Exergy analysis of solar radiation processes - Solar Energy Sciences and Engineering Applications - page 78

Madrid, Spain, the floor diameter is D f = 240 m and the chimney height H 3 = 195 m.

Other data are as follows:

S = 800 W/m 2 T gr = T 0 c D = 1 . 015 c p = 1000 J/(kg K)

κ = 1 . 4 η T = 0 . 7 R = 287 . 04 J/(kg K) H T = 1m

k ch − 0 = 7 W/(m 2 K) k d − 0 = 5 W/(m 2 K) H e = 0 . 3m

The computation results are shown in the bands diagram (Figure 2.4.16) in which
the values e are expressed in %, and the solar radiation energy arriving at the deck
E S 39 . 05 MW is assumed as 100%. This amount, reduced by the 5% reflection,

is distributed between five SCPP components; collector air, floor, deck, turbine and
chimney. The floor (black body) fully absorbs the solar radiation (95.00%) trans-
mitted through the deck and converts this radiation energy to the energy at the level
of temperature T f , eff . Part of this energy ( e f - d =

=

77 . 19%) radiates to the deck and

the rest e f - a = 17 . 81% is transferred by convection to heated air in the collector. The

power performed by the turbine is relatively small ( E P = 0 . 23 MW) mostly due to

the small mass flow rate of air ( m 276 kg/s) and due to small pressure drop during

the air expansion. The percentage power of the turbine e P =

=

0 . 64% represents the
energy efficiency η E of the SCPP. The exhausted energy (enthalpy) of air from chim-
ney is e a 3 =

20 . 75% whereas the exhausted potential and kinetic energies are small;

10 − 4 %, respectively. The other SCPP energy losses are
by radiation and convection heat transferred from deck and chimney to the sky and
environment. Solar energy reflected from the deck is assumed e R =

e p 3 = 0 . 52% and e w 3 = 3 . 87 ×

5 . 00%.
Exergy analysis is based on the exergy balance equations. Exergy B in these equa-
tions has the subscripts respectively to E in equations (2.4.76)-(2.4.80) for energy
analysis. The five separate exergy equations can be written for floor, deck, air in col-
lector, turbine and chimney. The exergy equations are analogical to energy equations
and differ by the additional members, B , representing the respective irreversible
exergy losses:

B S - f = B f - a + B f - d + B f (2.4.81)

B f - d = B d - a + B d - sky + B d -0 + B d - ch + B d (2.4.82)

B f - a + B d - a = B a 1 + B w 1 + B p 1 + B a (2.4.83)

B a 1 + B w 1 + B p 1 = B a 2 + B w 2 + B p 2 + B P + B T (2.4.84)

B a 2 + B w 2 + B p 2 + B d - ch = B a 3 + B w 3 + B p 3 + B ch -0 + B ch - sky + B ch - gr + B ch

(2.4.85)

Exergy of solar radiation can be estimated for the radiation temperature slightly
smaller than 6000 K, e.g., B S ≈ ψ · E S , where ψ ≈ 0 . 9.