Half a croissant, on a plate, with a sign in front of it saying '50c'
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I like this idea, only I think it should be run by the government.

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City-sized solar reflector

Cool down cities with reflective sheets held aloft by balloons
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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.

arvin, Apr 20 2010

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]

[link]






       Kites. Lots of kites.
RayfordSteele, Apr 20 2010
  

       Green roofs, lots of green roofs.
MechE, Apr 20 2010
  

       AOL free CDs, lot of AOL free CDs.   

       Sorry, wrong decade ...
Aristotle, Apr 20 2010
  

       If they could be directionally controlled, might they be aimed to reflect sunlight at a centralized array of solar panels to generate electricity?
swimswim, Apr 20 2010
  

       "It's 42 degrees C outside."
Where?
phoenix, Apr 20 2010
  

       //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.   

       <edit: Actually I note from his home page that [arvin] is located in Kharagpur, India, so my first surmise wasn't terribly far off.>
jurist, Apr 20 2010
  

       Hi,   

       I haven't updated my home page in a long time - I'm in Ahmedabad, India now.   

       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)   

       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.   

       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.
arvin, Apr 20 2010
  

       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.   

       This is why blimps are so large and have such tiny lightweight compartments at the bottom.   

       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
metarinka, Apr 20 2010
  

       //self enclosed sphere// can we make that airtight ?
FlyingToaster, Apr 20 2010
  

       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.   

       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.   

       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.   

       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.   

       This will work best if the generator disposes of waste heat using a cooling tower -- not using once-through cooling water from a river.   

       FT, If you make it airtight, then how will the people below it breathe?
goldbb, Apr 20 2010
  

       //airtight...breathe?// well... that's a switch: usually it's the cities make it hard to breathe outside of same.
FlyingToaster, Apr 21 2010
  

       It’s 4° out right now, and the sun’s been up half an hour.
Ian Tindale, Apr 21 2010
  

       [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).   

       [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.)   

       //It’s 4° out right now, and the sun’s been up half an hour.// [Ian], would you like to swap places?
arvin, Apr 21 2010
  

       // a 5km radius // ... // 810m in diameter // and a tesseract.   

       // It's 42 degrees C outside. // I live in Dubai and that's an average day June - August. I can get up to 50C.   

       Assuming this system worked perfectly, you'd just get a warm breeze blowing into the city from the surrounding warmer areas.   

       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?).   

       Assuming some curvature, 1km of mylar covering a road would make a serious trough reflector (at least 10MW).
marklar, Apr 21 2010
  

       [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!
hippo, Apr 21 2010
  

       I like this but i have another idea i thought this was going to be and i'm about to post.
nineteenthly, Apr 21 2010
  

       //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.
mouseposture, Apr 22 2010
  

       //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).
FlyingToaster, Apr 22 2010
  

       // 10,000 cubic meters of helium //   

       Use Hydrogen. Cheap and safe, assuming the baloons are heigh enough, and the support structure not flamable.
Inyuki, Apr 23 2010
  

       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..?   

       To produce enough helium we'll need several fusion power plants.   

       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.
saedi, Apr 25 2010
  

       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).   

       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.   

       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.   

       (All data from wikipedia)
arvin, Apr 26 2010
  

       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.
metarinka, Apr 26 2010
  

       link is broken arvin... its just ads after ads
mofosyne, Dec 10 2014
  
      
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