Strategies for a seasonal thermal energy storage (STES) cooler
STES = seasonal thermal energy storage.
I need a cold room for storing seedlings and bare root trees for 2-3 months of the year, from roughly the end of April to the middle of June. The temperature needs to be between 0.5 and 3. Humidity needs to be at 90% The cost of a conventional cooler system is unworkable, both because of the machinery, but also that I would need to upgrade my electrical service, as well as upgrade the power distribution line (700 feet) from the transformer to the proposed location.
The location is outside of Edmonton, Alberta, Canada. Spring melt starts in mid March. Snow is gone by mid April, although spring snows are frequent. Leaf day (when the poplars break bud) is usually the 10th of May plus or minus a week.
At present I’m considering a 24 x 24 foot space packed gravel floor, strawbale walls, clay/lime render with a vapour retardant paint on the outside. The rule of thumb I’ve gotten from the SB groups is that the warm side render should have 1/5 to 1/10 the permeability of the cold side.
A: It’s not clear to me if this is valid for a cold interior. The dew point is still inside the wall, but this is true for a warm interior too. How do bale walls dry out?
Cooth would be stored by digging a 16 foot wide 3 foot deep slope sided hole in the middle of the floor, lining with a pond liner, and filled with water. In winter the door would be left open, and a conventional 3000 cfm window fan used to move cold air into the room.
The pond would be decked, and the space above covered with 1″ thick OSB/Foam/OSB panels. These panels would be removed and stacked during the winter, and put in place in summer. (Yes, this means that I have storage at two levels. Most of this is boxes moved by hand. If it gets to the point where carts are needed, I can put permanent deck on the rest of the floor later.
We have an annual freezing index of 1750 C degree days. Ice thickness on lakes is approximated by t= c*sqrt(F) where t is the ice thickness in cm, F is the freezing index and c is a constant ranging from about 1.5 for a somewhat sheltered snow covered lake to as high as 2.7 for open lakes in windy areas. If I use 2, I get an ice thickness of between 80 and 90 cm.
If this fails to materialize, my backup plan is to put 1000 feet of plastic pipe in the bottom of the pond, a similar length on the north roof and circulate antifreeze. (Break pipe up into ~100 foot sections, and manifold)
B: Is this a workable strategy? In particular the ground temperature the first year is going to be its usual 10 C. I would like to avoid insulating the bottom of the pond, thinking that after a few years operation, I will have the local ground temperature close to freezing, allowing my ice to last longer. It also makes for simpler construction. Common frost depths here are 4-6 feet, and under driveways we put the waterline 8 feet down. My expectation is that the first year I would have shallower frost depth under the pond due to the delay of the pond freezing.
In operation the pond would be separated from the room by the insulated panels on the deck. Since it is below, there is little tendency for coolth to escape. A thermostat controls a louvered fan set into the deck. A ceiling fan keeps the air moving.
Foundation would be a rubble/ river run cobble filled trench, insulated with foam, but it’s not clear how much, or where to put it. Conventionally it would be on the outside to both keep the interior warm, and to keep the frost from heaving the foundation. Here, in operation, the inside is likely to have deeper frost than the outside, since there is no insulating layer of snow. At this point I’m thinking in terms of 2″ type II EPS that goes down 1 foot and sideways 2 feet so that there is in effect a 6 foot wide chunk of ground that the foundation sits in.
C: How should I detail the foundation.
The roof stumps me. Various sources says that since cooth doesn’t rise, the ceiling space doesn’t need to be any better insulated that the walls. The vapour barrier should be on the warm side, but it isn’t practical to get a continuous vapour barrier in between the trusses. I suppose in theory I could use A frame trusses and get a clear space across the roof, but I’m not confident in getting it right at the edges.
My present thought is to apply 6-8″ of type II EPS in two layers to the underside of the rafters, sealing the joints with spray can expanding foam. 6″ gives R24. Add another r12 of cellulose on top.
D: Is there a better way to do the ceiling insulation?
Finally: The humidity requirement creates an interesting situation: The vapor gradient in use is from the inside out, while the temp gradient is from the outside in. Very high humidity inside. Outside typical daytime values in spring run 30 to 60%
E: Does this affect the design?
F: what else have I overlooked.
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