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Solar Power
7
Wind-Electric Systems
Ian Woofenden
©2005 Ian Woofenden
Wind-electric systems may be the most captivating of the three main renewable
electricity technologies. Most of us just love to watch a wind turbine spin. But these
systems are also the most prone to problems, and can be more complicated and
expensive to install.
To get you started down the right road in using wind energy, this article will
outline the basic system components and types. It will help you understand the
systems better, so you will make better choices if you decide that wind energy is
right for you.
Off-Grid Wind-Electric System
Off-grid wind-electric systems are battery based. People
generally choose these systems because their home or other
energy use is not connected to the grid, and connection
would be expensive. Others prefer the independence of off-
grid systems, or live where utilities and governments make
it difficult to tie a renewable energy system to the grid.
Off-grid systems are limited in capacity by the size of the
generating sources (wind turbine, solar-electric array, fuel-
fired generator, etc.), the resources available, and the battery
bank size. Off-grid homeowners have to learn to live within
the limitations of their system capacity.
Wind
Generator
Meter
7
1
5
4
Dump
Load
Charge
Controller
Controller
Tower
3
2
6
Battery
Bank
Brake
10
home power 110
/ december 2005 & january 2006
System
Battery
wind
basics
1
Wind Generator
AKA: wind genny, wind turbine
The wind generator is what actually
generates electricity in the system.
Most modern wind generators are
upwind designs (blades are on the
side of the tower that faces into the
wind), and couple permanent magnet
alternators directly to the rotor (blades).
Three-bladed wind generators are most
common, providing a good compromise
between efficiency and rotor balance.
tilting the rotor up or to the side, or
by changing the pitch of the blades.
Electricity is transmitted down the tower
on wires, most often as three-phase wild
alternating current (AC).
It’s called “wild” because the voltage
and frequency vary with the rotational
speed of the wind turbine. The output
is then rectified to direct current (DC)
to charge batteries or to be inverted for
grid connection.
Small wind turbines protect themselves
from high winds (governing) by
2
Tower
A wind generator tower is very often more expensive than
the turbine. The tower puts the turbine up in the “fuel”—
the smooth strong winds that give the most energy. Wind
turbines should be sited at least 30 feet (9 m) higher than
anything within 500 feet (152 m).
Three common types of towers are tilt-up, fixed-guyed,
and freestanding. Towers must be specifically engineered
for the lateral thrust and weight of the turbine, and should
be adequately grounded to protect your equipment against
lightning damage. See my article “Wind Generator Tower
Basics” in
HP105
for information about choosing a tower.
Backup Generator
AC Breaker Panel
AC Breaker Panel
10
12
8
Inverter
9
Main DC
Disconnect
Note: This is a simplified diagram,
showing only major system components.
To Household Loads
11
To Household Loads
3
Brake
AKA: emergency
shutdown mechanism
4
Charge Controller
AKA: controller, regulator
A wind-electric charge controller’s primary function is to
protect your battery bank from overcharging. It does this
by monitoring the battery bank—when the bank is fully
charged, the controller sends energy from the battery bank
to a dump (diversion) load.
Most wind turbines have some
means of stopping the turbine
for repairs, in an emergency,
for routine maintenance,
or when the energy is not
needed. Many turbines have
“dynamic braking,” which
simply shorts out the three
electrical phases and acts as
a disconnect. Others have
mechanical braking, either via
a disc or drum brake, activated
by a small winch at the base
of the tower. Still others have
mechanical furling, which
swings the rotor out of the
wind. Mechanical braking
is usually more effective
and reliable than dynamic
braking.
Many wind-electric charge controllers are built into the same
box as the rectifiers (AC-to-DC converters). Overcurrent
protection is needed between the battery and controller/
dump load.
In batteryless
grid-tie systems,
there is no controller
in normal operation, since
the inverter is selling whatever
energy the turbine is generating. But
there will be some control function in
the case of grid failure, and there may
be electronics before the inverter to
regulate the input voltage.
grid-tie systems,
there is no controller
Grid-Tied Wind-Electric System
with Battery Backup
Connecting a wind-electric system to the utility grid with
battery backup gives you the best of both worlds. You have
the unlimited capacity of the grid at your disposal, and
you can send your surplus wind energy to the grid. When
the grid is down, you can still use your system, within the
limitations of the battery bank and turbine. Wind-electric
systems can be a much better match for utility backup
than solar-electric systems, since many grid outages are
caused by high winds. The drawback is that this is the most
expensive type of wind-electric system you can install.
Wind
Generator
Meter
7
1
4
5
Charge
Controller
Dump
Load
Controller
Controller
Tower
3
2
6
Battery
Bank
Battery
Battery
Brake
12
home power 110
/ december 2005 & january 2006
wind
basics
System
Battery
5
Dump Load
AKA: diversion load, shunt load
wind
basics
Solar-electric modules can be turned off—open
circuited—with no damage. Most wind generators
should not run unloaded. They will run too fast
and too loud, and may self-destruct. They must be
connected to a battery bank or load. So normally,
a charge controller that has the capability of
being a diversion controller is used. A diversion
controller takes surplus energy from the battery
bank and sends it to a dump load. In contrast, a
series controller (commonly used in PV systems),
actually opens the circuit.
A dump load is an electrical resistance heater,
and it must be sized to handle the full generating
capacity of the wind generator used. These
dump loads can be air or water heaters, and are
activated by the charge controller whenever the
batteries or the grid cannot accept the energy
being produced.
6
Battery Bank
AKA: storage battery
Your wind generator will produce electricity
whenever the wind blows above the cut-in
speed. If your system is off grid, you’ll need
a battery bank—a group of batteries wired
together—to store energy so you can have
electricity when it’s not windy. For off-
grid systems, battery banks are typically
sized to keep household electricity running
for one to three calm days. Grid-intertied
systems also can include battery banks
to provide emergency backup during
blackouts—perfect for keeping critical
electric loads operating until the grid is up
again.
Use only deep-cycle batteries in wind-
electric systems. Lead-acid batteries
are the most common battery type.
Flooded lead-acid batteries are usually
the least expensive, but require adding
distilled water occasionally to replenish
water lost during the normal charging
process. Sealed absorbed glass mat
(AGM) batteries are maintenance free
and designed for grid-tied systems
where the batteries are typically kept at a
full state of charge. Sealed gel-cell batteries
can be a good choice to use in unheated spaces
due to their freeze-resistant qualities.
Note: This is a simplified diagram,
showing only major system components.
AC Breaker Panel
AC Breaker Panel
10
Kilowatt-Hour
Meter
11
9
Inverter
8
To/From
Utility Grid
Main DC
Disconnect
To Household Loads
13
full state of charge. Sealed gel-cell batteries
can be a good choice to use in unheated spaces
due to their freeze-resistant qualities.
Inverter
To/From
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