Page 132 - Applied Photovoltaics

Fixed arrays
Fixed arrays are the most commonly used. The modules are placed on a support
structure, facing within 5° of north in the southern hemisphere (Standards Australia,
1999–2000b) and of south in the northern hemisphere, at an angle determined by the
requirement. For example, for the most constant output over the year, an angle of
latitude plus ~23° is used, which places the array at right angles to the sun’s rays in
mid-winter. A minimum tilt angle of 10q is recommended, to allow natural cleaning
of the array surface by rain (Standards Australia, 2002). Standards Australia (2002)
recommends the tilt angles given in Table 6.1.
Table 6.1. Approximate optimum array tilt angles for fixed arrays.
tilt angle relative to latitude (°)
latitude (°) constant seasonal load winter-peaking load summer-peaking load
5–25 +5 +5 – +15 –5 – +5
25–45 +5 – +10 +10 – +20 +10

Seasonally-adjusted tilting

The array angle can be changed manually against the horizontal axis (e.g. monthly or
seasonally), to allow for the changing solar elevation at noon. This is a relatively
simple way of increasing output and does not add significantly to the cost. Flexibility
in tilt angles for seasonal changes is marginally economical for small systems. For
mid-latitude locations, adjustment to the tilt angles every three months increases the
annual energy production by less than 5%. Such arrays need to be marked to indicate
either the tilt angle or the time of year suited by each position (Standards Australia,
1999–2000b).

Single-axis tracking
The array can be tilted automatically every hour or more frequently, along the vertical
axis, to follow the sun from east to west. Output can be increased significantly.
Theoretical estimates of insolation increases relative to optimally-tilted fixed arrays
for a range of sites, using meteorological data, found values of 29–37% (Nann, 1990).
However, it is not always possible to convert all the theoretical insolation gain into
output power due, for example, to mutual shading (Townsend & Whitaker, 1997). An
experimental study found that insolation on an array could be increased by 18%
compared to a fixed array by azimuth tracking around a vertical axis or by 11% using
a tilted-axis tracker (Helwa, 2000). A single axis tracking array is illustrated in
Fig. 6.8. Reinforced concrete foundations with anchor bolts are recommended and the
movement path should be carefully checked to ensure it is free from obstructions. The
system cost is higher, as is the maintenance requirement (Lepley & Hammond 1997),
so that their cost effectiveness for each particular application requires careful
consideration.

Two-axis tracking

Power output can be further increased by tracking the sun along both the north-south
and east-west axes. Helwa (2000) measured a 30% insolation improvement relative to
a fixed-tilt array. However, both the capital and maintenance costs can be high and

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