The radioactive decay of elements among the extremely high pressures deep in the interior of the planet generates heat that rises to the surface through Magma (molten rock) and through fissures and cracks.
Another reason why geothermal energy may not always be renewable is that patterns of geothermal activity in Earth’s crust shift naturally over time, so an area that produces hot groundwater now, may not always do so.
Because the oceans are continually heated by the sun and cover nearly 70% of the Earth's surface, this temperature difference contains a vast amount of solar energy which could potentially be tapped for human use.
The development of fuel cells and hydrogen fuel show promise to store energy conveniently and in considerable quantities and to produce electricity at least as cleanly and efficiently as renewable energy sources.
As this type of research continues, it is possible that we may find, in the future, that photobiological methods will serve as a significant approach for generating the hydrogen that will be used as the fuel we put into our cars, planes, buses, etc.