The term thermodynamic free energy denotes the total amount of energy in a physical system which can be converted to do work.
- Helmholtz free energy: the amount of thermodynamic energy which can be converted into work at constant temperature and volume. 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 work at constant temperature and pressure. This is the most relevant state function for chemical reactions in open containers.
In engineering fields related to power generation, free energy means an energy source available directly from the greater environment and which cannot be expected to be depletable by humans. Free energy in this sense may be categorised as renewable energy, although most renewable energy sources would not normally be called free energy sources.
- Free energy is energy which may be directly utilized (and returned) by a device from the surroundings (electromagnetic free energy is sometimes referred to as radiant energy).
- Free energy can also mean a primary energy source that is free (i.e. does not cost anything) for consumption. Examples include wind power, earth batteries, water power, and solar power. Remaining so-far-unexploited forces of nature which are well documented in scientific literature include atmospheric electricity, and atmospheric pressure change.
