Aerodynamics of Horizontal-Axis Wind Turbines

A wind turbine is a device for extracting kinetic energy from the wind. By removing some of its kinetic energy the wind must slow down but only that mass of air which passes through the rotor disc is affected. Assuming that the affected mass of air remains separate from the air which does not pass through the rotor disc and does not slow down a boundary surface can be drawn containing the affected air mass and this boundary can be extended upstream as well as downstream forming a long stream-tube of circular cross section. No air flows across the boundary and so the mass flow rate of the air flowing along the stream-tube will be the same for all stream-wise positions along the stream-tube. Because the air within the stream-tube slows down, but does not become compressed, the cross-sectional area of the stream-tube must expand to accommodate the slower moving air (Figure 1).

Although kinetic energy is extracted from the airflow, a sudden step change in velocity is neither possible nor desirable because of the enormous accelerations and forces this would require. Pressure energy can be extracted in a step-like manner, however, and all wind turbines, whatever their design, operate in this way.

Figure 1 The Energy Extracting Stream-tube of a Wind Turbine

The presence of the turbine causes the approaching air, upstream, gradually to slow down such that when the air arrives at the rotor disc its velocity is already lower than the free-stream wind speed.

The stream-tube expands as a result of the slowing down and, because no work has yet been done on, or by, the air its static pressure rises to absorb the decrease in kinetic energy. As the air passes through the rotor disc, by design, there is a drop in static pressure such that, on leaving, the air is below the atmospheric pressure level. The air then proceeds downstream with reduced speed and static pressure – this region of the flow is called the wake.

Eventually, far downstream, the static pressure in the wake must return to the atmospheric level for equilibrium to be achieved. The rise in static pressure is at the expense of the kinetic energy and so causes a further slowing down of the wind. Thus, between the far upstream and far wake conditions, no change in static pressure exists but there is a reduction in kinetic energy.

What about the world’s wind resources?

When evaluating different methods of power generation some of the main questions inevitably concern the fuel proposed. Is there enough of it? Is it in the right places? It is available when you want it? What does it cost? What emissions does it have?

What residue does it leave? For wind power generation, wind is of course the ‘fuel’ and these questions are as relevant as they are for any other source.

The last of these questions has the simplest answer: unlike fuels that are burnt, wind is free and clean. Questions about how much wind there is, where it is to be found, and when it is available, however, are very pertinent.

On a global scale numerous studies confirm the enormity of the resource, and how it could theoretically meet global electricity demand many times over. A recent addition to this sequence is a collaboration by researchers at Harvard University in the United States and VTT in Finland that concluded that ‘a network of land-based, 2.5 MW turbines, restricted to non-forested, ice-free and non-urban areas, operating at as little as 20% of their rated capacity could supply more than 40 times current worldwide consumption of electricity’.

A slightly earlier comprehensive study by researchers from Stanford University’s Global Climate and Energy Project based its conclusions on five years of data from the US National Climatic Data Center. Using an extensive set of surface and balloon measurements, they concluded that 13% of the sites tested had a good wind resource(Class 3) at 80 meters off the ground, and using one in five of these sites for power generation would allow wind energy to meet the world’s electricity demand (using the figure from the year 2000) seven times over.

Similarly, an earlier study in 2003 by the German Advisory Council on Global Change calculated that the global technical potential for energy production from both onshore and offshore wind was 270,000 T Wh per year. Assuming 10%–15% of this was realizable in a sustainable fashion, the resulting 39,000 T Wh would meet more than double the current global electricity demand. A literature search shows up numerous similar studies with broadly similar conclusions.

Each of the studies varies in its outcome, depending on the assumptions used. One variable concerns the size, capacity factor and rated power of the turbines used for estimations of wind power potential. In addition, the higher the turbine is mounted, the better the wind resource. Further, higher turbines are less prone to be affected by turbulence caused by obstructions, topography, surface roughness or thermal effects. In addition, advances in technology can not only increase the capacity factor of wind turbines, but also the range of wind speeds in which they can operate, thus broadening the range of sites at which they can be used.

Another variable concerns assumptions about the land areas on which turbines can be deployed. While most studies will rule out conservation areas, forests and urban sites, some types of agricultural land such as pastures are easily compatible with wind farms without constraining the overall wind potential of a region. Methodologies for assessing offshore wind resources also differ in terms of the underlying assumptions used.

An assumption needs to made concerning the areas in which wind farms can be built, both for practical reasons (maximum distance to shore, water depth etc), as well as taking into account environmental and regulatory limitations (nature reserve areas, shipping lanes, minimum distance to shore, etc).

Some new configurations that deploy turbines on floating structures and are thus suitable for use in deep water are at a preliminary stage of test deployment.

World wide 5 km Wind Map at 80m

These could dramatically increase the technically usable fraction of the offshore wind potential. Evidence from a large number of studies into the world’s wind resources suggests that there is no shortage of suitable sites for wind power development. However, it is worth noting that the rate of deployment of wind power in each county has largely been dependent on political will rather than resource criteria. Germany has a lower wind potential than many other European countries, yet its favourable political climate has led to rapid and large-scale deployment of wind power.

On the other hand, there are numerous parts of the world with a good wind resource – places such as Argentina, Russia and South Africa – where development of wind power has barely started.

Overall, it is clear that the wind’s energy offers a practically unlimited, clean and emissions free power source of which only a tiny faction is currently being exploited. There may be concerns about ‘Peak Oil’ but ‘Peak Wind’ is not a concept that need worry us!

Large and Small Wind Turbines are Different

Wind turbines are generally divided into two broad categories based on their rated capacity and their intended applications. Small wind turbines are typically less than 50 kW in size, but can be as large as 250 kW and are designed for use in residential, agricultural, small commercial and some industrial applications. In all of these applications, the turbine(s) are providing energy for the end user to offset the use of grid power.

Large wind turbines have rated capacities ranging from 660 kW to 1,800 kW (1.8 MW) and are designed for use in electricity generating power plants. Large turbines are typically deployed in wind farms and are intended to provide wholesale bulk electricity production for delivery on the local transmission network

Small wind turbines can be grid-connected for residential or industrial electricity generation or they can be used in off-grid applications such as water pumping or battery charging. Small turbines are typically installed as a single unit or in small numbers. The smallest turbines (with power ratings less than 1 kW) are normally used to charge batteries for sailboats, cabins, and small homes. Turbines with power ratings between 1 kW to 20 kW are normally used for water pumping, small businesses, residential power, farm applications, remote communication stations, and government facilities.

They are often found as part of a hybrid system that can include photovoltaic cells, grid power connections, storage batteries, and possibly back-up diesel generator sets. Turbines rated at 50 to 250 kW are used in light commercial/industrial, larger farms, and village power applications.

Small Wind Turbine Height Comparisons

Large wind turbines are most commonly deployed in large arrays of multiple turbines. Less common, but increasingly of interest to municipalities or electric cooperatives, large turbines are also installed in distributed generation applications that consist of a single or a few turbines connected directly to a distribution line. Many large wind turbine manufacturers are offering models in the 1 MW range.

Wind turbines as large as 1.8 MW are available for land-based applications. For offshore environments, manufacturers are testing designs in the range of 3-5 MW.

Large Wind Turbine Height Comparisons

Examples and information pertaining to both large and small wind turbines are included in this guide. Although there is some overlap, the majority of the technical issues, permitting requirements, and operating procedures are different for large and small wind turbine applications. Therefore, it is important to differentiate and address the large and small wind turbine applications separately rather than trying to address them with the same set of regulations or review processes.

Large-scale applications are likely to have more impact on a community.

Wind power applications, Grid connected or not?

Wind power applications, Grid connected or not? (on photo: The wind turbines, each with a rated power output of 3.2 megawatts (MW) and a hub height of 143 metres, are intended for the Poysdorf-Wilfersdorf III wind farm.)

There are perhaps four distinct categories of wind power which should be discussed.

These are: 

1. Small, non-grid connected
2. Small, grid-connected
3. Large, non-grid connected
4. Large, grid connected

By small, we mean a size appropriate for an individual to own, up to a few tens of kilowatts. Large refers to utility scale.

Small, Non-Grid Connected

Home wind turbines - designed for ease of use, they are smooth and quiet.

If one wants electricity in a location not serviced by a utility, one of the options is a wind turbine, with batteries to level out supply and demand. This might be a your home, a remote antenna and transmitter site, or a Third-World village.

The costs will be high, on the order of $0.50/kWh, but if the total energy usage is small, this might be acceptable.

The alternatives, photovoltaics, microhydro, and diesel generators, are not cheap either, so a careful economic study needs to be done for each situation.

Small, Grid Connected

Small wind turbine - Grid connected (Horizontal-axis turbine designed)

The small, grid connected turbine is usually not economically feasible. The cost of wind-generated electricity is less because the utility is used for storage rather than a battery bank, but is still not competitive.

In order for the small, grid connected turbine to have any hope of financial breakeven, the turbine owner needs to get something close to the retail price for the wind-generated electricity.

One way this is done is for the owner to have an arrangement with the utility called net metering. With this system, the meter runs backward when the turbine is generating more than the owner is consuming at the moment.

The owner pays a monthly charge for the wires to his home, but it is conceivable that the utility will sometimes write a check to the owner at the end of the month, rather than the other way around.

The utilities do not like this arrangement. They want to buy at wholesale and sell at retail.

They feel it is unfair to be used as a storage system without remuneration.

For most of the twentieth century, utilities simply refused to connect the grid to wind turbines. The utility had the right to generate electricity in a given service territory, and they would not tolerate competition. Then a law was passed that utilities had to hook up wind turbines and pay them the avoided cost for energy.

Unless the state mandated net metering, the utility typically required the installation of a second meter, one measuring energy consumption by the home and the other energy production by the turbine.

The owner would pay the regular retail rate, and the utility would pay their estimate of avoided cost, usually the fuel cost of some base load generator.

Large, Non-Grid Connected

Large, non-grid connected wind turbines installed on mountain

These machines would be installed on islands, high mountains or native villages in the far north where it is virtually impossible to connect to a large grid. Such places are typically supplied by diesel generators, and have a substantial cost just for the imported fuel.

One or more wind turbines would be installed in parallel with the diesel generators, and act as fuel savers when the wind was blowing.

This concept has been studied carefully and appears to be quite feasible technically. One would expect the market to develop after a few turbines have been shown to work for an extended period in hostile environments.

It would be helpful if the diesel maintenance companies would also carry a line of wind turbines so the people in remote locations would not need to teach another group of maintenance people about the realities of life at places far away from the nearest hardware store.

Large, Grid Connected

16 gigantic wind turbines stretch into the air. Every tower has huge, rotating blades and a height of close to 180 meters.

We might ask if the utilities should be forced to buy wind-generated electricity from these small machines at a premium price which reflects their environmental value.

Many have argued this over the years.

A better question might be whether the small or the large turbines will result in a lower net cost to society.

Given that we want the environmental benefits of wind generation, should we get the electricity from the wind with many thousands of individually owned small turbines, or should we use a much smaller number of utility-scale machines?

If we could make the argument that a dollar spent on wind turbines is a dollar not spent on hospitals, schools, and the like, then it follows that wind turbines should be as efficient as possible. Economies of scale and costs of operation and maintenance are such that the small, grid connected turbine will always need to receive substantially more per kilowatt hour than the utility-scale turbines in order to break even.

There is obviously a niche market for turbines that are not connected to the grid, but small, grid connected turbines will probably not develop a thriving market. Most of the action will be from the utility-scale machines.

Sizes of these turbines have been increasing rapidly. Turbines with ratings near 1 MW are now common, with prototypes of 2 MW and more being tested. This is still small compared to the needs of a utility, so clusters of turbines are placed together to form wind power plants with total ratings of 10 to 100 MW.

Why Bats Are Insanely Attracted To Wind Turbines?

Bat Kills at Wind Turbines

Recent studies have reported large numbers of bats being killed at wind farms in many parts of North America and Europe. Project monitoring has also discovered significant bat mortality at the Mexico La Venta II wind farm. Bat kills at wind turbines were first discovered in Australia.

Small numbers of bats were first recorded in the United States at wind power projects in California during bird monitoring.

Some time ago, an estimated 1,400-4,000 bats were recorded as killed during 2008 at the Mountaineer Wind Energy Center in West Virginia . High bat mortality at that site has continued since then.

The frequency and number of bat kills at wind turbines are much greater than for any other type of human-built structure.

Unlike birds, bats strike wind towers as well as telecommunications towers, buildings or power lines, but very infrequently.

Bat Mortality from Collisions and Barotrauma

The bat killed by wind turbine blades

Bats that fly too close to wind turbines are killed by either direct impact or from major air pressure changes around the spinning rotors.

While bats clearly are killed by direct collision with turbine blades, up to 50 percent of the dead bats around wind turbines are found with no visible sign of injury.

The cause for this non-collision mortality is believed to be a type of decompression known as barotrauma, resulting from rapid air pressure reduction near moving turbine blades.

Barotrauma kills bats near wind turbines by causing severe tissue damage to their lungs, which are large and pliable, thereby overly expanding when exposed to a sudden drop in pressure.

By contrast, barotrauma does not affect birds because they have compact, rigid lungs that do not excessively expand.

Bat Atraction to Wind Turbines

Many species of bats appear to be significantly attracted to wind turbines for reasons that are still poorly understood.

Here we’re gonna try to summarizes the more plausible scientific hypotheses that have been advanced to date. By contrast, birds are not normally attracted to wind turbines, and simply collide with them by accident.

The Eastern Red Bat Lasiurus borealisis typical of the migratory, tree-roosting bat species that are frequent casualties at some wind farms in North America.

Hypotheses for Bat Attraction to Wind Turbines

Various scientific hypotheses have been proposed as to why bats are seemingly attracted to and/or fail to detect wind turbines

The more plausible hypotheses include the following:

1. Auditory Attraction

Bats may be attracted to the audible “swishing” sound produced by wind turbines. Museum collectors seeking bat specimens have used long poles that were swung back and forth to attract bats and then knock them to the ground for collection.

It is not known if these bats were attracted to the audible “swishing” sound, the movement of the pole, or both factors.

2. Electromagnetic Field Disorientation

Wind turbines produce complex electromagnetic fields, which may cause bats in the general vicinity to become disoriented and continue flying close to the turbines.

3. Insect Attraction

As flying insects may be attracted to wind turbines, perhaps due to their prominence in the landscape, white color, lighting sources, or heat emitted from the nacelles, bats would be attracted to concentrations of prey.

4. Heat Attraction

Bats may be attracted to the heat produced by the nacelles of wind turbines because they are seeking warm roosting sites.

5. Roost Attraction

Wind turbines may attract bats because they are perceived as potential roosting sites.

6. Lek Mating

Migratory tree bats may be attracted to wind turbines because they are the highest structures in the landscape along migratory routes, possibly thereby serving as ren-dezvous points for mating.

7. Linear Corridor

Wind farms constructed along forested ridge-tops create clearings with linear landscapes that may be attractive to bats.

8. Forest Edge Effect

The clearings around wind turbines and access roads located within forested areas create forest edges. At forest edges, insect activity might well be higher, along with the ability of bats to capture the insects in flight.

Resident bats as well as migrants making stopovers may be similarly attracted to these areas to feed, thus increasing their expo-sure to turbines and thus mortality from collision or barotrauma.

9. Thermal Inversion

Thermal inversions create dense fog in cool valleys, thus concentrating both bats and their insect prey on ridge-tops.

Bat Species Most Significantly Affected

The bat killed by wind turbine blades

In North America, migratory bat species have been found dead at wind farms much more frequently than the resident (non-migratory) species, even in areas where the resident species are more common throughout the summer.

Eleven of the 45 species of bats that occur in North America north of Mexico have been found dead at wind farms, but most studies report that the mortality is heavily skewed towards migratory, tree-roosting species such as Hoary Bat Lasiurus cinereus, Eastern Red Bat Lasiurus borea-lis, and Silver-haired Bat Lasionycteris noctivagans.

While these three species are not listed as threatened or endangered under the U.S. Endangered Species Act, they are classified as of Special Management Concern at the provincial level in Canada. Although the globally endangered Indiana Bat Myotis sodalishas not yet been found dead at wind farms, potential new wind farms within this species’ remaining strongholds could possibly threaten it.

In Europe, 19 of the 38 species of bats found within the European Union have been reported killed by wind turbines.

Although migratory species are among the most numerous casualties, resident bats are also killed in substantial numbers, particularly in forested areas.

Turbine-related bat mortality has been found in every European country in which bat monitoring has been done, except for Poland where no dead bats were found during monitoring at two sites. The highest numbers of bat fatalities have been found in Germany and France, which is almost certainly due to the more extensive monitoring carried out in those countries.

What Is The Purpose Of Wind Farm Lighting?

What Is The Purpose Of Wind Farm Lighting?

Aircraft Warning Lights

Wind farm lighting includes:

1. Aircraft warning lights that are installed on tops of some turbine nacelles and
2. Night lights around wind farm buildings, parking lots, or other facilities.

Aircraft warning lights on top of tall telecommunications towers are known to be highly problematic for night-flying migratory birds (through not bats) during cloudy weather when the birds cannot navigate via the stars and are aĴracted to the lights, thereby colliding with the towers, guy wires, or each other.

Recent U.S. research indicates that white strobe lights with brief pulses are more bird friendly than solid or slowly pulsating red or white lights, since they seem to be less of an attractant to migrating birds on cloudy nights.

The windy sites most favorable for wind farm development often, though not always, have clear skies at night when night-flying birds are not attracted to artificial lights.

To minimize the risks that night-flying migrant birds will be attracted to wind turbines during overcast weather, aircraft warning lights would ideally be:

1. White strobe lights and
2. Placed a top a limited number of turbines to help show the wind farm outline, rather than on every one, consistent with national and local regulatory requirements.

In many low-risk cases far away from airports, such lights might not be needed at all; for example, U.S. law requires aircraft warning lights only on structures 200 feet or higher.

If aviation regulations provide flexibility regarding the type of aircraft warning lights to be used, it would also be important to consult with local residents regarding their aesthetic preferences, while also explaining the bird-friendly advantages of white strobe lights.

At the Mexico La Venta II project, only a small number of the 98 turbines have aircraft warning lights, which appears to be adequate for showing any pilots that might be in the area that there are multiple structures around, without potentially at-tracting numerous migrating birds to the turbines on overcast nights.

Night lighting close to ground level can also be managed to minimize bird and bat mortality.

Night lights around worker compounds, parking lots, and other wind farm facilities can attract to the wind farm migratory birds during cloudy weather, as well as bats and nocturnal birds (owls and nighjars) that seek out the flying insects that con-centrate around night lights.

In this regard, wind farm night lighting can be managed to minimize risks to birds and bats in ways that do not compromise worker safety and operational security.

These approaches include the use of:

1. Sensors and switches to keep lights off when not needed and
2. Lighting fixtures that are hooded and directed downward to minimize the skyward and horizontal illumination that could attract night-flying birds and bats to the vicinity of wind turbines.

Wind turbine lighting signalization

Wind Energy End-Use Applications

Wind Energy End-Use Applications (photo by SCA Svenska Cellulosa Aktiebolaget)

Markets

Wind energy markets can be classified based on the end-use application of the technology. Wind energy projects are common for off-grid applications.

However, the largest market potential for wind energy projects is with on-grid (or grid-connected) applications.

Off-grid applications

Historically, wind energy was most competitive in remote sites, far from the electric grid and requiring relatively small amounts of power, typically less than 10 kW.

In these off-grid applications, wind energy is typically used in the charging of batteries that store the energy captured by the wind turbines and provides the user with electrical energy on demand, as depicted in Figure 1.

Water pumping, where water, rather than energy, can be stored for future use, is also a key historical application of wind energy. The key competitive area for wind energy in remote off-grid power applications is against electric grid extension, primary (disposable) batteries, diesel, gas and thermoelectric generators.

Wind energy is also competitive in water pumping applications.

On-grid applications

In on-grid applications the wind energy system feeds electrical energy directly into the electric utility grid.

Two on-grid application types can be distinguished.

1. Isolated-grid electricity generation, with wind turbine generation capacity typically ranging from approximately 10 kW to 200 kW.
2. Central-grid electricity generation, with wind turbine generation capacity typically ranging from approximately 200 kW to 2 MW.

Figure 1 - 10 kW Off-Grid Wind Turbine in Mexico. Photo Credit to Charles Newcomber/NREL Pix

Isolated-grids

Isolated-grids are common in remote areas. Electricity generation is often relatively expensive due to the high cost of transporting diesel fuel to these isolated sites. However, if the site has good local winds, a small wind energy project could be installed to help supply a portion of the electricity requirements.

These wind energy projects are normally referred to as wind-diesel hybrid systems.

The wind energy system’s primary role is to help reduce the amount of diesel fuel consumption. A wind-diesel hybrid system is shown in Figure 2.

Figure 2 - 50 kW Isolated-Grid Wind Turbine in the Arctic. Photo Credit to Phil Owens/Nunavut Power Corp.

Central-grids

Central-grid applications for wind energy projects are becoming more common.

In relatively windy areas, larger scale wind turbines are clustered together to create a wind-farm with capacities in the multi-megawatt range. The land within the wind-farm is usually used for other purposes, such as agriculture or forestry.

Another common approach for wind energy project development includes the installation of one or more larger scale wind turbines by individuals, businesses or cooperatives.

A windfarm, as depicted in Figure 3, consists of a number of wind turbines (which are often installed in rows perpendicular to the wind direction), access roads, electrical interconnections and a substation, a monitoring and control system and a maintenance building for the larger farms.

The development of a wind energy project includes the determination of the wind resource, the acquisition of all authorisations and permits, the design and specification of the civil, electrical and mechanical infrastructure, the layout of the wind turbines, the purchasing of the equipment, the construction and the commissioning of the installation.

Construction involves the following:

1. Preparing the site,
2. Grading roads,
3. Building turbine foundations,
4. Installing the electrical collection lines and transformers,
5. Erecting the turbines, and
6. Construction of the substation and building.

Figure 3 - Components of a Windfarm in the United States. Photo Credit to Warren Gretz/NREL Pix

The wind resource assessment and approvals for a windfarm are often the longest activities in the development of the wind energy project. These can take up to 4 years in the case of a large windfarm requiring a comprehensive environmental impact study.

The construction itself can normally be completed within one year.

The precise determination of the wind resource at a given site is one of the most important aspects in the development of a wind energy project as the available wind resource at the project site can dramatically impact the cost of wind energy production.

In the case where a pre-feasibility study indicates that a proposed wind  energy project could be financially viable, it is typically recommended that a project developer take at least a full year of wind measurements at the exact location where the wind energy project is going to be installed.

Figure 4 shows the installation of a 40 m tall meteorologi-cal mast at the CANMET Energy Technology Centre – Varennes in Canada.

Figure 4 - Installation of a 40 m Meteorological Mast. Photo Credit to GPCo Inc.

For very small-scale projects (e.g. off-grid battery charging and water pumping), the cost of wind monitoring could actually be higher than the cost to purchase and install a small wind turbine.

In this case a detailed wind resource assessment would normally not be completed.

Science in the City: Altamont Wind Farms (VIDEO)

Southeast of San Francisco, on the way out to California’s Central Valley, thousands of wind turbines dot the landscape of Altamont Pass. Mounted both in rows and individually, machines with large propellers catch the wind, turning round and round at different speeds.

Learn how wind energy is generated and stored for use in this most peculiar area, and its impact on living things both near and far.

 

Cant see this video? Click here to watch it on Youtube.