Page 185 - Applied Photovoltaics

System design in practice
The steps typically taken in designing a system can be summarised as follows:

1. Determine the load in consultation with the user (consumer education
included).
2. Design initially for a photovoltaic-only system with non-critical availability
(95%) as per stand-alone system design procedures.
3. Locate the design on the hybrid indicator to ascertain the viability of a
photovoltaic-only system as opposed to a photovoltaic-diesel hybrid system.
4. For a photovoltaic-diesel hybrid system, downgrade the availability/array size
so that on a clear sunny day in summer, at the design tilt angle, the electrical
energy generated (allowing for subsystem inefficiencies) just matches or is
slightly less than that consumed by the load over a 24 hour period. The most
suitable design will probably correspond to a photovoltaic availability of
about 80%.

5. Select the diesel so that its rating matches the peak load. This will facilitate
battery charging whenever the peak load is not being drawn, while enabling
the generator to be always almost fully loaded when operating.
6. Select the inverter to supply the peak general loads. This will also determine
the system DC voltage.

7. Select appropriate deep cycle batteries that satisfy your design and also the
charging/discharging rates of the system.
8. Select an appropriate controller.

9. Discuss the system design with the user and the cost and system operation
implication of increasing or decreasing the photovoltaic component (by a
string at a time). If less photovoltaics is desired (to cut system costs), the
array tilt angle should be decreased to provide greater annual electrical energy
generation. This will boost summer generation at the expense of winter
generation but will reduce overall diesel operating time and hence O&M
costs.

Alternative approach for low capital intensive system
This is a typical method used by RAPS suppliers to provide a minimum cost system
design:

1. Initially design for a diesel-battery system and calculate the reduction in
diesel operating time per day corresponding to the inclusion of each string of
photovoltaic panels.
2. The user then determines the component mix.
3. The ultimate system design must be settled on before the appropriate balance
of system components (inverter, battery etc.) can be selected.

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