Need to find the best bang for the buck.  It takes energy to transform energy into another form for use.  The trick will be to find what form of energy is in an endless supply that can be economically converted to a usable form.  The economic part will continue to move upward as existing usable forms of energy are depleted.  Since CO2 is such a problem and many believe us humans are the main source of that problem, just find a way to use the energy in CO2 to provide an endless supply of energy.  REMEMBER, it takes energy to make energy.