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are three major types of lead acid batteries; deep
cycle, shallow cycle, and SLI (starting, lighting, and
ignition). SLI batteries are meant for automotive use
and are almost universally unsuitable for solar
electric applications.
Deep
cycle batteries are designed with the intention of
having a high percentage of their stored electricity
removed on a regular basis. On the other hand, shallow
cycle batteries are engineered to provide infrequent
stand-by service, with only 15-20% of their stored
power intended for use. Examples of deep cycle service
are golf carts and fork lifts, shallow cycle examples
would include hospital and telephone back-up
batteries. Deep cycle batteries are the best and
indeed the only suitable choice for solar electric
storage.
Battery
capacity is expressed in amp-hours at a given
discharge rate (X A/hr @ Y hour rate) to full
discharge. For example a Surrette KS 13 battery has a
capacity of 788 amp hours at the 72 hour rate. Another
way to express this would be to say that this battery
would be fully depleted if an 11 amp load was
continuously connected for 72 hours. (Rated battery
capacity divided by the hour rate determines the size
of load). The number of times a battery can be cycled
before it is worn out increases significantly as the
percentage depth of discharge (DOD) declines. A
battery which is required to deliver 50% of its power
will provide up to 30% more cycles than when it is
drained to 80% DOD.
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Synchronous Inverters
Synchronous inverters change DC power into AC power to
be fed into the utility grid. A power system with this
type of inverter uses the utility company as a storage
battery. When the sun is shining, your electricity
comes from the PV array, via the inverter. If the PV
array is making more power than you are using, the
excess is sold to the utility power company through a
second electric meter. If you use more power than the
PV array can supply, the utility makes up the
difference. This type of system makes the most sense
if you have utility power, because there are no
batteries to maintain or replace, but it has a very
long payback period and may not be cost-effective at
today's electric rates. Using a multifunction inverter
allows you to sell excess power to the utility, and
also maintain a battery bank for stand-by power in the
event of a utility power failure.
Multi-function
Inverters
A multi-function inverter
is connected to a battery bank, the utility power
lines, a standby generator and the house load center.
When batteries are in a charged condition, the SW
inverter supplies AC power to the house from the
batteries. If the batteries become discharged, the
inverter supplies the house loads from the utility
lines, while charging the batteries. If the batteries
become fully charged by another power source, such as
photovoltaic modules or a wind or hydroelectric
generator, excess power may be sold back to the
utility. If utility power fails, the inverter can
still operate, supplying critical loads. If a standby
generator is started, it can also supply power to
loads. The inverter will synchronize to the generator
and allow loads to be powered that are too large for
either the generator or inverter to supply alone.
Stand-Alone Inverters
Stand-Alone inverters
convert DC power stored in batteries to AC power that
can be used as needed. Selecting an inverter for your
power system based on the maximum load you will be
powering, the maximum surge required, output voltage
required, input battery voltage and optional features
needed. High quality stand-alone inverters are
available in sizes from 100 watts, for powering
notebook computers and fax machines from your car, to
8000 watts, for powering an entire house or small
commercial operation. The size of an inverter is
measured by its maximum continuous output in watts.
This rating must be larger than the total wattage of
all of the AC loads you plan to run at one time. The
size of the inverter can be minimized if the number
and size of the AC loads is kept under control.
Wattage of most AC loads can be determined from a tag
or label on the appliance, usually located near where
the power cord enters, or from the owner's manual. If
the inverter is expected to run induction motors, like
the ones found in automatic washers, dryers,
dishwashers and large power tools, it must be designed
to surge, or deliver power many times its rating for
short periods of time while these motors start.
Stand-alone inverters
are available with three basic power output waveforms:
square wave, modified square wave (sometimes called
modified sine wave) and pure sine wave. Synchronous
Inverters and Utility companies deliver a pure sine
wave.
Sine wave inverters
have a slightly higher cost, but they can operate
almost anything that can be operated on utility power. Sinewave inverters are available in sizes from
2500 watts to 5500 watts, and a pair of them can be
synchronized to deliver up to 11,000 watts. They are
an excellent choice for a 'whole house" inverter.
Output Voltage
Inverters should supply standard 120 Volt 60 HZ AC power, such as one
gets from utility companies and fuel-powered
generators. Most of them can be special ordered with
other output voltages and frequencies for use anywhere
in the world.
Interference
The
electronic circuitry in inverters may, in some cases,
cause problems with radio and television reception,
noise on telephones and buzz in audio equipment. Sine
wave inverters cause the least amount of interference.
Interference can be minimized by locating the
inverter very close to the batteries, twisting
together cables that connect the inverter to the
battery and locating the inverter away from appliances
that are susceptible to interference. All inverters
cause interference on AM radio! |
Saturday July 05, 2008 - 12:42 PM
