America is a nation of pavement. According to research conducted by the Lawrence Berkeley National Laboratory, most cities' surfaces are 35 to 50 percent composed of the stuff. And 40 percent of that pavement is parking lots. That has a large effect: Asphalt and concrete absorb the sun's energy, retaining heat - and contributing to the "urban heat island effect," in which cities are hotter than the surrounding areas. So what if there were a way to cut down on that heat, cool down the cars that park in these lots, power up those parked cars that are electric vehicles, and generate a lot of energy to boot? There is actually a technology that does all of this - solar carports. It's just what it sounds like - covering up a parking lot with solar panels, which are elevated above the ground so that cars park in the shade beneath a canopy of photovoltaics. Depending of course on the size of the array, you can generate a lot of power. For instance, one vast solar carport installation at Rutgers University is 28 acres in size and produces 8 megawatts of power, or about enough energy to power 1,000 homes. So what's the downside here? And why aren't solar parking lots to be found pretty much everywhere you turn? In a word, the problem is cost. They are mainly springing up in Arizona, New Jersey, Maryland, Massachusetts, and New York and most of all California. That's because these states offer an array of state financial incentives to support their development.