Mech 470 Group Report. Wind Energy

Transcription

1 Mech 470 Group Report Wind Energy Members: Chia Wei Yeh, Leo (XiaoHang) Fang, Date Submitted: 2016/2/23

2 Table of Contents Introduction... 3 Background Information... 5 Sustainability of Wind Energy... 6 Design Challenges of Wind Energy... 7 Methods of analysis... 9 Wind Characteristics... 9 Variation of Wind... 9 Wind density Betz limit Coefficient of Power Swept Area Limitations Future Plans References:... 13

3 Abstract Of all the renewable energy sources (except direct solar heat and light), wind energy is commonly recognized to be a clean and environmentally friendly renewable energy source that can reduce our dependency on fossil fuels, has developed rapidly in recent years. Its mature technology and comparatively low cost make it promising as an important primary energy source in the future. This paper compiles latest literatures about the development of wind technology. It will provide an overview of the technology itself, existing challenges of switching power supply from conventional fossil fuels to wind power, and some evaluation of the wind power sustainability. This paper also includes a modeling tool that provides a quick design model for wind farms. Moreover, it explains the potential of wind power as an ultimate alternative for fossil fuels. Introduction Nowadays, the need of energy has been increasing day by day with the population growth and the advancements of technology. In 2005 the worldwide electricity generation was T W h out of which 40% originated from coal, 20% from gas, 16% from nuclear, 16% hydro, 7% from oil and only 2% from renewable sources such as geothermal, solar, wind, combustible renewables and waste [5]. The current fuel mix has fossil and nuclear fuels contributing to nearly 70% of total power generation. Although great efforts have been putting into renewable energy source for the past decade, only about 18% of the world's energy demand is supplied from renewable energy source [1]. Combustible fossil fuels such as, coal is known to have the highest carbon dioxide emissions per kwh, as well as emitting other pollutants at high levels. Still, it continues to dominate the market due to its low cost and high availability, while at the same time challenging the principles of sustainability. If significant efforts are not made to reduce the amount of emissions produced, the number of coal fired power stations will continue to rise and in developing countries alone will produce more CO2 than the entire OECD power sector for the year 2030 [5]. Hence, the quest to develop renewable and clean energy sources, such as solar, wind and solar-hydrogen energy, is imperative and timely. Among these many renewable resources, wind power is the only one that offers a mature technique, as well as promising commercial prospects, and is now generally applied in large-scale electricity generation. In this article a comprehensive overview of the wind power technology as well as its sustainability will be presented. In addition, the current status, technologies and challenges are discussed in details. Last but not least a simple analysis tool used to provide a fundamental model for basic wind power system design will be developed and examined.

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5 Background Information It is safe to assume that wind energy is the earliest form of renewable energy used by humans. Wind power has a history of more than 3000 years starting from the first windmills built by Egyptians to pump water, and people began to use it to generate electrical power about 120 years ago. In 1941, a prototype of the modern horizontal axis wind turbine was built in US; at that time wind turbines were widely used to provide electricity to farms to which electric power lines could not reach. Although there has been some fluctuation of wind energy development due to the discovery of fossil fuels, global warming and environmental concerns has turned the focus back to the development of wind energy. Over the last decade, the world's wind power generation capacity has been growing rapidly, with an annual growth of about 30%. Wind energy is a form of energy caused by air movements. The energy transfers from the sun that unevenly heats up the earth, the warmer areas will rise due to lower air densities. The movements of air is resulted due to the local pressure gradient between the warm and cool air [17]. The natural of airflow is created which can be used to create work using mechanical systems. Wind carries a lot of kinetic energy. There are various ways of converting the kinetic energy in the wind to mechanical energy such as sails, windmills and more commonly wind turbines are widely used in today s life. Wind turbines are used to capture the kinetic energy in the wind to electrical energy. Among all types of wind turbines, conventional horizontal axis wind turbines are the most commonly used. There are three major components in these turbines shown in figure 1. The rotor component, which is approximately 20% of the wind turbine cost, includes the blades for converting wind energy to low speed rotational energy. When the wind passes through the turbine blades, the blade profile creates pressure gradient between upper and lower blade surfaces, which in turns creates lift force to spin the turbine [14]. The rotary motion is then converted into electrical energy. This process happens at the generator component, which includes the electrical generator, the control electronics, most likely a gearbox, and adjust-speed drive or continuously variable transmission component for converting the low speed incoming rotation to high-speed rotation suitable for generating electricity. The last component for the system will be the structural support component, which includes the tower and rotor yaw mechanism.

6 Figure 1: Components of Wind Turbine [22] In order to capture more wind power and to bring down the cost of renewable energy generation, engineers aim at more powerful and larger-scale wind turbines. Therefore, the sizes of modern wind turbines, including both blade length and generation capacity are becoming larger and larger. Typically, the rotor diameter in modern wind turbines ranges from m, and is rated between 500kW and 2 MW. As the size of the turbine increases, scientists also brought up rising environmental concerns of wind turbines. Therefore it is essential to study and analyze the sustainability of wind power technologies. Sustainability of Wind Energy Unlike other sources of energy (i.e. coal, gas and petroleum based fuel), wind energy generally has zero direct air pollution. Wind energy is popularly perceived as one of the cleanest sources of energy. Emissions especially carbon dioxide, nitrogen oxide and sulfur dioxide can be significantly reduced if wind energy can replace the current sources of conventional energy. Carbon dioxide is the major cause of global warming and the latter two pollutants are responsible acid rain and urban smog. According to the World Energy Commission, used of one million KWh of wind power can save 600 tones of CO2. A small amount of CO2 emissions is released by the wind energy during its construction and maintenance phases. However this amount of CO2 is much less compared to other fossil fuel based power plants and the amount of CO2 can actually be absorbed by forests through the process of photosynthesis.

7 According to American Wind Energy Association a typical wind turbine installed in the Perhentian Island currently with a rated capacity of 100 KW is estimated to reduce 168 tones of CO2 annually. Based on ecological footprint, that is the total amount of CO2 absorbed by 24 acres of forest per year. As mentioned before wind is generated by atmospheric pressure differences, driven by solar power, which produces zero NOx and SOx while operation. Due to the nature of Solar Power, wind energy is considered to be the most promising renewable energy. In order to consider the impacts of electricity generation to the environment and economy several authors completed the full life cycle analysis of wind energy generation technologies, which further confirmed the sustainability of wind energy. Design Challenges of Wind Energy Technology advances in wind energy have been dramatic, reducing costs from 30 cents per kwh in 1980 to the 3 5 cent range today. Power rating of the largest turbines has increased from 55 kw in 1980 to 4.5 MW in While producing electricity with wind size is a major factor. Tall towers, some over 400 ft high, can raise turbines to take advantage of stronger and less turbulent winds at high elevations. New materials such as E-glass/polyester allow blade lengths of 150 ft compared to 15 ft in This is significant because a wind turbine s capability to generate electricity increases by the square of the blade length. Therefore, seeking out new materials continue to be a big challenge for wind turbine designers. With increasing size of wind turbine as well as the emerging idea of offshore wind farms, the stability of the foundation as well as the blade itself also becomes a challenge. Locating wind turbines in the right place is important because the energy produced by a wind turbine is proportional to the cube of the wind speed. For example, the potential of a wind turbine located at a site with an average wind speed of 15 mph versus one located at a site where the average wind speed is 10 mph can produce 238% more electricity. The percentage of time a turbine can be in use during the 8760 h of the year (i.e., the turbine utilization rate) has increased from 36 38% to 40 43% at the best sites [1]. Modern turbines can operate at lower wind speeds more efficiently than older models. Furthermore, they can operate at wind speeds up to 50 mph, wind speeds that would have caused older models to be shut down or to fail. In addition to the wind speed requirements, rising concerns of both health and environmental impacts of wind turbine such as noise and visual impacts has made choosing the proper location of wind farm even more challenging. Another major design concern for wind farm is the wake effect loss between wind turbines. A wind turbine wake is a long trail of turbulent wind exiting the turbine with diminished wind speed. For wind turbines, wake effect relates to the wind speed deficit and diminished energy content wind possesses after leaving a particular utility-scale wind turbine. As wind flows through a turbine, the volume of air downwind of the turbine has a lower wind speed and higher turbulence than wind in the free stream.

8 The free stream is the air far upstream from a wind turbine that is traveling at its natural velocity and that has not yet been slowed down, deflected, or otherwise impacted by a wind turbine or other obstruction. Consequently, wind exiting a turbine contains less kinetic energy than does wind before passing through a turbine. This diminished, turbulent wind from an upwind turbine reduces the energy entering downwind turbines, thereby decreasing the downwind turbines overall energy output. Therefore while design the wind turbine farm it is essential to consider the wake effect loss. Computational fluid dynamics as one of the tools to evaluate the wake effect has shown promising results. Recently researchers have used large eddy simulation and a Lagrangian scale-dependent dynamic subgrid model to investigate the flow characteristics of the wind turbine wake [20]. However, modeling of wind farms using computational fluid dynamics (CFD) resolving the flow field around each wind turbine s blades on a moving computational grid is still too costly and time consuming in terms of computational capacity and effort. Moreover, the variability of wind energy produced electricity compared to other fossil fuels and nuclear power also creates great challenges for engineers. The current electrical grid in North America also limits the implementation of large-scale wind farms. With all these challenges in mind we developed a simple model for wind turbine design.

9 Methods of analysis With other energy generating alternatives, wind energy has begun to grow since sustainability has become the major concern for future development. The major take away for this technology is that it does not require fuel inputs; therefore, the operation of wind turbine does not create pollutions. It also reduces the reliance on other non-environmental friendly energy plants which directly reduces the overall pollutions. It is expected that six 65kW wind turbines can reduce CO2 emissions by 750 tonnes [15]. It is crucial to understand the basics of the wind technologies as it provides the fundamentals to design analysis. Wind turbine design analysis can be very involved, for example, the air foil of turbine blades requires detailed computational fluid dynamic simulations in order to optimize the performance. The method provided in this section allows one to provide a rudimentary model for basic design and introduce a more in depth approach methods that one can be applied for detailed analysis. A simple approximation of wind turbine power output depends on Betz limits, density of air, rotor sweep area, and wind speed. This simple analysis tool can be used as a preliminary study to see if it makes economic sense to install a wind turbine at given sites. Wind Characteristics The important aspect of wind turbine technology focuses on how much energy can be extracted from the wind. For example, wind quality can be affected by air density and the surrounding obstacles that cause disturbance to the wind flow. A lot of wind farms are built offshore because of the uniformity of wind and the tendency of wind blowing harder [8]. The source of wind is one of the key factor to wind turbine performance. For small turbines, it requires an average annual wind speed of at least 15 km/h to be cost effective [17]. The following provides some basic factors that influence the wind quality. Variation of Wind There are several ways to access the annual wind speed to help evaluate the potential sites for wind turbines. It can be calculated by measuring wind speed using anemometer over a certain amount of time or using a wind resource potential map [3]. However, measurements may not be accurate because external environment such as trees might affect the results. Moreover, the height of measuring site is different than the actual turbine height will also create some deviations to the data collected. The wind velocity varies with the height above ground [10]. The following formula can be used to correct the deviation from the height of measuring site to actual turbine and the external environments. V(Zr) = V(Z) ln(zr Z0 ) ln( Z Z0 ) [10]

10 Where V(Zr) = wind velocity at the turbine height Example: V(z) = 7 m/s at 10m Z0 = 0.1 Zr = 100 m V(Z) = Wind velocity at the measuring sites Z0 = correction factor for external environments ranging from [10] V(Zr) = 10.5 m/s at 100 m The above results shows that the height is an important factor to the wind turbine performance. The higher the tower, the speedier the wind which results in higher energy output. Figure 2: Wind speed vs. distance from the ground [10] Wind density The potential of wind energy is related to the air density because the higher the air density the higher the momentum energy can be transferred to the wind turbine. Moreover, the air density is related to temperature and elevation. In case of design, the elevation effects should be taken into consideration at higher elevation because air density varies significantly at higher altitude. Temperature should be taken into consideration if the wind turbine sites environment are too extreme [7]. If wind carries high density air, it allows more kinetic energy to be transmitted through turbine due to the conservation of momentum. It is shown that 16 percent more energy can be extracted at minus 20 degree than plus 20 degree [17].

11 Betz limit When wind passes through the wind turbine, some of the wind energy loses due to transforming kinetic energy to mechanical energy. The wind flow will stop if the wind energy is completely extracted. Therefore, it is impossible to extract all the wind energy to mechanical energy [12]. The Betz limit is a theoretical limit for wind turbine as it cannot extract all the wind energy. The maximum wind energy that can be extract from wind turbine is about 59 % [17]. It represents the maximum efficiency in theory a turbine can possibly reach. Betz limit is derived by using a simple disc as a turbine with airstream flows from one side to another. The derivation neglects the radial flow at the disc [19]. Conservation of momentum along with Bernoulli s equation are applied to the system. Equating the equations yields the efficiency. η = 1 ( 1 Ud Ud ) (1 + 2 Uu Uu )2 [19] Where Ud = velocity down stream Uu = velocity up stream By taking derivative of the efficiency equation, the maximum efficiency occurs at 59% [19] Coefficient of Power By definition the coefficient of power of a wind turbine is the ratio between the power outputs over total energy available in the wind. In another words, it represents the real turbine efficiency at specific wind speed [12]. Therefore, the real turbine efficiency will be lower than the Betz limit shown in previous section. Coefficient of power is a dimensionless number ranging from 0.25 to 0.45 depending on turbine s performance [7]. If we set the turbine to operate at its maximum performance which yields the following equation. Cp = power output = Pmax [9] Avaliable input AV 3 As indicated by the above equation, the performance of wind turbine heavily rely on the swept area of wind turbine and the wind speeds. More specifically, the coefficient of power is a function of blade geometry and tip speed ratio [18]. In terms of design, it is important to determine the relationships between the tunable parameters in order to reach a higher efficiency. As indicated in the figure below, there is a maximum value for the tip speed ratio to operate at the turbine s optimum. The maximum turbine efficiency occurs at tip speed ratio of 7.5.

12 Figure 3. Coefficient of power vs Tip Speed Ratio [12] Tip speed ratio is given as rotor tip speed / wind speed TSR = R ω V [9] Where R = radius of rotor, m ω = rotational speed of rotor, rad/s V = wind speed, m/s Swept Area The swept area of wind turbine is basically the rotor swept area which can be seen as a circle when the blades travel. More power will be generated through wind turbine if we increase the swept area. It is shown that by doubling the rotor blades radius, the power output of wind turbine will be quadruples [17]. One Dimensional Momentum theory The wind power can be found using momentum theory which is given as: wind power = 1 2 ρav3 [13] Where ρ = air density A = Swept Area

13 V = wind speed However, as we know previously there is a maximum limit the wind turbine can extract from the wind which is known as Betz limit. For industrial practices, a coefficient of power is introduce to represent the real efficiency of turbine. Therefore, the mechanical power is given as: mechanical power = 1 2 cp ρav3 [13] Where cp = coefficient of power The mechanical power equation presented above can be used to perform a preliminary analysis of wind turbine performance. However, if we want to obtain a more accurate results, a more advanced simulation tools are required. Limitations The method presented above is a rough estimation of power production of wind turbine. The equation presented above may yield a significant different result than the actual power. The approach shown above, does not consider the rotor blade geometry, which is extremely important when, comes to design. A more detail investigation is needed to capture the flow interaction between the rotor blade and the wind. For example, computational fluid dynamic software is required to get more insight to the real system such as wake behind the rotor and the drag losses. Moreover, the wind speed is another significant factor to the power prediction. As we know, the wind is fluctuating over period. Therefore, there will be high uncertainties in the wind power production. It may be more appropriate to incorporate probability density function to closely predict the wind velocity distribution to predict the power output. Moreover, there are also some internal losses occurring in the components of wind turbines, which should be included in the more detailed calculations. For example, the losses in the gearbox and the generator.

14 Future Plans At present, wind energy is a mature renewable energy source that has high potential to become a major primary source of energy in the future. Over the last decade, wind energy has developed by leaps and bounds. During this time, the global wind energy capacity has had an average annual growth of 29% [6] while most wind farms are onshore, offshore wind power is a comparatively new sector of wind energy that has attracted people's attention due to its many advantages over onshore wind. For example, the wind blows harder and stronger offshore, which provides greater productivity when larger turbines are installed. There are more productive areas available offshore, which makes large-scale wind farms possible. Moreover, the offshore wind farms will be far away from the city and human life, therefore environmental and health issues such as visual impact and noise can be ignored. Although offshore technology has many advantages, there are still many challenges to its successful implementation. First of all, the cost of constructing a wind farm offshore will be times greater than that onshore due to towers, foundations, underwater cabling and installation offshore itself is more difficult and expensive. [2]. The other challenge to offshore technology is how to settle the wind tower in water that lies deeper and further from the land, as wind speeds tend to increase with the distance from the shore and it is possible to harness more energy from the wind. Currently offshore wind power plants are restricted to waters shallower than 30m, and the water depth is, to a large extent, affected by the supported foundation of the wind tower such as ballast stabilized foundations, buoyancy stabilized foundations and mooring line stabilized foundations. [6]. Recent years have seen great development around the world for offshore wind farms. However as it is still a relatively new technology, additional tests are required to be conducted in the tough marine environment. With the development of wind power both onshore and offshore, the CFD method has been more and more frequently used in variety of wind project studies and has been used to predict environmental impact of wind turbines. With a good understanding of its environmental impact, wind energy can be a clean sustainable source of energy that can successfully replace fossil fuels.

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