Page 307 - Applied Photovoltaics

similar to those in the approach described above, but over a wide range of possible
design values and in conjunction with appropriately-treated average global insolation
data, curves have been generated that facilitate:
1. determination of battery capacity for a specified loss-of-load probability
(LOLP)
2. optimisation of array tilt angle
3. obtaining insolation data at the appropriate tilt angle

4. determination of the array size that, in conjunction with (1), provides the
required LOLP.

Four sets of the curves exist, each giving a different battery capacity in (1) for the
specified LOLP. When followed through (2)–(4), each set of curves provides for a
different system design (different array size/battery storage combination) with the
same LOLP. These four sets can then be analysed on the basis of cost to determine
the least-cost approach to satisfying system specifications.

Step 1—Define site-specific and application-specific parameters
x latitude
x horizontal insolation for worst month (usually June in Australia, December in
the northern hemisphere)
x daily energy demand
x LOLP required.
For example:
x latitude—30°N
x average daily horizontal insolation in December—3 kWh/m 2
x daily demand—5 kWh ac
x LOLP—0.001 (critical load such as a vaccine refrigerator).

If the average daily summer demand exceeds the average daily winter demand by
more than 10%, attention needs to be paid to the possibility of battery discharge
during summer (which may necessitate re-optimisation of tilt angle).
Step 2—Determine battery storage for each of the four designs
This is read directly from the appropriate nomogram, as a function of LOLP. Fig. G.1
shows this nomogram for design 2, showing that for an LOLP of 0.001, storage (S) is
5.80 days.
Similarly, looking at the appropriate nomograms, designs 1, 3 and 4 give storage
values of 3.49, 8.13 and 10.19 days, respectively. From the S values, the actual
battery capacity can be calculated from
S u L
CAP (G.2)
DODu Ș out

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