short solar chimney for pumping

[Kwazai]

Hi- just joined the forum.

I have a background in engineering but as you'll see its very rusty.

I been following the details on the 1000m chimney in Australia and have been really intrigued as to how bouyancy really works. I understand that the 100m on in manzannarres acheived  .01% efficiency and think I understand why. As a chimney gets taller the exit velocity is driven by the bouyant force. This velocity is greater than the bernouilli pressure drop across the chimney because the bouyant force acts primarily at the top. since the pressure drop occurs at both top and bottom a short chimney would leak to the point of not working. I ran some rough calcs to realize that the chimney (short one) would need to get larger as it went down towards the ground. I also realized that somewhere above 30FT you begin to get enough wind energy for charging batteries, somewhere above 100ft for true power generation. If the opening at the top is sized for continuous flow at the top of a short chimney (density pressure drop at bottom volume flow equal to bouyant force volume flow at the top) then a steady 'solar wind' could be gotten at the top of the chimney. I have been looking at using the inside of the roof of my house as a very cheap (though inefficient) solar collector and in trapping the heat would need some way to ventilate it  to avoid moisture problems- hence a short chimney(less than 20ft).

It would be nice to be able to pump water with the solar chimney as part of a solar heating system (not producing electricity). With a 10FT chimney sized for constant volume flow-with 7sqft exit (somewhere in the neighborhood of 3ft/sec velocity)  (assuming a 100% efficient horizontal ducted savonius-actually closer to 14% from what I've read) I would get 21rpm-,09ftlb torque on a three foot diameter savonius. I am not real familiar with the savonius- much less a horizontal one ducted to one side flow only so the efficiency number is based on a standard savonius. Given the efficiency numbers it doesn't seem worthwhile to do vs. a pv cell and pump for about the same cost. What I don't understand about bouyancy is this-

if I have a chimney sized for constant volume flow (read no leak at top) and put a large funnel in the bottom (chimney still sized for constant flow-like a big balloon with small hole at top-large hole at bottom) and connect the funnel to a manometer- I would read the pressure drop above the small hole at the bottom (small amount of head). if I fill the funnel to the large hole side with water-still connected to the manometer- I get the manometer reading much larger due to hydraulics. If these measurements are taken in a balloon that is not leaking I get the air pressure based on density. I am wondering if I have continuos flow chimney (trapped hot air but volume flow) and take the same measurements with a manometer will I get the manometer to read the bouyant force?

Hopefully there is someone out there who understands bouyancy.

tia

Mike

[picmacmillan]

http://www.eng-tips.com  i also am an engineer and if you don't stay current, you get lost in something like this....go to this website and let a few of the new brainiacs give this a shot .....there wa a lot of information in your post and it would take me a week just to understand it as it is not my specialty,...in fact i don't have a specialty maybe forces and vectors..maybe someone else here can get it?.....good luck on that though, i would be very interested to see what you come up with...while you were giving us the specs, somehow i was skeptical of the figures as towards volume, atmospheric pressure 14.7kpa? etc....anyhow, good luck, hope you can figure this out....pickster

[Kwazai]

these pics may help. the first is the assumptions for constant volume flow with bouyant force driving the velocity at the top and pressure drop (assuming no bouyant force) driving velocity at the bottom.  makes more since to put the 'turbine' at the top until you get real tall with the chimney.

the second pic is the part I don't understand- related to how the bouyant force behaves when moving- first part is just air at base of chimney, second part is water hydraulics- my assumption is that the differnce in density of the air is the only thing read in the second one if it is a balloon instead of a chimney- the questions what is it when the air is moving? (same?)

hopefully they are readable.

[picmacmillan]

well, i don't know how much further we are on this but,newtons second law of motion states that an object will change velocity if a force is acting on it....if i get you right, you are saying the air will excel out the pipe on to a turbine and generate enough energy to pump water....i see where you're going with the height of the pipe and the diameter....it is very hard to read the picture...i see force=mass x acceleration, and velocity= acceleration x time,...i also see force =prssure x exponential?....lost me there...see if you can get this picture a little clearer....without solving this, it appears our quandry is in something i have sometimes pondered , if you could dissipate gravity(14.7) behind or in front of an object not with turbulance, but with  0 gravity, acceleration as far as i know would be enormous....it would seem to me that this has already been solved, but i would think that there is a number of ways to collect energy in this application but in the confines of what would be practical in what we do here, it would not work(in my opinion)..it may even be possible to utilize the peltier effect with a heat source??..there simply is not enough volume to create more than a breeze...in the confines of the gennies we build here, you would have to create a minumum of a 7 km steady breeze to be effective, unless i am missing something here which is quite possible...i keep thinking of the torque, but ..i just better leave this to someone who still knows there stuff....nice idea though..very intriguing...pickster

[hobot]

I've been piqued by heat rising chimney to tap flow for power also but have NO background nor terminology just some simple ideas.

Almost moved into a south facing cove with serveral hundred feet drop

to the bowl like slope and rather stiffling heat in season. Kept

picturing a chain of black plastic duct or oil drums starting

out horizontal to run up to top of ridge and either stick a fan

in it or put a crockscrew/eggbeater turbine at the exit, part sticking down

into exit venturi and part sticking up in prevailing wind to add its part to

the equation. Thot also to make it a fire chimeny too with fireplace or other

heat source with hood over to collect.

To measure the buoyancy, I assume you mean cold vs hot air lift, maybe block

chimney to flow and use manometer tubes to tap pressure at top vs

the bottom? Lots of long tubing with a big loop somewhat nidway and

maybe lighter fluid than water to push about.

hobot

[Kwazai]

the top is force mass equation ,the bottom is bernouilli assuming there is no work done across it-pressure drop drives the velocity --balancing the two so the chimney doesn't leak from the top--constant volume flow for a given (temperature/ height) velocity. Like a trapped balloon. the lift of the balloon (assuming it has no mass) is a function of the volume (for a 3ftdia x 10fttall chimney-about 1.72 ounces).

The part that confuses me about bouyancy is that it only acts over an open area like the hole at the top and the hole at the bottom. if i put the manometer into a ballon 10ft tall full of hot air then i'll read the pressure differnce of the height of reduced density air.

 by hydraulics (not hydrostatics) a fluctuating pressure over a 3ft funnel of water would be this pressure drop (the fluctuation from chimney flow maybe?) over a larger area and moving a larger amount of water in the manometer. (with the upper limit (100%eff.?) at the bouyant lift 1.75oz.mass of water or with the upper limit being the movement of the water at the top of the funnel due to the density pressure drop of 10ft at 20ft (.04lbm/ft3).

from the calcs i did the pressure drop would actually exceed the water volume flow of the 1.72oz balloon lift. bouyancy just doesn't make a lot of sense- like having a bubble pump or something.

whether the height of the water in the funnel would be affected by the balloon lift mass or the pressure based on its height is what I don't get.

the reason I started looking at this is beacause my roof would make about 300sqft of low efficeincy solar collector with the simple addition of a radiant barrier-but in trapping the heat it would need to be ventilated. a standard house chimney is about 2x4 and an equivalent diameter is about 3ft, a 2x5 would yield about the right increase in area for a 10ft height of chimney (rough calcs-really rough (equiv.52" diameter)

the one plus and I hear this as a drawback with conventional wind electric is this solarchimney would be pretty steady flow- something like a bicycle wheel-poly sheet(turbine ventilator and get the wind too?) version of thegreenwindmill would run off of it. the torque I am assuming would be a function of the area of the windmill-at a ten foot-3ft/sec would be at best somewhere around 1.72oz( i am assuming) per 3ftcircles area.- a vertical version of an old farm windmill might work better-with the added bonus of being a nondirectional windmill with the blades tilted.

this same amount of heat could be radiated from a pvc pipe poly sheet greenhouse-feeding a pvc pipe chimney. I would guess that somewhere around an 18x24 greenhouse would feed a 3ftdia.x10ft fairly easily-with turbine on top.

the thing about electricity generation is batteries of one sort or another. cheapest battery i can think of is two ponds, one higher than the other.(my .02$)-wasn't sure where to post this topic.

Admittedly 1.72 oz. at 3ft/sec isn't much.

With a rectangular chimney something like a horizontal savonius might make more since. There is a physicist on the net that has some ideas about directional savonius turbines that would increase there efficiencies.

my engineering skills are pretty rusty so please correct me where I am wrong.

Mike

[Kwazai]

bigger pictures

somewhere around a thirty foot height would be needed for electicity with the turbine at the top-(somewhere around 6ft/sec velocity)

I keep trying to think of this like an upside down waterfall.

[Kwazai]

Started putting one together (10FT tall) after checking with the proffessor at the University of Florida for data on theirs. There calcs for the 'collector' part of the chimney confirmed what I was getting at in terms of constant volume flow. I built part of it last weekend- a pipe 'tipi' looking thing and am trying to track down a 10' x 50' roll of 1.2mil clear plastic (transmissivity is better than the 4mill vapor barrier stuff). The base of it is somewhere around 14FT diameter. From the 'rough' calcs I ran- this is the bare minimum that will turn my bicycle wheel 'savonius' (drag turbine- not actually savonius). I'll post pics when its put together. worse case is it doesn't turn the  'wheel' and I have to extend it up another ten feet and try again.

I looked at the pressure drop available from the top of the chimney to the bottom and if I understand the math correctly (bouyancy?) then I would need the bottom to be over a funnel of water to get anything other than the difference in the density of the air as a manometer reading. The water at the bottom of it should be affected by the height of the column of air rather than just the air density, the air alone would only be a density change. not sure if that is right or not , but still only talking .0837 psi as a manometer reading (way, way to small for my purposes- about 3 inches of water head).

L8r

Mike

[Kwazai]

not enough airflow to make it work. started rethinking the 'leaky balloon theory' and have something I plan to try. I need to find some info on making some cheap, cheap barometric dampers. I am going to put them at the top of a short chimney(refrig. box) and hopefully stop the chimney from 'leaking' at the top even with laminar flow velocity coming out. It should allow me to ratio the diameters and get some pressure drop at the bottom-not sure where the actual stagnation/static pressure drop will be-but at the very least will keep from heating air that doesn't run thru the inlet. ( Manzanarres efficiency was only .01%-will this increase it to .02%?) any ,just an update.

Mike

[Kwazai]

[Kwazai]

the way I invision it working (and this may be wrong thinking) is that the warm air would build up in the box until there is sufficient height of warm air to pop the damper open and exhaust some, at the point the damper would leak air into the box it would close, much like an 'open' stirling cycle exhausting the hot air. the displaced volume would be replaced thru the banki turbine at the bottom and the cycle start over again. I am assuming that the sealing area of the damper would need to at least match the area of its opening and from there be barometrically adjustable. The dampers could be made from the sides of a plastic milk carton with a damper hole diameter of 2" at the box and an overall diameter of 3", balanced at the centerline, with a couple of 45 degree bends so that the top 'flap' of the damper is above the top of the box and the bottom 'flap' of the damper is below the box top. when it tilts open it flows. a small piece of tape could be used to adjust the opening pressure. I invision it as a bunch of small dampers rather than one large one (variable volume flow )at the top.

the 'solar' side of it is that the box would have foil "wings"(dish/trough/etc.) to increase the collection area that impinges on two of the four sides-the other two being insulated.

anybody out there have an opinion/experience/etc. in regards to something like this.

Mike

[Kwazai]

complete rethink- feedback wanted-see pic

Mike

[Kwazai]

Anybody know where I can find cheap 24" diameter pipe?(30ft. lengths). I went back to the original assumptions and a 2ft diameter 30 foot pipe with a 20degC temp rise should net about 6 watts with a 15% efficient genny.(it would take 14ft diameter 50ft to get any real power(200watts) out of it-(with the turbine at the top.)The losses from the pipe flow have to be figured in for a turbine at the bottom.

30ft height-20degC deltaT should net a 2 to 3 m/s airflow velocity, 50ft should net a 5 to 6m/s airflow velocity.

I still have questions regarding bouyancy -if anybody has a better understanding of it. The natural draught doesn't appear to be affected by the diameter of the bottom of the chimney only by the exit height. I will be looking at natural draught pressure drops as they relate to diameters at some point in the future. any insight would be helpful.

Mike

[Kwazai]

more thoughts...