Wind power


Wind energy is a form of solar energy. Winds are caused by the uneven heating of the atmosphere by the sun, the irregularities of the earth's surface, and rotation of the earth. Wind flow patterns are modified by the earth's terrain, bodies of water, and vegetative cover. This wind flow, or motion energy, when "harvested" by the wind turbines, or modern-day windmills, can be used to generate electricity.

Since earliest recorded history, wind power has been used to move ships, grind grain and pump water. There is evidence that wind energy was used to propel boats along the Nile River as early as 5000 B.C. Within several centuries before Christ, simple windmills were used in China to pump water. In the United States, millions of windmills were erected as the American West was developed during the late 19th century. Most of them were used to pump water for farms and ranches.

By 1900, small electric wind systems were developed to generate direct current, but most of these units fell into disuse as inexpensive grid power was extended to rural areas during the 1930s. By 1910, wind turbine generators were producing electricity in many European countries.

Wind turbines, like aircraft propeller blades, turn in the moving air and power an electric generator which supplies an electric current. Modern wind turbines fall into two basic groups; the horizontal-axis variety, like the traditional farm windmills used for pumping water; and the vertical-axis design, like the eggbeater-style Darrieus model, named after its French inventor. Modern wind technology takes advantage of advances in materials, engineering, electronics, and aerodynamics. Wind turbines are often grouped together into a single wind powe plant, also known as a wind farm, and generate bulk electrical power. Electricity from these turbines is fed into the local utility grid for National and Regional applications and distribute to customers just as it is with conventional power plants. In the case of utilization at local or site specific levels, they could be used with a storage facility or integrated with other renewable source to provide electricity to a defined load.

Wind turbines are available in a variety of sizes, and therefore power ratings. All electric-generating wind turbines, no matter what size, are comprised of a few basic components: the rotor (or blade, is the part that actually rotates in the wind), the electrical generator and a speed control system (if applicable). Some wind machines have fail-safe shutdown systems so that if part of the machine fails, the shutdown systems turn the blades out of the wind or puts on brakes.

In applications for building rooftops, there are not too many choices of wind turbine models to choose from as most of the commercially available wind turbines are designed to be sitted in remote areas where the wind speeds are a lot higher due to less obstruction from the surroundings. Unlike the wind turbines used in wind farms with power generating potentials in the KW range, the roofmounted small turbines, are not mounted on towers and can have rotor diameters as small as 1 meter. The available selection of the small wind turbines for rooftop applications come from manufacturers like Southwest Wind Power Turbines and Marlec . Also another potential for use on building rooftops are ducted wind turbines. Although still at an experimental stage, it has good potential to be developed for commercial purposes.

An analysis of roof mounted wind turbines would require a few factors to be considered. Data on wind speeds and direction for the proposed site of installation would be one of the most important things to consider for an accurate analysis to be conducted. However, in coming up with a crude or preliminary estimations, not having information on the particular site in question, data could be obtained from the neareast Met. Station and by conducting a field survey for the site location and surroundings (obstructions) areas to be factored in as corrections to the weather stationís data.

Although wind power plants have relatively little impact on the environment compared to other conventional power plants, there is some concern over the noise produced by the rotor blades, possible interference with television reception, and sometimes birds have been killed by flying into the rotors. Most of these problems have been resolved or greatly reduced through technological development or by properly siting wind plants. Avian mortality remains an issue to be resolved since it is unclear if birds are attracted to wind turbines.


Ducted Wind Turbines

Ducted wind turbines are still at an experimental stage but show good potential for low power applications. It has undergone wind tunnel testing and built as a larger prototype for field testing. Results show that the device is best sitted at locations with a directional wind regime and can be integrated with larger structures, it is also quiet and robust.

This device is made of easily available/recyclable material ie. sheet metal/ducting and the location of the generator beneath the ducting does not obstruct air flow as it would for the conventional wind turbines.


From the wind tunnel testing, general information was obtained about various aspects of the wind turbine performance. By modifying the model geometry, the Power Coefficient Cp was observed to have changed. The most significant change (+30 %) in the Cp was observed when a roof spoiler was added to the ducting structure. Another observation in this trial was that the turbine was sensitive to wind direction but it would tolerate a misalignment of ±15°.

The similar model with some minor changes to construction was used in a field trial at the National Wind Turbine Test Centre in Myres Hill near Eaglesham. The ducted turbine was tested along side a conventional wind turbine and the former had shown better performance with a site average wind speed of 8 m/s compared to the wind tunnel testing. Also observed at a higher wind speed of about 25m/s was the robustness of the ducted wind turbine when it operated steadily while the conventional wind turbine suffered blade breakages.

In conclusion, the ducted wind turbine when tested both in the wind tunnel and out in the field had demonstrated the ability to produce substantial amounts of power. The turbine wind direction sensitivity was also observed to accomodate a tolerance of 30° misalignment before any significant drop in the Cp had occured. The construction of the ducted wind turbine other than being robust and contributed towards a quite operation, had a cleverly hidden turbine within the structure to minimize the visual impact if these units were mounted on building rooftops.

Reference

Grant, A.D., Nasr, S.A., Kilpatrick, J.; Development of a Ducted Wind Energy Converter, Wind Engineering, Volume 18 No. 6 1994; Multi-Science Publishing Co. LTD., UK.


Analysis of Roof Mounted Wind Turbines

In small scale wind turbines for use on building rooftops, the factors which needs to be considered before an analysis could be made includes the wind speed and direction . These data could be obtained from in-situ wind measurements or from the nearest meteorological station. The latter would require correction factors to be introduced to customize it to a particular location since wind speeds are site dependent ie. obstruction from surrounding areas could cause drastic changes. Therefore, care should be taken when selecting the data for this purpose.

For ducted wind turbines, which are designed to operate within a certain wind direction band, the data has to be checked to disregard all the ĎOut of Directioní wind speeds. Data validation is also done for the Cut-In speed of a wind turbine which is the minimal wind speed value a turbine would operate at.

It would be helpful to use a checklist/procedure to compile this preliminary information in order to analyze the power output of a wind turbine scheme. Results from the checklist would give a rough idea of what to expect before performing a power output calculation.

Power produced from a wind turbine is calculated based on the elementary actuator disc momentum theory,

	
P = ½ ·r· Cp · Area · V³ (Eqn 1)
Where: With the hourly wind speed data, the power could now be calculated using equation (1) above to obtain an hourly power profile. The analysis however, could also use statistical methods to come up with a power profile. The Weibull Windspeed Probability Distribution is a statistical method to determine the yearly wind speed profile for a particular location.

The formula,
H(V<Vp<V+dV) = (8760 (k/c)(V/c)(k-1))exp(-(V/c)k) dV (Eqn. 2)

where,

  • V - Wind Speed,
  • c - Windspeed normalization factor (m/s),
  • k - Shape Factor,
  • H - Hours of the year.


generates the Velocity Exceedence Curve which would then enable a relationship of velocity and time to be established. With this information, the power output could be calculated from equation (1) and the total energy in kW-hrs could then be determined with this knowledge of power and time.


Example of a Velocity Exceedence Curve

Example of a Power Vs. Time Curve

Another criteria often used in the analysis of wind turbines is the Capacity Factor of Capacity Coefficient which is defined by,

	
Cc=Energy total (annual) / Energy rated (annual) (Eqn 3)

If an economic analysis or a sensitivity exercise is to be carried out, the following factors would need to be considered,

	
Annual Repayment (£/yr)=Cr(1+r)n/(1+r)(n-1) (Eqn 4)
Energy Cost (£/kW-hr)=Annual Repayment/Energy total (annual) (Eqn 5)

Where,