The clean tech sector is continuing to defy the global economic downturn as investment flowing into the industry continues to accelerate.

That is the conclusion of a series of new reports each of which shows that despite concerns about the global economy and tightening credit conditions clean tech investment is growing rapidly.

The first report, from the United Nation's Environment Programme (UNEP), claims that global investment capital flows into renewable energy companies topped $100bn for the first time last year as high oil prices and government incentives stoked investor interest in the sector. Around $30bn of the total came from merger and acquisition activity led by major investment banks, further underlining the growing maturity of the sector.

The report said that the 2007 performance continued a rapid upward trend, which saw global investment in sustainable energy climb 43 per cent in 2006 to $70.9bn.

Writing in the foreword to the report Achim Steiner, executive director of UNEP, said that the soaring investment levels confirmed that renewable energy technologiues have entered the mainstream. "This is more than just interesting data," he said. "It is a powerful market signal to the arrival of an alternative future for today’s fossil-fuel dominated energy markets. Signals move markets, and the signal these investment numbers make is that [renewable energy] markets are becoming more liquid, more globalised and more mainstream."

The report comes as two recent rival studies of US venture capital clean tech investment confirmed the sector enjoyed a strong first quarter to the year.

According to reports at the Silicon Valley Business Journal, Ernst & Young has released a study based on data from Dow Jones VentureOne showing investment in clean tech firms during the first quarter climbed 18 per cent year-on-year to $571.6m, bucking a downward trend for the VC market as a whole that saw total investment slip seven per cent to $6.5bn.

It follows a recent MoneyTree report from PricewaterhouseCoopers which concluded that US venture capital investment in clean tech firms during the first quarter of the year reached $625m, an increase of 51 per cent on the first quarter of 2007.

Investment analysts said that the strong investment growth rates were set to continue throughout 2008 as high energy prices and government incentives and legislation drives increased interest in clean technologies. However, experts said that with concerns mounting about the health of the IPO market the shift towards later stage investments evident in the first quarter is likely to continue.