Exergy analysis of solar radiation processes - Solar Energy Sciences and Engineering Applications - page 51

Table 2.4.2 Responsive trends of output to change of some input parameters (for IR = 800W/m 2 ,
T S = 6000 K, ψ = 0.933,A S = 1m 2 ), (from Petela, 2010)
Mono-variant changes of input parameters and

Reference resulting outputs

Quantity Units value

1 2 3 4 5 6

Input
ε 0.9 1

W/(m 2 K)

k 3 5

T 0 K 300 270

Output
T K 517.8 519.9 504.2 514.8

W/m 2 q 653.3 659.6 1021 734.6

Q W 108.9 109.9 170.2 122.4

B Q W 45.79 46.49 68.94 58.23

η E

% 13.61 13.74 21.28 15.30

η B

% 5.34 5.42 8.04 6.79

2.4.1.3 Global warming effect

Solar radiation is the energy source for life on the Earth and this radiation establishes
the temperatures of the atmosphere and the Earth's surface. However, human activity
seems to change the conditions of the energy exchange between the Sun and Earth and
it seems that the observed tendency is a gradual increase of these temperatures. This
effect of temperature growth is called the global warming effect. Some exergy insights
are discussed in the present paragraph.
The atmosphere absorbs some of visible radiation directly from the Sun. From the
Earth the atmosphere receives some infrared radiation and exchanges convective heat.
The Earth, besides energy exchanged with the atmosphere, receives radiation from the
Sun and reflects some radiation to the space. Assuming that the extraterrestrial solar
radiation directed to the Earth renders 100%, the energy flow diagram for the thermal
equilibrium for any Earth temperature T 0 , can be shown schematically in Figure 2.4.4a.
It is usually estimated that the annual average temperature T 0 of the Earth surface is
about 14 ◦ C (287 K) and at this temperature the Earth's energy emission is

∼

192%.
The atmosphere, at a certain assumed effective temperature T A , emits energy to the
Earth (

83%).
The global warming effect is not a subject well exposed by the exergy concept
because the exergy relates to environment temperature T 0 with no regards of how high
this temperature is. For example, for any Earth temperature T 0 the exergy radiated
from the Earth's surface will be always zero, as shown in Figure 2.4.4b in which 100%
is assumed for the exergy of extraterrestrial solar radiation directed to the Earth.

∼ 138%) and space ( ∼

Example 2.4.1.4 To illustrate the global warming effect a very rough consideration
of exchanged radiative and convective heat fluxes are analyzed in a simplified model
of a polluted air layer between the space and the Earth's surface. The air layer of
transmissivity τ is assumed as a body of reflectivity ρ = 0 and of absorptivity equal to