Guardian: It has become the home improvement of choice for the environmentally aware, but erecting a wind turbine on the side of your house could create more carbon dioxide than it actually saves, a study into their performance will reveal today. David Cameron led the trend for "micro-wind" this year when he installed a turbine on the side of his west London home. But he may have been wasting his time and money. The Building Research Establishment Trust, which advises the government …