Page 68 - Alternative Energy Systems in Building Design

SOLAR POWER SYSTEM COMPONENTS 45

The amount of required autonomy time depends on the specific application. Circuit
loads, such as telecommunications and remote telemetry stations, may require 2 weeks
of autonomy, whereas a residential unit may require no more than 12 hours. Batteries
must be selected properly to store sufficient energy for the daily demand. When cal-
culating battery ampere-hours and storage capacity, additional derating factors, such
as cloudy and sunless conditions, must be taken into consideration.
In solar power cogeneration systems, when using storage batteries for electrical energy
storage, fluctuations in the dc power produced by PV systems caused by variations in
solar irradiance are regulated by electronic devices referred to as charge regulators.
Charge regulators are placed between the PV output and the battery banks to regulate
the amount of electric charge current, which protects batteries from overcharging.

INVERTERS

As described earlier, PV solar panels generate direct current, which can be used by
only 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 into alternating current. Although inverters usually are designed
for specific applications, the basic conversion principles remain the same. Essentially,
the inversion process consists of the following: In the wave-formation process, direct
current characterized by a continuous potential of positive and negative references
(bias) is essentially chopped into equidistant segments that are then processed through
circuitry that alternately eliminates positive and negative portions of the chopped
pattern, which results in a waveform pattern called a squarewave. Figure 3.12 shows
a single-line diagram of an inverter.

Waveshaping or filtration process A squarewave, when analyzed mathemati-
cally (by Fourier series analysis), consists of a combination of a very large number of
sinusoidal (alternating) wave patterns called harmonics. Each wave harmonic has a
distinct number of cycles (rise-and-fall patterns within a time period). An electronic
device referred to as a choke (magnetic coils) filters or passes through 60-cycle har-
monics, which form the basis of sinusoidal current. Solid-state inverters use a highly
efficient conversion technique known as envelope construction. Direct current is sliced
into fine sections, which are then converted into a progressive rising (positive) and
falling (negative) sinusoidal 60-cycle waveform pattern. This chopped sinusoidal
wave is passed through a series of electronic filters that produce an output current that
has a smooth sinusoidal curvature.

Protective relaying systems In general, most inverters used in PV applications
are built from sensitive solid-state electronic devices that are very susceptible to external
stray spikes, load short circuits, and overload voltage and currents. To protect the equip-
ment from harm, inverters incorporate a number of electronic circuitry configurations:

■ Synchronization relay
■ Undervoltage relay