The flaw is that using renewables for power demand means a dangerous reliance on the wind blowing and the sun shining. The ability to store excess power for use when supply is low is, to put it mildly, fundamental in any big move towards renewables.
Now Aquion Energy has installed, at an old Sony TV plant near Pittsburgh, an assembly line for non-toxic batteries. Samples are being sent out to potential customers for evaluation. $55 million has been raised by the company, which was formed in 2008, and production is expected in the Spring of 2014.
The article quotes Jay Whitacre, the Carnegie Mellon University professor of materials science who led the research effort, as saying that the new batteries cost as much as lead-acid batteries, the cheapest around, but last more than twice as long.
Sodium-ions instead of conventional lithium-ions are used. The battery is ideal for isolated locations or for use at power plant locations, but because sodium-ions are less efficient in storing by weight or volume than lithium-ions, the batteries are not practical in for example a car or a phone because of the bulk required.
There are five "World patent" applications by Jay Whitacre for Aquion Energy. The most recent, published in September 2013, is titled Large format electrochemical energy storage device housing and module.
Here is a video featuring Whitacre talking about the company.