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Urban Roof Reservoir Power Plant

Above the city, catches rain, provides fresh water, generates power.
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The above Summary just about describes it all. You build a flat transparent roof over the entire city, say 1 km above the ground, supporting it with thousands of columns. All available skyscrapers will of course be enlisted to help. Then add walls around the edge of the roof, going upward from the roof. The higher these walls, the thicker they and the roof need to be.

Let that basin now catch and hold rainwater. This becomes an extra City Reservoir for fresh water. When the water flows down from the roof/reservoir, it passes through turbines that generate a decent amount of power. The flow probably won't be so great, but the height of the water column will be super, compared to the height of the water behind ordinary hydro-power dams!

Whenever there is so much rain that the reservoir can't hold it, just generate more power and let the water flow through an aqueduct toward, say, some other city that wasn't so smart as to invest in a roof like this one, that needs the water because the rain they receive just flows away without being captured.

Since water is mostly transparent, but not entirely transparent, the city will still receive daylight, but it will be diminished somewhat. The city should be cooler, therefore, under this roof/reservoir.

So we get a degree of shading, an enhancement to water storage, some control of wasted rain-run-off, and some extra power.

The biggest problem I see has to do with trees and gardens and other plants, throughout the city. Plant leaves absorb a significant amount of red light and blue light (which is why they look green). The water above the city will let blue light through, but will absorb red light. The plants may suffer, depending on how much they actually use the red light they absorb. But it depends on just how deep is that roof/reservoir. A couple meters might have a relatively insignificant effect.

Vernon, Aug 17 2012

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       I'm not sure about the power generation. Average rainfall in the UK is something like 80mm/month, or about 1 metre per year. If the building is 30m tall and 30m square (say), it will capture 900 tons of water per year. If this falls through the full 30m, then the energy available will be 900,000*10*30 = 270MJ per year, or about 9 Watts.
MaxwellBuchanan, Aug 17 2012
  

       Sure, but cover all of London (1570 km^2), and you could generate a cool 15 megawatts. Granted, that's only about 1/60th of the output of your average nuclear plant, but it's FREE ENERGY (barring the cost of the structure, as well as the potentially disastrous effects on the environment of disrupting the water cycle, filtering out substantial amounts of sunlight, and creating what's effectively a giant greenhouse—not to mention the constant threat of structural failure causing some 1.5 gigatonnes of water to flood the city without warning).
ytk, Aug 17 2012
  

       //you could generate a cool 15 megawatts// Does that assume 100% coverage with 30-metre buildings, and 100% efficiency?   

       At the end of the day, you're still looking at 9 Watts per building. Given the energy requirements to make, install and maintain the apparatus, this would have a truly horrendous carbon footprint.   

       It would also be a financial disaster. Electricity costs at most £0.15/kWh in the UK (probably less for businesses). Hence, the savings in electricity costs over a year will be £1314 [EDIT: about £12 - not sure where the maths went wrong there!] for the building used as an example. The costs of installing a suitable roof reservoir and generator, and of the gear necessary to feed that into the main supply, I would guess would run to many tens of thousands of pounds. Maintainence costs per year are likely to far exceed the annual electricity saving.   

       Far cheaper to install a few square metres of solar panel, or a cheap caravan-style wind turbine.
MaxwellBuchanan, Aug 17 2012
  

       hmmm, how much energy could you get from a roof-top parabolic reflective solar steam piston engine I wonder?   

       // You build a flat transparent roof over the entire city, //   

       Solar gain ? Man-made impermeable inversion layer ?   

       // say 1 km above the ground, //   

       No problem there ... apart from the need for developing entirely new high-rise techniques, and the huge amounts of equipment, materials and manpower ...   

       // supporting it with thousands of columns. //   

       ... using staggering quantities of steel and concrete, which require vast energy input to manufacture, have then to be transported to site and assembled, and then in the curing process the concrete releases even more heat ...   

       // All available skyscrapers will of course be enlisted to help. //   

       ... adding a massive amount of dead load the structures were never designed to support.   

       // Then add walls around the edge of the roof, going upward from the roof. The higher these walls, the thicker they and the roof need to be. //   

       And the columns supporting them. And the amount of space at ground level taken up by the bases of the columns, and also space for the piles of skyscraper rubble, and the new graveyards for all the citizens crushed to death by collapsing overloaded skyscrapers.   

       Apart from that ...
8th of 7, Aug 17 2012
  

       Water does have a high emissivity for most wavelengths longer than visible light, so the temperature moderating effect would be great. And you would be well protected from orbital neutron bombs. You may need to breed more freshwater- tolerant manta rays, though.
spidermother, Aug 18 2012
  

       // you would be well protected from orbital neutron bombs. //   

       … while remaining tragically vulnerable to anyone at ground level with a pneumatic drill, a few kilos of HE, and an evil intent.
8th of 7, Aug 18 2012
  

       Ventilation of smoke, vehicle exhaust and general airborne pollutants trapped under the roof reservoir would soon become the overweening issue.
AusCan531, Aug 18 2012
  

       Evacuation of the dying and burial of the dead is more likely to be the" overweening issue" …
8th of 7, Aug 18 2012
  

       Well, people climbing up the towers to take a leak into the reservoir is likely to become the "overweeing issue".   

       Also, I see nobody has mentioned the fact that this would essentially ground all air traffic in the area.
ytk, Aug 19 2012
  
      
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