Environmental Engineering Reference
In-Depth Information
TABLE 7.5
Clear-Sky Irradiance at 40
◦
N Latitude
40
◦
Tilt
Horizontal
Vertical
Direct Normal
(MJ/m
2
)
(MJ/m
2
)
(MJ/m
2
)
(MJ/m
2
)
Date
Daily Total
21 March
21.02
26.44
16.84
33.09
21 June
30.05
25.24
6.92
36.08
21 September
20.29
25.28
16.08
30.73
21 December
8.87
18.54
18.68
22.44
Annual Average
20.06
23.88
14.63
30.58
Maximum Hourly
21 March
810
1027
656
968
21 June
958
911
309
879
21 September
785
987
630
914
21 December
451
867
829
898
Annual Average
751
948
606
915
collector, such as a window, the winter daily total irradiance is much greater than that in summer,
making a south-facing window a good source of winter space heat and a small source of summer
air-conditioning load. The focusing collector's daily irradiance is about 30% higher than the tilted
flat plate, which may be hardly enough in itself to justify the complication of moving the collector
to track the sun. On an annual average basis, there is only a factor of two difference between
the greatest and least values of the daily totals for the various collectors of Table 7.5. Similar
distinctions apply to the maximum noontime hourly irradiance, listed in the lower half of Table 7.5.
Under most circumstances, these values lie within the range 800-1000 W/m
2
, or 60-75% of the
extraterrestrial irradiance of 1367 W/m
2
.
The values of Table 7.5 apply only to clear-sky irradiance. The effect of clouds and atmospheric
dust will reduce the available irradiance to lesser values. Figure 7.6 illustrates how cloudiness,
in reducing the irradiance on a horizontal surface, has a greater effect on beam irradiance than
does diffuse, or scattered, light. The average value of the clearness index, the ratio of daily total
irradiance to the extraterrestrial value, varies with geographic location and season of the year. As
a consequence, seasonal or annual average irradiances can be as low as half of the values shown
in Table 7.5.
As will be seen below, only part of the incident solar energy flux can be collected by a well-
designed collector system. While a collector can absorb a high fraction of the solar irradiance, it
also loses heat to the ambient atmosphere, the more so as it attempts to store the collected energy
at a temperature higher than atmospheric. Practical solar collectors may average between 5 and
10 MJ/m
2
of collected energy per day at typical U.S. locations, or 2 to 4 GJ/m
2
per year. If this
energy were to be valued monetarily as equal to that for the heating value of fossil fuel, which
currently is in the range of 3-5 $/GJ, then the dollar value of the collected energy would be 6-
20 $/m
2
y. On the other hand, if it were valued as equal to the retail price of electric energy, which
is about 30 $/GJ, then the annual value of the collected solar energy would be 60-120 $/GJ. It is
for this reason that solar collectors are economically advantageous if they replace electricity for
the heating of domestic hot water.
