Small-Scale Wind

Small wind turbines generally have a lower energy output than large commercial wind turbines, but their size can differ significantly: So called Micro wind turbines may be as small as a fifty watt generator and generate only about 300 kWh per year. They are used for boats, caravans, miniature refrigeration unit, but also for fence-charging and other low-power uses. In comparison to that household-size turbines reach diameters of 9-meter, can have a rated power of 20kW and produce about 20000 kWh per year for homes, farms, ranches and small businesses.

Small units often have direct drive generators, Direct current output, aeroelastic blades, lifetime bearings and use a vane to point into the wind. Larger, more costly turbines generally have geared power trains, alternating current output, flaps and are actively pointed into the wind. Direct drive generators and aeroelastic blades for large wind turbines are being researched.

Small wind turbine technology

Nearly all small wind turbines today are upwind, horizontal-axis turbines, which means the rotor is spinning in front of the tower. There are some turbines using two blades, while the majority of the recent turbines comes supplied with a three-blade rotor, which in general terms makes the turbine run more smoothly and last a longer time. The prevalent blade materials are composite materials as fiberglass, while only a few products in this class still use wood. Instead of the yaw motors of the big wind turbines, small wind turbines often use tail vanes to point the rotor to the wind.

Micro and Mini wind turbines use generators based on permanent-magnet alternators. The magnets in the generator are conventionelly tied on the rotating shaft driven by the rotor, but their are also several wind turbines with the magnets attached in a case which rotates around the stationary part of generator. This inside-out design has two advantages: The blades can be bolted directly to the case containing the magnets and the magnets are pressed against the case wall by the centrifugal force. Contrary to this, conventionelly attached magnets on the rotating shaft of a generator have to be retained by sophisticated means.

In household-sized generators, besides permanent-magnet alternators conventional wound-field and induction alternators are used. Most small wind turbines generate a three-phase AC which can be rectified by a controller for battery charging applications. There are turbines with built-in controllers and also products with an external controlling entity.

Because of the sometimes demanding environmental conditions, the robustness of a turbine is a very important parameter, which can be estimated only very roughly: Experience has shown, that the weight of the turbine in relation to the area swept by the rotor can be used as a criterium. For example a turbine with a relative mass of 10 kg/m² should be more robust than a turbine with 5 kg/m².

To prevent damages caused by very high winds, every wind turbine needs a means for overspeed control. The preferred mechanism used by producers is furling or folding the turbine across a hinge so that the rotor swings towards the tail vane. In case of a passiv furling mechanism, the thrust of very high winds overcomes the restraining force which kept the the rotor towards the wind. The threshold value (wind speed) for this mechanism is caused by the design of the hinge, which connects the body of the turbine and the tail vane. Furling mechanisms are very common for micro and mini wind turbines, while many household-sized turbines pitch the rotor-blades for overspeed control and a few also use a combination of pitching and furling.

Common applications

Village Power: Potable Water

Small mechanical wind turbines have been used to drive pumps for potable water for a very long time. Today small electric wind turbines are a efficient alternative which can be used to supply people and livestock with underground water from a well. Creating a village water tap eliminates the need to carry water from distant sources and using underground water generally avoids common health problems. The size and capacity of the needed generator is proportional to population served and pumping height. For example a turbine with a capacity of 1 kW can supply approximately 200 people. An example for this application is given in the following table.

Table 1: Project examples - water-supply for people and livestock

Site	Application	Equipment	Performance	Cost	Installation
Niama, Morocco	Community Water Supply	Two Sites: 10 kW Wind; 18 & 24 m Towers; 15 & 26 Stage Submersible Pumps	70 m3 & 30 m3 of Water per Day	~$100.000, Including Tech. Assist. and Training, US-AID Funded	February 1990
Results: Supplies 4,000 people with 220% more water than original diesel pumps. Population decline has been reversed.

Village Power: Productive uses

Examples for productive uses in rural areas are irrigation, agroprocessing or ice-making. Small wind systems can be an excellent foundation for electrification, which at the same time increases income and chances for cost recovery of the system. The previously mentioned uses require more energy than pumping for drinking water. A turbine with a 1 kW generator can approximately support the work of 10 people in this case.

Table 2: Project examples for productive uses of small wind turbines in rural areas

Site	Application	Equipment	Performance	Cost	Installation
Oesao, Timor, Indonesia	Small plot irrigation	1.5 kW Wind Turbine with 18m Tower; 10 Stage Pump	~ 150 m3 of water per Day	~$11,000	July, 1992
Results: ~ 25 Additional Systems Installed, JICA & US-AID Funding

Village Power: Pre-Electrification

Pre-electrification means installing a small source of electricity for basic needs like lighting. Thus the costs for candles, kerosene or dry-cell batteries can be safed. The systems often work with micro wind turbines (25-120 Watts per household), have no grid connection, but generate a direct current that can be used for charging batteries. High efficiency fluorescent bulbs are often used to make the most advantage from the small lighting systems.

Table 3: Project examples - pre-electrification

Site	Application	Equipment	Performance	Cost	Installation
Tomenas, Timor, Indonesia	Battery Charging Station	7.5 kW BWC Wind Turbine with 30m Tower	Charges batteries for ~40 homes, plus powers productive uses (freezers, shop tools)	~$60,000	1997
Results:  Sustainable electrification which costs each family ~$2.40 per month.