Small wind turbine

Small wind turbines, also known as micro wind turbines, are used for microgeneration of electricity, as opposed to large commercial wind turbines, such as those found in wind farms. Small wind turbines often have passive yaw systems as opposed to active ones. They use a direct drive generator and use a tail fin to point into the wind, whereas larger turbines have geared powertrains that are actively pointed into the wind.

Small wind turbines typically produce between 500 W and 10 kW of power, although the smaller turbines may be as small as a 50 Watt auxiliary power generator for a boat, caravan, or miniature refrigeration unit, and the Canadian Wind Energy Association (CanWEA) defines "small wind" as high as 300 kW. The IEC 61400 Standard defines small wind turbines as wind turbines with a rotor swept area smaller than 200 m², generating at a voltage below 1000 Va.c. or 1500 Vd.c.

Design

Blades

Smaller scale turbine blades are usually in diameter and produce 0.5-10 kW of electricity at their optimal wind speed. The majority of small wind turbines are traditional horizontal-axis wind turbines.Gipe, Paul. Wind energy basics: a guide to home- and community-scale wind energy systems. ''Chelsea Green Publishing'', 2009. Accessed: 18 December 2010.
In some situations, vertical axis wind turbines have operational advantages in maintenance and placement due to their simplicity, however VAWTs are less reliable than HAWTs and less efficient at converting wind to electricity. The ratio between the speed of the blade tips and the speed of the wind is called the tip speed ratio. This should be kept at an optimal point of efficiency. A high lift-to-drag ratio will generally increase efficiency as well.

A range of synthetic materials including carbon fiber reinforced polymers, nanocomposites, and E-glass-polyester are available. Although natural fibers are susceptible to quality variations, high moisture uptake and low thermal stability that make them undesirable for larger blades, small turbines can still take advantage of them. Wood can be used, and the type of wood should be chosen based on availability, cost and growth time, average density, high stiffness, and breaking strain. Coatings are generally used to reduce moisture, and white enamel with primer has been found to be particularly effective. Sitka spruce (used in propellers), and Douglas Fir have been used in turbine blades. Nepal has used small blade turbines made of coated timber including Sal, Saur, Sisau, Uttish, Tuni, Okhar, pine, and lakuri wood. Beyond wood, bamboo-based composites may also be used in both large and small wind turbines due to their low density and carbon sequestration ability—which makes bamboo materials environmentally friendly. Furthermore, relative to wood, bamboo has higher fracture toughness, higher strength, lower processing costs and fast growth rate. Ongoing materials developments include bamboo laminates using resins and hybrid bamboo carbon-fiber materials. Hemp, flax, wood and bamboo are all candidate blade materials for small turbines.

Placement

Small wind turbines require a minimum wind speed to start generating, called the cut-in speed. This speed is typically around , although some small wind turbines can be designed to work at lower wind speeds. Turbines are often mounted on a tower to raise them above any nearby obstacles. One rule of thumb is that turbines should be at least higher than anything within . Better locations for wind turbines are far away from large upwind obstacles. Measurements made in a boundary layer wind tunnel have indicated that significant detrimental effects associated with nearby obstacles can extend up to 80 times the obstacle's height downwind. However, this is an extreme case. Another approach to siting a small turbine is to use a shelter model to predict how nearby obstacles will affect local wind conditions. Models of this type are general and can be applied to any site. They are often developed based on actual wind measurements, and can estimate flow properties such as mean wind speed and turbulence levels at a potential turbine location, taking into account the size, shape, and distance to any nearby obstacles.

A small wind turbine can be installed on a roof. Installation issues then include the strength of the roof, vibration, and the turbulence caused by the roof ledge. Small-scale rooftop turbines suffer from turbulence and rarely generate significant amounts of power, especially in towns and cities.

Wiring

The generators for small wind turbines are usually three-phase alternating current generators and the trend is to use the induction type, although some models utilize single-phase generators or direct current output.

After running the three phase AC wire through a slip ring and down to the receiving end, a three-phase rectifier is used to convert the AC to rectified DC for battery charging, especially in solar hybrid power systems. The rectifier should be mounted to a heat sink for cooling, with the option of adding a computer fan that is activated by a bimetal thermal switch for active cooling.

The DC end of the rectifier is then connected to the batteries. This connection should be as short as possible to avoid power losses, typically with a shunted digital wattmeter in between for monitoring. The batteries are then connected to a power inverter, which converts the power back to AC at a constant frequency for grid connectivity and end use.

Dynamic braking regulates the speed by dumping excess energy through a resistive load during high winds to prevent damage to the turbine. The dynamic braking resistor is often called a diversion load, or dump load. The dynamic braking is performed by a controller that is activated when the batteries go above a certain voltage, which turn on the dump load via a solenoid, or a DC/DC solid-state relay, the latter of which has the added benefit of "failing open". The controller has to be properly tuned to avoid parasitic oscillations, which can be achieved by adding a delay function, or by using a well designed stock PWM charge controller that supports a diversion function.

Cable resistant to UV radiation and temperature fluctuations, such as solar cable, should be used in cases where the wiring is exposed to the elements. The wire gauge across the whole system must be appropriate for the amount of current running through it. The resistance of the wire, which increases linearly with its length, should not create a voltage drop that is more than 2-5% of the total voltage drop.

Markets

Japan

In July 2012, a new feed-in tariff approved by Japanese Industry Minister Yukio Edano went into effect, promising to boost the country's production of wind and solar energy production. The country is aiming to increase renewable energy investment in part as a response to the Fukushima radiation crisis in March 2011. The feed-in tariff applies to solar panels and small wind turbines and requires utilities to buy back electricity generated from renewable energy sources at government-established rates.

Small-scale wind power (turbines of less than 20 kW capacity) will be subsidized at least 57.75 JPY (about 0.74 USD per kwh).

United Kingdom

Properties in rural or suburban parts of the UK can opt for a wind turbine with inverter to supplement local grid power. The UK's Microgeneration Certification Scheme (MCS) provides feed-in tariffs to owners of qualified small wind turbines.

United States

Small wind turbines added a total of 17.3 MW of generating capacity throughout the United States in 2008, according to the American Wind Energy Association (AWEA). That growth equaled a 78% increase in the domestic market for small wind turbines, which are defined as wind turbines with capacities of 100 kW or less. AWEA's "2009 Small Wind Global Market Study", published in late 2009 May, credited the increase in part to greater manufacturing volumes, as the industry was able to attract enough private investment to finance manufacturing plant expansions. It also credited rising electricity prices and greater public awareness of wind technologies for an increase in residential sale.

In 2019, much of the US demand for small wind turbines was for power generation at remote locations, and for purposes of site assessment for large scale wind power installations.

The U.S. small wind industry also benefits from the global market, as it controls about half of the global market share. U.S. manufacturers garnered $77 million of the $156 million that was spent throughout the world on small wind turbine installations. A total of 38.7 MW of small wind power capacity was installed globally in 2008.

In the United States, residential wind turbines with outputs of 2–10 kW typically cost between and installed ( per watt), although there are incentives and rebates available in 19 states that can reduce the purchase price for homeowners by up to 50 percent, to $3 per watt. The US manufacturer Southwest Windpower estimates a turbine to pay for itself in energy savings in 5 to 12 years.

The dominant models on the market, especially in the United States, are horizontal-axis wind turbines.

To enable consumers to make an informed decision when purchasing a small wind turbine, a method for consumer labeling has been developed by IEA Wind Task 27 in collaboration with IEC TC88 MT2. In 2011 IEA Wind published a Recommended Practice, which describes the tests and procedures required to apply the label.

Croatia

Croatia is an ideal market for small wind turbines due to Mediterranean climate and numerous islands with no access to the electric grid. In winter months when there is less sun, but more wind, small wind turbines are a great addition to isolated renewable energy sites (GSM, stations, marinas etc.). That way solar and wind power provide consistent energy throughout the year.

Germany

In Germany the feed-in tariff for small wind turbines has always been the same as for large turbines. This is the main reason the small wind turbine sector in Germany developed slowly. In contrast, small photovoltaic systems in Germany benefited from a high feed-in tariff, at times above 50 Euro-Cent per kilowatt hour.

In August 2014 the German renewable energy law was adjusted, also affecting the feed-in tariffs for wind turbines. For the operation of a small wind turbine with a capacity below 50 kilowatt the tariff amounts to 8.5 Euro-Cent for a period of 20 years.

Due to the low feed-in tariff and high electricity prices in Germany, the economic operation of a small wind turbine depends on a large self-consumption rate of the electricity produced by the small wind turbine. Private households pay on average 28 cent per kilowatt hour for electricity (19% VAT included).

As part of the German renewable energy law 2014 a fee on self-consumed electricity was introduced in August 2014. The regulation does not apply to small power plants with a capacity below 10 kilowatt. With an amount of 1.87 Euro-Cents the fee is low.

Manufacturing

DIY construction

Some hobbyists have built wind turbines from kits, sourced components, or from scratch. DIY wind turbines are usually smaller (rooftop) turbines of approximately 1 kW or less. These small wind turbines are usually tilt-up or fixed / guyed towers.

Do it yourself or DIY-wind turbine construction has been made popular by magazines such as OtherPower and Home Power.

Organizations as Practical Action have designed DIY wind turbines that can be easily built by communities in developing nations and are supplying concrete documents on how to do so.

Local manufacturing

Designs of DIY small wind turbines date back to the early 1970s, and were further developed by the back-to-the-land movement of the late 1970s in the United States and Europe.

Locally manufactured small wind turbines, being small-scale, low-cost, socially-embedded, adoptive to local contexts and based on the open sharing of knowledge, have been framed under or associated with the perspectives of appropriate or intermediate technology, convivial technology, degrowth, open design and open manufacturing.

See also

* WWEA (World Wind Energy Association)
* Wind turbine design
* Grid-tied electrical system
* Ram air turbine

References

Further reading

*

External links

information about small wind market
by WWEA

Fact sheet
from the American Wind Energy Association

Otherpower
a group of alternative energy enthusiasts

{{Wind power

Distributed generation