It's all Plain to Sea

[Rocketplumber]

Hello, new member on the list here, throwing out a nascent idea. A lot of the difficulty of earth tubes seems to be fighting the tendency to grow mold and mildew, and if the tubes are water permeable, saturated humidity at the air exit, which only makes the issue worse and can make for a very ill greenhouse.

Separately, I've been looking into hydronic radiant heating and cooling to apply to an annualized geo-solar house, a la Don Stephens. Combine the concepts- what if you bury cheap simple PEX tubes in the greenhouse soil and circulate water to couple the heat to the soil (at much lower power demand), then run the water through a standard A/C air handler to cool/heat the greenhouse air? Any condensation in the air handler can be fed into the rainwater cistern (of course you capture rain water off the greenhouse roof, don't you?), and in heating mode there is no moisture added to the greenhouse air. If any mildew grows in the air handler, the above ground device can be readily cleaned, or possibly have UV LEDs illuminating the air surfaces to keep them sterilized.

This decouples the air circulation and heat transfer tasks, and may allow each to be optimized separately. If large wall areas or radiant exchange panels are available, no fans might be needed at all, only some quite low power pumps for the water. [url][/url]

[Hex]

The exit air has fairly low humidity as the cold soil drops the RH to dewpoint on the way though the tubing. The soil does the exactly same job as the AC coils but without the high running costs.
You`ll need a fan to draw the same amount of greenhouse air through the A/C coils along with a pump to send it through the pex circuits.
The pex will transfer heat mainly by conduction to the soil, you`ll need a considerable amount of pex tubing to provide sufficient surface area.

[Rocketplumber]

To be more precise, the exit air will have dew point essentially equal to exit temperature, for 100% RH. Whether the heat exchange occurs in an earth tube or an air handler doesn't matter really.

Both earth tubes and water filled tubes transfer heat to the soil _entirely_ by conduction, but I'm pretty sure that water tubes can transfer more kW/$ of system than air filled earth tubes- air is a lousy heat transfer fluid. The issue is, does the cost of the liquid-air HX outweigh the cheaper in-soil heat exchange? If the GH has a large area wall that can have PEX tubing installed, the HX becomes free and only a circulation fan may be needed.

Since my plan is for a large greenhouse attached to a residence, with a low wall around the GH periphery, the system could simplify to a bunch of tubing in the wall and soil, a pump, and a big ass fan to stir the GH air so that it flows across the wall. The greenhouse, like the residence, would have a subsurface insulation cape reaching out 20' from the foundation to isolate the heat storage volume from the outside world.

I'm using LISA FEA to build a heat transfer model to see if fan-circulated air and radiation can provide enough heat transfer.

[Hex]

The shcs transfers sensible and latent heat directly into the storage medium in a single step process using a simple duct fan.
When you compare calculated conductive transfer based on tube surface area and air/soil temperature differential to the measured inlet/outlet air temperature differential of the shcs.. you`ll find that conduction constitutes only a small percentage of the total heat transfer taking place.

Its surprising how fast the heat is transferred. Its a good thing when you consider the greenhouse air temperature can rise rapidly with just a little sun, moving large volumes of air underground is inexpensive compared to running an AC to transfer the heat to the water and then use more energy to pump the warm water into the buried pex.

As it relies entirely on conduction you`d need a decent contact time (long tube lengths) to transfer the heat from the water so it returns to the AC unit at soil temperature.

[Rocketplumber]

I found someone who has already tried something very similar: http://www.solarbubblebuild.com/

He uses the bubble solution both to make the insulating foam, and as a heat exchange fluid (without blowing bubbles), flowed across the inner skin to cool and dry the GH air. The very thin wall skin allows the heat to flow with very little temperature delta. The condensate is kept separate from the surfactant solution and can be used for watering, very interesting for a desert dweller like me.

It does an excellent job of lowering humidity, see the plots at http://www.solarbubblebuild.com/log04.php

If he were to add a buried coil of tubing under the floor, he would have the full thermal benefit of SHCS as well as the insulation and light screening properties of the foam. I'm impressed with how the entire system can transport heat, control the GH humidity and lighting, insulate against cold nights, and conserve heat and light on winter days.

What I would add to his system:

* More, smaller bubble generators distributed along the peak, fed by a quieter, more efficient fan. These could be run continuously at lower rate to keep thick, even bubbles in place when they are desired. The steady flow of warm bubble solution also serves to move warmth from the underground storage back into the GH. Some of the solution can bypass the bubble generators to flow along the inner skin for more heat transfer.

* A 10' deep trench with plastic heat exchange tubing under the GH. This can couple the heat in the bubble solution into the soil very effectively.

* An insulation cape around the GH, reaching out 20' in all directions. This is straight out of John Hait's Passive Annual Heat Storage, and Don Stephenson's Annulaized Geo-Solar playbook.

* A "Big Ass" fan to stir the GH air, moving it both past the plants to encourage transpiration, and past the GH skin to lose/gain heat.

Hours later, more web browsing, found this site: http://www.solaroof.org/wiki/SolaRoof/HomePage -where they've been working on GH temperature and humidity control, but also struggle with inadequate thermal mass. Sort of a peanut butter and chocolate thing if they store that heat in the soil...

[Hex]

Hi Rocket Plumber
I followed Harvey`s bubblebuild progress with interest as he lives a little south of me in Sussex..it was quite a few years ago now. I was a member of the solar roof group on yahoo at that time.
As i recall, Harvey became disillusioned with the bubble insulation after investing a considerable amount of money and finding the bubbles were not providing the insulation values he had expected to see in a uk location.

The last i heard he had abandoned the bubble side altogether and his wife was using the tunnel for her culinary herb business as a regular double skin tunnel albeit with a large 18" airgap.

[mrhobbithhnet]

[mrhobbithhnet]

I've currently been investigating "reversing air flow" styles of UACT.

Turns out that as a typical unidirectional SHCS works through the day, the input side of the tubing matrix heats up first, leaving us with a growing length of 'dead' tubing to traverse before we reach tubing with enough delta T surface temps to take the air towards dewpoint effectively, if at all.

In the past, we have experimented with answers to this obvious limitation by keeping the tubing lengths long, so that the sub soil zone was long/large enough to have a Delta T opportunity far into the day's cycle of air moving and dewpoint heat transfer. Turns out that at the same time as we were approaching the solution this way, we were also depreciating the relative HP/effectiveness of the fans. Ie: the longer the tubes, the more losses in fan energy moving the air underground. Besides, if you have the same size fan, and just increase the tubing length, the air spends a proportionate longer time underground, effectively shortening it's time above ground extracting BTU's.

Research from the Chinese, Canadians, and Yonaton here in the states was indicating that higher volume RATES of air moving was the key - even if it was achieved by shortening the tubing lengths (effectively lowering (?!) the time air spent underground.) The point being - keeping the tubing temperature differentials as high as possible along as much tubing as you can was the key. And the best way to do that was to up the air speed on a relatively short UACT layout so that back pressures wouldn't stall the system at high speed.

Well, that bit of common sense aside - how about having two high volume fans, one on the entry and one at the exit. There would be double the fan hardware costs... but if they never run concurrently, the energy cost is the same as a one fan system AND the equipment lifetime doubles (they only run half as much as one fan running would run to move the same amount of air) So, in essence, you buy two fans at todays prices for say 20 years operation... instead of one fan now (at today's relatively low prices) and another in 10 years (at prices having risen to ten year out levels). That second fan would cost you significantly more than the first if inflation ticks along at a normal rate. SO - that means you actually save money in the long term. The fan running time for the two decades in the same, but the hardware costs could be significantly less numbers of dollars for the same produce volume.

The question then is: how do you arrange to have a dual fan system - where only one fan runs at a time, and while one is running, the other is not interfering in the flow... AND there isn't any expensive hardware or electrical controls to negate any potential savings you'd get from time shifting your future replacement fan expenses to today's present 'lower' pricing?