Page 45 - Solar Power in Building Design The Engineer's Complete Design Resource

SOLAR POWER SYSTEM COMPONENTS 15

In order to account for the average daily solar exposure time, design engineers refer
to world sunlight exposure maps. Each area is assigned an “area exposure time factor,”
which depending on the location may vary from 2 to 6 hours. A typical example for
calculating daily watt-hours (Wh) for a solar panel array consisting of 10 modules
with a power rating of 75 W in an area located with a multiplier of 5 will be (10 ×
75 W) × 5 h = 3750 Wh of average daily power.

Solar Power System Components

Photovoltaic modules only represent the basic element of a solar power system. They
work only in conjunction with complementary components, such as batteries, inverters,
and transformers. Power distribution panels and metering complete the energy conver-
sion process.

STORAGE BATTERIES

As mentioned previously, solar cells are devices that merely convert solar energy into
a dc voltage. Solar cells do not store energy. To store energy beyond daylight, the dc
voltage is used to charge an appropriate set of batteries.
The reserve capacity of batteries is referred to as the system autonomy. This varies
according to the requirements of specific applications. Batteries in applications that
require autonomy form a critical component of a solar power system. Battery banks in
photovoltaic applications are designed to operate at deep-cycle discharge rates and are
generally maintenance-free.
The amount of required autonomy time depends on the specific application.
Circuit loads, such as telecommunication and remote telemetry stations, may
require two weeks of autonomy, whereas a residential unit may require no more
than 12 hours. Batteries must be properly selected to store sufficient energy for the
daily demand. When calculating battery ampere-hours and storage capacity, addi-
tional derating factors, such as cloudy and sunless conditions, must be taken into
consideration.

CHARGE REGULATORS

Charge regulators are electronic devices designed to protect batteries from overcharg-
ing. They are installed between the solar array termination boxes and batteries.

INVERTERS

As described earlier, photovoltaic panels generate direct current, which can only be
used by a limited number of devices. Most residential, commercial, and industrial
devices and appliances are designed to work with alternating current. Inverters are
devices that convert direct current to alternating current. Although inverters are usually