Most current energy sources have significant problems, which will make them less and less usable as energy needs and environmental restrictions continue to advance. Even most so-called "green" or "renewable" energy sources cannot be made viable as long-term solutions, simply because they depend on sunlight as the origin of the energy that they provide.
Given that sunlight provides about 1kW/sq. yard, and the radius of the earth is about 6400 km, the maximum possible output of these "green" power sources is about 150000 terawatts. Given a world population of about 6 billion, this works out to about 25 MW per person. Now considering that the reason these power sources are considered "green" is that they don't have a substatial impact on the environment it becomes clear that not even a percent of this maximum output can be realized while maintaining this "green" status -- let's assume 0.01% coverage is acceptable, so the output can be up to 2.5 kW per person.
Now consider that charging an electric car takes about 1.2 MW, and driving it will use about 12.5 kW -- simply driving takes 5x the available energy for one person! Clearly, current "green" power is not a long-term solution.
Fossil fuels can provide higher power for a short time, as the act more as large energy reserivoirs which have been filled over the past few thousand years; however, there are two major problems: they cannot provide long-term power because those reerivoirs will quickly become depleted, and they are commonly understood to harm the environment (as well as being generally stinky and unpleasant to live near).
Current possibilities are nuclear and geothermal power. It is not currently known how scalable geothermal power is; perhaps it can scale to the required total power, perhaps not. Additionally, geothermal power requires that the local geography be at least somewhat cooperative.
The best currently known solution is nuclear. Waste doesn't have to be a problem, as you can either reprocess it to extract the valuble component elements (some of which can even be re-used as fuel again), or sell it to someone who wishes to use it. New plants can be built wherever there is sufficient water; on coastlines or at see, or on major rivers. Smaller single-community sized plants could likely even be built near small rivers, or perhaps even large streams.