Solar energy-harvesting party balloons

California’s Cool Earth Solar wants to replace expensive solar cells with larger versions of those crinkly, aluminized balloons you get at hospital gift shops.

Such balloons are made from metal-coated plastic. Cool Earth’s insight was that if you coat only one half of a balloon, leaving the other transparent, the inner surface of the coated half will act as a concave mirror. Put a solar cell at the focus of that mirror and you have an inexpensive solar-energy collector.