At first glance, clean technology, which either improves the efficiency or reduces the environmental impact of energy-related products, appears to be a booming industry. It is estimated to be worth $150bn now but will reach $600bn by 2020, a United Nations report says. Investments are being made in a huge variety of concepts and technologies. In one high-profile example, Sapphire Energy, a San Diego-based company that aims to make biofuel from algae, said last month that it had raised more than $100m from investors. New backers include Cascade Investment, owned by Bill Gates, Microsoft's founder. Eye-catching deals such as this are focusing attention on the algae industry, which is still in its infancy. But overall, the clean technology sector faces big challenges if it is to achieve the UN's growth predictions. Robert Hertzberg, co-founder of solar company G24 Innovations and a private investor, says small businesses in most of the clean technology industries are unable to access the funds they need to commercialise their products. "There are just not enough market-based incentives to commercialisation," he says. "There should be more incentives for people to invest . . . The smaller businesses are spending all their money on bankers and lawyers instead of engineers. That should be the principle focus." For investors, meanwhile, one of the problems is that achieving a return on investment for some clean technologies requires a leap of faith. There is money available for research and development, says Mr Hertzberg, but without sales there is no return. Some observers believe clean technology will follow a similar pattern to that of IT and the internet, which yielded huge returns for investors over short periods, especially in the dotcom boom at the end of the 1990s. However venture capitalists - mindful, perhaps, of the dotcom bust that followed from 2001 - are taking a cautious, longer-term approach because some of the technologies remain unproven. "[For entrepreneurs in this space] VCs do focus on the end user and do a lot of due diligence," says Peter Linthwaite, managing partner at Carbon Trust Investment Partners and former chief executive of the British Venture Capital and Private Equity Association. "You can see there is a lot of capital available for large-scale wind farms. But there is a lack of capital for small business to move to commercialisation of products in Europe. It still suffers from a lack of scale to turbocharge these businesses. " "It's a huge problem. You might get