Exergy analysis of solar radiation processes - Solar Energy Sciences and Engineering Applications - page 73

through the system and all the air parameters; temperature and pressure along the air
flow path.
The present study attempts to develop analysis of total SCPP process. The
complexity of such a thermodynamic object enforced many simplifying assumptions
and the main are:

(i) Floor has no heat loss to the soil; is perfectly insulated, and is perfectly black

(emissivity ε f

= 1). Thus, there is no solar energy reflected from the floor.

(ii) Deck material is prepared in such a way that it is almost perfectly transparent

for solar radiation (transmissivity τ d

=

0 . 95) and the remaining part (5%) of
solar radiation arriving at the deck is reflected. However, the deck material
absorbs perfectly (absoptivity α

1) low temperature radiation from the floor.
Thus, consideration of multi-reflected radiation fluxes is simplified. In addition,
the deck is thin enough that heat conducted through the deck occurs at zero
temperature gradient.

=

(iii) A certain “effective temperature'' T eff of floor or deck is applied, which

expresses potential to the heat transfer by conduction and radiation:

σ σT eff ) T 4 dA

A ( k average T eff + = k local TdA + (2.4.71)

A A

where k average and k local are the average and local convective heat transfer coef-
ficients, respectively, σ is the Boltzmann constant for black radiation, A is the
surface area and T is the local surface temperature.

(iv) The chimney material is perfectly black. The chimney wall is thin thus there

is no temperature gradient along the wall thickness and both sides of chimney
(inner and outer has the same temperature (average) constant along the chimney
height.

(v) Distribution of air temperature is represented by a certain effective temperature

defined according to equation (2.4.71), however with excluded radiative heat
transfer.

(vi) Air is considered as an ideal gas whose parameters fulfill the state equation;

p T , and the specific heat is assumed constant, (average, not varying

with temperature).

= ρ · R ·

(vii) Air is almost perfectly transparent for radiation, (transmissivity τ a ≈ 1 and

emissivity ε a ≈ 0). Air can exchange heat only by convection and conduction.

(viii) Air flow in the whole SCPP is frictionless. According to investigation by von

Backström and Fluri (2006), the relative air pressure drop r T during expansion
in turbine: “for maximum fluid power, the optimum ratio'' is 2/3, thus

p 1 − p 2 2

3 p 3 ≡

r T = (2.4.72)

p 1 −

The exemplary distribution of environment pressure and the pressure of air
along its flow within the SCPP is shown in Figure 2.4.15. The air pressure of
environment (solid thin line) drops from p 0 , at the zero level, to p 3 , at the level
H 3 of chimney inlet. The air pressure inside the SCPP drops from p 0 to p 1 at