Just to add, geothermal or ocean thermal energy could play a significant role in many parts of the world. There is a lot of on going research in this area. My take however, is that coal and coal-biomass firing technology will also be around for a while longer.

my 2 cents:

Nuclear seems to be the only existing, technically feasible, financially viable (but not financially attractive option, considering decommissioning cost in UK and Australian context)

A sad fact: every year, newly installed renewable energy sources cannot even cover the increased amount of global energy.

Bill Gates is investing in new nuclear technology research.

Many environmentalists also converted to believe that Nuclear seems to be the answer (documentary movie: Pandora's Promise)

Renewable energy is the way to go in terms of clean energy. However, the two renewable energy means cannot meet the drastic and continuous increase in the demand for energy with population. Solar and Wind are might be renewable but they are not a constant source which make them limited. The advantage here is improving on storage capacity to minimize waste.

The mix of Hydrogen, Biofuel and possibly hydrocarbon should be expected for powering machines in the nearest future. However, the use of hydrocarbon will be eliminated with time.

I agree with @Aaron Liu, Nuclear is a good alternative, but then a lot of research still have to be put into it.

In summary, I think the future energy source will be a mix of renewable and nuclear energy among other low carbon energy.