Mike Jay wrote:Hi Daren, welcome to the party! So I take it you will do two curved 20' pieces of pipe, joining at the peak? I built my curved trusses a bit beefier to handle snow load. I haven't had much snow to test it against but so far I'm very confident it can handle any snow it will encounter. I was also going with beefy trusses to handle the weight of all the insulation, siding and roofing on the north side without bulging out the south side. If I were to build another one of these greenhouses, I'd be very tempted to attempt an A frame with straight trusses. The curves probably added 1/3 to the timeline of the project and 60% to the brain power needed.
With your question about cubic feet of battery to glazing, what kind of battery do you have in mind? Are you thinking of a GHAT or climate battery with air pipes underground? I decided against that type of system due to our cloudy winter climate (My first Permies post asking about climate batteries in cloudy cold places). The systems seem to work well in sunny cold places (Colorado, Nebraska, great plains, etc). This year, I think we had about 15-20 sunny days between Nov 30 and today. A battery system couldn't gain enough on those days to give back heat on the other 50 (in my climate).
There is a Sunny John calculator for climate batteries that has been posted and lost and reposted. I think you can get info on sizing through this link: ecosystems-design.com
Regarding if a greenhouse heats up enough to charge the battery, my greenhouse on a frigid sunny day will heat up to 100+ in the middle of the day. That lasts from about noon till 3pm. So that's three hours of hot air to put into the battery. And 21 hours where that heat is needed to be drawn back out.
I think I'll do a poor man's climate battery by digging shallow trenches in the greenhouse (18" deep) to circulate hot air on sunny days. The main goals of that would be to store that heat when it happens (avoid opening the vents) and maybe heat up the roots of the plants by a degree or two. I wouldn't use it to return warm air to the greenhouse. We'll see if I actually do that...
Regarding the phase change, I haven't decided if I'm really going to do that or not. I think the 6 water barrels I had in there did help moderate the temps until they froze. I think until I know the temperature swing of the greenhouse when it's "done" I won't be able to pick a good phase change material. For instance, if the greenhouse swings from 30 to 70 degrees, a phase change material that melts around 50-60 would probably be ideal. It should melt most days and freeze most nights and hold the temp nicely. Using water as the phase change would only really kick into action at 32 degrees (too late to protect your tomatoes). Water, or any phase change material, or a stack of bricks, all act as thermal mass regardless of the temperature. That just slows down the temperature swing and takes away the peaks. So they're good too. I think between the footings and the cement blocks I have about 20,000 lbs of thermal mass. Add in the top 4" of topsoil and it's a bunch more. So if I add 30 barrels of water it will help and it will distribute the effect, but I don't know if it's worth the space it takes up. So that's a long way of saying, I don't know what you should do.
I managed to avoid the blower on my two layers. A 1.5" spacer keeps them apart except for one spot in the middle. But Solawrap or twinwall polycarbonate would be slicker than my installation.
I'm not sure the perfect answer for your endwall question. I think it depends on your goals for the greenhouse. If you're going to try to keep it warm through the winter at our latitudes, I think they have to be insulated. I'd only get a bit of light (and solar heat) through them for an hour a day. Then I'd bleed heat out through them for 23 hours a day. If you are just going for a much longer growing season, glazing part of them might start to make more sense. Maybe glaze the south half of each and insulate the north half. Another way to think of it is that at 10am the sun is hitting the outside of the E wall and not entering the greenhouse. But the sun that goes into the greenhouse and hits the solid W wall is reflected back towards the plants. If those endwalls were clear, that 10am sun would enter the E wall but the sun going through the greenhouse and hitting the W wall would escape and light up the snow outside. So am I gaining any sun if they were clear? And keep in mind the tremendous heat loss through those walls 24/7.
Regarding the compost heat, I haven't got it figured out yet. I hope to so that we can all heat our stuff in a wonderful way. But I fully understand your concern. My current mix is too slow of a "burn". I'm going to pull some out and change the mix and see if that fixes it for the second half of winter.
If you have enough ventilation for the summer (meaning a lot), then it's hard to have a really well sealed greenhouse. Mine has the equivalent of 13 doors on it. All but one are homemade and sealed with weatherstripping. I'll fix more leaks but it will always breath more than I want. So I wouldn't be too worried about wood heat for cold nights sucking up all the oxygen. Maybe if you're burning a face cord every three days... Plus the plants should be giving off oxygen.
As you get your plans together feel free to start a build thread like this one. You'll get lots of good info and we'll all get to watch your progress