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.

Classes of small wind turbines

Comparing small wind turbines

Wind turbines are usually compared by their rated power in W, kW or MW. It is important to look at these values with care, because there is no standard in rating the output of small wind turbine output. The most significant differences are revealed in the diverging wind speeds related to the rated output for a wind turbine. The power contained in a wind with 12,5 m/s is almost three times higher than at a wind speed of 9 m/s. Thus the rated power of a wind turbine given for a wind speed of 12,5 m/s is three times higher than the value given for a wind speed of 9 m/s. The same machine can be labelled with a very different rated power only depending on the wind speed which is used as basic value. It must be mentioned, that only at the windiest sites of the world turbines will operate for a significant time span at a wind speed of 12,5 m/s. At most sites such high winds are very rare. Looking at the rated power of a wind turbine one must compare the wind speed used in the rating procedure with the expected wind speeds at the site the turbine will be installed.

As an alternative the energy (in kWh) that is generated by a small wind turbine at a site with a given average wind speed can be used to compare the turbines. Finally the combination of rotor diameter and energy generated per year (at a reference wind speed) is a roughly reliable indicator for comparing small wind turbines.

Micro wind turbines

These wind turbines have a very small rotor diameter around 1 m or less and generate about 300 kWh per year at sites with an average wind speed of 5,5 m/s. They are typically used for low-power uses in remote areas (e.g. fence-charging, basic lighting, electricity for sailboats).

Mini wind turbines

Mini wind turbines typically have rotor diameters of 1,5-2,6 m an generate 1000-2000 kWh per year at sites with 5,5 m/s.

Household-size wind turbines

This term summarizes a much broader field of wind turbines depending on the very different size of 'households' and the related applications: Household-size turbines are suitable for the supply of homes, but also for farms, ranches and even for small businesses and telecommunications. Thus in this class rotor diameters of 2,7-9 m can be found and the generated energy per year ranges from 2000-20000 kWh for sites with 5,5 m/s.

Installation of small wind turbines

For micro and mini wind turbines tilt-up towers are the easiest and cheapest carrying constructions: A tube with a diameter dependent on the size and weight of the wind turbine it should carry is tilted over a hinge fixed in the ground until the tower is raised vertically. A so-called gin pole is used to raise the tower over the hinge. A gin pole is a second shorter mast at a right angle to the tower, which acts as a lever to reduce the lifting loads. Bigger household-size wind turbines require heavier lattice towers which can only be erected by a crane. The following paragraph focuses on raising a tilt-up tower, which needs only a few persons and relatively little experience and skills.

Common tower tubes consist of several pieces with lengths of a few meters; some smaller towers have separations of only two meters length allowing a comfortable delivery by standard parcel services. Tilt-up towers are anchored by guy cables, whose number and diameter depends on the height of the used tower. The anchors for the guy cables could be screwed into the ground, if the soil is medium-dense. Using these screw-anchors simplifies the installation of a tower a lot, but the stability of their placement must be proven with great care. For heavier wind turbines, bad ground conditions (very low or very high density) or very high wind areas conventional concrete anchors or power-driven screw anchors are required.

The power conductors carrying the generated power to the ground are threaded through the inside of the tower tube in most cases. Their attachment is a very important but often overlooked aspect of the installation: If the conducting cables are connected to the leads of the wind turbine only, the leads are likely to be pulled out and damaged by the weight of the conductor. For this reason a strain relief has to be used to support the weight of the cable. A strain relief is a fine-meshed wire net which can be put around the cable. The upper side of the strain relief is fixed at a point at the top of the tower. Additionally the connection between the wind turbine leads and the cable should be done by an in-line vibration proof compression connector.

Tools for the installation of small wind turbines

For the erection of a tilt-up tower with a wind turbine either an electrical winch or a type of hoist is necessary. A relatively light-weight, low-cost and very effective tool working like a hoist in principle is the so called griphoist. In contrast to a winch a griphoist does not furl the cable, wire or rope, but it pulls it directly through a hoist inside the tool. The griphoist is used manually by a lever and needs no supply of electricity. Combined with the low weight it is an ideal tool for raising tilt-up towers especially in remote areas, where an electrical winch and the necessary battery are not available. Besides practitioners prefer these tools because of the full and direct control the operator has over the raising process. The force needed by the operator to move the lever is a good indicator for the status of the raising procedure. In this way the operator is able to react on unusual tensions on the cable and can reduce the lift, when the tower is near to vertical.

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.