Exergy analysis of solar radiation processes - Solar Energy Sciences and Engineering Applications - page 84

turbine z P = 0 . 70% represents the eZergy efficiency of SCPP. Specificity of the

diagram in Figure 2.4.18 is showing the relatively large ezergies of air z a 1 =

b P =

8 . 96%,

z a 2 = 8 . 82% and z a 3 = 8 . 24%. As a result, the gravity inputs are: z Ga = 7 . 28% for

the air in collector, smaller for turbine z GT = 0 . 89% and the smallest for chimney

z Gch = 0 . 67%

More details of the energy and exergy balances, especially for different input
parameters, were analyzed by Petela (2010).

2.4.4 Photosynthesis
The very simplified threefold study of photosynthesis process was developed by Petela
(2008a) including a) an energy analysis (the energy conservation equation developed to
estimate the energy effects of the process); b) entropy analysis (the changes of entropy
were used to estimate the irreversibility of the component processes); and c) exergy
analysis (developed for thermodynamic evaluation of involved matters). In the present
paragraph only the outline of energy (a) and exergy (c) analyses is discussed based on
engineering thermodynamics to propose the methodology of exergetic consideration
of photosynthesis.
Photosynthesis is the process by which the energy of the photosynthetically active
radiation (PAR), i.e. within the wavelength range 400-700 nm, is used to split gaseous
carbon dioxide and liquid water and recombine them into gaseous oxygen and a sugar
called glucose. The photo-chemical reaction of photosynthesis cannot occur without
the presence of chlorophyll and is a complex two stage process. For the present analyses
only the following endothermic overall reaction of the photosynthesis is considered:

6H 2 O + 6CO 2 → C 6 H 12 O 6 + 6O 2 (2.4.96)

A simplified scheme of the considered system shown in Figure 2.4.19 is defined
by the system boundary and contains a leaf surface layer in which biomass is created
at temperature T . Diffusion of gaseous substances and convective heat transfer occurs
through the gaseous boundary layer at the leaf surface. The boundary layer is not
considered for radiation fluxes because it is assumed that air in this layer is transparent
to radiation. The leaf surface absorbs part of the incident solar radiation and emits
its own leaf radiation of temperature T . The absorbed radiation is expended on the
metabolism processes of the leaf and on maintaining the leaf temperature T above the
environment temperature T 0 .
Liquid water, at temperature T , from the leaf body enters the considered system.
A relatively small amount of this water is used for the assimilation of the CO 2 , which
diffuses into the leaf from the external environment. The large excess of water is tran-
spired in the form of vapor diffusing from the leaf to environment. Oxygen produced
during the photosynthesis also diffuses into the environment. The water vapor and
oxygen exiting the boundary layer, as well as CO 2 entering the boundary layer, have
environment temperature T 0 at the respective environment mole concentrations z H2O,0 ,
z O2,0 , and z CO2,0 .
Only the chemical and physical components of the energy and exergy of the sub-
stances are considered. Also only the overall effects described by equation (2.4.96),