Wind/Solar are definitely intermittent power. To be feasible they require backup capacity that is cheap and efficient to bring online quickly, such as natural gas power plants. Using coal plants for backup capacity is NOT a good idea because they can’t simply be powered up on-demand. We’re lucky in the US since we have massive reserves of natural gas. In the UK they have a real national security dilemma regarding wind-power because they don’t have large reserves of natural gas, so they have to get it from Russia...
There are conceptually two types of power plants: base load and peaking. The base load plants (eg coal, nuclear) are generally more efficient, but are not good at varying their outputs quickly to adapt to varying loads. Peaking plants (eg gas turbine) are generally less efficient, but can vary their outputs easily. In general, a power company will try to run a high percentage of base load plants and just use the peaking load plants to cover the variations. When you add wind, wave, solar, etc you now have uncontrolled variable sources to add to the mix. The power company is now forced to run a higher percentage of peaking plants to give them the ability to handle the additional variation, which, of course, lowers the overall efficiency of their fuel-based plants.
There are also technologies for power storage to smooth the flow, but they are generally impractical (eg batteries), expensive, and/or may have high environmental impact (eg pumped storage).
So one has to consider the entire system to make sure there is a net benefit.
Marty:
Yes, I am aware that Denmark has a high percentage of wind and Germany has a tax surcharge on power to fund solar and wind. Many tend to focus on just the purported advantages of renewable power sources--the entire system needs to be evaluated, not just some aspects of some parts and I am trying to fill in some of the broader perspective. For instance, does a green power source produce more energy than it took to manufacture it?
BTW, the Mar 2009 issue of Scientific American magazine has an article entitled "The Power of Renewables" which appears to summarize the prospects for a number of technologies. Might be worth looking at. (Just got the mag--haven't had time to read it yet.) There also appears to be more online (
www.sciam.com).
Doug