JFO - Part 5 - cd3wd405.zip - Selecting the right PV voltage...By Windy Dankoff

JFO - Part 5 - cd3wd405.zip - Selecting the right PV voltage...By Windy Dankoff
Selecting the right PV voltage
By Windy Dankoff
he independent home power sysT
tem is based on storage batteries and
direct current (DC electric power.
Batteries are low voltage modules that
may be assembled in 6, 12, 24-volt or
higher configurations. Voltage is the
electrical “pressure” at which the system operates, and part of the battery’s
job is to maintain this pressure at a
fairly constant level. Thus, a “12-volt”
battery maintains a working voltage
within the range of about 11 to 14.5
volts—a standard. A 12-volt appliance will run properly within this
range of electrical pressure.
While the voltage remains fairly
constant, the current (measured in
amps) varies according to the power
required by the appliance. As more
lights are turned on in your house,
more current is drawn from your batteries. A large bulb draws more current than a small one. Some appliances draw different amounts of current at different times; a circular saw
draws more current cutting 2” wood
than ½” wood, because the motor
works harder.
Twelve volts is the most common
standard for alternative energy homes
only because it is already a conventional standard—for vehicles! As we
progress to higher voltages, less current (amps) is required to deliver the
same amount of power (watts/horsepower). Wire, switches, and other
inline components are sized according
to the current they carry; the voltage
has little bearing on their sizing.
Therefore, a 24-volt home electric
system is less costly to wire—it
requires half the wire size, and less
labor to install. Control systems and
inverters contain components that the
current must pass through, so they too
can be smaller and less expensive in a
higher voltage system.
To confirm this for yourself, compare prices of 12 and 24-volt charge
controllers and inverters. The 24 volt
models handle far more watts per
dollar! Efficiencies also tend to
increase with higher voltage/lower
current. To see an extreme example of
relative wire sizes, look under the
hood of your car and see the big wire
that goes from the battery to the
starter. A typical circular saw requires
as much power as your starter, but
look at the little wire it uses! The saw
uses 120 volts, and requires 1/10 the
wire size to carry the current.
The common voltage standards for
independent-powered homes are 12
volts and 24 volts. Your choice of
standard is based on these factors:
1. Overall system size: Small, cabinsize systems standardize on 12 volts,
which offers the widest choice of
small DC appliances and small
inverters. Medium to large homes
generally cost less to set up on
24 volts, for the reasons below.
2. Inverter size: Inverter requirements beyond 2,000 watts or so indicate 24 volts, for lower cost per watt
and higher efficiency.
3. DC well pump or other large
motors: Motors above ¼ HP often
necessitate the use of 24 volts,
whether they are DC motors or AC
run by inverter. Large motors are
more efficient at higher voltages. High
current is required to start most
motors, so both wire and inverter need
to be oversized. So the potential
savings are especially great in going
to higher voltage for motor circuits.
4. Wiring distances: Long wire
runs from PV (photovoltaics, or solar
cells) or (especially) wind or hydro
generators, to a DC well pump, or to
other buildings, can be very costly at
low voltage/high current. The longer
the distance, the larger the wire must
be to reduce losses. So cutting the current in half by using twice the voltage
can cut your wire cost by nearly 75%.
5. Plans for future growth: If any
of the above indicate a requirement
for 24 volts in the future, set up for it
from the start so you won’t be left
with obsolete equipment (such as electronics) on a 24-volt system. High
quality 24-volt lights are nearly as
common as 12. Many large DC
motors and pumps are not available at
all in 12 volts, because the lower voltage motors are less efficient and require
costly, over-sized wire, breakers
and switches.
We do not go to 48 volts very often,
because we cannot get DC lights,
refrigerators, and well pumps at that
voltage. Most PV dealers and users
agree that DC power still has its place
for running the specialized, super-efficient DC appliances made specifically
for independent power. Direct use of
DC in well-engineered appliances
reduces both energy consumption and
inverter requirements.
We are maintaining 12 and 24 volts
as our DC home standard because it is
safer and less costly to use than higher
DC voltages. (1) Less battery cells are
required (they are 2 volts each) with
less connections between them. (2)
High DC voltage from batteries (120
volts) poses a serious shock hazard
(twice that of 120 volt AC). (3) High
DC voltage poses more fire hazard (it
causes much bigger sparks) than AC
power at the same voltage. Low voltage virtually eliminates these hazards.
120 volt DC is used in industrial
power systems, but generally not in
homes. Our use of high-efficiency
appliances and our elimination of
electric heating devices keeps power
consumption low, so wire sizes in our
DC homes need not be 5 or 10 times
oversized for low voltage!
A system dedicated to one specialized purpose need not conform to the
common 12 or 24-volt standard. When
a solar system is designed only to
power a well pump (with a motor
range of ½ to 1 HP) we may go to 60
or 120 volts DC if that optimizes
economy and efficiency.
Remember, the final product of your
energy system is not volts—it’s light,
water, communication, mechanical
energy, etc. The voltage selected
should be that which produces these
ends at the lowest overall cost, with a
high degree of safety and reliability. ∆
The Fourth Year
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