Page 143 - Applied Photovoltaics

7.5 SANDIA NATIONAL LABORATORY APPROACH
This approach (Chapman, 1987) was developed by Sandia National Laboratories in
the United States and is considerably more sophisticated than that presented in
Section 7.4, automatically incorporating many years of accumulated insolation data.
It overcomes the limitations listed for the approach considered above and is
applicable to any system with a fixed tilt array, allowing the designer to choose tilt
angles in the range from latitude – 20° to latitude + 20°.
The system design is based on a specific loss-of-load probability (LOLP) specified by
the designer. By definition, LOLP is the probability at any point in time that the load
will not be satisfied by the PV-storage system and, as such, is directly related to
availability (A), as previously dealt with, where

LOLP 1 A (7.1)
In reality, for a specified LOLP, there is a continuum of array size/storage capacity
combinations, where the relative costs and efficiencies of the constituent components
determine the least-cost approach. In general, the system cost increases approximately
exponentially as the LOLP approaches zero.
In the development of this model for system design, extensive work has been carried
out in correlating variability in insolation with average data commonly available. The
correlations have resulted from the study of about 24 years of hourly data and as such
should, on a statistical basis, provide accurate correlations. The latitude is a required
variable for the system design, indicating that the model makes use of theoretically-
calculated light intensities as a function of air mass throughout each day, in an
approach similar to that outlined earlier (whereby average global data can be
reasonably accurately converted into approximate direct and diffuse components).
An interesting outcome of the study that led to the formation of this model and
approach is that accuracy of system design is not lost by basing the design only on
data for the month with the lowest insolation levels over the year. This of course
greatly simplifies the design approach. In addition, through the use of calculations
similar to those in Section 7.4, 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 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.
In fact, four sets of the curves exist, each giving a different battery capacity in (1) for
the specified LOLP. Completion of (2)–(4) above results in each set of curves
providing 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 that satisfies system specifications.

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