Most renewable energy sources

Gibbs free energy, the amount of thermodynamic energy in a system which can be converted into work at a constant temperature and pressure.

Electromagnetic free energy, a.k.a. radiant energy

Free energy suppression, the notion that corporate energy interests suppress technologies that are claimed to provide cheap energy.

Helmholtz free energy, the amount of thermodynamic energy in a system which can be converted into work at constant temperature (called work content in chemistry).

Thermodynamic free energy, the total amount of energy in a physical system which can be converted to do work, in particular:

A perpetual motion machine, creating a source of energy that would violate the Second law of thermodynamics.

Energy with no or negligible feedstock cost, including solar power, telluric power, water power, and wind power