  Exergy is a very important thermodynamic quantity in several fields such as
economy, engineering, ecology and yet it has attracted little attention in pure
physics. One of the main problems of the currently used definition of exergy is
its dependence on an arbitrarily chosen reference state, which is the
thermodynamic state of a reservoir the system is supposedly in contact with. In
this paper, starting from a very general definition of exergy, a formula is
derived for the exergy balance of a general open continuous medium without any
reference to an external environment. A formula is also derived for the most
suitable thermodynamic parameters of the Earth atmosphere when seen as an
external environment in the usual exergy applications.
