  We present the relatively less known thermodynamic concept of exergy in the
context of ecology and sustainability. To this end, we first very briefly
outline thermodynamics as it arose historically via engineering studies. This
enables us to define exergy as available energy. An example of applying the
concept of exergy to a simple human process is next described. Then we present
an exergy analysis of Earth as a flow system, also concurrently describing
other necessary concepts. Finally, we briefly comment on the applicability of
exergy analysis to ecology and sustainability. Keywords: exergy - energy - work
- thermodynamics - heat transfer - flow processes
