Exergy analysis of solar radiation processes - Solar Energy Sciences and Engineering Applications - page 50

Figure 2.4.3 Exemplary effects of concentration of solar radiation (from Petela, 2010).

6000 K, the
efficiency η B can be determined from formula (2.4.8) and shown in Figure 2.4.3 (thick
solid line). The efficiency η B has a maximum about 5.34% which also corresponds to
value of a S

Assuming ψ = 0 . 933 like for the black radiation at temperature T S =

6.
The overall entropy growth determined from equation (2.4.10) for the data used in
the example is always positive ( > 0) and with growing a S diminishes to zero (

≈

= 0)

for a S 6000 K. For further growing of a S

the overall entropy growth becomes negative ( < 0) i.e. the further concentration of
solar radiation is impossible.
Based on the calculations the process of “de-concentration'' of solar radiation,
which would correspond to reducing a S below 1, is irreversible and can occur but heat
absorbed by the surface A is negative which means that the surface would be heated.
The data used in the present example were also used for the computation of the
results shown in Table 2.4.2 which illustrates the trends of the output data in response
to changes in some input parameters. The values in column 3 of Table 2.4.2 are consid-
ered as the reference values for studying the influence of the varying input parameters
in the output. Therefore each of the next columns (4 to 6) corresponds to the case in
which the input is changed only by the values shown in a particular column, whereas
the other input parameters remain at the reference level.
For example, column 4 corresponds to a change in the emissivity ε , which increases
from 0.9 to 1. This 10% ε increase causes the increase of: temperature T from 517.8
to 519.9 K, q from 653.3 to 659.5 W/m 2 , Q from 108.9 to 109.9 W, η E from 13.61 to
13.74%, B Q from 45.79 to 46.49 W and η B from 5.34 to 5.42%.
Column 5 and 6 can be similarly interpreted. For example increasing the heat
transfer coefficient k from 3 to 5 W/(m 2 K) causes increase in exergetic efficiency from
5.34 to 8.04%, which is the result of increased heat rate q from 653.3 to 1021 W/m 2 .

= 91,843 corresponding to temperature T =