Passive air conditioner will it work?

[random]

It's pretty basic really just dig a trench 50 feet or so long and bury a 4 inch diameter O-pipe in the bottom with 1 end leading into the house and 1 end at ground level. Of course cover the outside end in screen and build a Cupola type shelter above it.

On the end inside the house you then use a high velocity fan to pump the cold air into your plenum on your furnace and just set the fan on the furnace to run.

 

Sounds good in theory right? but will the ground around the O-pipe eventually warm? This is why I chose a trench rather than digging a hole and coiling the O-pipe so there would be more surface area and a longer run of cold earth. I was going to do this at my old house but never got around to it. Just looking to improve upon the idea.

[lemur]

I used to think a lot about passive air-conditioning methods like this. However, during the same period I kept nudging the thermostat setting higher until I eventually got to the point of turning the air-conditioner completely off. I find that my body adjusts to the average temperature in different situations and then deviations from that feel relatively hot/cold. So, for example, shade makes a giant difference on a hot day. It would be interesting to experiment with underground pipes and tunnels, though, and see what the air coming out of the duct feels like. If it gets cool enough to condense humidity, though, you would have to have a way to drain it.

[Cap'n Refsmmat]

I'd imagine that running air through an underground pipe would only cool it by a couple of degrees, since it won't radiate heat into the surrounding soil very easily. You'd want to maximize the air-soil interface surface area as much as possible, by having the air move through as much soil as possible or by splitting it into thin streams.

 

What you're really trying to build is a geothermal cooling system:

 

http://en.wikipedia.org/wiki/Geothermal_heat_pump

 

Even when pumping water and antifreeze through the pipes, which transfers heat much more efficiently, you can still see how much piping is required to cool a house:

 

http://en.wikipedia.org/wiki/File:3-ton_Slinky_Loop.jpg

[lemur]

Even when pumping water and antifreeze through the pipes, which transfers heat much more efficiently, you can still see how much piping is required to cool a house:

It would also be interesting to analyze how efficiently heat transfers into a house at different temperature differentials. This is probably a topic for its own thread, but it's interesting when you're talking about getting heat out of incoming air how much heat is coming into the house directly and how that relates to the amount of degrees difference between inside and outside.

[random]

The soil stays at 68 degrees I do believe (if I remember what the installer of our Geothermal heat pump said) 72 is the average normal temperature we keep our homes (I find 74 just right myself) so on a 90 plus degree day if you have a source of 68 degree air I do believe it would be welcome!!!

[Cap'n Refsmmat]

Well, here's the problem:

 

Suppose it's 90 degrees outside and the soil is at 68 or so. You pull 90-degree outside air into the pipe and run it through 50 feet of earth. Once it arrives at your house, it'll have cooled by a few degrees, to maybe 85 degrees. That's not really air-conditioning.

 

What you need is an efficient way of dumping heat from outside into the pipes, like a geothermal heat pump would do. Look at the Wikipedia article I linked to.

[lemur]

I wonder how much heat-exchange with the ground would occur if you built a house on top of a black cement slab or other foundation. Wouldn't the foundation act as a black-body absorbing heat from the warmer side and radiating it into the cooler side?

[Cap'n Refsmmat]

If it's directly in contact with the ground, conduction will be a far more effective heat transfer method than radiation.

 

If you lived in a country with no sunlight, you could paint your walls black and they'd radiate out the house's internal heat. In sunlight, though, they'll end up absorbing more energy from the sunlight than they would radiate from your house. (Ever stand outside on a sunny day in a black shirt?)

[ewmon]

Would the cool ground condense the humid air inside the dark tube and promote the growth of mold?

[lemur]

If it's directly in contact with the ground, conduction will be a far more effective heat transfer method than radiation.

 

If you lived in a country with no sunlight, you could paint your walls black and they'd radiate out the house's internal heat. In sunlight, though, they'll end up absorbing more energy from the sunlight than they would radiate from your house. (Ever stand outside on a sunny day in a black shirt?)

I don't know how to assess how much heat transfer would be due to conduction and how much to radiation. Are you saying that the absorption/emission of the floor is negligible compared to conduction, or just less?

[random]

<br />Well, here's the problem:<br /><br />Suppose it's 90 degrees outside and the soil is at 68 or so. You pull 90-degree outside air into the pipe and run it through 50 feet of earth. Once it arrives at your house, it'll have cooled by a few degrees, to maybe 85 degrees. That's not really air-conditioning.<br /><br />What you need is an efficient way of dumping heat from outside into the pipes, like a geothermal heat pump would do. Look at the Wikipedia article I linked to.<br />

<br /><br /><br />

 

ok so I read part of the article I understand your point ...........(oh ground temp is 60 degrees) ..........so perhaps a loop system would be better, instead of open ended. Hot air out from house loops around (lets increase to 100 feet of O-pipe length) and back inside. Are we getting somewhere now?

[Cap'n Refsmmat]

That would be more effective, yes, but I still think you'd want to run a liquid through the pipe instead, and use a heat exchanger on your side. Air won't conduct very much heat into the ground.

[random]

That would be more effective, yes, but I still think you'd want to run a liquid through the pipe instead, and use a heat exchanger on your side. Air won't conduct very much heat into the ground.

 

 

No that is defeating our purpose of a passive (cheap) system, We want mother nature to do as much work for us as possible. Perhaps the O-pipe needs more volume so the air can linger underground dissipating heat longer.?

[Cap'n Refsmmat]

Yes. You want a long pipe that is in contact with as much soil as possible, with air in the pipe for as long as is possible.

 

It may be more efficient to pump air from your house into the soil and back, rather than outside air; 90-degree outside air will take much longer to cool than 78-degree air from inside your house.

 

Some rough estimates:

 

Suppose you pump 1000 cubic feet of air through the piping each minute. (There are commercial products that can move air that quickly.) Suppose the air is cooled by five degrees Celsius while in the pipes.

 

Each minute, you can dump nearly 200 kilojoules of heat into the ground. That's nearly 11 megajoules per hour, which seems to be the equivalent of a 10,000 BTU window air conditioner, which some random online calculator tells me is good enough to cool a sunny 350-square-foot room with excellent insulation.

 

Caveats: You have to move a lot of air through the pipes each minute, while letting the air stay in the pipes long enough to be cooled by 5 degrees C. A 4-inch pipe 400 feet long would only give the air one minute underground, which I don't think is nearly enough to cool it down that drastically.

 

This is why I suggest using water. Water has a volumetric heat capacity over three thousand times greater, meaning you'd need 3000 times less water to dump the same amount of heat.

[random]

So I guess I could use a radiator system similar to what comes with an outdoor wood furnace, just a tightly looped 3/8 copper line with a fan behind it more or less and have the loop buried in the ground. they are 1 inch lines for infeed and outfeed so maybe with water the length of this could be substantially shortened?

 

Then along with some ventilating turbines in the roof and that would just about do it. Seeming as the average older house is too hot from poor ventilation in the attic.

[Cap'n Refsmmat]

Well, you want a well-insulated attic floor (to keep heat from moving through the attic floor and into the house) and an automatically activated ventilation fan; we have one at home that works off a thermometer and turns on automatically when the attic gets hot.

[random]

we had 24 inches of fibreglass insulation on the attic floor at our old place but it was still sweltering hot until we installed 2 turbines to exhaust the heat and ventilate the attic. So IMO the ventilation is much more important than superb insulation.

[Cap'n Refsmmat]

During the winter you don't want to ventilate the attic but insulate it well, because drawing in more cold air will just let the heat from your house leak into the attic.

 

Ventilating it well also doesn't protect you if it's already 100 degrees outside, since you'll have a 100-degree attic no matter how much you ventilate it.

[random]

No I need to differ with that opinion.......if it's 100 degrees outside and your attic isn't well ventilated it's going to be more like 130 in the attic as well if it is not well ventilated you cannot take advantage of the cool night air to dissipate the hot stagnant air of the day.

[Externet]

I would say cooling would be very satisfactory, this chart is important for your project, understand it :

 

-------> http://www.geo4va.vt.edu/A1/time-lag-vs-depth.gif

 

And light heating in winter as a bonus.

But I would use plain water in the pipe, would take a smaller pipe for the mass of flowing medium. Looping into a car radiator in the house ducting with a fan will pump the heat.

Now if you have a pond, river nearby or a water well, I would tap those sources instead of digging a trench that deep.

 

In a closed circuit, the faster the fluid velocity in the pipe, the more heat exchange. There is no such thing as slowing the flow allows more time for the heat to transfer.