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(Inspiration: It's 42 degrees C outside. And it's only
April!)
The idea is to deploy lightweight, semi-reflective
polygonal
plastic sheets over an entire city, supported by tethered
helium balloons, at 300m or so altitude (Just to clear the
tallest buildings). Adjacent sheets will have gaps
in
between - the idea is to reflect as much sunlight as
possible but not to trap air (or automobile exhaust). The
sheets will also have small holes to let water drain away
when it rains.
The balloons should be kept in place with lightweight
(nylon?) guy wires to keep them stable in moderate
winds.
They should also have feed tubes hanging to the ground
directly underneath, so that they can be topped up from
the ground periodically to compensate for helium loss.
The entire arrangement is light enough that if it
collapses
(due to a major storm, for example), it shouldn't cause
any
damage. All materials used have to be electrically
insulating and fireproof, but I think this can be
managed. The modular design will also allow parts of the
system to
be taken down for repairs/cleaning/replacement.
Needless to say, there won't be any cover over the
airport
and the aircraft approach paths.
The system can earn money as well in two ways - 1)
some
balloons can carry cellphone antennae (they will have to
be
larger to take the weight, plus with cables running to a
base station underneath, and have safety systems in
place
for reeling them to a controlled drop zone in the event of
a
major storm) and charge cellphone operators a rent.
2) I'll probably get fishboned for this, but this is far safer
and cheaper so I'll include it anyway - put advertisements
on the panels, either printed or projected onto them
from
below.
Here's an illustration
http://yfrog.com/j2solarreflectorg
How it might look with hexagonal panels and each balloon tethered at three points. [arvin, Apr 20 2010]
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Annotation:
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Green roofs, lots of green roofs. |
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AOL free CDs, lot of AOL free CDs. |
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If they could be directionally controlled, might they be aimed to reflect sunlight at a centralized array of solar panels to generate electricity? |
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"It's 42 degrees C outside."
Where? |
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//Where?// That's 107.6°F...[arvin]'s either standing out in an equatorial desert somewhere, or he just popped out of his exterior sauna and forgot to close the door behind him. |
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<edit: Actually I note from his home page that [arvin] is located in Kharagpur, India, so my first surmise wasn't terribly far off.> |
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I haven't updated my home page in a long time -
I'm in Ahmedabad, India now. |
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Turning this into a solar concentrator is a great
idea - I think an rough concave mirror can be
formed by just adjusting the heights of the
balloons. At the center of the city, a much higher
balloon-reflector system could get all that heat to
a ground-based generator (photovoltaic or
thermal) |
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Ahmedabad (and a lot of other cities) have a river
close to the center - this would make the
situation ideal for steam-based electricity
generation. |
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The only problem I see is that heat rejection from
a thermal generator in the center of the city
would partially negate the cooling effect of the
reflectors - but it might not be that significant. |
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I think you're great over estimating the lift capacity of helium,
assuming an ultralight material+ support structure of 500g a m^2 a 5km radius section would weigh 9800kg
You would need slightly over 10,000 cubic meters of helium at standard pressure to achieve this a balloon would have to be 810m in diameter. Or several smaller ones a fraction of that size. |
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This is why blimps are so large and have such tiny lightweight compartments at the bottom. |
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p.s: this idea was proposed by buckminster fuller except he used a self enclosed sphere that would heat up and rise from the temperature difference |
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//self enclosed sphere// can we make that airtight ? |
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This being the halfbakery, I feel the need to point out that hydrogen would be a more economical lifting gas than helium. Sure it's flammable, but so what? It's cheaper than helium, it's renewable (helium isn't), and if properly handled, can be safe. |
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One way to focus the light (for making solar power) would be to make each sheet into a Fresnel lens... or maybe a prism using the principle behind a Fresnel lens. |
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Or perhaps make the balloons themselves into the lenses/mirrors -- the outside of each balloon would be clear, with a circular reflector or lens dividing the balloon into two hemispheres. |
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As for the issue of heat rejection from a thermal generator... if the waste heat is hot enough, it will rise (due to buoyancy) into the upper atmosphere, and not significantly warm the city. In fact, it will create a cooling breeze, as cool low altitude air flows in to replace the rising air. |
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This will work best if the generator disposes of waste heat using a cooling tower -- not using once-through cooling water from a river. |
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FT, If you make it airtight, then how will the people below it breathe? |
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//airtight...breathe?// well... that's a switch: usually it's the cities make it hard to breathe outside of same. |
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[metarinka], thanks for the calculations. I think it
would be more practical to go with a large number
of smaller sheets, say about 1km radius. If we are
looking at hexagonal sheets with balloons at the
vertices, each sheet will be held up by the lifting
force of two balloons (6 balloons around a sheet,
each balloon contributing to 3 sheets); I think the
from your figures the volume per balloon would
come down to 200 m^3 or a radius of 5m. We'll
need one of those for every 1.5 sq km of cover
(plus more around the edges). |
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[goldbb]'s idea of fresnel lenses (actually, zone
plates would be easier to manufacture) would be
much easier than trying to make a parabolic dish
from flimsy sheets hold up in strong wind. It
wouldn't be possible to get a focal length of 300m
over an aperture of 50km though - it would work
better with a small PV or stirling cycle generator
under each sheet. (We could keep this at the
same spot as the tether landings, to minimize
footprint requirements.) |
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//It’s 4° out right now, and the sun’s been up half
an hour.// [Ian], would you like to swap places? |
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// a 5km radius // ... // 810m in diameter // and a tesseract. |
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// It's 42 degrees C outside. // I live in Dubai and that's an average day June - August. I can get up to 50C. |
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Assuming this system worked perfectly, you'd just get a warm breeze blowing into the city from the surrounding warmer areas. |
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The system may be of more benefit if it only targets hot spots, such as roads. The panels then don't need to be as high; you could simply suspend sheets of mylar between street lamps (how awesome would that be to drive under?). |
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Assuming some curvature, 1km of mylar covering a road would make a serious trough reflector (at least 10MW). |
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[arvin] I was in Ahmedabad at about this time last year and it was 42°C there then. I was presenting at a conference so I had to wear a suit as well - agh! |
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I like this but i have another idea i thought this was going to be and i'm about to post. |
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//Highschool pranksters with air rifles would just *love*
this// In the rural US, sure -- but that's easily solved. Just
avoid placing these in any locale where the road signs are full
of bullet holes. That would not, I think, exclude
Ahmedabad, or most cities, or, even most cities in the US. |
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//Highschool pranksters with air rifles would just *love* this//
I don't know of any air rifles that can fire vertically 300m... and neither do you... or shotguns... (I'm too lazy to calculate for hunting rifles, but certainly not rimfires). |
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// 10,000 cubic meters of helium // |
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Use Hydrogen. Cheap and safe, assuming the baloons are heigh enough, and the support structure not flamable. |
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Read above that 500g/sqm and radius of 5000m would give a mass of ~10 tonnes. It's much more like 40,000 tonnes. Which would need 40,000,000 cubic meters of helium. That sounds a lot, but if spread out over the whole surface would only be ~0.5m thick. Thus an enormous quilted silver blanket might do the job..? |
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To produce enough helium we'll need several fusion power plants. |
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As an alternative, we could keep a vast supply of air from the present atmosphere. In 100 years time, with an increased concentration of carbon dioxide, the air then would be heavier than today. That would give us a supply of 100 year old fresh air that floats. |
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I think if we remove the requirement of
withstanding rain, the reflective film can be made
much lighter - the Aluminized mylar used in solar
sail experiments is only 7 g/m2. Say we use
something that weighs 10 g/m2 (and needs to be
taken down when it rains). |
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A 500m radius sheet will weigh 7.8 tons, so each
balloon (assuming hexagonal sheets) will displace
about 4000
m^3, or spherical balloons with diameters of about
29m. |
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A 200 sq km city will need 500 of these balloons,
which would need 2 million m3 of helium. That is a
lot - the worldwide production of helium in 2008
was 169 million m3. I guess there's a reason it
hasn't been baked yet. |
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(All data from wikipedia) |
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Just so people have their numbers straight, 1 cubic meter of Helium lifts 1.03 Kg. So for every metric ton you need 1000 Cubic meters minus the weight of the lift envelope. |
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link is broken arvin... its just ads after ads |
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