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Wind Electricity Basics

Small wind-electric systems can provide electricity on remote, off-grid sites, or right in town connected to the utility grid. Although wind systems require more maintenance and need more attention than solar-electric or microhydro-electric systems, if you invest up front in good equipment, design, and installation, wind-electric systems can make economic and environmental sense. They also bring a great deal of satisfaction—there´s nothing quite like watching your wind generator convert a summer breeze or a winter storm into electrical energy.

How It WorksEoltec_6KW

Boiled down to its simplest principles, a wind generator´s rotating blades convert the wind´s kinetic energy into rotational momentum in a shaft. The rotating shaft turns an alternator, which makes electricity. This electricity is transmitted through wiring down the tower to its end use.

The blades use engineered airfoils, matched to the alternator, that capture the wind´s energy. Most modern wind generators use three blades, the best compromise between the highest efficiency possible (one blade) and the balance that comes with multiple blades. Together, the blades and the hub they are attached to are termed the rotor, which is the collector of the system, intercepting winds that pass by. Most turbines on the market today are upwind machines—their blades are on the windward side of the tower. A few downwind machines are available, but neither configuration has a clear performance advantage over the other.

In most small-scale designs, the rotor is connected directly to the shaft of a permanent magnet alternator, which creates wild, three-phase AC. Wild, three-phase electricity means that the voltage and frequency vary continuously with the wind speed. They are not fixed like the 60 Hz, 120 VAC electricity coming out of common household outlets. The wild output is rectified to DC to either charge batteries or feed a grid-synchronous inverter. In most designs (up to 15 KW in peak capacity), the rotor is usually connected directly to the alternator, which eliminates the additional maintenance of gears. In systems 20 KW and larger, as well as some smaller wind systems (like the Endurance, Tulipo, or Aircon), a gearbox is used to increase alternator speed from a slower turning rotor.

The blades must turn to face the wind, so a yaw bearing is needed, allowing the wind turbine to track the winds as they shift direction. The tail directs the rotor into the wind. Some sort of governing system limits the rotor rpm as well as generator output to protect the turbine from high winds. A shutdown mechanism is also useful to stop the machine when necessary, such as during an extreme storm, when you do not need the energy, or when you want to service the system.

How Wind Turbines are RatedWhisper_200

Wind turbine rating is a tricky affair. While solar-electric module or microhydro-electric turbine production can be predicted fairly realistically based on rated output, this number is very misleading with wind turbines. Why? Because rated output is pegged to a particular wind speed, and different manufacturers use different wind speeds to determine rated output. Also, the power available in the wind varies with the cube of its speed, so small increases in wind speed result in large increases in power available to the rotor. A 10 percent increase in wind speed yields a 33 percent increase in power available in the wind. Conversely, this means that a turbine rated at 1,000 watts at 28 mph might produce only 125 watts or less at half that wind speed, 14 mph.

So what´s a wind turbine buyer to do? Ignore the peak output and the power curve. Look for the monthly or annual energy numbers for the turbine, estimated for the average wind speed you expect or measure at your site. These will be given in KWH per month (or year) in the manufacturer´s specifications for each turbine. Energy is what you´re after, not peak power! If, for example, you are looking for a turbine that can produce 300 KWH per month, and you know that you have a 10 mph average wind speed at the proposed turbine height, you can shop for a turbine that is predicted to generate that much energy in that average wind speed.

If you can´t get energy production estimates from the manufacturer or a turbine owner, look for a different manufacturer. This is basic information that any manufacturer should supply. However, knowing a turbine´s swept area may also help you calculate the annual energy output for the wind turbine. All other things being equal, ″there´s no replacement for displacement.″ Hugh Piggott gives a rough formula for calculating output based on average wind speed and swept area in his HP102 article. Jim Green at the National Renewable Energy Lab (NREL) developed a similar formula: annual energy output (AEO) in KWH = 0.01328 x rotor diameter (ft.) squared x average wind speed (mph) cubed.

A turbine´s revolutions per minute (rpm) at its rated wind speed can give you some idea of the relative aerodynamic sound of the machine, and also speaks to longevity. Slower-turning wind turbines tend to be quieter and last longer. High rpm machines wear out components, such as bearings, much faster. In addition, the faster blades move through the air, the greater the possibility that they will waste some of that energy as sound from the blades.

How To Choose A Wind TurbineProven WT 2.5

Trying to keep an inexpensive wind generator running can be an uphill battle that you´ll soon tire of. But expect to pay more for a better machine—it´s a tough job to design and manufacture a long-lasting, small-scale wind generator.

The bottom line: Buy a turbine that has a very good track record and a good warranty—five years is preferable but not always available in the small wind industry. A warranty is one indication of the manufacturer´s confidence in their product, and their intention to stand behind it.

Real-world reports from users carry even more weight than a warranty, so search for people who own the model of turbine you´re considering buying, and get the straight scoop from them about performance, durability, reliability, and maintenance issues.

Note that a number of the wind turbines listed here are relatively new introductions with not very much customer run-time in North America. These turbines include the ARE, Eoltec, Kestrel, and Skystream. We recommend that you contact either your local wind turbine installer, or the manufacturers or importers and find out how many of these machines are actually operating in North America. Then contact the owners, and inquire about their experience and satisfaction with both the machine and the manufacturer or importer.

Some manufacturers make only battery-charging machines, and may offer a variety of turbine voltages. Others produce machines intended to connect to grid-synchronous inverters without batteries. One machine even includes an inverter integrated with the turbine itself. Make sure you´re buying a machine that is appropriate for your intended use.

When you look at prices, keep in mind that just buying a wind turbine will not get you any wind-generated electricity. You´ll also need most or all of the components mentioned elsewhere. Also budget for equipment rental, like a backhoe and crane, concrete and rebar, electrical components, shipping, and sales tax. Unless you do all of the work yourself, also factor in installation labor expenses. These costs can add up significantly, so make sure that you research and understand all of the associated expenses before committing to a purchase. Many people are quite surprised to learn that the wind turbine cost can range from only 10 percent to as much as 40 percent of the entire wind system´s expenses.

Small-scale wind energy is not for the half-hearted, uninvolved, or uncommitted, and probably not for folks who never change the oil in their vehicles (or are willing to spend the bucks to hire someone to do the tower work). The North American landscape is littered with failed installations: Designs not fully thought-out or tested, machines bought because they were cheap, and installations that required more time and money for repairs than they ever yielded in electricity generated. Many of the failures were the result of wishful thinking and too little research. That said, there are tens of thousands of happy wind-electric system owners. These owners did their homework—purchasing, designing, and installing rugged and well-thought-out systems on adequately sized towers. In addition, they are either committed to maintaining the systems, or to hiring someone to do this regular work.

While many first-time wind turbine buyers may be looking for a bargain, second-time wind turbine buyers are seeking the most rugged machine they can afford. You can avoid a painful "learning experience″ by focusing on durability, production, warranty, and track record, and not on price alone, or on peak output. You don´t want to depend on the low bidder for something as important to you as your long-term energy investment.

See also the following Home Power feature articles:

How To Buy a Wind-Electric System
Wind Turbine Buyer´s Guide
Estimating Wind Energy
Wind Generator Tower Basics
Wind-Electric Systems Simplified



Wind-Electric System Types

Off-Grid Wind-Electric Systems

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.

The following illustration includes the primary components of any off-grid wind-electric system with battery backup. See our Wind-Electric System Components section for an introduction to the function(s) of each component.

Off-Grid Wind Electric System

See also the following Home Power feature articles:

Watts in the Wind

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.

The following illustration includes the primary components of any grid-tied wind-electric system with battery backup. See our Wind-Electric System Components section for an introduction to the function(s) of each component.

Grid-Tied Wind-Electric System with Battery Backup

See also the following Home Power feature articles:

The First Small Wind System in Lassen County

Batteryless Grid-Tied Wind-Electric System

Connecting to the grid without batteries is the most cost-effective and environmentally friendly way to go. You eliminate batteries, which are costly, require maintenance, and carry a significant efficiency penalty. The only drawback of batteryless systems is that when the grid is down, your system shuts down. But in most grid-serviced areas, utility outages are only a few hours a year—a small inconvenience to endure for the efficiency, environmental friendliness, and thriftiness of these systems.

Batteryless grid-tie systems may see increased performance (sometimes dramatically) from the wind turbine compared to battery-based systems. This is because the inverter´s electronics can match the wind´s load more exactly, running the turbine at optimum speed, and extracting the maximum energy.

The following illustration includes the primary components of any batteryless grid-tied wind-electric system. See our Wind-Electric System Components section for an introduction to the function(s) of each component.

Batteryless Grid-Tied Wind-Electric System

See also the following Home Power feature articles:

At Last....Simple Wind Grid-Tie
Betting the Farm—Wind Electricity Pays Off
Farming the Wind

Direct-Drive Batteryless Wind-Electric System

These are the least common wind-electric systems, typically used for water pumping. A turbine is matched to a pump, often through an electronic controller. When the wind blows, water is pumped to an elevated tank, a stock-watering tank, or directly to the land to irrigate. These systems can be simple and cost effective in the right situation. Direct-drive systems are also used for heating, which can be a good match, since it´s normally colder when it´s windy. But heating is a big load, so large turbines are needed.

The following illustration includes the primary components of any batteryless grid-tied wind-electric system. See our Wind-Electric System Components section for an introduction to the function(s) of each component.

Direct-Drive Batteryless Wind-Electric System



Wind-Electric System Components

Understanding the basic components of an RE system and how they function is not an overwhelming task. Here are some brief descriptions of the common equipment used in grid-intertied and off-grid wind-electric systems. Systems vary—not all equipment is necessary for every system type.

Wind Generator
Tower
Brake
Charge Controller
Dump Load
Battery Bank
System Meter
Main DC Disconnect
Inverter
AC Breaker Panel
Kilowatt-Hour Meter
Backup Generator

Wind GeneratorWind 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.

Small wind turbines protect themselves from high winds (governing) by 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.

See also the following Home Power feature articles:

Wind Turbine Buyer´s Guide
How to Buy a Wind-Electric System
Anatomy of a Wind Turbine

TowerWind 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 Wind Generator Tower Basics in HP105 for information about choosing a tower.

See also the following Home Power feature articles:

Wind Generator Tower Basics
What the Heck? Gin Pole

Brake
AKA: emergency shutdown mechanism

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.

See also the following Home Power feature articles:

Anatomy of a Wind Turbine

Charge ControllerCharge Controller 1
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.

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.

See also the following Home Power feature articles:Charge Controller 2

Under Control: Charge Controllers for Whole-House Systems
What is a Charge Controller?
Get Maximum Power From Your Solar Panels with MPPT
What The Heck? Charge Controller







Dump LoadDump Load
AKA: diversion load, shunt load

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.

Battery BankBattery 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 absorbent 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.

See also the following Home Power feature articles:

Top 10 Battery Blunders and How to Avoid Them
Flooded Lead Acid Battery Maintenance
Battery Box Basics

System MeterSystem Meter
AKA: battery monitor, amp-hour meter, watt-hour meter

System meters can measure and display several different aspects of your wind-electric system´s performance and status—tracking how full your battery bank is, how much electricity your wind generator is producing or has produced, and how much electricity is in use. Operating your system without metering is like running your car without any gauges—although possible to do, it´s always better to know how much fuel is in the tank.

See also the following Home Power feature articles:

The Whole Picture: Computer-Based Solutions for PV System Monitoring
Mutichannel Metering: Beta-Testing a New System Monitor
Control Your Energy Use & Costs with Solar Monitoring

Main DC DisconnectMain DC Disconnect
AKA: battery / inverter disconnect

In battery-based systems, a disconnect between the batteries and inverter is required. This disconnect is typically a large, DC-rated breaker mounted in a metal enclosure. This breaker allows the inverter to be quickly disconnected from the batteries for service, and protects the inverter-to-battery wiring against electrical fires.

See also the following Home Power feature articles:

What The Heck? Disconnect


InverterBattery-Based Inverter
AKA: DC-to-AC converter

Inverters transform the electricity produced by your wind generator into the AC electricity commonly used in most homes for powering lights and appliances. Grid-tied inverters synchronize the electricity they produce with the grid´s "utility grade" AC electricity, allowing the system to feed wind electricity to the utility grid.

Grid-tie inverters are either designed to operate with or without batteries. Battery-based inverters for off-grid or grid-tieInverter 2 systems often include a battery charger, which is capable of charging a battery bank from either the grid or a backup generator during cloudy weather.

See also the following Home Power feature articles:

What’s Going On—The Grid? A New Generation of Grid-Tied PV Inverters
Off-Grid Inverter Efficiency



AC Breaker PanelAC Breaker Panel
AKA: mains panel, breaker box, fuse box

The AC breaker panel is the point at which all of a home’s electrical wiring meets with the provider of the electricity, whether that’s the grid or a solar-electric system. This wall-mounted panel or box is usually installed in a utility room, basement, garage, or on the exterior of the building. It contains a number of labeled circuit breakers that route electricity to the various rooms throughout a house. These breakers allow electricity to be disconnected for servicing, and also protect the building’s wiring against electrical fires.

Just like the electrical circuits in your home or office, an inverter’s electrical output needs to be routed through an AC circuit breaker. This breaker is usually mounted inside the building’s mains panel, which enables the inverter to be disconnected from either the grid or from electrical loads if servicing is necessary, and also safeguards the circuit’s electrical wiring.

Additionally, for their use, utilities usually require an AC disconnect between the inverter and the grid that is for their use. These are usually located near the utility KWH meter.

Kilowatt-Hour MeterKilowatt-Hour Meter
AKA: KWH meter, utility meter

Most homes with a grid-tied wind-electric system will have AC electricity both coming from and going to the electric utility grid. A bidirectional KWH meter can simultaneously keep track of how much electricity you´re using and how much your system is producing. The utility company often provides intertie-capable meters at no cost.





Backup GeneratorBackup Generator
Backup Generator AKA: gas-guzzler, "the Noise"

Off-grid wind-electric systems can be sized to provide electricity during calm periods when the wind doesn´t blow. But sizing a system to cover a worst-case scenario, like several calm weeks during the summer, can result in a very large, expensive system that will rarely get used to its capacity and will run a huge surplus in windy times. To spare your pocketbook, go with at least two sources of energy. Wind-PV hybrid systems are often an excellent fit with local renewable resources. But a backup, fuel-powered generator still may be necessary.

Engine-generators can be fueled with biodiesel, petroleum diesel, gasoline, or propane, depending on the design. Most generators produce AC electricity that a battery charger (either stand-alone or incorporated into an inverter) converts to DC energy, which is stored in batteries. Like most internal combustion engines, generators tend to be loud and stinky, but a well-designed renewable energy system will require running them only 50 to 200 hours a year or less.


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