This started as wind trade - and I have been remiss recently for not keeping upto date on these things (here). Below is a link from a web page made up from a write up of mine (here in PPRUNE) of a few years back - it is probably still in the archive. It says most of what the original query wanted as an answer and is still as true today as it was a few years back when I wrote same. The laws of physics not having changed over much since then
http://www.iasa-intl.com/wtt.html
MG