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Frogwatch 04-24-2012 12:19 PM

Solar chimney
 
Here is an idea I will try to make for sailing. In hot weather with no shore power, staying on your boat can be miserable if there is no wind to allow a windscoop to be used. Any air flow thru the boat can make all the diff in whether you use her or not.
So, I want to build a "solar chimney" that will mount on a forward hatch. The bottom half will be either white fabric or even silvered reflective fabric and the top will be transparent material to pass the IR sunlight. Inside this upper part will also be a concentric black absorber. The top part would be raised using the forward halyard and it would have a conical cap to prevent rain from falling in it.
Sunlight will heat the air in this upper part causing it to rise producing a draft in the boat.
The trick will be to maximize the thermal differential between the top and bottom parts. Minimizing heating in the bottom is easy with silvery or white fabric. Getting good heating of the top part is harder. I'd probably make half of it silvered to reflect heat back into it while the front half is transparent. However, I am not sure I need the heat absorber to make maximum contact with the air to get good heating or if sunlight heating the air directly will do it.

I'll begin experimenting when I finish working on my super efficient solar still for making fresh water.

shogan50 04-24-2012 02:45 PM

Re: Solar chimney
 
No first hand experience with this, but I'm a mechanical engineer and took heat transfer 101 almost 2 decades ago. You can't heat the air directly with sunlight as it is transparent. You can largely only heat it by getting it in contact with a hot surface. Also, you don't need hot and cold parts, just make sure your hot parts are all at greater elevation than the cabin you want to cool. With that, just a black tube, as long as you can practically make it should work well. You might do a little research and find the "solar emmisivity" value for various materials (colors) (may be difficult to find.) Just because it is black doesn't necessarily mean its solar emmisivity value is low. An easy comparison can be had by laying candidate materials on a horizontal surface in the sun and measuring their temperature with an IR (laser) thermometer after the temperatures have reached steady state. An absorbing tube in a clear tube would be more efficient because you wouldn't have as much convection on the outside of the tube, but I'd be surprised if it was worth the trouble. Just make your tube taller to compensate.

My $.02

MedSailor 04-24-2012 03:08 PM

Re: Solar chimney
 
I like your idea and look forward to your results. What Frogwatch says makes sense and I especially like the idea of a black tube inside a clear tube. As I imagine the system, I think your biggest hurdle will be that it works too well, and the induced convection of air (the stated goal) will produce convective cooling of the tube itself, thus slowing the desired airflow.

How about a black tube inside a white tube with water in-between? Think sun shower (bag of water with one clear side and one black side). Water has an extraordinary specific heat capacity and won't be easily cooled by the air moving through the tube. You could even add hot water to kick-start it, though based on my experience with sun-showers it wouldn't take long to get it started. Also, if you make the inside tube longer than the outside tube, and sized it to fit a standard deck-plate hole, you'd have an easy way to stick it on the deck.

Keep us posed!

MedSailor

PS. Apparently this idea is quite ancient:

LakeSuperiorGeezer 04-24-2012 04:21 PM

Re: Solar chimney
 
I have a brother who works for the National Park Service. The Park Service has tried the solar chimney on both chemical and composting toilets. Engineers tried testing this to see what was going on as it did seem to work. What they found was that the wind accounted for the air movement. If you are going to do this, I would suggest testing it out first.Maybe a really tall chimney would work, but the boom and/or sails could be a problem. You would probably be better off with a battery powered fan with solar charging of the battery.

Barquito 04-24-2012 05:00 PM

Re: Solar chimney
 
Possibly another "thermal chimney" would be for higher latitude sailors (such as on Great Lakes) to, in some way, bring cold water from the thermocline layer (maybe 15 meters down) up to cool the boat. 'I'm looking at you, scientists'.

hellosailor 04-24-2012 09:00 PM

Re: Solar chimney
 
Anodize keel-stepped mast matte black. Solar chimney.

There's also more wind aloft, putting a turbine cap on it will work the same way it does on building vent caps, sucking up more air.

Frogwatch 04-24-2012 09:44 PM

Re: Solar chimney
 
I thought of using the mast that way but I sorta think that the smaller diameter might produce too much internal drag. Besides, mine is deck stepped. Try it, I'd like o know if it works. Instead of anodizing, you might just paint the top 15' or so black.

Barquito 04-25-2012 02:28 PM

Re: Solar chimney
 
Quote:

What exactly is it the great lake sailors need COLD water for???
I thought they already had surplus of that.
We just need to get all the ingredients in the right place. Toward the end of August, the air temp may be 90F with no wind, and the water temp... not 90F. A bit like geothermal heating. Just need to drop a tube unto the water and figure out how to passively get cool water to flow up to a heat exchanger in the smoldering hot cabin.

hellosailor 04-25-2012 02:58 PM

Re: Solar chimney
 
"Passively".
And that's where the perpetual motion schemes fall apart,nothing happens passively contrary to entropy.
"Swamp coolers" are old technology but you need yards and yards of piping or terracotta or whatever your heat exchange material will be on the "swapping" end. And then you need an active pump to haul up the water, and a way to clean the marine life out of the plumbing, or maintain filters to do that, and if the water is salty you'll need cupronickel or something else besides copper or aluminum.

Still, if you're in fresh water, don't have zebra mussels, and do have a power supply, water misting and swamp coolers both work. Might invest in a reflective awning or something simpler first though.

ebs001 04-25-2012 03:20 PM

Re: Solar chimney
 
Years ago I saw A/Cs adverised that used cold water pumped from below then through through a heat exchanger with a fan. I had forgotten about them until I saw this thread. I tried to google them but came up with nothing. They would probably be easy enough to build.


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