Small wind turbines, also known as micro wind turbines or urban wind turbines, are wind turbines that generate electricity for small-scale use. These turbines are typically smaller than 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. They usually produce between 500 W and 10 kW, with some as small as 50 W. The Canadian Wind Energy Association considers small wind turbines to be up to 300 kW, while the IEC 61400 standard defines them as having a rotor area smaller than 200 m2 and generating voltage below 1000 Va.c. or 1500 Vd.c.
Design
Blades Turbine blades for small-scale wind turbines are typically 1.5 to 3.5 metres (4 ft 11 in – 11 ft 6 in) in diameter and produce 0.5-10 kW at their optimal wind speed. Most small wind turbines are horizontal-axis wind turbines, but vertical axis wind turbines (VAWTs) may have benefits in maintenance and placement, although they are less efficient at converting wind to electricity. To optimize efficiency, the tip speed ratio (the ratio of blade tip speed to wind speed) and lift-to-drag ratio should be kept at optimal levels.
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 (a grass) 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 must reach a certain wind speed, called the cut-in speed, to start generating electricity. This speed is usually around 4 metres per second (8.9 mph), but some turbines can work at lower speeds. To avoid obstacles, turbines are often placed on towers at least 9 m (30 ft) above anything within 150 m (490 ft). Better locations for turbines are far from large upwind obstacles, as wind tunnel studies show significant negative effects from nearby obstacles can extend up to 80 times the obstacle's height downwind, although this is an extreme case. Another option for placing a small turbine is using a model based on actual wind measurements to predict how nearby obstacles will affect local wind conditions at the potential turbine location, considering the size, shape, and distance to the obstacles. Small-scale rooftop turbines can be installed on a roof, but may face issues such as vibration and turbulence caused by the roof ledge, which can impact their power generation. These turbines often struggle to generate significant amounts of power, particularly in urban areas.
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 is a technique used to regulate the speed of a turbine by discharging excess energy through a resistive load during high winds to prevent damage. The controller, activated when batteries reach a certain voltage, turns on the load using a solenoid or solid-state relay, the latter of which has the added benefit of "failing open". Proper tuning of the controller is important to prevent parasitic oscillations, which can be achieved through a delay function or using a stock PWM charge controller with 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).
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