Environmental Engineering Reference
In-Depth Information
(vi)
The surroundings of the considered leaf consists only of the surfaces at temper-
ature
T
0
and of absorptivity
α
0
=
1, (the Sun's surface area is neglected as being
seen within a relatively very small solid angle).
(vii)
Mixtures of substances in the system are ideal; the components do not mutually
interact. Therefore, mixture properties are the respective sums of the component
properties. For example, the biomass contained within the leaf structure is an
ideal solution of solid C
6
H
12
O
6
and water.
(viii)
The environment air contains only N
2
,O
2
,CO
2
and H
2
O. The dry environment
air contains 79.07% N
2
, 20.9% O
2
, and 0.03% CO
2
. The sum of all mole
fractions of the air components is:
z
N
2
,0
1, where
z
H2O,0
is determined by the relative humidity
ϕ
0
of the air and the saturation
pressure
p
s0
for the environment temperature
T
0
:
z
H2O,0
+
z
O
2
,0
+
z
CO
2
,0
+
z
H
2
O,0
=
=
·
p
s0
. In terms of
radiation it is assumed that the diatomic gases have transmissivity 100% and
the concentration of the triatomic components (CO
2
and H
2
O) is relatively
small and they also have transmissivity 100%.
ϕ
0
(ix)
The considered leaf has uniform temperature
T
; there is no heat transfer
within the considered surface layer. According to the evaluation by Jørgensen
and Svirezhev (2004) there is a several degree difference
T between the leaf
temperature and environment temperature.
(x)
The liquid water required for photosynthesis is available in sufficient amount.
(xi)
In the considered conditions the rate of sugar production is limited by the
effectiveness of diffusion of gases; not by the reaction kinetics depending on
temperature.
(xii)
The generated sugar has only chemical exergy
b
ch
resulting from chemical reac-
tion (2.4.96). The component of the sugar exergy gained as a result of ordering
(structure of the biomass in according to genetic plan) is neglected.
It could be expected that some simplifications may not qualitatively affect the final
conclusions although the quantitative results could be affected remarkably.
The considered substances in the system (Fig. 2.4.19) are gaseous CO
2
,O
2
,H
2
O
(assumed to be ideal), liquid water and the leaf substance (biomass). The enthalpies of
the gases are zero because at the system boundary they have environment temperature
T
0
. However, for the liquid H
2
O the water vapor is the reference phase. Therefore,
the enthalpy of liquid water is equal to the sum of the negative value of the latent heat
of vaporization at temperature
T
0
and of the temperature difference
T
=
T
−
T
0
multiplied by the specific heat of water.
The generated biomass is assumed to be a mixture of liquid water and sugar.
The enthalpy of this mixture is calculated as the sum of the component enthalpies.
(For liquid and solid bodies the enthalpy is practically equal to the internal energy).
The enthalpy of sugar is the sum of the physical enthalpy and devaluation enthalpy. The
physical enthalpy of sugar is calculated for the temperatures range from
T
0
to
T
at the
constant specific heat
c
SU
=
430
.
2 kJ/(kmol K).
The values of devaluation enthalpies
d
n
for the standard parameters (pressure
p
n
=
101
.
325 kPa and temperature
T
n
=
298
.
15 K) are tabulated by Szargut et al. (1988).
The varying of the devaluation enthalpy within the considered temperatures range is
negligible.
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