Wave and Wind Power Generation
Wind Physics Basics
Wind is atmospheric air which is in motion
Energy Source
The earth surface absorbs solar radiation differently. Some parts absorb more radiation than other. This creates pressure zones. As a result of pressure, air moves from high-pressure zones to low-pressure zones. This causes atmospheric air motion known as wind. Wind is a renewable source of energy. It is constantly available free of charge (Bhattacharyya, 2011).
Spatial scales
Atmospheric air motion can be categorized depending in terms of spatial scale. Four spatial scales identified include planetary scale, synoptic scale, mesoscale, and microscale. There are various types of spatial scales of wind. Planetary scale occurs as a result of global air circulation. Synoptic scale occurs due to atmospheric air motion resulting from weather systems. Mesoscale is air motion due to air circulations which are thermally induced or local topographical air circulations. In micro-scale, this is air circulation in a small area such as urban topographic.
Wind types
Several wind types have been identified. This include planetary circulations (jet streams, trade winds, and polar jets), geostrophic winds, thermal winds, gradient winds, topographic winds (anabatic/katabatic winds), downslope windstorms (Foehn/Bora/Chinook), land breeze/sea breeze, downdrafts/convective storms, typhoons/hurricanes, tornadoes, dust devils/gusts/microbursts, nocturnal jets, and atmospheric waves. Each of these wind types can be harnessed for power generation. However, while some can be effectively used to generate power, others it is quite difficult to extract power from some of them.
Major Parts of a Wind Turbine
The major parts of a wind turbine (See Fig.1 and Fig.2) are rotor blades, anemometer, shaft, nacelle, gearbox, generator, electronic control unit, yaw drive, controller, brakes, pitch, rotor, wind vane, yaw motor, tower, and electrical equipment (US Department of Energy, 2016).
Wind Power Fundamentals
The quantity of power that can be derived from atmospheric wind can be estimated using the fundamental equation of wind power. This amount depends on three factors: the amount of air (volume), the speed of air (velocity), and the mass of air (density) flowing through an area of interest (flux).
Atmospheric wind possesses kinetic energy given by:
KE=12 mv2 (1)
Where:
m = mass of air
v = speed of air.
Mass flux or mass of air flowing in a given time is given by:
m=dmdt (2)
Therefore, power in KE per unit time is given by:
P=12 mv2
The power (P) that can be extracted from the wind is a function of the surface area of the blades intercepting the wind (A), the air density (⍴), and the cube of the instantaneous wind speed or velocity (v3) (Garsch and Twele, 2011; Gipe, 2009).
In fluid mechanics, the mass flow rate is a product of density and volume flux and given by:
dmdt= ⍴Av (3)
Thus:
P=12 ⍴Av3 (4)
Cube of velocity
Air density
Swept area (A) of the rotor (where A= πr2)
Efficiency of Wind Power Extraction
Not all wind flux flowing perpendicular to the swept areas of the rotor is converted to power. It has been mathematically proved that it is only a fraction of the energy which is extracted.
Betz Limit & Power Coefficient
The ration of the power extracted by a wind turbine to the total power possessed by a wind resource is known as Power Coefficient, Cp.
Cp=PTPW (5)
Where:
PT = power extracted by the turbine
PW = total power contained in the wind resource
Therefore, the output power of a wind turbine is given by (Garsch and Twele, 2011):
P=12 .CP.⍴.A.v3 (6)
The maximum possible value of Betz limit, CP has been found to be 16/27 or 0.59. The maximum power extracted by a conventional wind turbine from the wind is therefore 59%.
Wind Turbine Power Curve
A wind turbine does not extract a constant amount of power throughout the year. This is partly due to characteristics of the wind turbine and partly due to site characteristics. The part of the year the wind turbine generator is operating at the maximum rated power is known as a capacity factor.
Capacity factor, CP=Average outputPeak output ≈30% (7)
It has been found out that a CP of 0.3 is good for a site.
Major Parts of a Wind Turbine
The major parts of a wind turbine are:
Foundations
Towers
Blades
Generators
Towers
Towers are classified into two major categories: self-supporting towers and guyed towers. Self-supporting towers are usually used for large turbines, and they are made from tapered tubular steel. For small-scale wind turbines, three types of self-supporting towers are used: wood poles, lattice towers, and steel poles. Guyed towers are further categorized into two: guyed lattice and guyed pole towers. For a given height, self-supporting towers are more costly than guyed towers. That is why guyed towers are preferably for small wind turbines (Tong, 2010).
Foundations
A strong foundation is one of the most important aspects of wind turbine installation. Foundation for offshore and onshore can be different. For offshore applications, the most commonly used foundations are gravity-based, monopile, triple, tripod, floating, and jacked. For shallow depths, monopole and gravity foundations are used. However, for greater depths located far from the coastline, jacket, triple, and tripod foundations are usually employed. For on-land wind turbine installations, the most preferred wind turbine foundation configuration include multiple, slab, and monopile (Wagner and Mathur, 2013; Wu, Lang, Zargari, and Kouro, 2011).
Generators
One of the most important components in a wind power generation is the generator. A generator is a device used to convert mechanical energy of the wind to the electrical energy. There exist many types of generators used in wind power systems. Unlike generators used in conventional energy conversion systems, wind turbine generators have to work under fluctuating load conditions. In small wind turbines, DC generators of a few Watts to a kiloWatts are used. However, large-scale wind turbines are equipped with two or three-phase AC generators (Sathyajith, 2006). Fixed-speed induction generators are widely used in wind power systems. There are two types of induction generators: wound-rotor generators and squirrel cage generators. Fixed-speed induction generators are popular in wind power energy systems because are not only simple but also durable in construction. They are also economical in costs and easy to connect to the main grid (Rivkin, Randall, and Silk, 2014).
Wind Turbines
The modern wind turbine technology is dominated by two primary types of wind turbine designs: the horizontal-axis wind turbines (HAWT) and the vertical-axis wind turbine (VAWT). However, VAWTs designs are rare. The one commercially in operation is the Darrieus turbine which resembles an egg beater.
Vertical-Axis Wind Turbine (VAWT)
In a vertical-axis wind turbine, the shaft is positioned in a vertical axis, perpendicular to the ground. Unlike, HAWTs, VAWTs are always aligned with the wind (See Fig.3). Consequently, VAWTs do not need any adjustment whenever there is a change in the wind direction. However, the disadvantage of VAWTs is that they do not move by themselves. Their electrical system provides them with a boost needed to start them. Since they do not use a tower, guy wires are usually used making their rotor heights lower. This implies they operate in heights with lower winds due to turbulence arising from ground interference. VAWTs are therefore less efficient when compared with VAWTs. One advantage is that because most of the major equipment are situated near the ground level and allow for easier installation and servicing. But the turbine occupies a larger base (footprint) and therefore not desirable in farming areas. VAWTs are mainly applied in small-scale power requirements such as pumping of water in remote locations.
Horizontal-Axis Wind Turbine
Horizontal-Axis Wind Turbines (HAWTs) are different from VAWTs because their shafts are installed in parallel to the ground (See Fig.3). Unlike the VAWTs, which are constantly aligned with the direction of the wind, HAWTs have to use yaw-adjustment mechanism to constantly align the blades to the wind. A tower is used to raise the position of the turbine high up the ground, and therefore HAWTs take up very little ground space and ideal for farming areas or locations where there are other activities taking place at the ground level.
Advantages and disadvantages of Horizontal wind turbines
Horizontal wind turbines are preferred over other types of turbines because they exhibit higher efficiency. The wind turbines are also capable of turning the blades without external input. They are also in high demand because of their low ratio of cost-to-power. However, the generator and gearbox have to be mounted in a tower, and such an arrangement restricts servicing. The need to design for a tail drive or yaw makes their designs more complex (Sumathi, Kumar, and Surekha, 2015).
Siting of wind turbines
The most difficult task is to find a place to position the tower and the wind turbine. Of utmost importance is to site the tower in a place where there is plenty of wind and where it is acceptable to the community. There are two primary rules which must be followed when siting a wind turbine. The first rule is that the wind turbine should be exposed to the wind as much as possible. It should be situated in an area free of obstructions such as vegetation, buildings, and other natural features. The second rule is to use a reasonably tall tower. Towers which are too short lead to wind turbines being positioned at heights dominated by wind turbulence. Wind turbines can be mounted on rooftops. But still, they should be installed high enough to avoid areas of turbulence. Siting in rooftops should also consider the vibrations and visual effects they generate. Siting should also consider the impact of noise in the surrounding. Some wind turbines are noisier, or certain conditions might escalate noise generation. Further, siting should also consider the impact it has on nature such as the likelihood of harming birds and bats. There are other considerations such zoning and approval requirements by various stakeholders (Gipe, 2009).
Siting wind turbines require similar and additional considerations to siting for onshore installations. Best favorable wind conditions are the first step to site identification. Site availability is another factor. It is also important to consider issues of visibility and distance from the shore. Because power may need to be transmitted to users in other locations, ideal sites need to be closer to power demand sites. Proximity to power distribution centers is also another factor. Further, they should be installed where impacts to existing shipping routes are minimal. Other factors include the need to avoid interfering with dredged channels, telecom installations, local airports, under-sea gas and cable lines, and flight path of birds (Malhotra, 2011).
Environmental Impact Assessment
Environmental Impact Assessment (EIA) for wind turbines is carried out to ensure that once installed the turbine(s) will not cause adverse impacts on the community and other ecosystem. Some of the major environmental concerns about wind turbines include electromagnetic interference and consequences to air safeguarding, visual and landscape impact, wider global environmental impacts, archeological impact, noise impact, hydrological impacts (groundwater protection), harm to birds and bats, construction and infrastructure impact, danger to airborne vehicles such as aircraft, ornithological and ecological impact, public access/recreation/safety, and shadow flicker (Stevenson, n.d.; Tester, 2005). To assess the impacts, potential construction activities or structures must be identified first. This is followed by the establishment of a baseline for the currently existing environment. Then a predicted magnitude of impact on the baseline resource is done. If there is a prediction of likely impacts, mitigation measures are put in place. Finally, an analysis of the significance of the impact is performed. However, a significant impact does not always indicate that the proposed project is unacceptable.
Regulations in Wind Energy Systems
There are guidelines and regulations which ensure that wind power systems development adheres to the best practice in the industry. Attributes of best practice include safety, socially sustainable, economically sustainable, environmentally sustainable, and reliable. The guidelines cover all the stages followed in the development of wind energy systems. It begins with guidelines pertaining identification of stakeholders as well as approving of pathways. The guidelines and regulations also cover aspects of site planning and site operations. For site planning, critical issues include site selection, project feasibility, detailed assessment, planning and approvals of environmental issues, and decisions regarding development application. Site operation issues include construction, wind farm operations, and decommissioning (Clean Energy Council, 2006).
Wave Energy Systems
Wave energy technology is a new concept, and some projects are nearing commercialization. The wave energy systems extract energy from oceanic waves and convert it into electrical energy. It has been found that the power in a wave is directly proportional to the period of its motion and the square of its amplitude (Peppas, 2008; Twidell and Weir, 2006). There are three main operating principles of the wave energy converting systems: the oscillating body converters, overtopping converters, and oscillating water columns. In oscillating body converters, the forward and backwards, the up and down, or the side to side motion of the waves is utilized to generate electricity. Overtopping converters use stored energy in reservoirs to drive energy while in oscillating columns, the changing pressure in trapped air columns is used to drive the turbines (IRENA, 2014).
Wave energy systems either extract energy directly from pressure fluctuations under the surface or waves on the surface of ocean water. However, wave energy systems have limited applications in the world since areas rich in wave energy can only be found in northern Canada, Australia, coasts of Scotland, and southern Africa (BOEM, 2016; US DOE, 2013).
Principle of Operations
Offshore Systems
A wave energy conversion device converts wave energy into electrical energy. The system can be either onshore or offshore. Offshore systems are installed in deep water up to 40 meters below the surface. Extracting energy from deep water requires the use of sophisticated energy extracting mechanisms. For example, a Slater Duck system powers a pump that generates electricity using the bobbing motion of the waves. Other offshore systems use hoses linked to floats that ride on the waves. As the float rises and falls, the hose is stretched and relaxed and thereby pressurizes the water which ultimately used to turn the turbine. Certain seagoing vessels specially built for such purposes have been used to extract energy from offshore waves. The floating platforms generate energy by funneling via internal turbines and then direct back into the sea (US DOE, 2013).
Onshore Systems
Onshore systems are constructed along on shorelines, and they operate by extracting energy from ocean breaking waves. Some of the technologies employed include oscillating water columns, tap chains, and pendular devices. Oscillating water column comprises partially submerged steel or concrete structure with an opening to the sea under the water line. As waves enter and leave the structure, it compresses and depressurizes the air column. Tapchans are tapered channels which feed reservoir which is constructed on the cliffs occurring above the sea level. As they move towards the reservoir, the wave height increases and eventually falls into the reservoir where the stored water is used to run a turbine. A pendulor device consists of a rectangular-shaped box-like structure with a hinged opening facing the sea. The entry and exit of waves cause the hinged flap to swing back and forth. The swinging motion is used to power a hydraulic pump linked to a generator (US DOE, 2013).
Environmental and Economic Issues of Wave Energy systems
Careful site selection needs to be performed to minimize environmental impacts. This includes the need to maintain the scenic beauty of the shoreline and avoid situations where the systems can alter the flow patterns of sediments in the floor of the oceans. Although the initial cost of constructing the wave energy systems, their operation, and maintenance costs are low since water is free (US DOE, 2013). Wave energy has been found to exert little environmental impacts. The major environmental impact is a reduction in wave climate. However, so much is unknown about the environmental impacts of wave energy systems such the long-term impacts of construction, operation, noise, and electromagnetic fields produced by power transmission cables (IRENA, 2014).
Other Ocean-based Energy Systems
Besides wave energy, two other ocean-based energy systems are tidal systems and thermal systems
Tidal systems
Tidal systems utilize rise and fall of tides to generate electricity. A dam or barrage is constructed so that during high tide rise, oceanic water is trapped in an estuarine basin but during the ebb, the receding ocean water is used to generate electricity (Twidell and Weir, 2006) as shown in Fig.4.
Ocean Thermal Energy
Ocean Thermal Energy Conversion devices utilize oceanic temperature differences to produce electricity. A working fluid is made to circulate in a closed circle. The fluid absorbs heat energy from warm water via a heat exchanger. The expansion of the fluid turns a turbine which then drives a generator. The cold water is used to cool the working fluid. Then the circle repeats (Twidell and Weir, 2006). 25 degrees Celsius or more degrees is the preferred temperature difference. Temperature difference creates currents under the water. The currents are used to drive a turbine. The generation of power can be represented as shown in Fig.5
Combine Wind and Wave Energy Technology
Wind blowing over the water surface creates waves. Both the waves and the wind can be harnessed to generate power. However, energy in waves has been found to be varying more smoothly than in the wind (Sorensen, 2011). An interesting future technology could be a combination of wave and wind energy technologies in one platform (See Fig.6) to create a hybrid platform. This technology is preferable where there are abundant wind energy and wave energy.
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