3.2 The Concept of Energy
45
E = E
E
+ E
A
(3.7)
E E and E A can both be described as state variables. Whereas exergy is the
maximum useful work that can be taken from a reversible process, anergy is the
share, that cannot cause work (Figure 3.5). “The concept of exergy allows for
the comparison of energies given in different forms such as electrical energy,
heat provided at different temperatures, or energy stored in chemical bonds. The
quantity exergy is only well defined when a reference state, the ambient state, is
given” [KS2015, p. 11]. This state defines the pressure ρ and the temperature T, as
well as the concentration of chemicals in the atmosphere. The conversion process
of primary energies E prim into useable energy E end like electric power involves a
degree of energy loss.
Figure 3.5 Energy conversions [KS2015, p. 12]
Using the first-law efficiency η 1 , the ‘energy loss’ in the transformation process
can be defined as the ratio between final and primary energy:
η 1 =
E end
E prim
; 0 < η 1 < 1
(3.8)
with E prim primary energy
E end
useable energy