In the context of thermodynamics, "availability" and "available energy" refer to related but distinct concepts.
- Availability: Availability, also known as exergy, is a measure of the maximum useful work that can be obtained from a system as it reaches equilibrium with its surroundings. It represents the potential of a system to perform work. Availability takes into account both the internal energy of the system and the external conditions, such as temperature and pressure, of the surroundings.
Availability is influenced by irreversibilities within the system, such as friction, heat transfer across finite temperature differences, and chemical reactions that are not fully reversible. These irreversibilities decrease the potential to extract work from the system. Therefore, availability is a more comprehensive and useful measure than just the internal energy of the system.
- Available Energy: Available energy refers specifically to the portion of the total energy of a system that can be converted into useful work, disregarding the external conditions. It is a measure of the energy that is available to do work, excluding any energy lost due to irreversibilities.
The available energy of a system depends on the internal energy and the state variables, such as temperature and pressure. However, it does not consider the impact of external factors or irreversibilities on the system's ability to deliver work.
In summary, availability takes into account both the internal energy and the impact of irreversibilities and external conditions on the potential to perform work, while available energy focuses solely on the portion of energy that can be converted into useful work within the system, without considering external factors.