MARSHALL BRAIN, HowStuffWorks.com | Posted: Monday, April 14, 2008 12:00 am
We hear so much about carbon dioxide and global climate change, and every day it seems the price of gasoline is rising again. You might be wondering if there is anything we can do to solve these problems.
One possible answer is solar energy. And it turns out that there are some gigantic solar projects getting underway in the U.S. The largest of these is a 500 megawatt plant in California. That's enough power to run a small city.
The sun's energy is free and abundant. Each square yard of Earth's surface receives about 1,000 watts of free energy on a bright, sunny day. In other words, the earth receives thousands of times more energy from the sun than humans will ever need. All we have to do is find inexpensive ways to convert all of this light into electricity, and our energy problems are solved.
There are several ways to use solar power. One of the simplest is a solar water heater. In China there are so many solar water heaters on residential rooftops that it saves the country from having to build 40 power plants. There are also large photovoltaic panels that convert sunlight directly to electricity. You frequently see these panels on rooftops, especially in California.
But large-scale projects use concentrating solar collectors. There are four ways for concentrating collectors to work, but all of them have one thing in common: they use mirrors to concentrate sunlight.
One of the simpler systems is a parabolic trough collector, which looks like a half tube mirrored on the inside. Sun reflects off the mirror and hits a pipe running down the center of the tube. Inside the pipe is mineral oil that reaches temperatures as high as 500 F degrees. This hot oil is used to boil water, creating steam that drives a turbine and electrical generator.
Another simple mirror system uses a parabolic dish lined with mirrors. These collectors look something like large satellite TV antennas. They reflect the sun's light to a point, and at that point there may be a Stirling heat engine. A Stirling engine converts heat to motion, and the motion drives an electrical generator.
Finally there are the tower systems. A tall tower sits in a field of large, moveable mirror panels. The mirrors rotate so that they reflect their sunlight at the top of the tower, which gets incredibly hot. This heat can boil water, creating steam that drives an electrical generator.
Solar power does have one big disadvantage - there is no sunlight at night. But engineers are working on ways to store power overnight to eliminate this problem.