Sad is an evocative three-letter acronym standing for seasonal affective disorder, a type of winter depression that afflicts those starved of sunlight. It is also an emotion many investors will be feeling as a global recession bites and defaults snowball. But sunshine may come to lighten investment woes if renewable energy sources are behind the next economic boom.

Venture capital investments in clean technologies globally have taken off in the last few years, rising from just over $1bn (€775m) at the end of 2003 to $6.5bn in only three quarters so far this year, according to green investment organisation The Cleantech Group. That is already above last year's total investment of $6bn, itself a record.