Page 142 - Applied Photovoltaics

x there is some self-discharge of the batteries
x dust often partly obscures light penetration.
Using the initial array size and the modified insolation data from (4), the
ampere-hours generated throughout the year can be calculated. In these
calculations, allowance needs to be made for loss owing to dust coverage,
assumed to be in the vicinity of 10%, although this may be an overestimation
for the impact of dust. An Arizona study (Hammond, 1997) found that for
modules at normal incidence to the sun, soiling causes a maximum of 3% loss
between periods of rain but that the loss increased with incidence angle, to
4.7% at 24° and 8% at 58°. Bird droppings, however, can have a more serious
impact.
The electricity generated can then be compared to the amount consumed by
the load throughout the year. When calculating the load consumption,
allowance needs to be made for self-discharge of the battery, usually set at
about 3% of the battery charge per month.

Assuming the batteries are at a full state of charge in summer, the state of
charge of the batteries throughout the year can be determined.
6. Optimising array tilt angle—Retaining the same array size, the above
procedures can be repeated with small variations in the array tilt angle until
the depth-of-discharge of the batteries is minimised. This represents the
optimal tilt angle.
7. Optimising array size—Using the optimal tilt angle, the array size can be
optimised, in conjunction with the depth-of-discharge of the batteries, by
using successive approximations of array size in conjunction with the above
procedures.
8. Summarise the design.
An example of the above procedure is provided in Appendix G. Its limitations are
threefold:
1. A mechanism for determining battery capacity needs to be devised and used
in conjunction with this approach, and even then the array-storage
combinations need to be able to reflect changes in array costings relative to
those for the storage.

2. The techniques are only useful once the direct and diffuse components are
known.
3. The iterative nature of the calculations requires use of a computer.

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