<p>Most attention on the rapid rise and importance of renewable energy has focussed on wind and solar power. But numerous other forms of renewable energy are available, some commercially exploited today, some in development and some more speculative. What role could they play in future? Are they just a useful adjunct to wind and solar, or do they make an indispensable and differentiated contribution? What technical, commercial, policy and environmental barriers do they have to overcome to realise their potential?</p>
