Exergy analysis of solar radiation processes - Solar Energy Sciences and Engineering Applications - page 9

delivered to the system is equal to the energy E 0 of this matter leaving the system, to
performed work W and to exchanged heat Q :

E = E 0 + B + Q (2.2.1)

The algebraic overall entropy growth for the reversible process is zero:

Q
T 0 =

S 0 − S + 0 (2.2.2)

When equation (2.2.2) is used to eliminate Q from equation (2.2.1) then the general
formula for the exergy B of matter is:

B = E − E 0 − T 0 ( S − S 0 ) (2.2.3)

For example when the considered matter is a substance then energy E is interpreted
as the enthalpy of the substance. In the case of radiation the appropriate radiation
energy has to be taken for E as discussed in the next paragraphs.
The thermodynamic value of a matter determined by energy E is significantly
different from the value determined by exergy B . This fact enables the interpretation
of the value of a matter, as well as the process, from two viewpoints and the deeper
understanding of the considered thermodynamic problem. However any engineering
designing should be based on the results from energy calculation.
The exergy concept is also applied to the phenomena of energy exchange which
are work and heat. Exergy of work, by definition, is directly equal to the work. Work,
beside mechanical work, can also appear as work performed by an electrical current
or magnetic field, etc.
Regarding heat, the thermodynamic concept of a heat source is applied. A heat
source is defined as the body, at given temperature, which can absorb or release
infinitely large amounts of heat without a change in the body temperature. Exergy
B Q of heat Q at temperature T is measured by the change of exergy of the heat source
at temperature T absorbing heat Q . The maximum work which could be performed
by heat Q is determined as the work performed in the ideal Carnot engine cooperating
with two heat sources: one at temperature T and other at environment temperature
T 0 . Using the definition of Carnot efficiency:

Q T

− T 0

B Q = (2.2.4)

T

T 0 has practically zero value and from
formula (2.2.4) the exergy B Q of such heat is zero. This is a typical illustration of
interpretation advantages of exergy over energy (heat). The value B Q can be recognized
either as a positive input to the system at T > T 0 or a positive output from the system
if T < T 0 .

For example, heat Q at temperature T =