Renewable Energy - Energy and the Environment - page 157

1.0

0.8

0.6

0.4

0.2

0

0 0.2 0.4 0.6 0.8 1.0

Daily total radiation
Daily exraterrestrial

Figure 7.6 The ratio of daily diffuse to total radiation as a function of the ratio of daily total to
extraterrestrial radiation, showing that cloudiness increases the diffuse portion of total radiation.

above the horizontal, and vertical (windows). On the other hand, focusing collectors may move
in the vertical and azimuthal directions to direct the collector aperture to be normal to the solar
direction. Each type of collector receives a different amount of solar energy.
Table 7.5 lists the clear-day solar irradiance at 40 ◦ N latitude 16 incident upon four common
types of collectors: a horizontal surface (first column); a south-facing flat plate tilted 40 ◦ above
the horizontal (second column); a vertical south-facing surface (third column); and a focusing
collector that is oriented both vertically and horizontally to collect direct (or beam) irradiance
(fourth column). The daily total of energy incident upon these collectors is listed in the upper part
of the table for four days of the year: the spring and fall equinoxes (21 March and 21 September)
and the summer and winter solstices (21 June and 21 December). On the equinoxial days the sun's
position in the sky is essentially identical, while the solstices mark the annual extremes in solar
irradiance. The lower half of the table lists the maximum hourly solar irradiance, which for the
collectors selected occurs at local noon. The annual average value of these items is also listed in
the table.
Several important conclusions about clear-sky solar irradiance may be drawn from the entries
in Table 7.5. For the horizontal collector, the summer daily total is more than three times the
winter value. 17 By tilting the collector to 40 ◦ , the difference between summer and winter is much
reduced, to a ratio of 1.5:1, so that the daily total is more nearly uniform year-round. For a vertical

16 This is the median latitude for the lower 48 U.S. states, which generally lie between 30 ◦ and 50 ◦ N latitude.
A lower latitude will experience a greater solar irradiance and a lesser variability between summer and winter,
and vice versa for a higher latitude.
17 The daily irradiance on a horizontal surface maintains the surface atmospheric temperature level. The
difference between summer and winter irradiance accounts in part for the atmospheric temperature difference
between these seasons.