Cooling system to maintain 0.5 °C at 95% humidity

Question

I am looking for some guidance on a redneck cooling system.

I need to store seeding trees at a temperature just above freezing, and at a humidity of close to saturation.

I only need the system for about 2 months of the year.

At present I use snow.

• Let the room chill all winter so frost is deep in the ground.
• Add 2 feet of snow at the end of winter.
• Use a cheap box fan to circulate air, blowing it against the snow face.

My limited understanding of heat exchangers is that they operate at a colder temperature that the set point of what they are cooling.

I think that a conventional air cooler would have constant problems with ice forming on the heat exchanger, since even a few degrees colder than the room will be below freezing. I inquired with a company called 'Cool Bot' that has a controller that uses a conventional air conditioner to cool a well insulated room. They agreed that my application would not be suitable due to ice formation.

The second thought is to use a freezer. Chill brine or antifreeze to some cold temperature, and run it through flexible pipe laid directly on the floor. If the floor had a liner so that in effect it was a 3" deep pond, then the seedling boxes can rest on pallets above the pond with the same fan circulating air against the pond.

I don't think a freezer moves enough BTUs for this. Substitute insulated tank + chilling unit. (Old electric water heater for the tank?). Propane tank. (means no salt.)

Now the problem becomes one of control.

My thought is to use a chest freezer outside, verify that it is water proof, and fill it with a solution that won't freeze at a given setting of the freezer. I think I would aim at about -10 to -15 °C. Fill the freezer with the solution. (Brine ok? Freezers inside are made of plastic.)

Run a coolant loop that has some large number of feet of tubing in the freezer with holes drilled in the lid for pass through. (0.700 ID x 1/16" wall thickness black polyethylene plastic is commonly used for irrigation, and is rated at 30 psi. 0.625 water line is rated at 75 psi, but wall thickness is twice the size, which would increase the amount of pipe needed for the same thermal transfer.

Addressing comments:

Precision of temp control

0.5C is a goal. But I have an electronic relay that registers to tenth of a degree, and calibrating it by putting it in a jar in an ice water bath is straight forward.

Trees are normally stored over winter an between -1 and -2 C. This is the temp that maximizes their survival, and leaves the most amount of sugars intact for spring growth. When I get them they have been out of the freezer long enough to start to thaw. Refreezing them is a bad idea. So you can rephase the goal as "As close to but not below freezing as possible."

Similarity to Walk in Cooler

There are two key differences: Most walk in coolers do not make an attempt to keep humidity at close to 100%. And most coolers do not attempt to keep the temperatures very close to freezing.

The other issue is one of cost. A 500 square foot walk in cooler not only costs north of a couple hundred grand, but it requires more power to operate than my entire farm electrical service (7 KVA) This for a system that is only used a few weeks a year. This is why this is "redneck engineering".

This is why the extreme thickness walls for insulation. This could also be done with a foot of styrofoam, but straw bales are about $25 each. A 24x24 internal space building only requires 42 bales, and they are the support walls too. There is a whole building literature about strawbale construciton.

Tree storage temp.

A tree in the wild routinely gets roots far colder than 0 C. I am not an expert on the physiology, but the trade practice is just below freezing. Dormancy is not a switch that is turned on and off. I suspect (but don't know) that dormancy requires a set of cues of light/day length/temperature to trigger. Since trees are harvested usually in late fall, storing them just below freezing balances them not getting the usual sequence of cues. Most growers do not grow entire freezer fulls of a single species harvested at a particular time, so this may be a general compromise.

I do know that respiration rate (metabolism) of trees in cold storage increases very rapidly with temperature. A day at 5 C can use more stored energy than a week at 1 C.

Moding a conventional cooler.

See comment above about costs.

Strawbale heating

Do NOT get straw bale mixed with hay bale. Much different critters. Hay is mowed green. allowed to dry some, then baled. If baled too wet they can spontaneously combust. If baled at moderate temps, they can heat up some.

The process isn't really ripening. It's bacteria taking advantage of a good culture medium, and generating heat as a byproduct.

Straw has much lower nitrogen content (protein) and there is no rush to bale it quickly. So it tends to be baled very dry. Respiration levels are low. I've yet to hear of a spontaneous straw bale fire.

Questions:

• Is this a workable solution?
• What is the unit I need called?
• How do I size it?
• Pointers to calculating the amount of heat exchange pipe I need
• What is the best way to control it so that I don't freeze my trees by mistake, nor allow them to get well above freezing?

In passing: Building details:

24 x 24 interior dimensions. 32x32 exterior. Roof dimension 40x40 Proposed new cold room will be built out of 3x4x8 foot straw bales, laid brick style, with the 4' dimension being the width of the wall, and using several inch layer of lichtlehm (light clay) between bales to adjust for bale irregularities. Bales run about R1.5 to R2 per inch, so the wall insulation value is R72 to R96 depending on whose lies you believe.

4" vertical styrofoam to reduce underground thermal transfer. (Cut a trench as deep as reasonable with a ditch witch). Usual strawbale construction footings.

Roof sized for 2 feet of blown cellulose insulation. Roof has large overhangs (4 feet) to protect the walls from rain.

Answer 1

What you are describing is a freezer kept just above freezing. That part isn't hard.

Yes, cooling cold must be colder than the thing they are cooling to work. Heat only flows form how to cold. That's fine, it just means that the evaporator coils will freeze up. This is desirable, you need ice to maintain humidity. This is how standard non-self-defrosting freezers maintain humidity in the chest to avoid freezer burn.

Now, I don't know if anyone has off-the-shelf equipment designed to maintain 95% relative humidity. This is very close to being fog and probably means you need to be constantly adding moisture to the system. I think you will need to experiment in how exactly to keep the humidity levels high. .5 degrees is also a very tight window to maintain. You are bumping against the accuracy capability of your instruments, and this also disallows the standard off-on nature of refrigeration systems. I suppose a variable-speed compressor system could be designed, like is use for high-efficiency Air Conditioning systems.

Honestly, a highly insulated enclosure that you feed ice into is probably your best best.

Answer 2

At near-freezing temperatures, the amount of water vapor in the air to get 95% relative humidity is tiny, this is no problem.

I recommend filling 55 gallon poly drums on pallets with water, park them outdoors to get them near freezing, then move them into your storage unit with a pallet lift. Put the plants in the center and surround them with the drums. Use a small electric fan to keep the air moving very slightly inside the structure.

The cold water forms a tremendous "thermal flywheel" that releases heat as the temperature outside rises and soaks it up as it falls. with the huge amount of insulation you have, this should work passively except for the fan which does not need to be big, and could run off a solar cell array (solar fans are are used in motorhomes and trailers).

Experiment will tell you how many drums of water will be needed. Start with 20 and leave room for 10 or 20 more.