Page 133 - Applied Photovoltaics

few large systems are currently installed. However, smaller, light sensitive trackers,
with one or two modules on a pole, as in Fig. 6.8, are now being used for water
pumping systems around Australia (Solar Energy Systems, 2004).
As more tracking systems are developed and tested, their relative cost and reliability
are expected to improve. Their use for larger PV systems may then increase,
especially for ground-mounted grid-connected systems where maintenance is less of
an issue than it is for remote installations.

Figure 6.8. Passive tracker for a small PV array (reprinted with permission of
Sandia National Laboratories, 1991). These are no longer manufactured owing to
their use of ozone-depleting chemicals.

Concentrator arrays
Concentrator arrays use optical lenses and mirrors to focus sunlight onto small areas
of high efficiency cells (Swanson, 2003). The high cell costs are allayed by relatively
low cost apertures. Since direct solar radiation is the most effectively-concentrated,
concentrator systems are suitable only for sites with characteristically high levels of
direct versus indirect radiation. Accurate tracking of the sun is necessary for these
systems, particularly when the concentration ratio is high. Tracking tends to increase
the intercepted insolation but, with concentration, the trade-off is the ability to access
only the direct fraction (see Chapter 1). The overall result depends on the clarity of
the sky at the site. Luque et al. (1995) compared the radiation available to fixed flat
plates tilted at latitude angle, with direct normal radiation at several locations in
Europe and the USA. In some locations with clear skies, such as Albuquerque, the
direct normal insolation exceeds that on the fixed tilted planes but for others, such as
Pittsburgh, it is less.

A large (220 kW p ) remote solar power station has recently been commissioned in the
north-west corner of South Australia (Australian Greenhouse Office, 2003b). It
consists of ten mirrored parabolic reflectors concentrating by 500 times onto silicon
2
cells. Each dish uses about 130 m of curved mirror sections. The solar system is
connected to a diesel grid, which supplies power to a remote community. Another

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