Page 145 - Applied Photovoltaics

6. Component sizing, allowing for full battery recharging within, say, 14 days
and for ability to provide gassing charge:
x Select first estimate for array tilt angle (see Table 6.1) and estimate
insolation for that plane (see Chapter 1).
x Size battery according to required days of autonomy, desired daily and
maximum depth-of-discharge, daily energy demand, surge demand and
maximum charging current.
x Check whether battery size is determined by the required storage capacity
or the power demand, which depends on the system parameters,
especially the required days of autonomy.
x Size the array and configure into strings to supply the chosen fraction of
the load energy, the remainder to be supplied by a genset or otherwise.
x Specify, with the aid of worksheets, inverter(s), regulator(s), battery
charger, genset, battery ventilation.
x Choose cable sizes, with aid of Appendix C of the Standard.
Other standards and guidelines are also available and some are listed in Appendix E
of this book.

7.7 SYSTEM DESIGN SOFTWARE
A large number of both commercial and freely available computer programs for
sizing and simulation of performance of stand-alone and grid-connected photovoltaic
systems have become available in recent years (Knaupp, 2003; Silvestre, 2003;
National Renewable Energy Laboratory, 2004). Some directly interface with
databases of insolation and other meteorological information. Different programs use
different algorithms for carrying out the calculations and vary in the transparency of
the methods and in the provision of intermediate results for verification. Of course,
these programs cannot be reliably used without checking the results by other means
and local standards and guidelines still need to take precedence.
Sizing programs, sometimes in spreadsheet form, help calculate the system
components for a given loads, insolation, orientation and, in the case of stand-alone
systems, days of autonomy. They aid selection of modules, cables, batteries and
power conditioning electronics. Simulation programs, additionally, simulate the
system behaviour. The reader is referred to Knaupp (2003) and Silvestre (2003) for
brief descriptions of a selection of available programs.

EXE RCI S ES
7.1 (a) What is a ‘hybrid’ system when considering photovoltaics?
(b) Discuss the advantages and disadvantages of hybrid systems over
purely photovoltaic-powered systems.
(c) Which applications are best suited to hybrid systems?
7.2 (a) Give examples of your telephone utility’s uses for solar cells,
referring to specific installations in your country.

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