For a plane rotated about a north-south axis parallel to the earth's axis, with continuous adjustment, 9 is equal to cos(9) = cos(6)

Time (hours)

FIGuRE 2.13 Daily variation of solar flux: tilted N-S axis with tilt adjusted daily.

Time (hours)

FIGuRE 2.13 Daily variation of solar flux: tilted N-S axis with tilt adjusted daily.

Time (hours)

FiGuRE 2.14 Daily variation of solar flux: polar N-S axis with E-w tracking.

Time (hours)

FiGuRE 2.14 Daily variation of solar flux: polar N-S axis with E-w tracking.

This configuration is shown in Figure 2.10b. As can be seen, the collector axis is tilted at the polar axis, which is equal to the local latitude. For this arrangement, the sun is normal to the collector at equinoxes (6 = 0°) and the cosine effect is maximum at the solstices. The same comments about the tilting of the collector and shadowing effects apply here as in the previous configuration. The performance of this mount is shown in Figure 2.14.

The equinox and summer solstice performance, in terms of solar radiation collected, are essentially equal; i.e., the smaller air mass for summer solstice offsets the small cosine projection effect. The winter noon value, however, is reduced because these two effects combine. If it is desired to increase the winter performance, an inclination higher than the local latitude would be required;

but the physical height of such configuration would be a potential penalty to be traded off in cost effectiveness with the structure of the polar mount. Another side effect of increased inclination is shadowing by the adjacent collectors, for multi-row installations.

The slope of the surface varies continuously and is given by tan(|3) = -tanL- (2.25a)

The surface azimuth angle is given by

where cos(0') = cos($) cos(L) + sin($) sin(L) cos(z) (2.25c)

0 if

1 otherwise

2 [-1 if z < 0° HORIZONTAL E-W AXIS WITH N-S TRACKING

For a plane rotated about a horizontal east-west axis with continuous adjustment to minimize the angle of incidence, 9 can be obtained from (Kreith and Kreider, 1978; Duffie and Beckman, 1991), cos(9) = - cos2(6)sin2(h) (2.26a)

or from this equation (Meinel and Meinel, 1976):

The basic geometry of this configuration is shown in Figure 2.10c. The shadowing effects of this arrangement are minimal. The principal shadowing is caused when the collector is tipped to a maximum degree south (6 = 23.5°) at winter solstice. In this case, the sun casts a shadow toward the collector at the north. This assembly has an advantage in that it approximates the full tracking collector in summer (see Figure 2.15), but the cosine effect in winter greatly reduces its effectiveness. This mount yields a rather "square" profile of solar

Time (hours)

FIGURE 2.15 Daily variation of solar flux: horizontal E-w axis with N-S tracking.

Time (hours)

FIGURE 2.15 Daily variation of solar flux: horizontal E-w axis with N-S tracking.

radiation, ideal for leveling the variation during the day. The winter performance, however, is seriously depressed relative to the summer one. The slope of this surface is given by tan(|3) + tan($)|cos(z)| (2.27a)

The surface orientation for this mode of tracking changes between 0° and 180°, if the solar azimuth angle passes through ±90°. For either hemisphere,

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