Observers might be forgiven for thinking that so-called clean technology's moment in the sun has passed. Over the last two years, many clean-tech equity indexes have performed poorly. In Europe, solar power took a hit after the European Commission decided to phase out subsidies for renewable energy by 2017. The installation of solar panels fell by nearly 60 percent in Germany in 2013 and by 70 percent in Italy. Meanwhile, in the United Kingdom, less than 30 percent of early-stage venture-capital-funded clean-tech deals were financed.

The truth is that we have been here before. The convulsions in the clean-tech sector are simply symptoms of a cycle that characterizes emerging technologies: excitement, inflated expectations and consolidation — ultimately followed by stability and the resumption of growth. Indeed, underlying recent developments are signs of a much more significant transformation: Clean tech is becoming commercially viable.

Confidence in the clean-tech sector's future is rooted in the need for sustainable solutions for a planet that is supporting an ever-wealthier population. Over the next 20 years, the number of middle-class consumers is expected to rise to some three billion, from 1.8 billion today. Their new lifestyles will require resources, including energy.