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Ideal suntrap curve

 
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I'm planning to build some sun traps from layers of sod (per this thread).  I'm not sure what curvature to put on the sun trap.  Does it depend on your latitude?  I'm at 45 degrees north if it matters.

My goal would be to grow plants that live in zone 5 up here in zone 4a.  Maybe peach trees, lavender, etc.  

The berm would be 5' high and 4' thick and very steeply sided.  So it should catch lots of sun when the sun is at low sun angles.  My initial guess was to make a 15' long curve with a 10' radius.  That means if you draw a line across the ends of the curve (farthest south points), the northernmost part of the sun trap would be recessed in from that line by about 3 feet.

I'm thinking of doing three sun traps and then letting the structure curve to the north for a shade trap.  Plus then if we get more material we can transition into hugelkulture and run that to the north.  This is wrapping around a community garden so blocking the wind is a bonus feature once we turn to the north.  But for now I need this to run E/W for sun trapping...

Any thoughts on the curve?  Too shallow, too wide, etc?
Berm-shape.png
berm shape
berm shape
 
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https://www.suncalc.org/#/43.2346,-90.4368,3/2020.12.21/19:02/1/3

This website might be helpful. I would think that knowing the rising and setting location on your horizon at the low point would be useful information. But the more I thought about it the more I thought my instincts are wrong. That site is a cool tool to help visualize the sun at different times of year though
 
Mike Haasl
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Thanks S!  Yeah, I'm not sure if I need or want sun right away in the morning.  Or if it's more important to give the plants a protected nook than it is to catch maximum sun.  And is too much sun a problem since we have snow cover to protect low growing plants anyway?

I don't really know so that's why I figured I'd ask.

In conjunction with this berm, I may add wind blocks coming off the points of the curves of evergreens or woody perennial bushes.  I might also make "living snow fences" of short shrubs that help trap snow to cover the plants in the sun trap.  Lots to try but it's hard to rearrange the berm once it's up
 
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Mike, no solution formulated in my head at the moment, just a couple thoughts.  For the parts of the year when you want to keep your plants in dormancy it can help them a lot to keep both the wind and sun off them.  To me this would translate into wrapping the berms around them to a larger degree.  I'd probably start with calculations of how many hours of light you want on them for how many months and then maximize the degree of wrap and relative wall heights in that wrap distribution based on that.  Should help with wind blocking as well as with radiation loss/gain.
 
Mike Haasl
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Good points Greg!  This is a bit of an experimentation and demonstration site so maybe I should have a whole series of curves of different radii.  The tracker S linked to showed that the back of the suntrap would see sunlight from early Feb onwards.  So that tells me I probably want to curl them up a bit tighter and/or go deeper...
 
s. lowe
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So I suppose I should post my initial thoughts  as well, can't hurt and I'll be curious to hear you alls thoughts.

Create the curve so that the east and west end point to the rising and setting points on your horizon at the winter solstice. This would mean that there would be sunlight landing inside the curve for the whole day then. It would also make the curve fairly tight which would allow for more wind blockage but would create morning and evening shade later/earlier.

My second thought was, align the curve so that the sun is falling inside of it all day starting at some point several weeks before you hit your risk of frost. Maybe the midpoint between equinox and solstice? Maybe just a month? two weeks? before your average first frost.
 
Mike Haasl
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I'm also suspecting that the curve of the suntrap creates a few different growing conditions.  Trapping solar energy in the fall and spring might help a plant that can handle snow pack but not exposed cold November nights.  Size of the recess and wind protection may be as important as the sun trapping.
 
s. lowe
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Orientation in relation to prevailing wind may be a consideration as well. For instance, if your wind typically comes out of the west you might have to find a balance between capturing that late sun and shielding against a strong wind. Although that could.be managed with a shorter berm that would push the wind up and over just enough without blocking too much solar energy.

I'm witnessing the effects of massive solar gain in real time this year. I have a nopal cactus that has lived in a half wine barrel with me for about 4 or 5 years. It has always lived along the south facing wall of my house and has always done alright considering that my cool damp climate is not cactus territory. We moved last summer and the new south facing wall gets better sun exposure, has better protection from wind, has a west facing wall nearby that reflects a good.bit of heat, and has the drier vent right on it. The cactus has exploded and is flowering and might actually produce some ripe prickly pears! This is an extreme case but good for demonstrating what is possible
 
I guess I've been abducted by space aliens. So unprofessional. They tried to probe me with this tiny ad:
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