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What does an inverter do?
Inverters convert
DC (typically low
voltage) DC into AC (at 230V 50Hz) as required for conventional appliances.
There are generally two types of inverter available: stand alone and grid
connect.
 Grid connect inverters
Grid connect inverters are
supply driven - they provide all the power supplied from a DC source
to the grid or mains. Grid connect inverters are usually
optimised for one specific type of generator, e.g. PV and generally
operate at a higher DC voltage than stand alone inverters.
For more information see:
Sunny Boy or
Windy Boy
inverters.
Grid connect inverters should
NOT be connected to batteries and stand alone inverters should NOT be
connected directly to PV, wind turbines or the grid.
 Stand alone inverters
Stand alone, or battery
supplied, inverters are demand driven - they provide any power or
current up to the rating of the inverter and assuming that there is
enough energy in the battery.
For more information see:
OutBack,
Studer
or
Sunny
Island inverters.
Smaller systems with few appliances
may have only DC power, but recent advances in inverter design, efficiency, and
reliability have increased the potential of wind/solar systems considerably.
With the use of modern high
efficiency AC lighting the majority of, if not all, loads can be operated on AC especially
in larger installations.
We tend to use both AC & DC
where each is most effective and economical - many DC appliances use less power than their
AC equivalents (especially refrigeration, lighting & electronics) - but DC appliances
tend to be harder to find & more expensive.
Where does any excess
energy go?
This depends on whether the system
is stand alone or whether it is grid connected.
Storage batteries are the
heart of all stand alone wind/PV or inverter electrical systems. By storing excess
energy when the
wind or sun is strong, they offer a reliable source of electricity which can be used when
solar or wind power is not available. Their function is therefore to
balance the outgoing electrical requirements with the incoming energy supply.
Batteries are also able to provide
short term power output many times higher than the charging source output.
For grid connected inverters energy is fed back into the grid.
For more information see:
Batteries
Who needs a generator?
In typical domestic situations, for
most of the day, loads are very small - perhaps a few lights and other appliances. For a small proportion of the time,
however, large loads such as washing machines, electric kettles, etc. must be powered.
Sizing a renewable energy system to
meet this peak demand is, in most cases, prohibitively expensive (at least initially).The optimum way to
incorporate a wind and solar energy is for these to supply the low loads required for most
of the day, and allow a generator to start up automatically to meet the small proportion
of loads for which a large capacity is required.
In such systems, batteries allow
power to be available 24 hrs/day but means that the generator need only run for short
periods to charge the battery.
For more information see:
Generators
Efficiency
Modern electronic inverters
are very efficient over a wide range of outputs, but some power is required simply to keep
the inverter running (the standing losses) and they are less efficient when running small
loads.
Consequently, sizing the inverter
for its required purpose is extremely important.
-
If it is undersized, then there will
not be enough power - demanding more than their limit will shut them off.
-
If it is oversized, it will be much
less efficient (due to the standing losses) and more costly to buy and run.
A load seeking circuit is normally
included to ensure that battery power is conserved for useful purposes by automatically
switching the inverter on and off as loads are applied or discontinued. (NB. Older type
rotary inverters which use a DC motor to turn an AC generator have typical efficiencies of
around 25% due to the mechanical standing losses).
Sizing
In inverter sizing the most
important factor is peak power consumption: the peak power demand should not exceed the
rated peak output of the inverter.
This is difficult when it is
possible for many devices to consume power at the same time, and is further complicated by
any electric motors in the system.
Some types of electric motors
require three times as much power to start them as is required to run them. If two or more
motors are started at the same time the surge power demand is much higher than the average
demand. Consequently, the inverter should be sized to be able to at least start the
largest motor in the system and measures taken to ensure that all motors do not start at
the same time.
Proper energy management can reduce
peak demand, and so the inverter can be sized closer to the average power demand, thereby
increasing the system's efficiency and reducing hardware costs.
Siting
Inverters should be located
in a dry, non-condensing, clean, ventilated, environment.
Vented lead acid batteries can
produce corrosive vapours and when on charge produce an explosive mixture of hydrogen
& oxygen. So good ventilation is required for the battery, particularly at a high
level to allow any hydrogen to disperse.
Preferably, the battery should be
in it's own cubicle, vented to the outside. If this is not practicable, don't mount the
inverter directly above the battery or directly adjacent to it.
In order to minimise the voltage
drop in the connecting cables to the battery, these should be kept as short as possible
and of sufficient size.
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