Exergy analysis of solar radiation processes - Solar Energy Sciences and Engineering Applications - page 57

Figure 2.4.9 Plate under a canopy with a vacuum between the canopy and the plate, S = 700W/m 2
(left) and S = 1000W/m 2 (right), (from Petela, 2010).

temperature T p , the energetic efficiency η E decreases whereas the exergetic efficiency
η B is at maximum.
The third possible situation, shown in Figure 2.4.6c, is the same as in the previous
situation (b), except that between the plate and canopy is a vacuum, thus in this space
heat convection does not occur. The energy balance equations for the plate and the
canopy are:

S = Q + E p − c (2.4.25)

E p − c = E c − sky + E c − 0 (2.4.26)

The energetic efficiency η E and exergetic efficiency η B are determined respectively
also from formulae (2.4.18) and (2.4.19). Figure 2.4.9 shows the calculation results
for the two different values of irradiance, S 1000 W/m 2 .Asin

situations (a) and (b), also in situation (c), with the increasing plate temperature T p
the energetic efficiency η E decreases whereas the exergetic efficiency η B is at maximum.
The comparative discussion of the three models (Fig. 2.4.6) can be summarized
as follows. The irradiated black plate (a), the plate under the canopy (b) and the plate
under the vacuum and canopy (c), were considered under simplifying assumptions of
extreme values of surface properties to better emphasize the canopy idea. The compar-
ison of Figures 2.4.7, 2.4.8 and 2.4.9 illustrates benefits of application of canopy for
increasing effect of trapping solar radiation. The amount of exergy (practical value) of
absorbed heat grows gradually through the three considered situations from (a) to (c).

700 W/m 2

= and S =

2.4.1.5 Evaluation of solar radiation conversion into heat

Solar radiation can be converted to heat in many various applications. Generally,
in each application the solar radiation is absorbed by certain designed surfaces at a
temperature controlled by appropriate amount of heat extracted. Unfortunately the
high quality of solar energy, e.g., measured by exergy, is significantly degraded during
a conversion to heat.
A simple introduction to evaluation of the non-concentrated solar radiation poten-
tial for heating and determination of heat temperature can be considered with use of
the model of 1 m 2 of absorbing surface shown in Figure 2.4.10. The considered surface