especially sucky is that in the winter, when you really need it inflated, there’s no sun to keep the batts charged.
Our reality: Most of the time it is not inflated. That’s because we still have to carry 70lb 6v batteries back and forth, and it just doesn’t need to be inflated 100% of the time. We turn it on for windy and snowy days and nights. I was a nervous Nellie at first about it, but the first winter it saw was one of the worst for snowload ever in the Maritimes, and it handily evaded Greenhouse collapse disorder. I tightened up the plastic much more assiduously than usual for an inflated GH, to quite smooth, and cold, there’s hardly any slack to flap. In the heat of the summer sun, I’ll have to reevaluate how often it needs to be inflated, and perhaps dedicate a panel to it. Then the battery-carrying might be eliminated or limited to the wintertime.
The moral of the story: think hard about inflating vs. not, before you buy.
May 2015- We dedicated a panel to it. Built a simple frame with legs. It rotates manually:) It’s working really well, now that the summer time sun is here – now we just leave GH inflated all the time, as it was intended to be. It’s a bit of a waste for a 120W panel, perhaps, from our home system, but then, maybe it will be just right for the shorter days of winter and be not such a waste.