 WIND farms in Britain generated practically no electricity during the recent cold spell, raising fresh concerns about whether they could be relied upon to meet the country's energy needs.  Despite high demand for electricity as people shivered at home over Christmas, most of the 3,000 wind turbines around Britain stood still due to a lack of wind.  Even yesterday, when conditions were slightly breezier, wind farms generated just 1.8 per cent of the nation's electricity - less than a third of usual levels.  The failure of wind farms to function at full tilt during December forced energy suppliers to rely on coal-fired power stations to keep the lights on - meaning more greenhouse gases were produced.  Experts feared that as the Government moved towards a target of generating 30 per cent of electricity from wind - while closing gas and coal-fired power stations - cold, still winters could cause a problem in the future.  Prof Michael Laughton,  