Wind Energy

Overview

Wind energy is one of the most widely applicable renewable energy resources. It can be utilized anywhere in the world, wherever the wind blows with a strong and consistent force, by using wind turbines.

A wind turbine captures the wind’s energy to generate electricity. Wind turbines provide high-yielding, economical and clean power, which makes it one of the most attractive source of energy for both large-scale and small applications. This has lead it to become the world’s fastest-growing source of renewable energy.

Wind Turbine Designs

Although all wind turbines operate on similar principles, several varieties are in use. The main types include horizontal axis turbines and vertical axis turbines.

Horizontal Axis Design

Horizontal axis wind turbines (HAWT) are the most common turbine configuration. They consist of a tall tower, atop which sits the main rotor shaft and electrical generator. The wind turns the blades, which turn the shaft connected to the generator which produces electricity. Most horizontal axis turbines have three blades. The tower allows them to take advantage of the stronger and less turbulent wind.

Horizontal-axis wind turbines must be pointed into the wind. Small turbines are pointed by a simple wind vane, while large turbines generally use a wind sensor coupled with a servo motor. Most large turbines have a gearbox, which turns the slow rotation of the blades into a quicker rotation that is more suitable to drive an electrical generator.

Vertical Axis Design

Vertical axis wind turbines (VAWT) have their main rotor shaft arranged vertically. They are not as widely used, mainly because of the inherently lower power coefficient. The VAWT is typically about 40% less efficient than the HAWT in energy production, because half of the turbine is moving in the wrong direction. VAWTs must therefore be built twice as large as HAWTs to make the same amount of power. They also need to be built very sturdy, translating into higher cost and more weight. Because of the added weight, VAWT systems usually have to operate near the ground where there’s not as much wind. They also take up more space on the ground and produce wavy power pulses.

Advantages of the VAWT design include that the turbine does not need to be pointed into the wind to be effective. This is beneficial on sites where the wind direction is highly variable, for example when integrated into buildings. A second advantage is that the generator and gearbox can be placed at the ground, using a direct drive from the rotor assembly to the ground-based gearbox, hence improving accessibility for maintenance. They are also cheaper to install and maintain and have lower levels of noise.

Wind Turbine Placement

There are two main factors to consider when making a site for wind turbines. These are the location where the turbines are located and the other is the height the towers will have.

For greatest efficiency, wind turbine need to be placed in areas with enough sustainable wind speed, the ideal conditions being a minimum of about 8 mph for at least 18-20 hours per day. Such a place is generally found in open areas that are not obstructed by trees, hills or buildings in the line of the prevailing winds. For example, on the top of ridges and hills, on a flat plain or near a coastline.

To find out if wind power works for your location, call a local weather station or airport to find out what your average wind speed is. Exact estimates of the speed of the wind are critical when evaluating the potential of wind energy in any location. Wind resources are characterized by a scale of wind classes according to their speed, that extends class 1 (lowest) to class 7 (highest). Betz’s law states that the energy content of the wind varies with the cube of the average wind speed, so doubling the wind speed increases the energy production 8 times.

Wind speed increases with height, so wind turbines are typically mounted on a high tower to capture the most energy. At 30 meters or more aboveground, they take advantage of the faster and less turbulent wind. It is not uncommon to see a 5% increase in energy production for every 5 meter of additional tower height above 30 meter. The additional cost for the extra height is only around 2-3%. Large scale wind turbines have towers ranging from 60 to 90 meters. The high tower is also used to avoid the turbulence caused by the unevenness of the surface through which the wind is blowing before reaching a turbine. The turbulence otherwise steals large amounts of power and causes unnecessary fatigue for the wind turbine. If there are obstacles within 200m, the turbine should be at least 10m higher than those obstructions.

Wind over water is both stronger and more consistent, but at present onshore wind is more economical than development offshore. Small units can be placed on rooftops. Wind speeds within the built environment are generally much lower than at exposed rural sites, however the roof will redirect the wind towards the turbine which can double the wind speed.

Reliability

The wind does not always blow and is highly variable. Wind turbines typically perform best between 7 and 30 mph wind speed. At higher wind speeds the turbine turns away from the wind and breaks the blades in order to protect itself from the wind. Without this safety feature the turbine may burn up the generator or have bits start flying off. If your home isn’t connected to the national grid you need to store excess electricity for use during calm days, for example in batteries or using a pumped storage system.

Durability

A well-maintained wind turbine should last more than 20 years. Unlike conventional power plants, a wind turbine is exposed to highly variable, harsh weather conditions and constantly changing loads. They also have moving parts that suffer high mechanical stress from these conditions. Because of this, wind turbines have lower lifetime expectancy and higher maintenance costs than other power plants.

Maintenance

Maintenance checks and repairs are necessary every 1-2 years, and will generally cost around 1.5-2% of initial investment per year. Wind turbines can be insured and typically have a 5 year warranty.

Price

The cost of a wind turbine system will depend on its size. A small scale system (<100 kW) can cost up to $3000 to $5000 per kilowatt, while mid-scale systems (100-1000kW) range from around $1500 to $2500, and the largest scale utility wind farms (>1MW) cost $800 to $2000 a kilowatt. Added to this is the cost of the tower, foundation, control electronics, cabling and installation of it all. Building-mounted turbines cost less to install than pole-mounted ones. Here are some ballpark cost ranges for complete, installed wind energy systems.

Mass production and technology advances are making turbines cheaper, and many governments offer tax incentives to spur wind-energy development. Wind energy is competitive with new coal and new nuclear capacity, even before any environmental costs of fossil fuel and nuclear generation are taken into account.

Performance

The nameplate rating of a wind turbine specifies the theoretical maximum effect. Since wind speed is not constant, a wind turbine’s annual energy production is never as much as the sum of the generator nameplate ratings multiplied by the total hours in a year. The ratio of actual productivity in a year to this theoretical maximum is called the capacity factor. Typical capacity factors are 20–40%, with values at the upper end of the range in particularly favorable sites.

By multiplying the nameplate rating with the number of hours in a year (24*365) and the capacity factor (30% in average), you get an estimate for the annual energy production.

A quantitative measure of the wind energy available at any location is called the Wind Power Density (WPD). It is a calculation of the mean annual power available per square meter of swept area of a turbine, and is tabulated for different heights above ground. WDP is rated by class. Classes range from Class 1 (200 watts/square meter or less at 50 meters altitude) to Class 7 (800 to 2000 watts/square meter). Commercial wind farms generally are sited in Class 3 or higher areas, although isolated points in an otherwise Class 1 area may be practical to exploit.

Energy Cost

The average cost of generating electricity from large scale wind turbines is around 3-4p per kilowatt hour, competitive with new coal (2.5-4.5p) and cheaper than new nuclear (4-7p). Domestic wind energy is not as cheap. As an example, let’s calculate the energy cost for a 10kW wind turbine.

The capital and installation (C&I) cost for a 10kW wind turbine is about 63 000 dollars. The lifetime of the turbine is 20 years and the maintenance cost of the turbine is 2% of C&I per year. We then have a total lifetime maintenance cost of $25 000. The total cost for the turbine under its lifetime is therefore 63 000 + 25 000 = 88 000 dollars.

The turbine has a rated capacity of 10kW and, assuming it is well positioned, a capacity factor of 30%. The annual energy production is therefore 10 * 30% * 365 * 24 = 26 300 kWh. During its 20 year life the turbine will generate 26 300 kWh * 20 = 526 000 kWh.

And so, the total cost per kWh is 88 000 / 526 000 = $0.167/kWh. This is cheaper than the kWh cost in most countries even at todays rates, without taking into account the rapidly increasing energy prices. Note that in this example, the cost of having to lend the money was not included.

Advantages

• Widely applicable – Wind energy can be captured anywhere in the world where the wind blows.
• Free fuel source – Wind is free, so operational costs are nearly zero once a turbine is erected.
• Ecology – Wind is a clean source of renewable energy that produces no pollution.
• Small surface area – Wind turbines require only a small area of ground space for the turbine foundation. While a wind farm may cover a large area of land, many land uses such as agriculture are compatible, with only small areas of turbine foundations and infrastructure made unavailable for use.
• Moderately high reliability – A well-placed wind turbine produces electricity 70-85% of the time.
• Low energy cost – Electricity costs are competetive with traditional electricity generating technologies.

Disadvantages

• High maintenance – The maintenance costs for wind power are high at 2% per year of initial investment.
• Low durability – The life expectancy of wind turbines are comparatively low at 20 years.
• Noise – Wind turbines need to be at least 200m away from any residential property to fall below background noise levels of 35db.