This concept is to create a mile-diameter shiny aluminized Mylar film and to be able to move it around over a city, at a very high altitude of about twenty miles. Some of the sunlight is blocked from ever getting to the people and the buildings of a city, and also a significant amount of that sunlight is even immediately reflected back upward, to outer space, in a process of very slightly increasing the reflective Albedo of the Earth in that tiny location.
This graphic is very crude simply showing the large, mile-diameter flat shiny aluminized reflector (surrounded by a nearly invisible hula-hoop-like ring of hollow Mylar filled with buoyant Hydrogen gas) (which should be far higher in the light blue sky than shown here) over its own nearly mile-wide shadow due to the Sun being far above it. The blackness of the shadow on the dark green ground or ocean is also not quite as dark in reality.
I Engineered an eight-inch-diameter plastic Mylar tube that is 16,000 feet long, where the two ends are joined, and inflated by Hydrogen gas. This results in the tube inflating to become a huge hula-hoop-like ring, a mile in diameter.
Those dimensions and the characteristics of Hydrogen displace about 800 pounds of atmospheric air with about 100 pounds of Hydrogen. This creates a Buoyancy of around 700 pounds of lift.
The Mylar film is so thin that the entire ring of Mylar only weighs around 180 pounds.
The Mylar ring actually starts out as 32 separate 500-foot lengths of 8-inch Mylar tubes which are raised by a conventional Hydrogen balloon up above the disruptive motions of the atmosphere, where the 32 lengths are joined together by a special 'Coupler' fitting so they will become airtight and joined at both ends to become a ring.
From the Hydrogen balloon, at maybe 50,000 feet altitude, pre cut rolls of aluminized Mylar sheeting are then uncoiled and pulled across the ring diameter to eventually cover over the entire interior area of the ring with the shiny reflective surface.
There are four large 'ceiling fans' on the four sides of the giant ring, to be able to move the entire device East, West, North or South, by remote control from the ground. The Mylar film of the reflective sheet weighs another 300 pounds. There is also a spare tank of Hydrogen gas on board and some simple computer equipment. The total weight of everything is expected to be just over 600 pounds, where the Buoyancy of 700 pounds Lift is easily sufficient to maintain the entire device at the desired 100,000 feet altitude.
The Sun is very large, around 864,000 miles in diameter, and so there is a complex shadow on the ground 20 miles below the one-mile-diameter device. The full dark Umbra shadow is around 4200 feet in diameter, and the partially dark Penumbra is larger, around 5600 feet in diameter.
The 'Couplers' are small diameter couplings between each of the 32 Hydrogen-filled sections, which could be remotely closed and sealed in the event that a micro-meteorite might puncture one section of the three-mile-long surrounding tube. This should still keep the entire structure level and stable.
Regarding a smaller scale prototype sample device, say that someone owned an island in a tropical location. Currently, the presence of the ocean moderates summer heat, but the directing beating Sun can still be uncomfortable. During a particularly hot stretch in August, imagine that the owner could remotely move such a device to an appropriate location between the Sun and his island and house, having those 'ceiling fans' adjust its position to keep blocking the Sun from mid-morning to the late afternoon. The island residents could be wonderfully cool and comfortable, even without having to operate expensive air conditioning equipment that uses up massive amounts of fossil fuels in supplying the needed electricity.
A truly GREEN solution to a summer problem where conventional air conditioning equipment would not even be necessary.
The actual point of this device is not really for the convenience of billionaires such as Sir Richard Branson who can afford to own tropical islands. Instead, an even larger such 'Sun-blocker and reflector' is contemplated such that a city might move an existing device over New York City or Chicago or Beijing when a weather report predicts a 90 degree Fahrenheit day coming for the city. Imagine the SAVINGS in not needing to have all those millions of air conditioners running constantly all day! Imagine workers being able to work outdoors on such days without danger of heatstroke?
Amazingly, this entire (prototype) device seems likely to only cost a few thousand dollars, and only fairly common materials and components should be necessary. (Once the main structure is assembled, the conventional Hydrogen balloon can return to the ground.)
C Johnson, Theoretical Physicist, Physics Degree from Univ of Chicago