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Thermal Dynamics?  RSS feed

 
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I understand the basics of how a draft is created with a rocket stove

Does anyone know where I can find resources to create a draft or breeze in an enclosed Courtyard?  I am attempting to design a Permaculture garden demonstration at a local middle school, but the teachers tell me the air can become so hot and stagnant, that nobody would want to work or enjoy the garden.  My goal is to work on understanding the thermal dynamics of the space in order to place shade in the correct position to create a slight draft or breeze by appropriately placing hot and cold spots.
Thank you for any help or direction.
I tried to attach a Photo of the courtyard
PHPumpkin-Courtyard.jpg
[Thumbnail for PHPumpkin-Courtyard.jpg]
Courtyard Photo
 
Posts: 622
Location: Thunder Bay, Ontario, Canada
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You need to create a temperature differential between one end of the yard and the other. There will naturally be one already (North side vs South side), so the ideal solution is to enhance what's already there.

Trees transpire during the day and release a lot of hot, moist air into the atmosphere. Planting trees at one end of the courtyard might be enough to create a sort of thermal chimney where hot air rises out of the courtyard, and cool air gets sucked in at the other end.

But there are lots of variables so in reality there are many other factors to consider like the size and shape of the surrounding buildings, prevailing wind strength / direction, etc. You also have windows and the potential of obstructing people's view / cutting natural light to consider.
 
Posts: 225
Location: Abkhazia
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I have not played with thermodynamics on that scale, so I can't speak from experience. That being said…

Hot air rises, cold air sinks. We know that already.
The question is: Where does the cold air come from?
the answer is probably water and plants.

So if you turn one side of the area into an area with a lot if plants and open water, the water will evaporate, cooling the area down. The air will then stay where it is, as the above air is warmer.
While this is nice in the summer, a warm area would probably be nice during winter: Dark surfaces with some thermal storage.

If you combine both as you suggested, the air will cool down over the cold part, sink to the ground and move to the warm area where it heats up and rises. An air pump.
 
Kanulaf Zuk
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I love this place! 

Thank you both, that gives me a great place to start.  I will do my best to identify any wind patterns in the area, make notes on the orientation and shape of the courtyard, and then set my goals to identify the optimal places to create a heat chimney of sorts with trees, and then a cool area with a body of water.  

Thanks again!
 
pollinator
Posts: 176
Location: Ashhurst New Zealand
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Cooling towers have been around for millenia. If you can build something with a decent internal cross section, you have two options:

1. Have some sort of lightweight, air-permeable media in panels near the top, like coir, wood excelsior, rattan, or open weave fabric. Drip water onto the media so that it's always damp. Air crossing this surface will evaporate some of the water and give up heat in the process, and the cooler air inside the structure will sink and set up a draft. The cool air then spills into the courtyard and lowers the temperature. This only works if you don't have high humidity, and as such is mostly a drylands solution.

2. Paint the tower a dark colour (or plaster with a mix that has a lot of charcoal mixed in). Solar heating will move air up the tower and pull stagnant air from the courtyard. If you can provide a source of cooler air at ground level to replace what's being sucked out, you've nailed it. An underground duct would be ideal.
 
pollinator
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Location: Los Angeles, CA
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As I look at that picture, a couple of things are evident.

First, there is a tremendous amount of thermal mass to capture heat and hold it.  It's not just the sun that is making that place hot, but the bricks and concrete hold onto that heat and make it hotter and hotter as the day goes on.  The air in the "well" is trapped and the temp continues to rise.  I would imagine that with that much thermal mass, it would stay warm late into the evening.  You can probably go out there at midnight and the walls will still be warm to the touch.

Second, where would cool air even come from?  Any air moving into that space would likely have to pass over the roof-tops of the buildings that surround it, and then drop down into the "well" that is created by the walls of the buildings.  So the air will already be pre-heated even before it drops down into the well.  Ever walk on a flat roof?  Ever walk on a flat roof that has some form of black-tar roofing?  It'll be 120 or more up on a roof like that.  That is where your "cool" air is coming from.

Third, there doesn't appear to be any sort of cross-ventilation.  That space creates a pocket of air that isn't being exchanged.  Think of a similarly sized field without walls around it.  How long would it take for the air on that field to be exchanged?  Maybe 5 seconds?  Just a little breeze and all the air quickly exchanges.  But in the space you've shown us, the air sits stagnant and isn't exchanged. 

In classic "the problem is the solution" permaculture, I'd pick plants that thrive in such intolerable conditions.  Tomatoes and peppers come to mind.  But I'd look for a way to increase the humidity of the space.  Perhaps you could lay a long stripe of plastic down in a shallow swale that runs the length of the courtyard and fill it with water.  That way you'd hopefully increase the moisture/humidity in the air.  Tomatoes like hot muggy air.  But I'd stay away from plants that thrive in cool breezy locations: lettuce, brassicas, etc.  The west and south walls will get a bit of shadow during the hottest part of the day.  You might be able to plant some stuff in those locations where they will not get baked by the hot sun.  I'd plant heat loving nut trees like almonds or pecans.  In 10 years, that will help bring down the ambient temperature by 20 degrees or more.

If you planted about 200 tomato plants and trained them to grow up a trellis, maybe that would help absorb some of that sun and keep the ground from getting too hot, thus keeping the overall ambient temperature of the space a few degrees cooler.  Or if you installed misters (like they use on patios in Palm Springs or in Arizona), that might keep things a bit cooler.  But as it stands right now, a crop of carrots, potatoes and beets are going to get scorched in that courtyard.
 
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