Page 81 - Solar Power in Building Design The Engineer's Complete Design Resource

SOLAR POWER SYSTEM CONFIGURATION AND CLASSIFICATIONS 51

Stand-alone PV system arrays are connected in series to obtain the desired dc volt-
age, such as 12, 24, or 48 V; outputs of that are in turn connected to a dc collector
panel equipped with specially rated overcurrent devices, such as ceramic-type fuses.
The positive lead of each PV array conductor is connected to a dedicated fuse, and the
negative lead is connected to a common neutral bus. All fuses as well are connected
to a common positive bus. The output of the dc collector bus, which represents the col-
lective amperes and voltages of the overall array group, is connected to a dc charge
controller, which regulates the current output and prevents the voltage level from
exceeding the maximum needed for charging the batteries.
The output of the charge controller is connected to the battery bank by means of
a dual dc cutoff disconnect. As depicted in Figure 3.3, the cutoff switch, when turned
off for safety measures, disconnects the load and the PV arrays simultaneously.
Under normal operation, during the daytime when there is adequate solar insolation,
the load is supplied with dc power while simultaneously charging the battery. When
sizing the solar power system, take into account that the dc power output from the PV
arrays should be adequate to sustain the connected load and the battery trickle charge
requirements.
Battery storage sizing depends on a number of factors, such as the duration of an
uninterrupted power supply to the load when the solar power system is inoperative,
which occurs at nighttime or during cloudy days. Note that battery banks inherently,
when in operation, produce a 20 to 30 percent power loss due to heat, which also must
be taken into consideration.
When designing a solar power system with a battery backup, the designer must
determine the appropriate location for the battery racks and room ventilation, to allow
for dissipation of the hydrogen gas generated during the charging process. Sealed-type
batteries do not require special ventilation.
All dc wiring calculations discussed take into consideration losses resulting from
solar exposure, battery cable current derating, and equipment current resistance
requirements, as stipulated in NEC 690 articles.

STAND-ALONE HYBRID AC SOLAR POWER SYSTEM
WITH GENERATOR AND BATTERY BACKUP
A stand-alone hybrid solar power configuration is essentially identical to the dc solar
power system just discussed, except that it incorporates two additional components, as
shown in Figure 3.4. The first component is an inverter. Inverters are electronic power
equipment designed to convert direct current into alternating current. The second
component is a standby emergency dc generator, which will be discussed later.

Alternating-current inverters The principal mechanism of dc-to-ac conversion
consists of chopping or segmenting the dc current into specific portions, referred to as
square waves, which are filtered and shaped into sinusoidal ac waveforms.
Any power waveform, when analyzed from a mathematical point of view, essen-
tially consists of the superimposition of many sinusoidal waveforms, referred to as
harmonics. The first harmonic represents a pure sinusoidal waveform, which has a unit
base wavelength, amplitude, and frequency of repetition over a unit of time called