where small, standalone PV systems have this kind of connection between the
panels and battery.
A reverse diode, or electronic component that allows current flow in one
direction only, is used to connect the PV module and battery. The reverse diode
prevents any current from moving from the battery to the PV module at night. If
not for the reverse diode, the modules would draw current and act as a load. The
load would drain the battery. These products are also used to hold a charge in car,
truck, and RV batteries.
Direct coupling is not commonly used in larger systems. It is not recom-
mended without a serious understanding of all the details in the match between
array and battery. Direct coupling is not a good choice for almost any residential
and commercial applications.
Another safer and more reliable method for eliminating battery losses is to
incorporate charge controllers. Charge controllers guard the battery from over-
charging and help to ensure that the battery does not over discharge. An MPPT
charge controller helps to maximize available solar energy.
Measuring Electricity Consumption
The hardest part of designing a standalone PV power system is determining the
amount of energy that will be used. Most areas of the United States are not
77 degrees and sunny year-round. San Diego may be, yes, but not Des Moines.
One of the first things a PV designer needs to do is calculate or find accurate
data on solar irradiation for the project geographic location at different times
during the year. Many organizations have this data available for public use on the
Internet. The second step is to gather information on the distinct electrical loads
that will require power, the equipment and appliances that will consume energy,
their hours of operation, and how much energy they will require during
operation.
If the standalone PV system is intended for everyday use, then it is wise for
the PV designer to discuss alternatives to complement the standalone system.
This may include a different size battery bank, the addition of a generator or an
increase in its size, or suggesting a hybrid design utilizing wind or hydroelectric
power sources. Whatever the options being considered, always include cost esti-
mates of those systems with or without them.
Sizing the PV Array
After determining the estimated electricity consumption, the PV designer needs
to determine the PV array size needed to produce the power required for the load.
Once the PV type is selected, the designer can determine how many modules will
be needed to meet the needs of the home.
126 ADVANCED PHOTOVOLTAIC INSTALLATIONS
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