Wind Turbines and Wind Energy Buzz

In order to understand how wind energy turbines work the wind term should be clarified first. Wind is moving air and is caused by differences in air pressure within our atmosphere. The sun heats the earth at different rates depending on whether an area is below clouds, in direct sunlight, or covered with water. The air above the warmer areas heats up, becomes less dense, and rises. The rising air creates a low pressure area. Cooler air from adjacent higher pressure areas moves to the low pressure areas. This air movement is wind. The greater the difference in pressure, the faster the air flows.
Then the kinetic energy of the wind due to its speed is captured by the turbine and is converted to mechanical energy. However the more kinetic energy a wind turbine pulls out of the wind, the more the wind will be slowed down. This is due the Betz law. The more kinetic energy a wind turbine pulls out of the wind, the more the wind will be slowed down as it leaves the back side of the turbine.

The ratio between the power we extract from the wind and the power in the undisturbed wind is then:
(P/P0) = (1/2) *(1 – (v2 / v1) 2) * (1 + (v2 / v1))

Where:
P stands for the power extracted from the rotor
P0 stands for the total power when with no rotor blocking the wind
V1 stand for the air speed in front of the rotor
V2 wind speed after the passage through the rotor plane

A more detailed exlpaination of the  can be found here: http://www.rensmart.com/Information/Betz%27Law

New wind turbines are much different with the predecessors although the basic idea remains; to harvest the wind and generate energy. After all the moving power remains the wind.

The wind energy projects can now be classified in two major categories,  mainly based on the end-use application use.

• On-grid applications: the wind energy system feeds electrical energy directly into the electric utility grid. The on-grid can be either central-grid application or an isolated-grid application
• Central-grid: when wind turbines are clustered together to create a wind farm.

The energy production from the wind depends on several key factors mainly of the wind characteristics.
The wind power density is the amount of energy in the wind passing through the area swept by the wind turbine blades in a unit of time. A key aspect of wind power density is its dependence on wind speed cubed. This means that the power contained in the wind increases very rapidly with wind speed; if the speed doubles, the power increases by a  factor of eight.

In practice, the relationship between the power output of a wind turbine and wind speed does not ollow a cubic relationship. Typically, wind speeds greater than 15 km/h are needed before a wind energy system can begin to generate electricity. This is known as the “cut-in” speed.

Factors effecting the wind energy production:
• The “cut-out” speed, usually around 70 km/h, is where the system stalls to protect itself from damage.
• The winds variability. To accurately predict the performance of wind turbines, one needs to know not only the average wind speed at a particular location but also how wind speed varies over time.
• Wind speed dependence on height. Wind speed tends to increase with height in most locations, a phenomenon known as wind shear. The degree of wind shear depends mainly upon on two factors, atmospheric mixing and the roughness of the terrain.
• The spatial variations. Wind resource characteristics can differ greatly between nearby locations. For obvious reasons, the strongest winds usually are found in well-exposed locations. In addition, terrain features such as hills and ridges can accelerate the wind as it passes over them.

The wind plant ideally should be located close enough to the electric grid in order to reduce development costs and energy loose.
Wind turbine design is dictated by a combination of technology, prevailing wind regime, and economics. Wind turbine manufacturers optimize machines to deliver electricity at the lowest possible cost per kilowatt-hour (kWh) of energy.
Almost all wind turbines producing electricity consist of rotor blades which rotate around a horizontal hub. The hub is connected to a gearbox and generator, which are located inside the nacelle. The nacelle houses the electrical components and is mounted at the top of the tower.
On the other hand vertical axis wind turbines found in built up areas and cope well with turbulent winds.  In vertical axis wind turbines (VAWT) the axis of rotation is perpendicular to the wind stream and the ground. The basic theoretical advantages of a vertical axis machine are:

• The generator, gearbox etc. may be placed on the ground, and a tower is not essential for the machine
• A yaw mechanism isn’t needed to turn the rotor against the wind.

The use of vertical axis wind turbine (VAWT) is considered more feasible for the project rather than the use of horizontal axis wind turbines (HAWT)  mainly due to the fact that the former are more “manageable” in terms of service / maintenance and cope significantly better with turbulence wind.

In general high turbulence levels created form fast moving vehicles considered to reduce performance and induce excessive stress on a HAWT. In the VAWTs the rotor needs to be positioned into the wind direction by means of a tail or active yawing by a yaw motor. Additionally, HAWTs are sensitive to the changes in wind direction and turbulence which have a negative effect on performance due to the required repositioning of the turbine into the wind flow. On the contrary, in VAWTs changes in wind direction have fewer negative effects on this type of turbines because it does not need to be positioned into the wind direction.

Identify location – Site access

In order for a project to be economically feasible and environmentally friendly, the most appropriate place for setting up the wind farm needs to be selected. Sites are analyzed with respect to road access, grid connectivity, area availability, site conditions, wind resource, visual impact, land ownership and noise issue.

Wind Resource Assessment

The wind speed study would include the average wind speed for various times of the year and the frequency at which the wind blows at that speed. A series of thorough examination of the wind data potential (i.e wind maps), meteorological data and proximity to transmission lines need to take place.  Of course, this wind map (and others like it) is simply a generalization of the entire state and does not reflect local wind conditions. Before a wind turbine could be erected a feasibility study on the site must be conducted. This would include the erection of an anemometer to measure the speed of the wind at the height at which the turbine will be operating.

Transmission lines

Obviously, the wind turbines have to be connected to the electrical grid.
For smaller projects, it is therefore essential to be reasonably close to a minimum 10-30 kilovolt power line if the costs of extending the electrical grid are not to be prohibitively high.

Electrical Power Collection System – Energy produced from the turbines is collected in a medium-voltage (approximately 25-35 kV) power collection system consisting of belowground cabling within the turbine rows and above-ground power lines from the turbine rows to the main substation . The interconnection point to the utility line can be colocated in the substation or it can be physically separated and located adjacent to the utility line. In general, wind energy projects are positioned within 1 to 10 Kilometers from the high-voltage transmission line to minimize costs associated with the interconnection.

Substation and Interconnection – For most wind energy projects, electrical energy produced by the turbines passes through a substation where it is metered and the voltage is increased to match the voltage of the utility grid. Plant isolation breakers, power quality monitors, and protective equipment are also present in the substation to protect  both the electrical grid and the wind turbines. A system of switches and overhead infrastructure is used to connect the substation to the utility’s power lines.

Ground conditions

The ground conditions at the site should be examined to consider whether construction of the foundations for the wind turbines, the erection of the machines and the provision of access roads is practical and economic.

Foundation & logistics

In general, the foundation design is based on the weight and configuration of the proposed turbine, the expected maximum wind speeds, and the soil characteristics at the site.
“Installation process. An appropriate platform (or equivalent structure) must be prepared in advance to interface with the Windrotor (at the customer‘s charge). The proper elevation of the rotor above ground level depends upon its surroundings and geographical conditions. It is recommended to position the rotor between 8 to 20 m above ground level. The static load calculated for a wind speed of 56 m/s at sea level and at the top of the mounting structure will be 2000 N for WRE.005, 9000 N for WRE.030 and 19000 N for WRE.060 for example.”
Source: http://www.genasyspowersystems.co.uk/Document_download/Ropatec_Catalog_Windrotor.pdf

Space Occupation

The wind environment in particular determines the number of turbines required and in turn the distance between the turbines. The better the wind the fewer the turbine needed and the shorter the cabling and other base case equipment, which minimizes the environmental footprint and improves long-term project viability.
Wind farms or wind parks often have many turbines installed. Since each turbine extracts some of the energy of the wind, it is important to provide adequate spacing between turbines to avoid excess energy loss.

Noise

The unit used to describe the intensity of sound is the decibel (dB). Audible sounds range from 0 dB (“threshold of hearing”) to about 140 dB (“threshold of pain”). The normal audible frequency range is approximately 20 Hz to 20 kHz. The A-weighted scale, denoted as dB (A), approximates the range of human hearing by filtering out lower frequency noises, which are not as damaging as the higher frequencies. It is used in most noise ordinances and standards.
To provide a frame of reference, rustling leaves have a decibel level of 10 dB (A); suburban expressway at 90 meters, 60db (A); large truck pass by at 15 meters, 90dB (A); and aircraft takeoff, 120 dB (A). Also Table 9 provides a list of typical noise levels in the environment.

Wind turbine noise

In general wind turbines generate noise as every machine does. The noise from the wind turbine is divided into two major categories depending on the noise origin. These are:
1. Mechanical noise caused by the gearbox and the generator (tonal sound),
2. Aerodynamic noise caused by the interaction of the turbine blades with the wind

Modern wind turbines are machines producing little or no noise at all in comparison to their predecessors and to their rated power they produce. And this due to the fact that wind manufactures quickly realized that the noise problem needed to be dealt with and stared producing quieter machines. As a result the noise from the gearbox and the bladeswas reduced by careful attention to the design and manufacture of the components and also the noise for the generator minimized with good sound insulation within the turbine head.
On the other hand, wind farms are always located where the wind speed is higher than average, and the “background” noise of the wind tends to “mask” any sounds that might be produced by operating wind turbines.
Over and above that the VAWTs tend to generate less noise than the HAWTs mainly due to the fact that the blades do not create the whooshing noise that occurs with HAWTs when the blade pass close to the mast at each revolution. On the other hand due to the proximity of human activity, these applications could potentially result in noise complaints.

Aesthetics

The visual effect of wind turbines may be one of the most debated topics in the reviews of wind farm proposals. There is a wide variety of views on the aesthetics of wind turbines. The proposed site and its surrounding landscape, public attitudes, land use practices, and individual perspectives influence those views. When evaluating the visual impacts of wind energy projects, the essential question is not whether people will find them beautifully or not, but instead to what degree then may affect the important visual resources in the surrounding area. To some, the blades are an eyesore; to others, they’re a beautiful alternative to conventional power plants.
Since aesthetic judgments are subjective, responses from the public to a proposed wind farm can vary considerably.

In general, the visibility of a particular wind system will depend on many factors, including tower height, proximity to neighbors and roadways, local terrain, and tree coverage, public acceptance and knowledge of renewable energy technologies. What ever the surrounding environment is the developer should try to reduce the visual impact as
much s possible. The main visual aim of a wind farm layout should be to convey a sense of clarity.

Interference of a wind turbine with electromagnetic communication systems

Wind turbines in some areas can reflect electromagnetic waves (mainly due to the moving blades), which will be scattered and diffracted. This means that wind turbines may interfere with telecommunication links.

Wind Turbines and Birds

Wind energy’s ability to generate electricity without many of the environmental impacts associated with other energy sources (air pollution, water pollution, mercury emissions, and greenhouse gas emissions associated with global climate change) can significantly benefit birds, and many other plant and animal species.
However the populations of many bird species are experiencing long-term declines, due not only to the effects of energy use, but many other human activities. Especially in highways birds and bats sometimes die as a result of collisions with vehicles traveling the project roads.
On the other hand, the wind turbines per se are responsible only for a small portion of the total number of bird causalities caused by human builds.
However the place of the wind farm and wind speed generated from the passing cars it is possible to affect the mortality ratio. The variable speed turbine is a more serious threat as there is a correlation between the speed of rotation and the number of birds killed. Up to 80% of birds can fly through the rapidly rotating blades of variable speed turbines and remain unharmed (Winkelman 1992b). Birds have much more time to evade the blades of
a fixed speed turbine (www.windshare.ca/documents/EA_draftscreeningdoc.pdf).

Economic – Socioeconomic Impact of Wind Turbines

Several human activities have to be suspended within a wider area in order to mitigate the unwanted environmental interferences of the wind farm operation As with most business ventures, wind energy projects create jobs. In general, the employment opportunities associated with a wind power plant are in construction, operations and maintenance (O&M), and manufacturing. It should be noted that most wind energy jobs are in the manufacturing, construction and installation fields, with relatively few jobs in ongoing operations and maintenance.