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Energy at no cost, like mechanic energy which drives wind mill, or light in solar cell which is transormed into DC electric current, i.e. wind power, water power, telluric power, and solar power

Helmholtz free energy, the amount of thermodynamic energy which can be converted into work at constant temperature. In chemistry, this quantity is called work content

Gibbs free energy, the amount of thermodynamic energy in a fluid system which can be converted into non-mechanical work at constant temperature and pressure. This is the most relevant state function for chemical reactions in open containers

Renewable energy, although most renewable energy sources would not normally be called free energy sources

energy which may be directly utilized (and returned) by a device from the surroundings (electromagnetic free energy is sometimes referred to as radiant energy)

a primary energy source that is free (i.e. does not cost anything) for consumption. Examples include wind power, water power, telluric power, and solar power

Free energy (DNA) is the free energy pertaining to DNA

Free energy suppression is the notion that corporate energy interests deliberately suppress technologies that may provide energy at very little cost. Remaining so-far-unexploited forces of nature which are well documented in the scientific literature include telluric currents, atmospheric electricity, earth batteries, and pressure system changes

The energy from fantastical forces considered perpetual motion. These devices utilize quantum vacuum perturbation, quantum vacuum energy, rotating magnets, as well as some purported methods to crack hydrogen