Full tracking

For a two-axis tracking mechanism, keeping the surface in question continuously oriented to face the sun (see Figure 2.10a) at all times has an angle of incidence, 9, equal to cos(9) = 1 (2.22)

or 9 = 0°. This, of course, depends on the accuracy of the mechanism. The full tracking configuration collects the maximum possible sunshine. The performance of this mode of tracking with respect to the amount of radiation collected during one day under standard conditions is shown in Figure 2.11.

Polar axis

(a) Full tracking Z

(a) Full tracking Z

(c) N-S horizontal

FiGuRE 2.10 collector geometry for various modes of tracking.

(c) N-S horizontal

Polar axis

Geometric Plane Design

(d) E-W horizontal

(d) E-W horizontal

FiGuRE 2.10 collector geometry for various modes of tracking.

Solar Geometry

Time (hours)

FiGuRE 2.11 Daily variation of solar flux, full tracking.

Time (hours)

FiGuRE 2.11 Daily variation of solar flux, full tracking.

Solar Declination Angle
Declination angle (Degree) FiGuRE 2.12 Number of consecutive days the sun remains within 4° declination.

The slope of this surface (|3) is equal to the solar zenith angle ($), and the surface azimuth angle (Zs) is equal to the solar azimuth angle (z).

TILTED N-S AxiS wiTH TILT ADJuSTED DAILY

For a plane moved about a north-south axis with a single daily adjustment so that its surface normal coincides with the solar beam at noon each day, 9 is equal to (Meinel and Meinel, 1976; Duffie and Beckman, 1991)

For this mode of tracking, we can accept that, when the sun is at noon, the angle of the sun's rays and the normal to the collector can be up to a 4° declination, since for small angles cos(4°) = 0.998—1. Figure 2.12 shows the number of consecutive days that the sun remains within this 4° "declination window" at noon. As can be seen in Figure 2.12, most of the time the sun remains close to either the summer solstice or the winter solstice, moving rapidly between the two extremes. For nearly 70 consecutive days, the sun is within 4° of an extreme position, spending only nine days in the 4° window, at the equinox. This means that a seasonally tilted collector needs to be adjusted only occasionally.

The problem encountered with this and all tilted collectors, when more than one collector is used, is that the front collectors cast shadows on adjacent ones. This means that, in terms of land utilization, these collectors lose some of their benefits when the cost of land is taken into account. The performance of this mode of tracking (see Figure 2.13) shows the peaked curves typical for this assembly.

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