My roll-up solar air heater is putting out about 2200 watts of power, in the form of heated air, in the mid-day October sunshine, 37 degrees north latitude.
But absent some sort of magic, that’s not enough. Not on its own, anyway. To keep my composter working through the winter, I’d have to build an insulated box for it, and pipe the hot air into that box.
I’m not even sure that would work. And even if it did work, I’m not sure it’s worth the effort.
Background: 2200 watts of output.
I compost kitchen scraps, mostly so that I can use the resulting compost in my vegetable garden. I use a little “tumbling” composter, as pictured below.
Source: Amazon.
Despite the fact that this composter is made in Canada, it doesn’t work at all in the winter. As the outside temperature drops, the composting stops. This is a common problem, faced by anyone who composts a small amount of material in a cold climate. The heat of decomposition isn’t enough to keep (say) a 40-gallon tumbling composter warm during the winter.
This year, I’m trying to solve that problem by heating the composter.
As discussed in Post #1863, it’s not worth going to a lot of effort to do this. At best, I’ll generate an extra cubic foot of compost per year. And if I just throw the scraps into the garbage, there’s no great damage done. Fairfax County VA incinerates them (along with all the rest of our household waste) and uses the resulting heat to generate electricity.
In the spirit of doing the least possible, I started by tossing a sheet of clear (ish) plastic over the composter. That helped heat the top plastic surface, but didn’t do much more than that.
I then amped up my game, using my cheap-n-cheesy roll-up solar heater (Post #1858) to blow hot air underneath the plastic. That’s pictured above.
I took a moment (and a thermometer, and an anemometer) to measure the output of that solar heater, around solar 11 AM, end of October, sunny day, 37 degrees north latitude.
I had to check the calculation twice before I believed it.
But, near as I can tell, that’s somewhere in the neighborhood of 50% efficient. (That is, somewhere around half the solar energy impinging on the top of the heater gets blown out the end of the unit in the form of hot air.) Which, based on my quick review of the scholarly literature on solar air heaters (Post #1857), is about ballpark for how well this should work.
In short, that combination of reasonable efficiency, and large size, makes for a pretty good heat output.
If the sun shines.
For the three or four hours a day that this will be in full sunlight, in its current location.
Is that enough heat energy? That’s the next question.
An insulated box, necessary but maybe not sufficient.
Can this heater provide enough energy to keep the composter working in the dead of winter? It’s not clear that I can answer that question without actually trying it out. But I can rule out some simple solutions that won’t work, just by asking what the average 24-hour temperature increase would be, if all the heat from that solar air heater piped to a box containing the composter.
January is the coldest month in my area, with an average temperature of just over freezing. So, the question is, can this indirect solar heater keep the keep that composter at (say) 72F, when the outdoor temperature is (say) 32F, or 40F over the ambient temperature?
A quick-and-dirty calculation, ignoring all kinds of practicalities, assuming a completely best-case 100% efficient scenario, shows that this won’t keep the compost warm enough with just a plastic sheet draped over the composter. But it’s not completely implausible that this might work if I surrounded the composter with a box made out of 1″ foam board insulation, and piped the output of the solar air heater to that box.
This ignores the heat absorbed in raising the temperature of the compost itself. But it’s enough to rule out simply piping the hot air under the plastic and hoping for the best.
The upshot is that if I want to go this route, I have to make an insulated enclosure for the compost. It’s not clear this is worth the effort.
I can rule out some other obvious solutions. I’m not going to bring that composter inside, even into an attached, unheated garage, owing to the smell, mess, insects, and so on. And I’m not going to bury kitchen scraps in some large outdoor pile of yard waste, because that attracts rats, among other things, in this area.
Conclusion
This has helped to clarify the situation.
My roll-up solar heater works as expected. But it would have to be paired with an insulated enclosure for the composter, to have any hope of keeping that composter working continuously over the winter. Even with that, there’s no guarantee it will work.
I have plenty of scraps of foam board. It’s just a question of whether I want to go to the trouble of cobbling up an “insulated composter cabinet”, out of those scraps and whatever else is handy, to give this a try.
Or I could just toss my kitchen scraps in the garbage, over the winter, like a normal person.
I’ll have to think about it.
