This is what I don't get in this story. How on Earth are plastic balls cheaper than just covering the thing? One would think that a tarp would use less material and be easier to manufacture than the equivalent number of plastic balls.
EDIT: Thanks for all your answers. I guess the heatwaves we have right now in Poland are negatively affecting my imagination :(.
How do you suspend the tarp? What about when it rains on the tarp, increasing its weight substantially? Repair the tarp when sections get damaged? UV resistance over its lifetime?
Using barges/boats and dragging it from shore-to-shore is very slow going, and any suitably durable tarp (i.e. waxed canvas or some plastic) is going to be HEAVY.
The tarp would ideally need to be reflective, like a white roof on modern buildings. The rain issue is the easiest one to solve, since the tarp could be slightly perforated to allow drainage.
I wonder if a permanent structure over the lake would be easier than a tarp? Like literally building a roof.
Either way the balls seem like a solid approach.