Exergy analysis of solar radiation processes - Solar Energy Sciences and Engineering Applications - page 41

Figure 2.3.4 Bands diagram of exergy balance interpretation including gravity input in case G > 0, (from
Petela, 2010).

0; there is no change in the traditional exergy, and it means that the
work of substance during theoretical expansion at altitude H (considered in paragraph
2.2.3.2) to obtain the equilibrium of densities has no accountable importance.
The exemplary calculation and analysis including gravity input in the case of the
waste combustion products in a chimney were carried out by Petela (2009b). The
other example of calculation of gravity input, for adiabatic expansion of air in a
turbine and for drawing air through a throttling valve followed by a fan are discussed
by Petela (2009a). Further application of gravity input interpretation is also discussed
in paragraph 2.4.3.

In case of G =

2.3.4 Process efficiency
2.3.4.1 Carnot efficiency

By observation of nature we know that the continuous generation of a useful effect
(e.g. work or heat) or conversion of energy is possible only in a situation when at least
two heat sources of different temperatures are available. Such situation can then be
utilized in an installation in which a fluid (e.g. gas, liquid or photon gas) is circulating
and cyclically exchanging heat and performing work.
In a search for the best effectiveness of the cycle process, which would occur
reversibly without any losses, the ideal model was established by Carnot (1824).
The real cycles could be then designed possibly close to the model by application
of different “carnotization'' attempts. The model cycle should consist of only ideal
(reversible) processes. Thus the cycle processes of releasing and absorbing heat should
occur reversibly (at an infinitely small temperature difference between the heat source
and circulating fluid) and the flow of fluid should be frictionless. The other cycle pro-
cesses, during which work is generated or consumed, should also occur reversibly
(at constant entropy) which is possible if the fluid does not exchange heat (adi-
abatic) with the surroundings when with no friction (isentropic) it expands or is
compressed.
Based on the energy conservation law the net work W performed in the Carnot
cycle is equal to heat absorbed Q I and released Q II by fluid: W

= Q I − Q II . The