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Help me figure out some temperature/airflow issues in my passive solar greenhouse...  RSS feed

 
Matt Smith
Posts: 181
Location: Central Ohio, Zone 6A - High water table, heavy clay.
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So we're currently working to insulate and "finish" the interior of our 12"x48" passive solar greenhouse. We have two 12" gable vents near the apex of the roof on each end. When the interior heats up, it tends to push hot air out these vents at a pretty good rate (had my face up by the vents when working on it last summer and it was like having the air from an oven blasted in your face).

We're in the process of making the structure as airtight as possible for heat retention purposes. As I'm finding and sealing small air gaps I've been noticing that the building tends to pull air in through them, even when it's cloudy/cold and the temperature difference between inside/outside is relatively low.

In thinking about this process, I've decided to try and add some "cooling air intake tubes" to the greenhouse to provide another means of passive temperature regulation when it starts to really heat up in warmer months. These would be long tubes (probably HDPE drainage tubes, about 6" or 8" in diameter) that would run the length of the greenhouse, buried beneath the soil of the raised bed that runs along the greenhouse on the mostly shaded north side, and then connect to the interior of the greenhouse near the floor in the center of the structure.

My theory here is that the natural tendency of the greenhouse to expel air through the gable vents and draw air in from other places would pull warm air in from the outside through the tubes, cooling it somewhat as it traveled through the moist shaded soil. The tubes would be screened on each end to prevent unwanted fauna getting inside.

Below is a *ROUGH* diagram of how I'm envisioning this working. Please note there will be a row of water-filled 55-gallon drums along the back wall by where the tubes enter to act as thermal mass.



So, with this, some questions I'm throwing out in the hopes of getting some more informed opinions than my own.

1) What do you all think of this plan in general? Am I missing anything major?

2) What is likely to happen in the tubes regarding moisture? We tend to have pretty hot and humid summers, and my understanding is that pulling hot moist air into a cooler environment will cause condensation. How will this affect air temperature?

Any other thoughts, ideas or comments would be greatly appreciated!
 
Josef Theisen
Posts: 236
Location: SE Wisconsin, USA zone 5b
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There are 5 basic elements to a passive solar design.

1. The Aperature. Allows solar energy to enter the structure. You have this one covered with the triple walled poly glazing. Awesome!

2. The Collector. Usually a dark colored surface that converts sunlight into heat. From what I have seen of your building you seem to be using the dark floor as a collector. I think it would be a good Idea to also use dark colored barrels to collect max heat for the winter months.

3. Thermal Mass. Stores the heat energy for later use. I am glad to hear you are adding barrels as this will be an important factor in storing enough heat to make it through the long winter months. Water and concrete are the two most common choices for thermal mass.

4. Distribution. Allowing the floor and the barrels to radiate the stored heat back out is the simplest form of distribution, but can be very effective.

5. Control. This is the thing I see lacking in your design. Many designs rely on a properly sized overhang to control which time of year the sun hits the collector, but you don't have that option since your entire South roof is the collector. I worry that the heat will be too intense in the late Spring and summer. It only takes one hot day to kill an entire greenhouse full of plants. Movable insulation is an option but you have to be there to put it on at the right time. Personally I see nothing wrong with adding active elements if it makes your design work better, and a thermostatically controlled louvered vent fan could do the trick. Another option is to add roof vents that open with a hinge at the top. There are automatic vent openers on the market for around $100 that are pretty increadible. They operate on a piston that looks like a storm door closer. Inside is a metallic fluid that expands and opens the vent at a pre set temp, so they require no external power and operate on the heat you are trying to get rid of. Some even have temperature adjustments within a small range. Combined with some type of low intake, they make great insurance against overheating.

The underground intake tubes are an interesting concept. Combining geo-thermal with passive solar. I don't know enough to comment much on it other than to say I suspect your top vents are too small to be totally passive.

I recommend getting a digital thermometer/hygrometer that recoreds the highest and lowest points and experimenting with different configurations of benches, barrels ect. I also recommend treating the wood framing inside to protect it from swelling, rotting, or warping in the high humidity enviorment. But take my advice with a grain of salt as I don't have any practical experience in passive solar or greenhouses beyond reading, planning, and dreaming.
 
Matt Smith
Posts: 181
Location: Central Ohio, Zone 6A - High water table, heavy clay.
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Josef, many sincere thank you's for taking the time to review and respond to my post.

Regarding:

2/3 Collector/Thermal mass - We are planning to install a double stack of 55-gallon drums (painted black, filled with water) all along (6" away from the wall surface, to be precise) the back wall to act as our main means of collection and significant thermal mass. We'll have around 50 in total. I will be adding additional means of collection and thermal mass as space and layout permit.

4- Distribution - We are planning on what you describe, and also the installation of radiant heat barrier along all interior walls to prevent the radiant heat from escaping for as long as possible.

5- Control - You are absolutely correct, and as best I can tell at this early stage, the weakest spot in our design is the ability to regulate the higher temps in the hotter months. Having not yet installed the thermal mass, it's difficult to tell how they will help moderate the greenhouse temps either way (cooling down or heating up). The good news is, we have the time to progress in steps, with measurement and consideration in between.

A few notes on your comments:

- Most importantly, we are intending to use this greenhouse primarily during fall, winter and spring, to help bridge the gap between temperate growing seasons. If we had to forego using the greenhouse in the hotter parts of the year, we would be willing to do so as we have plenty of outdoor growing space to utilize during that time.

- We are keeping open the idea of using electric exhaust fans to some degree if need be (although this would not be ideal). I am going to try and utilize every possible method to reduce the need for this. Even if we do need some fans, I'm hoping to make enough progress that we could utilize efficient solar fans. And like I said above, our goal for this greenhouse is not summer use.

- We designed the angle and relative pitches of the roof to collect the most light in winter and shed the most light in summer relative to the angle of the sun at those times. Even so, your comment about the lack of overhang on the south facing roof side is quite valid. We may yet opt to add some sort of overhang over a small portion of the top of the south facing roof to further help shed direct summer sun. Hopefully we can do so in a way that does not interfere with solar collection in the winter. I need to get the barrels in first and then do some measurements.

- We may find that it makes sense to install shade cloth or alumi-net to physically reduce the amount of sunlight the building absorbs in the hotter months.

- I have also considered adding some automatic solar powered roof vents to the north facing roof to vent excess heat. This would involve making holes in the existing roof, but we can do it if needed.

- We are painting the wood framing inside with high quality white paint, to help deflect/diffuse light and to increase longevity in a humid environment. All the insulation we're using is closed-cell and will not absorb moisture.

- I am worried about the smaller size of my top vents as well. We have some room to make them bigger if need be, or add other ones.

- I will be buying a digital temperature logger so that I can chart out the indoor/outdoor temperatures once we get everything installed.

I remain hopeful that the "cooling intake tubes" idea holds some merit.






 
R Scott
Posts: 3349
Location: Kansas Zone 6a
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There are basically two ways earthtubes are used in greenhouses

Open loop, like yours, which is what Earthships do as well--although earthships use a LOT more mass than most people realize. It isn't just the tires and their dirt, but several feet (varies based on climate) of dry dirt behind the tires before the insulate and waterproof. It works, but you can over-cool your mass in cold weather if you don't have enough. Even on cold days you will be creating enough heat in the greenhouse to need to vent, and will be pulling heat out of the mass to do it when you really don't want extra heat.

Closed loop work by vent fans drawing hot air from the top of the greenhouse, then pushing it through tubes in the floor and bed to put the heat into the mass. They do this all day every day (usually on a timer or solar-powered fan). Then they have separate exhaust fans or automatic vents to control overheat temps, drawing air directly into the greenhouse air.

Tubes often are a condensation problem. The closed loop systems use this to an advantage--that phase change stores a HUGE amount of energy. They use perferated pipe in gravel, so any condensation is released into the gravel to re-vaporize as humidity when it warms up, absorbing the extra heat in the process.
 
Matt Smith
Posts: 181
Location: Central Ohio, Zone 6A - High water table, heavy clay.
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@R Scott - Thank you as well for taking the time. I was unfamiliar with the common terminology and now I'm searching and learning helpful information regarding earthtubes!

If I am reading your post correctly, you're discussing a system that both heats and cools. I am intending on utilizing this system for cooling only, and likely sealing off the tubes in the winter months and relying on the design of the greenhouse and our thermal mass to take care of the heating aspect.

I am reading on wikipedia that denser soils (check) and moist soils (check) are best for this sort of thing. So that's good for me.

It seems that the non-smooth (aka common drainage) pipe might be best given that my tube length runs are relatively short (under 25 ft) and I want maximum thermal exchange. Also that condensation can be a bigger issue with non-smooth tubes.

Now I'm wondering if I should be using perforated drainage tubes so that any excess condensation can help water my garden beds as opposed to building up in the tubes...
 
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