Renewable Energy Sources?
These are sources that essentially cannot get depleted, at least while the Sun can still shine and the Earth's radioactive isotopes have not all decayed. But we need not worry about either running out for the next several billion years.
So what are the possibilities?
Geothermal uses the Earth's heat, which is generated by the decay of the Earth's radioactive isotopes, which include K40, Th232, U235, and U238. However, the Earth's internal temperature does not increase very fast with depth, with the main exceptions being volcanoes. So one has to use hot springs and the like, and these are not very widely distributed. An alternative to that involves esssentially creating one's own hot springs. Drill a long way down, then pump in some water to create some cracks. Drill into those cracks to recover the heated water, and continue with adding more water.
The other source, the Sun, can be tapped both directly and indirectly in a variety of ways; it must be pointed out that most of the ways have trouble as a result of how diluted sunlight is. Sunlight also has the problem of being easily blockable by clouds, especially in the sorts of areas where many people prefer to live, and by being blocked by the Earth for about half the time at every place on it.
A simple way of tapping solar energy is to put solar water heaters on one's roof.
One can get around the dilution by focusing sunlight with mirrors; one might even use the concentrated heat to run a tubine/generator setup with that to generate electricity.
An alternate approach is the solar tower or solar chimney approach -- a tall chimney surrounded by a tower skirt, big area covered with glass or transparent plastic that gets heated by the Sun. Air inside it will try to rise, and the skirt will direct the heated air into the chimney, where it can power some turbine/generators.
And one can also generate electricity directly with photovoltaic cells.
Less directly, the Sun's heat makes the Earth's air and water circulate, making winds, waves, and a temperature gradient in the ocean -- all of which can be utilized and sometimes have been.
Tidal power might seem associated with waves, but the tides' uitlimate power source is the gravity of the Sun and the Moon combined with the rotation of the Earth, and that source is also not likely to run out for several billion years.
Back to solar, i note that plants also capture solar energy; their chlorophyll and other photosynthetic pigments are inside antenna complexes that act like tiny photovoltaic cells. And one can then use the plant material as fuel: biomass. One can burn it, or else bake it or ferment it to produce more convenient fuels.
I note in passing that fossil fuels are largely fossilized biomass. But it is being consumed at something like a million times the rate at which it is foruming. And in some cases, it is even worse. Most of the Earth's coal deposits formed before the Triassic Period, about 200 million years ago, which is when termites evolved. They started eating the dead trees, keeping them from forming coal deposits. Their ancestors were early cockroach-like insects, and some present-day cockroaches like eating rotting wood.
Nuclear energy is based on another sort of fossil fuel: uranium and thorium, which are produced in the cores of massive stars as they collapse to become neutron stars, and perhaps also by binary neutron stars in their final stages of inspiral. And that fossil fuel was frozen into the Earth when it formed 4.5 billion years ago. Fusion relies on hydrogen, helium, and perhaps lithium isotopes, which are leftovers from the Big Bang. But hydrogen has the nice feature of being MUCH more common than uranium.
That overview being done, I concentrate on some practical issues.
First, many of these alternatives are best adapted for producing electricity. This is no real problem for stationary customers like homes and offices and factories and the like, but moble customers are another story entirely. It is rather difficult to get good energy densities in batteries, energy densities comparable to those of combustible fuels. Thus, electric cars have not been very feasible, and the most successful electric vehicles to date have been electric locomotives and railcars, which "cheat" by getting their electricity from extra rails or overhead cables. Electric trolleybuses get their electricity in the same way, from pairs of overhead cables. And electric ships or airplanes are much less feasible.
So while the drive-everywhere lifestyle may become less and less feasible over time, we may still be able to enjoy much of what electricity has made possible for us.