Page 175 - Applied Photovoltaics

x desire for independence and low running costs
x avoidance of overhead wires in environmentally sensitive areas.
Stand-alone photovoltaic power systems that require availabilities in excess of 90%
often need considerable oversizing of the array for summer months and sunny days to
ensure adequate electricity generation in the winter months and for periods of
prolonged cloudy weather. This oversizing is very costly, but is necessary for many
applications in remote areas, where maintenance and refuelling are difficult and
expensive.

In comparison, where humans are in close proximity and are able to do at least
rudimentary maintenance and refuelling, more flexibility can be exercised in
photovoltaic system design. PV-only availability can therefore be reduced, usually
resulting in vastly more efficient use of photovoltaics throughout the year. For
example, to maximise the efficiency of photovoltaic use in the system, it is necessary
that all generated electricity be used either directly by the load, or be stored in the
batteries. For a stand-alone photovoltaic system design, this involves considerable
downsizing of the photovoltaic component to the linear region of the photovoltaic
availability versus photovoltaic cost curve of Fig. 7.1. The corresponding reduction in
availability necessitates incorporating an additional energy source to supplement the
photovoltaic generation, so that in combination, high system availability is achieved.
House design and orientation, as well as selection of appropriate energy sources and
appliances for different end uses, such as cooking, heating and lighting, are of critical
importance when using a RAPS system (see Fig. 9.1 and Standards Australia, 2002).
The topic of passive solar house design will not be covered in detail in this book, but
excellent information is available from the Australian Greenhouse Office (2004b)
web site.

Figure 9.1. A home based on energy efficient passive solar design principles for
the southern hemisphere. The directions north and south are reversed for the
northern hemisphere.

With regard to appliances used in RAPS households (Australian Greenhouse Office,
2004c), some general rules apply:

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