Wind

Introduction

Wind power is a potential source of energy for electrical power in rural and remote areas. Wind-driven turbines capture mechanical energy, which can be converted to electrical energy through a generator. Small wind systems are considered to be those turbines with a generating capacity of less than 100 kW.

Small wind turbines for off-grid use are commercially available in a wide range of sizes and ratings. For small applications of the type needed for standalone ICT applications, wind turbines of 100 W to a few kilowatts are appropriate. Although 90% of global wind power capacity is installed in Europe and the United States, usage in other countries and regions is growing. India is one of the top five wind energy markets in the world, and China has installed more than 150,000 small wind turbines (Ref).

Resource

The faster the wind speed, the more energy there is in it. The energy in wind has a cubed relationship to the wind speed, a squared relationship to the turbine radius, and a direct relationship to the duration of the wind. Roughly speaking:

Wind Energy a (wind speed)3 x (radius of turbine)2 x time

The cubed relationship between wind energy and wind speed means that small increases in wind speed yield significant increases in energy output. Air flowing at 5 meters per second can yield twice the energy of air flowing at 4 m/s, and nearly 5 times the energy as air flowing at 3 m/s.

It is not uncommon for wind speed to have very large daily and seasonal variations and to be influenced by local terrain and microclimates. If the wind is very strong only for a few hours a day or a few months a year, wind power alone may not provide enough continuous power to support an ICT facility that operates year-round.

Components

The typical components of a small wind system include:

• A rotor, which comprises the blades and the hub of the wind turbine. For small-scale ICT applications, rotors of less than four meters in diameter are usually sufficient.
• A generator, typically housed in the turbine, to convert mechanical energy to electrical energy.
• A tower 10 to 20 meters tall, which supports the turbine far above the ground to capture higher wind speeds. The tower should be high enough so that the bottom of the rotor is at least 10 m above any turbulence-producing object within 100 m.
• A solid foundation and/or guy wire to prevent the wind turbine from toppling in high winds.
• A safety disconnect switch located between the generating components and the system electronics.
• A controller and/or regulator.
• A battery bank.
• An inverter to power AC loads, if necessary.

Because wind turbines typically produce larger fluctuations in voltage and current than PV systems, they may require controllers and other system components designed specifically for small wind systems.

Location

The process of deciding on the site for setting up a wind turbine is called siting. The suitability of a particular site is affected by the duration and regularity of wind flows over the course of the day and the year. Large wind farms use complex computer models for siting. However, smaller wind turbines can be set up by using some rules of thumb, such as:

• Elevated areas such as hilltops receive more wind.
• Gentle sloping in the prevailing direction of the wind tends to increase wind speed but a very steep slope leads to turbulence that can impact the lifespan of the turbine.
• Ideally, there should be no hindrance to the flow of wind about the turbine within a distance of about 100 times the diameter of the rotor.
• The bottom of the rotor should be at least 10 rotor lengths higher than any surrounding obstacles.
• Local vegetation may be indicative of the existence of a wind resource and the prevailing direction of the wind.

Due to losses in efficiency from transporting energy over distances, wind systems should be located within a reasonable distance of the point of energy use, i.e. within about 100 m for a typical school-sized system.

Operation & Maintenance

Small turbines are specifically designed for high reliability and low maintenance. Today’s state-of-the-art small wind turbines have adopted simple designs with three to four moving parts and automatic operation. Operating under normal conditions, they typically require infrequent maintenance. However, particularly harsh environments can require more robust designs and more expensive equipment, with a risk of increased maintenance costs as well.

Environmental Impacts

Wind power is a clean, renewable source of energy that produces negligible pollutants during normal operations. As with PV systems, battery disposal and recycling is the main environmental issue. Large wind turbines are known to generate noise that may negatively affect nearby species of animals, and there are also instances of flying birds colliding with turbines. However, noise and bird collisions are not typically associated with the smaller systems discussed here. Potential environmental impacts such as these can be evaluated on a case by case basis.

Costs

Small wind turbines are relatively inexpensive due to manufacturing economies of scale. Typical prices for the turbine only are in the range of $1.90 / Watt for a 500 W unit with a rotor 1.7 m in diameter, and $1.60 / Watt for a 10,000 W unit with a 7 m rotor.

Initial investment in small wind systems typically ranges from $2,000 to $8,000 per rated kW, including installation costs. Operation and maintenance costs tend to be in the range of $0.01 per kWh. Based on life cycle costs, wind is usually less expensive than PV for locations with an adequate wind resource. However, wind turbine efficiencies and cost per kilowatt vary from product to product.

Viability

The cost effectiveness of a small wind system is dependent on the amount of wind available. Many parts of the world have excellent wind resources. Sites with average annual wind speeds of at least 3.5 m/s are more likely to be viable than those with lower average wind speeds. Annex 1 provides several starting points for obtaining wind resource data and maps for different countries.

Where there are sufficient energy resources to meet off-grid energy needs through either wind or PV systems, wind power tends to be the more cost-effective option. The more challenging aspects of a small wind project may include building the tower, maintaining a large area of clearance around the turbine throughout the lifetime of the system.

When evaluating wind projects, it is critical to assess the wind conditions at the proposed turbine site. Sites with an intermittent or irregular wind resource may benefit from the use of a hybrid energy system. A hybrid system integrates two or more energy generation technologies, such as wind power and solar PV, or wind power and a diesel generator. Hybrid energy systems are addressed in the Battery Backup Systems section.

Related Web-based Resources

• Energy Alternatives - Wind (web site)
• Small Wind Systems Tutorial (Mike Bergey, Village Power Conference Workshop, 2000.)