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would this work--solar chimney sucking through tube, conductive butt-cooler

 
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Basically, combine the concepts of cool-the-person-not-the-room and the passive solar chimney drawing air.

I'm thinking, what if you could sit on the cold coils of the air conditioner...
and the air conditioner were powered by passive solar energy rather than photovoltaic...

| |
| |
\/
-------------h


The "h" is a chair that the air is sucked through as it goes out the chimney
The area of chimney-ness above the V is black on the north, open glass on the south and top, like your usual solar chimney.

I considered a wider chimney with mirrors reflecting sun at a tube, but then the tube would heat up and conduct heat along its length (if it's copper) and down to your butt, undoing the benefits.
So it could be glass perhaps and painted on one side, but that's a bit fragile.

But in any case, the question is if this system would have anywhere near enough energy to do anything useful whatsoever.

Maybe it could be combined with a solar chimney cooling in the usual way, drawing the hot air off the person.

Does this already exist? is it actually focusing energy more where it needs to be or less?
How drying is a solar chimney (would it dehydrate the person if they're sitting in its path all day?)

 
Joshua Myrvaagnes
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so far I know:

--there is a usb butt-cooler that is/was sold at one time, but no reviews of it from a web search.  usb is max. 7.5 watts.  I don't know if it actually worked or not.  It sold out though, I think they said.  Thanko company, in Japan.
--there is potentially 1,000 watts of power that would hit the solar chimney per square meter of it on a sunny day...if it's big, say 1 square meter on its top and then 5 meters high, sun angle where we are at zenith in summer = 60 degrees elevation, so those 5 meters will each get a minimum of 2500 watts and, at sunset, it will actually climb almost to 5,000, plus the top will get at zenith 700 watts = 2200 to 5000 watts over the course of the afternoon.  If all of that energy goes into heating the air (some heats the chimney itself instead, but hypothetically, to find the upper limit), then you have 2200 watt fan or 5,000 watt fan, that would be a good bunch of power.  Uncomfortable to have that blowing in your face, but maybe OK if it's blowing past your butt through a copper tube somehow.

(Keeping copper tube form breaking, I dont' know, maybe it has to be in a hot water bag/bladder with enough pressure that the butt doesn't quite hit the tube.)

Friction of the air against the tube will add a lot of heat.

 
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A novel approach.
How much heat do you expect from friction?
 
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I would expect you'd be better off putting the chair in the intake, wherever you are drawing cool air into the system..

Not only would it be a cooler breeze for the person sitting there, now you can locate the entrance to the solar chimney at a high point, removing the hottest air from the room/building, rather than some of the coolest...
 
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Traditional Passive Cooling during the day work by increasing airflow(wind) and thus effectively increase the rate of evaporation.
Additionally the inside of house usually have higher temp and humidity so by simply exchanging the volume of air we feel better, thus can be done with a whole house fan in the roof. That way the stratified extra hot&humid air (showers, boiling/cooking) can be sent outside.

But to answer your question I am going to make a couple assumptions below:
Heat added to conductive tube via human = 200W or 682BTU
Constant Temp of Tube under human = 80F
Temperature of cold air in tube = 70F
Temperature Delta = 10F
Now we just need to know the flow rate (cfm) need to keep the tube at 80F. The bigger the chimney the faster the flow rate.
We will also have to consider how we are going get a temperature 70F for the incoming air. I assume there is some earth tube buried 8ft below in some 60F earth. The higher the the airflow, the longer the length/surface area of buried earth tube has to be.

Heat Removed = Airflow x deltaT x 1.08
Airflow = Heat / (deltaT x 1.08)
Airflow = 682/(10 x 1.08)
Airflow = 682/10.8
Airflow = 63cfm
(If the temperature difference between the cold air and the tube was only 1F vs 10F we would have needed a airflow of 630cfm going thru the conductive tube/chair
 
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I'd wonder how hot the makeup air that would have to come in to account for the solar chimney outflow would be. Would you lose more temperature due to hot air coming in? Depends on your climate....
I lived in the desert most of my life, we always watched air intake anytime we vented, as that was the worst part of cooling.
 
Joshua Myrvaagnes
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Thanks for the replies.

My assumption is not a buried coil, for various reasons, but just drawing from the shady side of the house.

Calculating based on some numbers of cubic-feet-per-minute per watt for industrial exhaust fans, (between 4 and 20 cfm per watt) 4,000 watts would be 16,000 cfm to 80,000 cfm.  (So, the higher figure of 630 cfm needed, it's an order of magnitude higher.

Through a ten-foot wind tunnel surrounding our victim at their workdesk, that would be 1.3 miles per minute or 90mph wind upper case scenario, or 18 miles per hour in the lower.  The 90 mph wind would be enough to knock you over, that's a hurricane; the lower figure would be uncomfortable after a few hours.

_These are theoretical figures, but just trying ot get a baseline read on how much air draw happens in a large solar chimney._.

I think putting some of the draw into this wind tunnel would be workable, and then funneling the rest into the butt-cooler tube would make some sense.

If it needs to be cooler than 80 degrees for it to work, that throws a monkey wrench into things.  I had assumed 98.6 degrees for the temperature of the human.  I have a bony butt, so it's hitting the bones themselves and cooling those.  For more endowed butts, I'm imagining it's close to contacting some blood vessels, through a bit of insulation....does the 80 degrees figure have a source or is that speculation? can you say more what you're thinking in terms of that part?

If there could be a slight bit of evaporative cooling element in this that would be great, but I'm designing for New England which is usually muggy in the summer when it's hottest--50-90% humidity from a recent glance at the government weather data.

I'm thinking a cooper tube is less workable than a couple of copper pipes, and then latex rubber hoses.  Wider == less friction with air passing.  
 
Joshua Myrvaagnes
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The air inside the house seems to really heat up; we don't have shade on the south side of the house here.  Basically, this is a rather speculative idea overall, but imagine you can't grow a tree or vine on the south side, or put up a shade over the house, and you aren't allowed to dig down underground to bury pipes either...can you build something up aboveground that beats a standard air conditioner?  I don't know how I'd hide a giant solar chimney above the house but just pretend...

I guess one question I also have is what percentage of the cooling effect of a fan on us (our bodies) is from evaporation (sweat evaporating) and what is from having our own heat blown away? if the air is even hotter than I am, it won't cool me down, but I've always assumed that the air is not hotter than 98.6...I guess when we do have 100-degree days then it's hotter air.

The other approach to this whole thing would be somehow using the energy of moving/heated air to effect compression, move that air to the butt-cooler, and then allow it to decomress there.  A passive solar mechanical air conditioner.  I get stuck in trying to find out how materially a compressor actually works, though, the internet is very vague.

Pearl Sutton wrote:I'd wonder how hot the makeup air that would have to come in to account for the solar chimney outflow would be. Would you lose more temperature due to hot air coming in? Depends on your climate....
I lived in the desert most of my life, we always watched air intake anytime we vented, as that was the worst part of cooling.

 
S Bengi
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I used 80F for the temperature of the Tube/chair because, I envision that being the temperature of your house. (Actually it should be a bit cooler than your house temperature due to the cooling airflow). And that you effectively want it to stay at that same temperature while it take the heat from your body and send it outside to the chimney area vs just store you heat until it is as hot as you or hotter (due to airflow friction/etc). And yes there would be a temperature difference between you(~100F) and the tube/chair(~80). And I am using nice round numbers so that I can do the math in my head.

Air Temp at intake = 90F in the shade
Tube/Chair Temp = 91F
Temperature Difference = 1F
Heat Removed from 98.6F Human = 200W or 682btu
Airflow need to remove 682btu with a temperature difference of 1F = ?

Airflow = Heat/ (deltaT x 1.08)
Airflow = 682/ (1 x 1.08)
Airflow = 682/1.08
Airflow = 631cfm

It makes more sense to put your face/neck right in front of the air intake as it enters the house, you will remove more heat via increase evaporation that by conduction.

As for the chimney side of your equation. I don't know the deficiency of the chimney solar collector. But the average for ones that are sold for solar water heaters is 40%, I am sure that one that I make would be closer to 20%. which means that they are just about even, at mid-day full sun, no cloud.
 
Joshua Myrvaagnes
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Thanks.  So you're saying the efficiency of the solar chimney is probably 20%, correct?  


S Bengi wrote:I used 80F for the temperature of Tube/chair because, I envision that being the temperature of your your house. (Actually it should be a bit cooler than your house temperature due to the cooling airflow). And that you effectively want it to stay at that same temperature while it take the heat from your body and send it outside to the chimney area vs just store you heat until it is as hot as you or hotter (due to airflow friction/etc). And yes there would be a temperature difference between you(~100F) and the tube/chair(~80). And I am using nice round numbers so that I can do the math in my head.

Air Temp at intake = 90F in the shade
Tube/Chair Temp = 91F
Temperature Difference = 1F
Heat Removed from 98.6F Human = 200W or 682btu
Airflow need to remove 682btu with a temperature difference of 1F = ?

Airflow = Heat/ (deltaT x 1.08)
Airflow = 682/ (1 x 1.08)
Airflow = 682/1.08
Airflow = 631cfm

It makes more sense to put your face/neck right in front of the air intake as it enters the house, you will remove more heat via increase evaporation that by conduction.


---so convection-evaporation works better in general for cooling?  

As for the chimney side of your equation. I don't know the deficiency of the solar collector. But the average for once that are solar for solar water heaters is 40%, I am sure that one that I make would be closer to 20%. which means that they are just about even, at mid-day full sun, no cloud.



The beauty of it is that if it gets cloudy, the temperature should drop slightly anyway, if the sun comes out full strength then the cooling increases too.

Is there any other way of "cutting out the middleman" for cooling?

 
Joshua Myrvaagnes
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OK I finally found an "how a compressor" works explanation that actually explains how a compressor works--it's basically a motor rotating, it seems, and lots of little check valves push the fluid/gas stuff into the chamber.  You could get that same effect to some extent with a turbine.  If its axle went down to the bottom of the solar chimney, it would be in the cooler air, and closer to where the compressed coolant needs to flow to (the butt needing cooling).  

I'm just puzzled by the thanko invention, did it even work at all? 5 watts? I think it would take a Japanese speaker to track down that info.
 
Joshua Myrvaagnes
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What is the effiicency of a standard air conditioner?  It is not measured in percent--it is stated that it moves X units of heat away from the interior per Y units of energy, and these ratios are generaly higher than 1 (seems impossible, but the air conditioner's moving the heat around, not actually cooling it in net impact).

The standard ac has an efficiency of about 10% of the theoretical maximum:

Assuming an outdoor temperature of 95 °F (35 °C) and an indoor temperature of 80 °F (27 °C), the above equation gives (when temperatures are converted to Kelvin or Rankine) a COP of 36, or an EER of 120. This is about 10 times more efficient than a typical home air conditioner available today.  (wikipedia)

how does this compare to a regular fan just blowing the hot air out the window, or blowing heat off of you out the window?

 
Joshua Myrvaagnes
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A lot of friction and energy loss in the standard AC I'd think, moving parts, a "swash" that moves 5 pistons up and down to compress the regrigerant.  I imagine if you could just build up air pressure in a single chamber and pressurize that, you could theoretically make tank (like a propane tank, for example) full of super-compressed air and then open a valve to get cooling when you want to use it, for example for nighttime cooling.
---
WIth 20% of the 16,000 cfm you have 3200 cfm still.
With 20% of 80,000 it's 16,000 cfm.

That's still a lot of airflow.

It's still a 5 mph wind (lower figure) to 27mph (higher) through ten square feet of sitting area.

It occurs to me that the radon problem if you do bury tubes underground is less harmful if the system pass through a butt cooler and out the top of the chimney, vs. directly into the person's face.

 
S Bengi
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Sadly it looks like the airflow will be a lot less as in only 16cfm(0.0077m3/s) with a 1m by 1m solar collector.

The efficiency of unglazed transpired type solar collector was observed as 38.41 % under 0.0077 m3/s airflow rate,


https://www.sciencedirect.com/science/article/pii/S1877705818301930#:~:text=In%20developing%20countries%2C%20solar%20drying,application%20for%20the%20food%20preservation.&text=The%20efficiency%20of%20unglazed%20transpired,of%20304.24%20W%2Fm2.
 
Joshua Myrvaagnes
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I'm starting to get where hte inefficiency goes, where the gap in my thinking was.

The majority of the energy goes to heating air that then disperese out above the top of the chimney.  That is work wasted, in a sense.
To some extent it helps draw the air up to have heat above the top of the chimney, but there's diminishing returns, and diffusion.
Comparatively, a bit  of energy in an electric fan is lost to friction heat, but not nearly as much, and the fan does a lot less heating of the air just to move it.
The electricity conversion efficiency of a solar tower is about 1% at large scale (using turbines--about 80% efficiency).  So the energy-to-air-movement ratio is approx. 100 to 1 also.
That inefficiency is not terrible, since it costs nothing ot create the other 99% of energy, but it would be a tempting thing to try to put it to better use.  Solar dehydrator is obviously a much better use.  Clothes dryer is another potentially...
Putting that hot air to use as a compressor (via a membrane pushing against another sealed chamber) is another possibility where the heat is more "used" than if it's allowed to escape.

Over all, putting the wind tunnel around the butt or chair of hte person working at a desk, vs. just trying to suck the hot air out of their whole house, may be an improvement over the original solar chimney cooling design.  This does sacrifice the secondary use as fire safety measure (sucking smoke out of the whole house) but in an emergency you could probably easily knock down your wind tunnel to allow it to suck smoke out of the house.

The wind tunnel wouldn't be a 60 mph wind, more like 0.6.

In the paper you sited S. Bengi, I'm not sure whether the airflow they refer to is the ambient wind airflow or something else, it's written a bit confusingly.  But the efficiency they site of about 40% seems to refer to efficiency in drying/dehydrating, rather than drawing air.

This has helped me understand solar chimneys a lot better.  Thanks for helping everyone.
 
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