Small Scale Wind Augmentation Device





Wind energy is one of the most reliable and abundant sources of renewable energy on this planet. The human kind has used it for centuries as there are estimates that people were using this 
type of energy as far as 4,000 years ago. In the year 1700, it was employed by the king of Babylon for irrigation purposes (Ackermann & Kuwahata, 2013). The wind was also for grinding grains by several farmers. It is also surprising to note that one of the earliest windmills had the vertical axis of rotation. The people use braided mats or sails for rotating these windmills across the vertical axis. These windmills had an operational advantage as they were independent of the direction of the wind.

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History of Wind Power

The wind power was initially utilized by humans in the form of irrigation pumping, sailboats and garn milling. The history of wind power can easily be tracked back to the ancient 6,000 years ago. The sails, ships and sailboats were used on the Nile River though its water, in Egypt. The earliest known use of wind power was represented by the sailboats. Although as per today’s standard, it is considered to be a rudimentary use of wind but the technology impacted the development of the current sail-type windmills (Hansen, M. O., 2015). The first documented proof of the existence of such mills dates back to 12th century. However, the ability to study the wind power across the horizontal axis was the first to measure in the 20th century (Kuwahata, 2013).
There are two types of wind turbines. One is built with high speed and is intended to generate electricity whereas the other one is built for low wind speeds (Wahab et al. 2008). The major components of Wind turbines include a rotor with a set of blades with a rotor hub which is seen to deflect the airflow by creating a force on the blades. Due to this, a torque is produced on the shaft and the rotor is seen to rotate around the horizontal axis attached with a generator and a gearbox. All this is present inside the nacelle present with other important electrical parts with the tower end. Electricity is generated moving down from the tower to the transformer which is then converted from voltage (usually 700V, 33000V for countrywide grid, 240 V for the personal use). The core windmills are termed as horizontal axis wind turbine (HAWT) and the vertical axis wind turbine (VAWT). The windmills using the horizontal axis of rotation are relatively new as compared to vertical axis ones. 

United States and Wind Power

Due to increasing high-energy costs, the government and the people are becoming highly concerned towards global warming, pollution and feel financially strapped at the same time (Bollen, Hers & Zwaan, 2010). With around 30 percent of the total world reserves, the United States have a vast coal supply (Hook & Aleklett, 2010). The annual production of United States is almost twice as compared to India and lacks only behind China (Hook & Aleklett, 2010). But it has been noticed that the US reserves are concentrated only in few states and have considerably decreased from 29.2 MJ/kg 23.6 MJ/kg in the period of 1950-2007 due to movement of subbituminous western coals by US (Hook & Aleklett, 2010). The production of one kilowatt hour (kWh) of electricity from coal is seen to produce 1kg of Carbon Dioxide (CO2), more than 200 grams of waste amounts of several metals and ash, seven grams of nitrogen oxides, sulfur oxides and particulates are released such as Methane (CH4), hydrogen sulfide (H2S), hydrocarbons ethane (C2H6), etc. (Stracher & Taylor, 2004). 

Benefits of Wind Power

According to Klingenberg (1996), CO2 levels were around 345,000 parts per billion in 1996 as compared to 290,000 parts per billion decades ago (Graedel & Crutzen, 1989). According to the U.S. Department of Energy, 2013, the dependency of the fossil fuels can be reduced using the wind energy as there is no emission of greenhouse gases. Although the generation of electricity is not just one cause of increasing CO2 level in the atmosphere but it contributes to a major share of CO2 emission (Graedel & Crutzen, 1989). Therefore the reduction of greenhouse gases in the atmosphere can be done by environmentally friendly electricity generation using wind power. The tunnel attachment of the wind has the ability of capturing low speeds of the winds especially at the lower altitudes which might lead to deduction in the wind turbines and will lead to over improvement of the efficiency. A vital step can be taken by the wind power in the field of renewable source for production of energy.  The current status of wind energy is promising. The fourth edition of Global Wind Energy Outlook claimed that the wind energy constitutes 3.5% of global electricity production. The share in power generation is estimated to reach 12% by the year 2020 (Kuwahata, 2013). At the end of the year 2012, the total global production of the Wind Energy was 282 Giga Watts showing an increase of 44% from the year 2011 (Kuwahata, 2013). The installation of wind energy had reached its peak during the year 2012, but after the same year, the total production of wind power generation has decreased due to several reasons. In fact, the installation of wind energy for electricity generation falls to 18.7% which is the lowest in the past two decades. 
The United States has also gained the global momentum, and the growth rate in the country was estimated to be around 27.9% during the year 2012. The total electricity produced in the United States from wind energy was 140 terawatt-hours constituting the 3.5% of the total energy produced from entire sources (Armentrout & Armentrout, 2009). The US Department of Energy (DOE) has accelerated its efforts to install wind and water power generation capability of the country.  The DOE also issues loans and guarantees to install wind electricity generation. The DOE has estimated to supply 20% energy from the wind generation by the year 2030 (Armentrout, 2009).

Adoption of Wind Energy

The main concerns for adoption and investments in the renewable energy technologies along with agreeing to the implementation policies for their support include environmental, security and economic concerns. The primary objectives for the adoption of wind energy on the global scale include the reduced risk from rising and volatile energy costs, energy security and decreased carbon emissions and other pollutants. One of the primary reasons for the adoption of wind energy worldwide is the fact that it is a highly environment-friendly source of energy. It is also estimated that in future conventional sources of energy such as fossil fuels might run out. The world has to look for alternate sources of energy to meet the energy demands of the future generations (Crossley, 2000).
The most important factor towards the adoption of the Wind Energy as energy production source as it is the cleanest forms of energy. The use of wind power as a source of power ensures that there is no emission of carbon dioxide and other harmful gasses into the atmosphere and thus it is safer to use as compared to fossil fuels and nuclear power stations. The main concern, however, is to increase the output and efficiency of wind energy generation keep the costs under check. 
The wind power generation will be able to meet the increasing demand of the energy felt worldwide. The adoption of wind power can be beneficial in enhancing the diversity in the supply market of the energy and help in securing long-term sustainable supplies of the energy. The emission of local and global atmospheric emissions will be reduced. Furthermore, in order to meet the specific demands of the energy services new commercially attractive options would be available along with creating tremendous job opportunities. The production of renewable energy especially from the sustainable natural sources is seen to contribute to the sustainable development and considers the needs of the future generation. Wind power will promise out to be the best alternative and the most suitable supplement towards the traditional energy sources especially in the developing countries. According to the studies, wind energy has made the considerable amount of contribution to the field of grid power installed capacity and can easily emerge out to be one of the options for mitigating pollution (Dincer, I., 2000). Due to wind power being environmental friendly and renewable in nature, wind energy can be converted to electricity. 

Wind Power and Augmentation

The primary challenge being faced by the urban wind energy is the lower mean speed of the wind especially in the urban environment owning to the increased roughness in the surface of the free streams and reduction in the heights of the wind turbines on being installed. The incoming wind is seen to encounter higher turbulence intensity which is seen to be next to the lower mean wind speed. However, the harvestation of the wind turbines can take place in the well sited acceleration of the flows. This can result in over 70 percent of the increase in power. 
The wind turbines and its performance can be increased using a plethora of methods. According to Phillips (2003), a myriad of research have been performed in the field of power augmentation of the wind turbines. Some of the famous examples of these attempts in power augmentations are cylindrical obstruction concentrators, tip vanes on the rotor blades, diffusers in which the rotor is located and vortex type augmentation devices. According to him, the diffuser augmented wind turbines were considered to be the best suitable action amongst all. 
The building and maintaining of the large scale wind shrouding is seen to be expensive however, its proper use can act as a great source of renewable energy providing an incentive for methods that can improve the technology (Foote, 2011). According to a study by Dakeev (2014), a cone shaped wind guide system is present in the shrouding system which results in over 60 percent of the power outcome with respect to the traditional bare wind turbine. Studies like Dakeev (2010) and Kosasih & Tondelli (2012) suggested that the wind augmentation devices are seen to influence the generation of power significantly.  
Various research also revealed that the shroud having an angle of 30 degree will connect to the wind turbine and will lead to delivering of an improved power efficiency and performance as compared to the augmentation devices installed at various angles and positions. A study calculated that the shroud must accelerate the wind speed by 2.1 factor where the power output will be augmented by almost three times. 
It has been stated by the Betz Law that no wind turbine would be able to capture over 59.3 percent of the kinetic energy present in the wind. Around 80 percent of Betz limit is seen to be achieved by the utility scale wind turbines at their highest peak. According to Hansen, 2000, when the wind turbine is enclosed within the shroud, the power generation is seen to be improved beyond the Betz limit.
The intended objective of this study is to develop an augmented wind device which can improve the efficiency of the small-scale experimental wind turbines. Testing of the wind turbine with the help of shroud and cone might facilitate in increasing the air flow to the turbine and thus increase the overall efficiency. The performance of the device at various speeds and output powers will be examined numerically and experimentally. The power output of the wind turbine will be compared using shrouded wind turbine using cones with the turbines which are not using the cones.

Literature Review

Various endeavors have been made in order to make the best of the all the natural resources provided to us for generating energy. Various projects have been performed for increasing the efficiency of the power generation and the main concern being the increasing of throughput of the wind present in the tunnel and on concentrate fully on the turbine and its blades. As various limitations have been seen in the efficiency of a conventional wind turbine. Therefore, various augmentation have been performed for overcoming the weaknesses of the wind tunnel. 
A dynamic wind shroud has to be built with sensors that can help in determining the optimal relationship between the angle wind shrouding adjusts and wind velocity owning to the constantly changing velocities of the wind and to establish better generation of power. A critical factor for increasing the power generation and its efficiency is to determine the optimal design of the wind augmentation. The study aims to build and test a wind augmentation device which can be used in improving the efficiency of the power generation in the experimental small-scale wind turbine. 
The wind turbine when tested along with the shroud and a cone helps in allowing the incoming airflow coming directly from the wind turbine in order to generate a greater output. The study first discusses the various types of wind turbines present and then focuses on the mall-scale wind augmentation device for increasing the efficiency of the wind power. The wind turbines can be divided into sub sections and can be classified on the basis of various criteria. 

Classification or Types of Wind Turbines

The classification of the wind turbines is done on three main parameters. These parameters are:

  1. On the basis of orientation of axis of rotation (Crossley, 2000).

  2. The component of aerodynamic forces which are responsible for its rotation (Crossley, 2000).

  3. The basis of energy generation capability (small, medium or large) (Crossley, 2000).

Vertical Axis Vs Horizontal Axis Turbines

There are mainly two kinds of wind turbines when they are classified on the basis of axis of rotation. As their name suggests, the vertical type of the wind mills rotate perpendicular to the ground known as the vertical axis wind turbine (VAWT) while the horizontal type rotates parallel to the ground known as horizontal axis wind turbine (HAWT). Currently there are three popular designs of wind mills (ERIKSSON, BERNHOFF, & LEIJON, 2008).

  1. Savonius VAWT.

  2. Curved Blade VAWT.

  3. Straight Blade VAWT.

Svonius rotors typically have two or half drums attached to the central shaft in the opposite directions and are seen to function similar to those water wheel that utilize drag forces. Curved blade VAWT consists of two or more curved blades attached to the central vertical shaft. At last, the Straight Blade VAWT has variable pitch angles (LEIJON, 2008). In the horizontal axis of rotation, the rotor shaft rotates parallel to the ground. The electric generator is attached to the turbine rotor via primary and secondary shafts.
It is seen that the VAWTs function quiet near to the ground and not present in the nacelle as compared to the HAWTs and are beneficial in enabling the proper placement of heavy equipment like the generator and gearbox. However, as the winds are seen to have lower speed at the ground level, less amount of power is generated. VAWTs are seen to be more advantageous that the HAWTs as they are packed closely and are present inside the wind farms allowing more amount of space. VAWTs are omnidirectional, quiet and generate low amount of force on the support structure. Further they can be implemented on the structures present at height and can be controlled easily (Riegler, H., 2003).
Whereas the Horizontal Axis Wind Turbines are seen to be positioned at the right direction so as to work in the most efficient manner. The HAWTs that are positioned away from the direction of the wind are termed as the downwind turbines due to their location with respect to the towers and are then pushed in an appropriate orientation. 

Large Vs Small Scale Wind Turbines

Generally the wind turbines can also be divided on the basis of their intended application and rated capacity.  The definitions of Large and Small scale wind turbines are not clearly elaborated in the literature present on the wind energy. Small scale wind turbine was defined by its capacity to meet the demands of individual households. The capacity of the small wind turbines is less than 50 kW size and sometimes are as large as 250 kW (these are designed for agricultural, residential, industrial applications, small commercial etc.) and are aimed at providing energy to the end user for offsetting grid power and its use.
Small scale wind turbine was defined by its capacity to meet the demands of individual households. The problem is the fact that the energy demand of a family varies with respect to time and place. The average energy demand of an American home is 10 kW while for the European houses this requirement is decreased to 4 kW. This requirement reduces to 1 kW in China (Howard, 2012).  Based on the energy demands from a small scale wind turbine following types can be shortlisted.

  1. Micro-scale Wind Turbines (µSWT) having a rotor diameter of ≤ 10 cm.

  2. Small-scale wind turbine (SSWT) having rotor diameter of ≤ 100 cm.

  3. Mid-Scale Wind Turbine (MSWT) having rotor diameter of ≤ 5m.

Whereas the large wind turbines are seen to have the capacities in the range of 660 kW to 1,800 kW (1.8 MW). These are typically designed for generating electricity in the power plants. These are used for deploying the wind farms and can be used for providing wholesale bulk electricity production and can be delivered to the local transmission network. 

Definition of Terminologies

There are some basic terminologies related to wind mills and wind energy which are defined in this section:

  1. Wind Generator: It is a device which is responsible for capturing wind force for providing the rotational motion. This is responsible for producing power along with the wind alternator or generator.

  2. Wind Turbine: It is a machine which is responsible for capturing the wind force and is also called as the Wind Generator in cases where it is used for production of electricity. 

  3. Leading Edge: it’s the point located at the front of air foil, this point has the maximum curvature.

  4. Trailing Edge: it located at rear end of air foil, having maximum curvature.

  5. Chord Line: Line connecting the leading edge and trailing edge.

  6. Chord Length (c): Simply the length of chord line.

  7. Suction Surface: upper surface of airfoil. The main attributes attached to this surface are higher velocities and lower static pressures.

  8. Pressure Surface: lower surface of an airfoil having lower velocity and high static pressure.

  9. Maximum Thickness: it is regarded as maximum thickness of an airfoil measured in perpendicular to the chord line.

  10. Setting Angle: The angle between the plane of the rotation of the blade and the blade Chord is known as setting angle. It is also called the blade or the Pitch angle. The blade with is carved along with the Twist can have various setting angles at the Tip as compared to the Root.

  11. Shaft: The part of the turbine which rotates continuously and is present in the center of the wind generator is called the shaft. It is responsible for transferring the power. 

  12. Tower: the structure of the wind generator which is responsible for supporting it is called the tower. It is usually situated at the top and high in the air.

  13. Torque: The turning force of the generator which is also equal to the force times radius. 

Apart from the nomenclature of the airfoil there are some terminologies related to the wind turbine. These are:

  • Power Coefficient: it is a measure to determine the efficiency of the wind turbine which is used by the wind power industry. Power Coefficient can be defined as the ratio of electric power been generated by the wind turbine and the total amount of wind power being flowing at the certain wind speed in the blades of the turbine. It is used for representing the combined efficiency of the components of the wind power system which includes the shaft, gear train, turbine blades, power electronics and generators. It is defined as amount of mechanical power produced by the wind turbine compared to the available wind energy (Kadar, 2013).

  • Betz’ Law: This law is used to calculate maximum mechanical power produced by the turbine while placed in an open wind flow. The law states that even most of the efficient turbines can extract only 59.7% of the total kinetic energy of the wind as 100 percent of the wind energy generation is impossible due to the structure and mechanism of the wind turbines to convert the wind energy into 100 percent of the kinetic energy (Kadar, 2013). 

  • Torque Coefficient: it is used to calculate the shaft torque produced by the wind turbine. High value of torque coefficient allows the wind to operate at lower wind speed.

λ = speed ratio of the turbine.
a.    Shrouded Turbines
In past few years, it has been concluded that the output power and efficiency of the wind turbines can be improved with the help of shrouded turbines compared to bare turbines. The shrouded tidal turbines are one of the revolutionary idea in the tidal stream technology where the turbine is seen to be enclosed in the venture shaped ventuduct and produces atmosphere with low amount of pressure in the turbine. This helps the turbine to operate at high amount of efficiency as compared to the traditional turbine where the volume of flow is seen to be increased over the turbine.
One of the most advanced designs of the wind turbines is known as diffuser augmented wind turbine (DAWT) (Ohya & Karasudani, 2010). It is also known that these turbines can extract more output power from the same geometry as compared to the conventional designs of the wind turbines. According to estimates, two-fold power augmentation can be achieved by using two wing profiled rings mounted on the rotor. The DAWTs offer various advantages as compared to the traditional turbines which include factors like minimal top speed, increased augmentation, rotor diameter for increasing the rounds per meter and is less yaw sensitive as compared to the HAWTs.
The shrouded wind turbine can improve the efficiency by 1.7 times compared to simply bared turbines. The terms shrouded, ducted, and turbine enclosed in a diffuser. The energy confined in a tidal current is directly related to density and area of cross-section. The cube velocity can be derived using the following equation.
The energy is converted into velocity by using the tidal current, and thus the power generated in the horizontal axis tidal current turbine is dependent on the current speed. The velocity to gain maximum efficiency should be around 2 m/sec. However, the global tidal velocity available ranges close 1 m/s. To obtain efficient output power from such low velocity, a bigger turbine system is needed (Karasudani, 2010). The installation of larger turbine system is not cost-effective, and thus the velocity can be increased by using diffuser augmented tidal turbine. The diffuser augmented tidal turbine is highly famous among the researchers due to the following aspects:

  • Better spatial coherence

  • Less sensitive to turbulence

  • Better resistant to fatigue

  • Less sensitive to jaw

  • Same power generation at lower torque

  • Lower fluctuating blade loads

  • Enhanced mass flow

  • Higher possible rotational speed, so reduced gear ratio

  • Less noise

  • Reduction of tip losses  (Nasir Mehmood, Z. L., & Khan, J., 2012).

The diffuser, in this case, acts as devices to increase the tidal speed and operates on a principal that a minimum increase in the speed can result in increased output power of the turbine. The diffuser augmented turbine produces same output power with a relatively small diameter compared to the naked turbines. Thus the installations of diffuser augmented turbines are more efficient economically compared to conventional turbines (Karasudani, 2010). Although the high cost of DAWTs counter balance its benefits due to high cost of material, transportation, fabrication and installation. Furthermore, the DAWTs are seen to be highly susceptible to the effects from environment like the temperature fluctuations, windborne particulates etc. Due to the flow separation, the diffusers are also seen to encounter aero elastic instabilities. 
There are several advantages to the installation of the diffuser augmented wind turbines. These can be summarized in the following points:

  • 1.    The diffuser augmented wind turbines can extract more energy compared to conventional wind turbines. These turbines are more efficient as compared to conventional turbines due to their increased flow manipulation and removal of tip losses (Toshimitsu, 2012).

  • 2.    The sound pollution caused by using this type of turbines is far less than the conventional turbines (Toshimitsu, 2012).

  • 3.    Weed growth a problem faced by the conventional turbines can be eliminated by the use diffuser augmented wind turbines (Toshimitsu, 2012).

  • 4.    The diffusers can be manufactured using low-cost materials. The current cost to benefit ratio of these turbines can result in developing more efficient diffusers in the future (Toshimitsu, 2012).

However, there are some limitations related to the use of diffuser augmented wind turbines which can be summarized in the following points:

  • 1.    To gain high efficiency from the turbines, small clearance should be present between shroud and blade tip. These require manufacturing and installations of very complex shapes which are very expensive and complicated (Toshimitsu, 2012).

  • 2.    The inner and outer profiles of the shrouds are tough to fabricate (Toshimitsu, 2012).

  • 3.    The diffuser augmented turbines operate at higher RPM which can cause vibration issues.

  • 4.    The diffuser augmented turbines have more drag as compared to the conventional turbines and thus require additional support structure (Toshimitsu, 2012).

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