Small scale wind power
This rooftop-mounted urban wind turbine charges a 12 volt battery and runs various 12 volt appliances within the building on which it is installed.
Small wind generation systems with capacities of 100 kW or less are usually used to power homes, farms, and small businesses. Isolated communities that otherwise rely on diesel generators may use wind turbines to displace diesel fuel consumption. Individuals purchase these systems to reduce or eliminate their electricity bills, or simply to generate their own clean power.
Wind turbines have been used for household electricity generation in conjunction with battery storage over many decades in remote areas. Increasingly, U.S. consumers are choosing to purchase grid-connected turbines in the 1 to 10 kilowatt range to power their whole homes. Household generator units of more than 1 kW are now functioning in several countries, and in every state in the U.S.
Grid-connected wind turbines may use grid energy storage, displacing purchased energy with local production when available. Off-grid system users either adapt to intermittent power or use batteries, photovoltaic or diesel systems to supplement the wind turbine.
In urban locations, where it is difficult to obtain predictable or large amounts of wind energy, smaller systems may still be used to run low power equipment. Equipment such as parking meters or wireless internet gateways may be powered by a wind turbine that charges a small battery, replacing the need for a connection to the power grid.
Types of wind turbines
Wind turbines can be separated into two types based by the axis in which the turbine rotates. Turbines that rotate around a horizontal axis are more common. Vertical-axis turbines are less frequently used.
Horizontal-axis wind turbines(HAWT) have the main rotor shaft and electrical generator at the top of a tower, and must be pointed into the wind. Small turbines are pointed by a simple wind vane, while large turbines generally use a wind sensor coupled with a servo motor. Most have a gearbox, which turns the slow rotation of the blades into a quicker rotation that is more suitable to drive a generator.
Since a tower produces turbulence behind it, the turbine is usually pointed upwind of the tower. Turbine blades are made stiff to prevent the blades from being pushed into the tower by high winds. Additionally, the blades are placed a considerable distance in front of the tower and are sometimes tilted up a small amount.
Downwind machines have been built, despite the problem of turbulence, because they don’t need an additional mechanism for keeping them in line with the wind, and because in high winds, the blades can be allowed to bend which reduces their swept area and thus their wind resistance. Since turbulence leads to fatigue failures, and reliability is so important, most HAWTs are upwind machines.
There are several types of HAWT:
- These four- (or more) bladed squat structures, usually with wooden shutters or fabric sails, were developed in Europe. These windmills were pointed into the wind manually or via a tail-fan and were typically used to grind grain. In the Netherlands they were also used to pump water from low-lying land, and were instrumental in keeping its polders dry. Windmills were also located throughout the USA, especially in the Northeastern region.
- Modern Rural Windmills
The Eclipse windmill factory was set up around 1866 in Beloit, Wisconsin and soon became a huge success building mills for farm water pumping and railroad tank filling. Other firms like Star, Dempster, and Aeromotor also entered the market. Hundreds of thousands of these mills were produced before rural electrification and small numbers continue to be made. They typically had many blades, operated at tip speed ratios (defined below) not better than one, and had good starting torque. Some had small direct-current generators used to charge storage batteries, to provide a few lights, or to operate a radio receiver. The American rural electrification connected many farms to centrally-generated power and replaced individual windmills as a primary source of farm power by the 1950′s. They were also produced in other countries like South Africa and Australia (where an American design was copied in 1876). Such devices are still used in locations where it is too costly to bring in commercial power.
Water pumping rural windmill in Germany.
In Schiedam, the Netherlands, a traditional style windmill (the Noletmolen) was built in 2005 to generate electricity. The mill is one of the tallest Tower mills in the world, being some 42.5 metres (139 ft) tall.
Common modern wind turbines
Turbines used in wind farms for commercial production of electric power are usually three-bladed and pointed into the wind by computer-controlled motors. This type is produced by Danish and other manufacturers. These have high tip speeds of up to six times the wind speed, high efficiency, and low torque ripple which contributes to good reliability. The blades are usually colored light gray to blend in with the clouds and range in length from 20 to 40 metres (65 to 130 ft) or more. The tubular steel towers range from about 200 to 300 feet (60 to 90 metres) high. The blades rotate at 16-22 revolutions per minute, and a speed increaser gear box steps up the speed of the generator. Some models operate at constant speed, but more energy can be collected by variable-speed turbines which use a solid-state power converter to interface to the transmission system. All turbines are equipped with high wind shut down features to avoid over speed damage.
Video of small wind turbine being installed
Installation of a Gaia 11 kW wind turbine; making the foundation, transport and mounting of the turbine, lifting the tower and making the electrical connections. At the end of the video turbine is installed and spinning.
- Blades are to the side of the turbine’s center of gravity, helping stability.
- Ability to wing warp, which gives the turbine blades the best angle of attack. Allowing the angle of attack to be remotely adjusted gives greater control, so the turbine collects the maximum amount of wind energy for the time of day and season.
- Ability to pitch the rotor blades in a storm, to minimize damage.
- Tall tower allows access to stronger wind in sites with wind shear. In some wind shear sites, every ten meters up, the wind speed can increase by 20% and the power output by 34%.
- HAWTs have difficulty operating in near ground, turbulent winds.
- The tall towers and long blades up to 90 meters long are difficult to transport on the sea and on land. Transportation can now cost 20% of equipment costs.
- Tall HAWTs are difficult to install, needing very tall and expensive cranes and skilled operators.
- Supply of HAWTs is less than demand and between 2004 and 2006, turbine prices increased up to 60%. At the end of 2006, all major manufacturers were booked up with orders through 2008.
- The FAA has raised concerns about tall HAWTs effects on radar near Air Force bases.
- Their height can create local opposition based on impacts to viewsheds.
- Downwind variants suffer from fatigue and structural failure caused by turbulence.
Cyclic stresses and vibration
Cyclic stresses fatigue the blade, axle and bearing material failures were a major cause of turbine failure for many years. Because wind velocity often increases at higher altitudes, the backward force and torque on a horizontal-axis wind turbine (HAWT) blade peaks as it turns through the highest point in its circle. The tower hinders the airflow at the lowest point in the circle, which produces a local dip in force and torque. These effects produce a cyclic twist on the main bearings of a HAWT. The combined twist is worst in machines with an even number of blades, where one is straight up when another is straight down. To improve reliability, teetering hubs have been used which allow the main shaft to rock through a few degrees, so that the main bearings do not have to resist the torque peaks.
Video of wind turbine collapsing in a storm
When the turbine turns to face the wind, the rotating blades act like a gyroscope. As it pivots, gyroscopic precession tries to twist the turbine into a forward or backward somersault. For each blade on a wind generator’s turbine, precessive force is at a minimum when the blade is horizontal and at a maximum when the blade is vertical. This cyclic twisting can quickly fatigue and crack the blade roots, hub and axle of the turbines.
Vertical-axis wind turbines (or VAWTs) have the main rotor shaft running vertically. Key advantages of this arrangement are that the generator and/or gearbox can be placed at the bottom, near the ground, so the tower doesn’t need to support it, and that the turbine doesn’t need to be pointed into the wind. Drawbacks are usually pulsating torque that can be produced during each revolution and drag created when the blade rotates into the wind. It is also difficult to mount vertical-axis turbines on towers, meaning they must operate in the often slower, more turbulent air flow near the ground, resulting in lower energy extraction efficiency.
Darrieus wind turbine
“Eggbeater” turbines. They have good efficiency, but produce large torque ripple and cyclic stress on the tower, which contributes to poor reliability. Also, they generally require some external power source, or an additional Savonius rotor, to start turning, because the starting torque is very low. The torque ripple is reduced by using 3 or more blades which results in a higher solidity for the rotor. Solidity is measured by blade area over the rotor area. Newer Darrieus type turbines are not held up by guy wires but have an external superstructure connected to the top bearing.
Gorlov helical turbine
Essentially a darrieus turbine in a helical configuration. Patented in 2001. It solves most of the problems of the Darrieus rotor. It is self-starting, has lower torque ripple, low vibration and noise, and low cyclic stress. High reliability is expected from tested or matured designs. At least two wind turbine products are on the market as of 2007, including the Turby wind turbine and the Quietrevolution wind turbine. Most importantly, the GHT is an excellent turbine for zero-head hydropower, and appears to be a much needed ecologically benign and affordable solution for micro-hydropower. It is up to 35% efficient, which is competitive with the most efficient VAWT’s.
A subtype of Darrieus turbine with vertical, as opposed to curved, blades. The cycloturbine variety have variable pitch to reduce the torque pulsation and are self-starting. The advantages of variable pitch are: high starting torque; a wide, relatively flat torque curve; a lower blade speed ratio; a higher coefficient of performance; more efficient operation in turbulent winds; and a lower blade speed ratio which lowers blade bending stresses. Straight, V, or curved blades may be used. Recently , this type of turbine has been advanced by former Russian rocket scientists who claim to have increased the efficiency of the VAWT up to 38% . A company , SRC Vertical Ltd. has been formed , and has begun selling the new turbine .
Videos of two Russian designs
Savonius wind turbine
These are drag-type devices with two- (or more) scoops that are used in anemometers, the Flettner vents (commonly seen on bus and van roofs), and in some high-reliability low-efficiency power turbines. They are always self-starting if there are at least three scoops. They sometimes have long helical scoops to give a smooth torque. The Banesh rotor and especially the Rahai rotor improve efficiency with blades shaped to produce significant lift as well as drag. A new variety uses sails that can open or close with changes in wind speed.
You can read more about their product on their web site www.quietrevolution.co.uk
- Easier to maintain because most of their moving parts are located near the ground. This is due to the vertical wind turbineâ€™s shape. The airfoils or rotor blades are connected by arms to a shaft that sits on a bearing and drives a generator below, usually by first connecting to a gearbox.
- As the rotor blades are vertical, a yaw device is not needed, reducing the need for this bearing and its cost.
- Vertical wind turbines have a higher airfoil pitch angle, giving improved aerodynamics while decreasing drag at low and high pressures.
- Mesas, hilltops, ridgelines and passes can have higher and more powerful winds near the ground than up high because of the speed up effect of winds moving up a slope or funneling into a pass combining with the winds moving directly into the site. In these places, VAWTs placed close to the ground can produce more power than HAWTs placed higher up.
- Low height useful where laws do not permit structures to be placed high.
- Smaller VAWTs can be much easier to transport and install.
- Does not need a free standing tower so is much less expensive and stronger in high winds that are close to the ground.
- Usually have a lower Tip-Speed ratio so less likely to break in high winds.
- Does not need to be pointed into the wind, can turn regardless of the direction of the wind.
- They can potentially be built to a far larger size than HAWT’s , for instance floating VAWT’s hundreds of meters in diameter where the entire vessel rotates , can eliminate the need for a large and expensive bearing .
- Most VAWTs produce energy at only 50% of the efficiency of HAWTs in large part because of the additional drag that they have as their blades rotate into the wind. This can be overcome by using structures to funnel more and align the wind into the rotor (e.g. “stators” on early Windstar turbines) or the “vortex” effect of placing straight bladed VAWTs closely together (e.g. Patent # 6784566).
- There may be a height limitation to how tall a vertical wind turbine can be built and how much sweep area it can have. However , this can be overcome by connecting a multiple number of turbines together in a triangular pattern with bracing across the top of the structure . Thus reducing the need for such strong vertical support , and allowing the turbine blades to be made much longer .
- Most VAWTS need to be installed on a relatively flat piece of land and some sites could be too steep for them but are still usable by HAWTs.
- Most VAWTs have low starting torque, and may require energy to start the turning.
- A VAWT that uses guy wires to hold it in place puts stress on the bottom bearing as all the weight of the rotor is on the bearing. Guide wires attached to the top bearing increase downward thrust in wind gusts. Solving this problem requires a superstructure to hold a top bearing in place to eliminate the downward thrusts of gust events in guy wired models.
- While VAWTs’ parts are located on the ground, they are also located under the weight of the structure above it, which can make changing out parts near impossible without dismantling the structure if not designed properly.
Wind turbines on the Lake Erie shore at Lackawanna, New York
The American Wind Energy Association (AWEA) has a huge amount of information on wind power an in particular small wind power. www.awea.org/smallwind/