Renewable energy. Once things get bad enough the world will wake up and renewables will become huge.
Is wealth going to be made on installing rather than manufacturing hardware? It's also hard to see how e.g. SolarCity protects its margins from competition with other installers. Customer acquisition is a big chunk of costs right now, but if SolarCity spends a bunch on ads to get Americans thinking about rooftop solar that helps competing installers too. Look at the Australian market for a glimpse at the future (much lower installation costs, much higher penetration, broad consumer awareness without further advertising).
EDIT: my own best guess is that if there's going to be a breakout company that achieves high margins on cleantech it's going to be in the area of storage. That's because a patented approach that can seriously lower storage costs or increase cycle life (which amounts to practically the same thing for stationary storage) is an advantage that can't be immediately cloned. OTOH I do not expect protected margins for companies that are lowering costs by scaling, e.g. Tesla's battery gigafactory. Historical reminder: at the beginning of the 2000s China had no solar PV industry to speak of. In less than a decade Chinese manufacturers went to dominating the market because IP barriers were minimal and they could scale larger very quickly once there was a state commitment.
I think the question you're asking is "how does a millionaire/billionaire multiply their net worth in a decade or two", while that question I'm answering is "how do I go from salary slave to capitalist class in a decade or two"
"Last year marked a turning point for renewables. Led by wind and solar, renewables represented more than half the new power capacity around the world, reaching a record 153 Gigawatt (GW), 15% more than the previous year. Most of these gains were driven by record-level wind additions of 66 GW and solar PV additions of 49 GW."