Exergy analysis of solar radiation processes - Solar Energy Sciences and Engineering Applications - page 54

Figure 2.4.6 The three typical situations considered in study of the canopy effect (from Petela, 2010).

absorbing solar radiation. The real greenhouse is built of transparent walls through
which solar radiation penetrates and heats the ground inside and then the confined air
gets heat from this ground. Based on the rough analogy the term 'greenhouse effect'
is referred also to the process in which the infrared radiation is exchanged between
the atmosphere and the Earth's surface. These two processes differ because in a real
greenhouse air is trapped whereas the environment air, when warmed from the ground,
rises and mixes with cooler air aloft. The analogy could be eventually recognized in
the fact that the glass roof of the greenhouse traps the infrared radiation to warm the
greenhouse air and the greenhouse roof plays the role of the huge and thick layer of
the atmosphere.
It is estimated that in the absence of the greenhouse effect the Earth's surface
temperature would be decreased from about 14 to about 19 ◦ C. It is believed that the

recent warming of the lower atmosphere is the result of an enhancing of the greenhouse
effect by an increasing of amount of gaseous, liquid and solid ingredients of different
radiative properties than air.
Using exergy, the effect of the canopy located above a considered surface to screen
the surface from the direct solar radiation can be considered, based on the three
situations presented schematically in Figure 2.4.6.
A black horizontal plate, of surface area A , located on the Earth's surface can
be exposed to direct solar radiation as shown schematically in Figure 2.4.6a. In the
thermodynamic equilibrium state the irradiance S is spent on heat Q extracted at
constant plate temperature T p and on the convective ( E p − 0 ) and radiative ( E p -sky ) heat
fluxes from the plate to the surroundings. The plate temperature T p is controlled by
the appropriately arranged amount of heat Q . The energy balance equation for the
plate is:

−

S = Q + E p -sky + E p − 0 (2.4.15)

where

Aσ ( T p − T sky )

E p -sky = (2.4.16)