Exergy analysis of solar radiation processes - Solar Energy Sciences and Engineering Applications - page 13

Figure 2.2.1 Exergy components.

reference level, which is the surface of the Earth. The “other'' possible components,
e.g., nuclear, or interfacial tension, are rarely used and are excluded from the present
consideration. The sum ( B ph +

B ) of most important components for substance
considerations; physical exergy B ph and chemical exergy B ch is called thermal exergy
B and according to the general formula (2.2.3) is determined as:

B ch =

B = H − H 0 − T 0 ( S − S 0 ) (2.2.12)

where B is the thermal exergy of the considered substance at enthalpy H and entropy
S , and H 0 and S 0 are the enthalpy and entropy of this substance in eventual state of
thermodynamic equilibrium with the environment parameters.
The thermal exergy B expressed by equation (2.2.12) is for a substance passing
through the system boundary. The exergy B of substance can be positive or negative
(e.g. for each medium flowing through the pipeline there could be a sufficiently low
pressure at which thermal exergy B is smaller than zero). Especially for air, such neg-
ative exergy can happen for example when the air temperature is not much higher
relative to environment, and the air pressure is lower than atmospheric. However
always positive is the exergy B s of any part of the system which remains within the
system boundary under pressure p and occupying volume V :

B s = B − V ( p − p 0 ) (2.2.13)

where p 0 is the environment pressure. The concepts B and B s suggest an analogy to
enthalpy and internal energy.
In a particular case if a space of volume V is empty, then the pressure p = 0, so

B 0 (because there is no substance), then from formula (2.2.13) the exergy of the

lack of substance is:

=

( B ) p = 0 = Vp 0 (2.2.14)

As shown later, a similar effect regarding lack of radiation (for T → 0) in formula

T 0 /3 in the vessel and the value from

(2.2.31) shows the finite exergy value b b , S = a ·

formula (2.2.42) for emissions.