Page 141 - Applied Photovoltaics

7.4 A SIMPLIFIED PV SYSTEM DESIGN APPROACH
Telecom Australia, now Telstra, was instrumental in developing PV powered
telecommunications systems for use in remote regions of Australia and hence in
developing Australia’s PV market. Telstra now has over 3 MW p of PV installed
across the country, mostly in PV battery systems, with a few PV diesel hybrids
(McKelliff, 2004). Although Telstra now uses sophisticated spreadsheet models for
system design and has developed its own solar resource maps, its original simplified
system design approach (Mack, 1979) forms the basis of the more iterative models
now used and represents a conservative approach, where high availability is needed
and in which array size is optimised as a function of the battery capacity.
1. Load determination—To specify the load as accurately as possible, and
hence achieve a system design that optimises components and costs, the
following information is needed:
x nominal system voltage
x range of voltages able to be tolerated by load
x average load per day
x load profile throughout the year.
For a microwave repeater station, for example, the voltage may be 24 ± 5 V,
the average load 100 W (current = 4.17 A), and the required storage 15 days.
2. Select battery capacity—For telecommunications loads, the design approach
is quite conservative, allowing for 15 days of battery storage to give very high
availabilities. For the example given above, this would be
4.17 A × 24 h × 15 days = 1500 Ah.
3. First approximation of tilt angle—This is based on site information and
usually involves selecting a tilt angle 20° greater than the latitude. For
example, for Melbourne, which is at latitude 37.8°S, the first approximation
for tilt angle is 37.8° + 20° = 57.8°.
4. Insolation—From the available site insolation data, the actual insolation
falling on the array at the selected tilt angle can be estimated. An example of
typical insolation data throughout the year falling on a horizontal plane in
Melbourne is provided in Appendix G. Using this insolation data, sample
calculations are provided for determining the actual corresponding amount of
insolation that will fall on the photovoltaic array when tilted at an angle of
57.8°. An assumption made in these calculations is that the diffuse component
of the insolation data is independent of tilt angle. This is a reasonable
approximation, provided the tilt angle is not too great.
5. First approximation of array size—As a rule of thumb, the initial array size
2
in peak amps (1 kW/m ) is selected to be five times the average load current.
This figure is large because:
x the sun does not shine at night
x there is reduced light intensity during mornings, afternoons and periods of
cloudy weather
x the batteries have a limited charging efficiency

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