Wind Energy System

Wind energy relies, indirectly, on the energy of the sun. A small proportion of the solar radiation received by the Earth is converted into kinetic energy, the main cause of which is the imbalance between the net outgoing radiation at high latitudes and the net incoming radiation at low latitudes (IPCC, 2011). The Earth’s rotation, geographic features and temperature gradients affect the location and nature of the resulting winds (Burton et al., 2011). The use of wind energy requires that the kinetic energy of moving air be converted to useful energy. As a result, the economics of using wind for electricity supply are highly sensitive to local wind conditions and the ability of wind turbines to reliably extract energy over a wide range of typical wind speeds.

According to IEA (2013) wind power deployment has more than doubled since 2008, approaching 300GW of cumulative installed capacities, led by China (75GW), the United States (60GW) and Germany (31GW). Wind power now provides 2.5% of global electricity demand – and up to 30% in Denmark, 20% in Portugal and 18% in Spain. Its roadmap targets 15% to 18% share of global electricity from wind power by 2050, a notable increase from the 12% aimed for in 2009. It has therefore set a new target of 2 300GW to 2 800GW of installed wind capacity will avoid emissions of up to 4.8 Gt of CO2 per year.

Wind is South Africa’s cheapest renewable energy source. Smit and Smit (2003) contend that South Africa has a more moderate wind resource along the coastline. The expected average wind speed is lower than northern Europe and United States. The South African wind resource is currently estimated between 500 to 1 000MW. According to them Eskom executed its first case study and derivable experiences on wind energy at Klipheuwel Wind Farm, the first wind farm in sub-Sahara Africa. Klipheuwel is about 50km north of Cape Town. The wind farm consists of a Danish Vestas V47 660kW, V66 1.75MW and one French Jeumont J48 750kW wind turbines with a combined capacity of 3.16 MW. It was formally opened on 21 February 2003. An early estimate for Klipheuwel capacity factor was 22% which compares favourably with rural Germany and California where average estimates are also around 22% to 23%.

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South Africa is blessed with excellent wind resources and her wind power has moved from the planning to the execution phase, and is becoming one of the most vibrant new wind markets globally. After taking a decade to install the first 10 MW of wind power, the industry in South Africa is currently developing between 3,000 MW and 5,000 MW of wind power, of which 636 MW is under construction and a further 562 MW approaching financial close. In addition, there is a long term energy blueprint giving wind a significant allocation, about 9,000 MW of new capacity in the period up to 2030 (GWEC, 2012). Figure 2.12 illustrates the trend of wind power installation in South Africa.

Figure 2.11: Total installed wind power capacity (GWEC, 2012)

Blaine (2014) has reported that Eskom’s Sere wind farm, near Vredendal on the West Coast (Western Cape Province), has erected three of its planned 46 turbines and is on track to deliver first power by the end of 2014. Once completed, the plant will add 100MW to the national grid and contribute to saving nearly 6Mt of GHG emissions over 20 years. The completed wind farm would have 46 Siemens 2,3VS-108 turbines, each generating 2.3MW and positioned over an area of 16km².

2.3.2.1 Brief History and Early Wind Energy System Applications

The idea of using wind, a natural source, is not new because people have used technology to transform the power of the wind into useful mechanical energy since antiquity. Along with the use of water power through water wheels, wind energy represents one of the world’s oldest forms of mechanised energy (Redlinger et al., 2002). According to them though solid historical evidence of wind power use does not extend much beyond the last thousand years, anecdotal evidence suggests that the harnessing of mechanised wind energy pre- dates the Christian era. Stiebler (2008) concurs that the history of windmills goes back more than 2000 years. Indeed, humans have been using wind energy in their daily work for some 4000 years (Molina and Alvarez, 2011). But Patel (1999) contend that the first use of wind power was to sail ships in the Nile some 5000 years ago. The use of wind power is said to have its origin in the Asian civilisations of China, Tibet, India, Afghanistan and Persia

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(Redlinger et al., 2002). In their view the first written evidence of the use of wind turbines is from Hero of Alexandria, who in the third or second century BC described a simple horizontal-axis wind turbine. It was described as powering an organ, but it has been debated as to whether it was of any practical use other than as a kind of toy. They also assert that more solid evidence indicates that the Persians were harnessing wind power using a vertical- axis machine in the seventh century AD and from Asia the use of wind power spread to Europe.

The Europeans used wind power to grind grains and pump water in the 1700s and 1800s while the first windmill to generate electricity in the rural USA was installed in 1890 (Patel, 1999). In 1700 BC, King Hammurabi of Babylon used wind powered scoops to irrigate Mesopotamia. Some other civilizations, like the Persians (500-900 AD), used the wind to grind grain into flour, while others used the wind to transport armies and goods across oceans and rivers. Sails revolutionized seafaring, which no longer had to be done with muscle power. More recently, mankind has used the power of the wind to pump water and produce electricity (Molina and Alvarez, 2011).

They have been used predominantly for grinding cereals and for pumping water. Important examples of more recent times are the Dutch Windmills which appeared in different variants and were erected in large numbers in the 17th and 18th century in Europe. Another memorable development of the 19th century was the Western Mill, found in rural areas especially in the USA up to the present day. Modern constructions of wind energy converters were developed in the 1920s, but it was not before the 1980s that they found professional interest as a prominent application of renewable energies (Stiebler, 2008). Today, large wind-power plants are competing with electric utilities in supplying economical clean power in many parts of the world (Patel, 1999).

2.3.2.2 Modern Wind Turbine

According to Molina and Alvarez (2011) the beginning of modern wind turbine development was in 1957, marked by the Danish engineer Johannes Juul and his pioneer work at a power utility (SEAS at Gedser coast in the Southern part of Denmark). His R&D effort formed the basis for the design of a modern AC wind turbine – the well-known Gedser machine which was successfully installed in 1959. With its 200kW capacity, the Gedser wind turbine was the largest of its kind in the world at that time and it was in operation for 11 years without

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maintenance. The robust Gedser wind turbine was a technological innovation as it became the hall mark of modern design of wind turbines with three wings, tip brakes, self-regulating and an asynchronous motor as generator. Foreign engineers named the Gedser wind turbine as ‘The Danish Concept’. The so-called “Danish concept” that was very popular in the eighties, refers to the transformation of wind energy into electrical energy using a simple squirrel-cage induction machine directly connected to a three-phase power grid (Molina and Mercado, 2011).

Wind turbines come in two broad categories: the horizontal-axis turbine whose blades appear similar to aeroplane propellers, and the vertical-axis turbine whose long curved blades are attached to the rotor tower at the top and bottom and have the appearance of an eggbeater (Redlinger et al., 2002). Vertical-axis turbines have not lived up to their early promise, and today virtually 100 per cent of existing turbines use the horizontal-axis concept.

Figure 2.13 shows the components in a modern wind turbine with a gearbox; in wind turbines without a gearbox, the rotor is mounted directly on the generator shaft. The rotor is the heart of a wind turbine and consists of multiple rotor blades attached to a hub (Molina and Alvarez, 2011). It is the turbine component responsible for collecting the energy present in the wind and transforming this energy into mechanical motion.

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Figure 2.12: Basic components of a modern, horizontal-axis wind turbine with a gearbox (IPCC, 2011)

Modern wind turbines, which are currently being deployed around the world, have three- bladed rotors with diameters of 70m to 80m mounted atop 60m to 80m towers (Lindenberg

et al., 2008), as illustrated in Figure 2.13. But according to Molina and Alvarez (2011),

currently most rotors have three blades, a horizontal axis, and a diameter of between 40 and 90 meters. In addition to the currently popular three-blade rotor, two-blade rotors are also used to be common in addition to rotors with many blades, such as the traditional wind mills with 20 to 30 metal blades that pump water. They have also noted that over time, it was found that three-blade rotor is the most efficient for power generation by large wind turbines. In addition, the use of three rotor blades allows for a better distribution of mass, which makes rotation smoother and also provides for a “calmer” appearance

The three blades are attached to a hub and main shaft, from which power is transferred (sometimes through a gearbox, depending on design) to a generator. The main shaft and main bearings, gearbox, generator and control system are contained within a housing called the nacelle.

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2.3.2.3 Basic Design and Operating Principles of Wind Turbines

Rotor blades being a crucial and basic part of a wind turbine means that the design of the individual blades also affects the overall design of the rotor. Generating electricity from the wind requires that the kinetic energy of moving air be converted to mechanical and then electrical energy, thus the engineering challenge for the wind energy industry is to design cost effective wind turbines and power plants to perform this conversion (IPCC, 2011). Molina and Alvarez (2011) posit that the rotor blades take the energy out of the wind; they “capture” the wind and convert its kinetic energy into the rotation of the hub. The profile is similar to that of airplane wings. Rotor blades utilise the same “lift” principle: below the wing, the stream of air produces overpressure; above the wing, the stream of air produces vacuum. These forces make the rotor rotate.

The amount of kinetic energy in the wind that is theoretically available for extraction increases with the cube of wind speed (IPCC, 2011; Lindenberg et al., 2008). This means that a 10% increase in wind speed creates a 33% increase in available energy. However, a turbine only captures a portion of that available energy as shown in Figure 2.14.

Modern large wind turbines typically employ rotors that start extracting energy from the wind at speeds of roughly 2.5 to 4m/s (cut-in speed). The Lanchester-Betz limit provides a theoretical upper limit (59.3%) on the amount of energy that can be extracted (Burton et al., 2011). A wind turbine increases power production with wind speed until it reaches its rated power level, often corresponding to a wind speed of 11 to 15m/s. According to Abad et al. (2011) wind turbines are designed to produce electrical energy as cheaply as possible and to yield maximum output at wind speeds around 15 meters per second. In their view it does not pay to design turbines that maximize their output at stronger winds, because such strong winds are rare. But in the case of stronger winds, it is necessary to waste part of the excess energy of the wind in order to avoid damaging the wind turbine. Therefore at still- higher wind speeds, control systems limit power output to prevent overloading the wind turbine, either through stall control, pitching the blades, or a combination of both (Abad et

al., 2011; Burton et al., 2011). Most turbines then stop producing energy at wind speeds of

approximately 20 to 25 m/s (cut-out speed) to limit loads on the rotor and prevent damage to the turbine’s structural components.

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Figure 2.13: Typical power output versus wind speed curve (IPCC, 2011; Lindenberg et al., 2008)

Wind turbine design has centred on maximizing energy capture over the range of wind speeds experienced by wind turbines, while seeking to minimize the cost of wind energy. According to IPCC (2011) as described generally in Burton et al. (2011), increased generator capacity leads to greater energy capture when the turbine is operating at rated power (Region III). Larger rotor diameters for a given generator capacity, meanwhile, as well as aerodynamic design improvements, yield greater energy capture at lower wind speeds (Region II), reducing the wind speed at which rated power is achieved. Variable speed operation allows energy extraction at peak efficiency over a wider range of wind speeds (Region II). Finally, because the average wind speed at a given location varies with the height above ground level, taller towers typically lead to increased energy capture.

According to Molina and Alvarez (2011) the maximum wind speed (or survival speed), above which wind turbines are destroyed, is in the range of 40 to 70m/s. Also the most common survival speed of commercial wind turbines is around 60 m/s. The foregoing shows that wind turbines have three modes of operation namely:

 Below rated wind speed operation

 Around rated wind speed operation (usually at nominal capacity)

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If the rated wind speed is exceeded the power has to be limited. Therefore, all wind turbines are designed with a power control that achieves this goal and avoids a run-away situation. Consequently, the power limitation may be done by some of the three following methods (Hansen, 2005; Molina and Mercado, 2011), namely:

 stall control (the blade position is fixed but stall of the wind appears along the blade at higher wind speed),

 active stall (the blade angle is adjusted in order to create stall along the blades) or

 pitch control (the blades are turned out of the wind at higher wind speed).

Figure 2.15 shows a generic qualitative power curve for a variable-speed pitch-controlled wind turbine, with four zones and two areas. The rated power Pr of the wind turbine (that is,

the actual power supplied to the grid at wind speed greater than Vr) separates the graph into

two main areas. According to Camacho et al. (2011) below rated power, the wind turbine produces only a fraction of its total design power, and therefore an optimization control strategy needs to be performed. Conversely, above rated power, a limitation control strategy is required to forestall the inherent damage.

Figure 2.14: Power curve of a wind turbine and control zones (Camacho et al., 2011) Therefore, Camacho et al. (2011) have highlighted the different wind turbine power limitation measures or controls as follows:

 For passive-stall-controlled wind turbines, in which the rotor blades are fixed to the hub at a specific angle, the generator reaction torque regulates rotor speed below rated operation to maximize energy capture. Above a specific wind speed, the

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geometry of the rotor induces stall. In this manner, the power delivered by the rotor is limited in high wind conditions courtesy of a particular design of the blades that provokes loss of efficiency.

 In pitch control, the power delivered by the rotor is regulated either by pitching the blades toward the wind to maximize energy capture or by pitching to feather to discard the excess power and ensure that the mechanical limitations are not exceeded. At rated operation, the aim is to maintain power and rotor speed at their rated value. To achieve this, the torque is held constant and the pitch is continually changed following the demands of a closed-loop rotor speed controller that optimizes energy capture and follows wind speed variations. In contrast, below rated operation there is no pitch control; the blade is set to a fine pitch position to yield higher power capture values while the generator torque itself regulates the rotor speed.

 Active stall control is a combination of stall and pitch control. It offers the same regulation possibilities as the pitch-regulated turbine but uses the stall properties of the blades. Above rated operation, the control system pitches the blades to induce stall instead of feathering. In this technique, the blades are rotated only by small amounts and less frequently than for pitch control.

Generally according to Lindenberg et al. (2008) the speed of the wind increases with the height above the ground, which is why engineers have found ways to increase the height and the size of wind turbines while minimizing the costs of materials. The increase in wind speed with elevation is referred to as wind shear. They authors have noted that there has been a long-term drive to develop larger turbines as a direct result of the desire to improve energy capture by accessing the stronger winds at higher elevations. Although the increase in turbine height is a major reason for the increase in capacity factor over time, there are economic and logistical constraints to this continued growth to larger sizes.

2.3.2.4 Classification of Wind Energy Systems

Wind energy systems could be classified based on several criteria such as position of rotational axis of the turbine, power output (capacity), speed, type of coupling between the mechanical and electrical parts, the nature of the rotor and stator, and even method of integration unto the grid.

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Based on axis position, wind turbines are classified as the horizontal axis and vertical axis turbines. Horizontal axis wind turbines (HAWTs) are more common than vertical axis wind turbines (VAWTs). The horizontal axis turbines have a horizontally positioned shaft, which helps ease the conversion of the wind’s linear energy into a rotational one.

In terms of capacity modern wind turbine technology can be classified into three main categories: large grid-connected turbines, intermediate-sized turbines in hybrid systems, and small stand-alone systems (Khaligh and Onar, 2010; Redlinger et al., 2002). According to them large grid-connected wind turbines, in the size range of 150 kW and above, account for by far the biggest market value among wind turbines. The size of commercially available grid-connected wind turbines has evolved from 20 – 50kW range in the early 1980s to the 500 – 800kW range most common in the late 1990s. Turbines in the 1 – 2MW size range have been commercially available since 1997. Also intermediate-sized or medium wind turbines in the 1 – 150kW range can operate in hybrid energy systems combined with other energy sources such as diesel, small-scale hydro, photovoltaics, and/or storage systems. According to Khaligh and Onar (2010) medium wind turbines usually provide between 20 and 300kW installed power having a blade diameter of 7–20 m, and the tower is not higher than 40 m. They are usually used to supply either remote loads that need more electrical power or commercial buildings and are directly connected to the load through DC/AC power electronic inverters. Furthermore, small ‘stand-alone’ wind turbines of less than 1kW for water pumping, battery charging, heating and so on represent the third turbine category. The most commercially successful in this category are very small wind turbines in the 25–150 watt range with rotor diameters of 0.5 to 1.5 metres. They are designed for low cut-in wind speeds of generally 3–4 m/s (Khaligh and Onar, 2010). Such small wind turbines are widely used for battery charging at remote telecommunication stations. Yachts also often carry a very small (less than 1 kW) wind turbine for battery charging which can be used for television sets, communication systems and small refrigerators. However, Lindenberg et al.

(2008) posit that until recently, three-bladed upwind designs using tail vanes for passive yaw