to the Solar CollectorBrainstorming and Development
Water Project 1
Water & Space Heating
Hi Mattie and everyone, In answering those question I'll go ahead and provide some further site info and construction details. Bear with me if I seem to ramble around before i get to the point. In reference to insulation on the top it is almost always true that the glazing is the weakest thermal point in a greenhouse. It certainly should be. There's only so much one can do to slow the heat loss at nighttime. I can look it up, but I think the 10mm twinwall polycarbonate I'm using for glazing is about R-3 or so. It cost ~350$ just for the 3 panels on the S side. The N and W-walls will be insulated as well as possible under the plywood and galvanized siding. I just looked at a chart that showed cardboard about as good as fiberglass or denim batting per inch...haven't settled on insulation yet but need to decide soon. I did put up some R-13ish polyiso board on the West wall already, and just picked up a big roll of reflectix bubblewrap sheeting for 8$ at the ReSource yard. A pallet planter filled with moist soil affixed to the interior W. wall will absorb a lot of BTU's during the day. That spot gets some of the best exposure year round. When it radiates it back out at night it will help moderate the temperature drop out in the central planting area. I'll have to find something heat tolerant to plant in it for the summertime when it's still is going to see a lot of sun. Maybe some Rosemary could make a go of it. The water mass above ground on the E. side will perform similar function. The old windows are decent, but nothing like the insulated wall on the West. The triangle shaped glazing around the sides of the windows actually consists of two layers of the twinwall polycarbonate seperated by a 2x4 width (actually 3.5" i think). So those are much better insulating than the old windows are, but do transmit less light. The 330gal Water cube can hold ~2750lbs of water though so its a MUCH more significant mass. When I installed the cube on its used fencepost supports before winter I dug a pit a couple feet down below it and filled it with donkey manure and worms. Instead of worms though it could be filled with a "hot" compost pile with plenty of nitrogen to add some heat to the equation in October/November just as winter starts to show. The pathways under those boards are similarly trenches filled up with compost, and were it done purposefully would radiate heat effectively to the surrounding grow beds.Regarding insulation of the heat mass, Yes, The hope is also definitely to benefit from the sheltering effect of the earth that closely surrounds the greenhouse. The North sloping sight was leveled only just enough to fit the structure in. Thus the S & W sides have significant earth sheltering for the underground heat mass. On the N & E walls the exterior grade is much lower due to the original slope of the site. I placed an additional 4" layer of earth that surrounds the insulation boards on these sides. It is held in place with some scraps of plywood, metal roofing, and a couple used siding panels that were donated by my neighbor. I'll put a couple pictures up that show some the earlier construction and the extra buffer of dirt surrounding the insulated heat mass. The buffer is actually exposed inside the structure right now, but I'll cap it well with insulation when I do the wall, which itself will conceal the buffer layer. If I left it exposed to the interior it would provide a thermal bridge for the heat to circumvent the insulation and be transferred more easily out the N sidewall. Frost line lies somewhere in the 3-4ft range around here depending on microclimate and how fast you hit bedrock. I'm expecting the greenhouse itself to create a significant "thermal island" and the frostline of the directly adjacent soil to be affected such that the 4ft. insulation boards will reach easily below that line. I don't know if it will be enough to tap into the average 50* temp of deep soil or not, I think you'd have to be deeper.Overall the system is designed to build up and carry a big load of thermal energy into winter, and slow its loss as much as possible. It is as well insulated as possible at every turn. In order to maintain the underground storehouse of heat as long as possible the idea is to only use the fan to draw heat out of the ground during cold nights just enough to keep certain parameters. Allowing the heat that simply radiates up from the mass to suffice whenever possible. For example keeping the air temp above 32* shouldn't require active transfer until the temps really drop outside. Keeping active draw to a minimum should be able to maintain its thermal inertia fairly well, at least I hope. The temperature tolerance required for dwarf trees and other perennials is two fold. Generally plants can take colder air temps for a short period if their roots do not get too cold. For many trees and "mediterranean" climate perennials, like figs and pomegranates, as long as the roots stay around 25* they can handle temp dips down to the teens while in a state of dormancy for the winter. This means the temp of the heat mass (soil/roots) is as important to consider as the air temp. Probably more so. I think this greenhouse will at least be able to keep the heat mass up to 25* without trouble. If i can get that hot air panel mounted on the roof securely, I have no doubt it will at least be suitable for figs and other "mediterranean" plants. I've found nurseries claiming citrus and even avocado varieties that can survive down to 20*F or so. An avocado tree would be amazing. Might even be room for that and a citrus. they'll need regular pruning and maintenance to keep under the cieling of just ~7ft or so, but I think it may be workable.Don't plan to plant any expensive trees until its performance has been evaluated for a winter or two, but may have a go with a pomegranate or a fig right out of the gate. I haven't decided if the little fig I got from Jerome will go in the ground out here or just in a big container on the back porch, where it would definitely be fine. Kale outside survived -17* below and several days that never saw temps above 0* in November. There was a blanket of maybe 2 inches of snow covering them during that. Other hardy winter crops should also survive easily inside this thing year round. The short days of december/January might not provide enough sunlight for them to really grow much, but they should remain harvestable during this time.I have no doubt the interior airspace will heat right up on a cold sunny day in January, but I don't have much clue numberwise just how efficient the transfer to heat mass will be. BTU captured vs loss, or the common % used for panel collectors would be cool to figure out someday...The hoop structure is a good shape to minimize exposed surface above ground to sub zero winter temps and also withstand frequent high winds. It also sheds snow well and importantly it will match the existing architecture to which it is adjacent. Did I mention it was easy, quick, and cheap to frame up? Used 5/8" rebar as the hoops @ ~10$ each 20ft stick. Got all of them erected in just about an hr. Should have bent them a little more precisely, but my neighbor and I just bent each one as it went up. First one end was clamped in place, each hoop end anchored to another 4+ft piece of buried rebar with 3 steel clamps and ~8" overlap. The attachment points were pre-bent to approximately match the final curve. Then my neighbor Bob pushed from the free end while I stood in the middle (marked w/tape) and made sure they went up straight and bent evenly, almost. They really bent quite easily. I could have corrected the variance better had I realized there was an issue right away. They are all pretty close though and I didn't realize how out of phase the panels were joining up until I would have had to dismantle too much to get back down to the "skeleton". The bit of variance isn't really any issue on the insulated half and where the two glazing panels join out of phase there is a strip of special tape that manages to bridge the small gap OK. By the time the ridge"pole" of 2x4's and the two 4"x6" endposts got bracketed it really felt strong, and now, almost all the way covered, it hardly even flinches when the high winds howl. It's got just enough room inside for 1 or perhaps 2 dwarf trees or similar large perennials like a fig or pomegranate. I hope it will stay cozy inside, but it will definitely be a constraint on what plants are gonna fit in.In regards to summer cooling, You bring up the excellent point of adjusting designs to suit the locations dependent variables. This one is located in a semi-arid, cold winter climate, with a very short (~90-100)day frost free summer growing season. Site is in Colorado's Rocky mountains @ ~7kft elevation, ~40N Lat. Steep grades and many microclimates are found in the mountainous canyon lands surrounding. Avg yearly precip around 15-18 inches or thereabouts. Ponderosa Pines predominate amongst junipers and a few other species of conifers. Patches of aspen are a local attraction for the fall colors. Lots of wild roses and currants in the understory. a few types of Cactus are seen on drier, usually S facing locations. As mentioned I recorded -17*F during that early cold spell back in november and last month saw the season low hit -22* while frequent sustained winds of 30-40mph pummeled what seemed like non-stop for several days at a stretch. A friend on the sunny side of the street (across the canyon) says his monitor records gusts at 100mph 2-4 times per year average. Feels like a wind tunnel often enough. In this context you can see why "winterizing" is primary concern, and summer cooling not so much. The shading from the N wall should be sufficient to mitigate any serious overheating potential. Around here Most houses with decent design do not need any cooling in summer. Just open the windows at night and we might see temps drop to the low 60's or even 50's after a scorching summer day at 90-100+*F. Daily fluctuation of 40+* is common in exposed areas. The sun can really get cookin up here. I'd love to see what irradiance measures in real time under clear skies. Certainly the typical estimate of 300btu/hr/ft^2 is a very conservative number at this site.We benefit from ~300 days per year with sunshine, about half of those considered "clear" days, and the other half "partly sunny". Colorado is certainly known to have great conditions for solar. PV panels are very much "In" fashion statewide. I found a calculator http://www.degreedays.net/ that says ~7,400* heating degree days typical in this area for a base temp of 65*. But that's suitable for a home. For a base temp of 45* the heating requirement is reduced to ~2,600*. A base temp of 32* requires just 987*days. The low insulation of the glazing would probably raise those numbers a bit, but I think overall it will be good to go. On a typical cold winters day, under sunny skies, temps often stay somewhere below freezing, but above zero. Of course they drop much lower during the occasional cold spells, winter storms and such. Its unusual for significant cloudcover to linger more than a couple days at a time though. On most days the solar gain should be easily able to get the internal temp up to a comfortable range, and I suspect some extra heat to suck underground for storage even in mid-winter. I do plan on installing that 4x8 hot air panel on top just N of the apex too which will definitely result in some excess for storage. Any excess heat stored during the day will radiate at night from directly below the plants. This will create a rather significant microclimate at soil(and below) level even without any active draw from the fan. Whew, sorry if that's a dense reply, but please do let me know what you think. Happy to hear more questions, they really help me to consider all the different angles.Joe
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