Exergy analysis of solar radiation processes - Solar Energy Sciences and Engineering Applications - page 19

Figure 2.2.3 Exergy of photon gas as function of temperature.

=

300 K.
The exergy of radiation is always non-negative and the zero value is achieved only at
T

For example, Figure 2.2.3 shows b b , S as a function of temperature T at T 0

= T 0 .

2.2.5 Exergy of radiation emission
During radiation of substantial bodies (solid, liquid or some gaseous), a part of their
energy (e.g. internal energy or enthalpy) transforms into the energy of electromagnetic
waves of a length theoretically from 0 to

. These waves can travel in a vacuum
because they do not require any medium for their propagation. Independently one can
also imagine this radiation process as the energy non-continuously emitted in form
of the smallest indivisible energy portions called photons. If the energy of a body is
not simultaneously supplemented from an external source, then temperature of the
radiating body decreases. The phenomenon of such radiation is called emission, which
is the key problem in study of radiation, especially its simplified models.
Temperature of radiation is always equal to the temperature of the emitter. Accord-
ing to the Stefan-Boltzmann law the energy e b of emission of a black surface at
temperature T is:

∞

σT 4

e b = (2.2.32)

10 − 8 W/(m 2 K 4 ) is the Boltzmann constant for black radiation.
Emissivity of the emitter, e.g., the emissivity of a solid surface, determines the sur-
face ability measured by the rate at which the black radiation is produced. The perfect
gray surface of emissivity ε< 1 emits black radiation of an amount determined by
the emissivity ε . If the density of emission e b expresses the amount of the emitted
black radiation energy from 1 m 2

where σ = 5 . 6693 ·

e b )
expresses the amount of the emitted black radiation energy from 1 m 2 of gray surface,
at the rate ε :

of black surface at ε = 1, then density e ,( e = ε ·

εσT 4

e = (2.2.33)