RET II Wind Energy - RET II Sommersemester 2020 PDF

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Wind Energy – IntroductionThe incoming wind (fuel) carries kinetic energy, which is then transformed into mechanical energy thanks to the rotor’s rotation. An electrical generator transforms the mechanical energy into electricity. Extracting the kinetic energy from the flow, reduces the wind speed b...

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Wind Energy – Introduction

The incoming wind (fuel) carries kinetic energy, which is then transformed into mechanical energy thanks to the rotor’s rotation. An electrical generator transforms the mechanical energy into electricity. Extracting the kinetic energy from the flow, reduces the wind speed behind the turbine, which is a natural consequence because behind the turbine, the wind flow carries a reduced kinetic energy. Consequently, the wind turbine after the first one produces less electricity. The region behind the first wind turbine is called wake and it’s characterized by lower wind speed, but also higher fluctuations of the wind speed itself (=turbine fluctuations). The wind turbine after another one not only produces less electricity but is also subjected to a higher fatigue loading 1. The wake expansion becomes larger as is moves downstream because the speed is slowed down, and the cross-section increases. There is also a change in pressure as the wind moves towards the rotor. The pressure grows and increases (in front of the rotor the pressure is higher than the ambient pressure), then the rotor disc accesses a sort of discontinuity (jumpdiscontinuity) behind the disc (lower pressure than the ambient pressure). The pressure after the rotor slowly recovers as it moves downstream and equals the ambient pressure. This jump in pressure generates a push of the air over the rotor (=thrust force2, which is essentially pushing the turbine backwards). As the air interacts with the rotor, first kinetic energy is transformed into mechanical energy. This is possible, because the rotor is realized with a certain number of plates (typically 3), which are essentially wings interacting with the airstream, generating forces that in the end generate a clockwise torque3 (torque causes rotation). Because of the principle of action and reaction, the air implies a torque on the rotor, then the rotor applies an equal opposite torque on air. Because of this, the air after the rotor swirls in the opposite direction. The idea of using wind is not new at all and it goes back over 1,000 years. Initially, wind was used for mechanical work (grinding grain, water pumping, powering tools). The idea of using wind for electricity generation goes back over 100 years and has gone through an interesting development process. First recorded windmill was found in Afghanistan/Persia (ca. 945 AD) and similar mills were developed in China to irrigate rice fields (~1045 AD), which is the precursor to the Savonius Rotors. The way of designing windmills evolved dramatically in Europe in ~1500 (specific for milling grain or pumping water), with the development of the today’s wind turbines. The machines were improved over a few centuries, which became very sophisticated, with very clever systems (e.g., gearing in cast iron or wood). Thousands of American style windmills (wind pumps) are still in use, because they are simple and robust, with passive control systems. Modern wind turbines, which are commonly used today, are the horizontal-axis 4three-bladed upwind5 variable-speed 6 wind turbines. There are various typologies depending on generator type, location of main bearings, gearbox, brake, etc. The blades are essentially “wings”, which interact with the wind flow. Their main role is generating the torque. Most of the turbines are pitch controlled, meaning that the blade is not connected rigidly 7 with the center of the rotor (the hub). But there is a relative degree of 1 2 3 4 5 6 7

Schwingbelastung Schubkraft Drehmoment Rotor spins about in axis, that is horizontal to the terrain Wind first hits the rotor, then the tower (downwind would be vice versa) Does not spin at a constant velocity steif

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freedom (relative rotation between the blade and the hub), which allows the blade to rotate about its own main axis (pitch axis). The rotor is connected to the hub through the drive drain8 , which is then connected to the generator (both are housed in the nacelle9 ). The nacelle is connected to the tower through a yaw system 10 (the nacelle can rotate vertically about an axis passing through the tower. It allows the rotor to point out in various directions to follow the wind changes). First main concept for wind turbines is the turbine with gearbox. Most common (intern) configurations are, that the hub is connected to a low-speed shaft11 (because the rotor spins at lower speed). The shaft is connected to a gearbox. The gearbox increases the shaft’s angular velocity to a much higher value. This high-speed shaft is then connected to the generator (induction-generator). It is a very proven configuration, but main disadvantage is that the gearbox is heavy weighted, very complex (in terms of designing – it has many components with many moving parts) and therefore costly. The turbine typically is designed to last 20-25 years. If the gearbox is not properly designed or maintained, it must be replaced, which is very costly. Because of the complexity of the gearbox and the drive drain, another configuration has been proposed and widely used. The so-called direct-drive turbine has no gearbox ad the rotor is directly connected with the generator. The generator is typically based on permanent magnets. One big advantage is the missing gearbox; however, the permanent magnet generator requires special materials (rare earths – are costly and not always available). Additionally, this generator becomes very heavy for very large machines (especially for offshore applications). These two configurations are the most common ones, there is no winner, both have their own niches/applications. Other concepts are: • Two-bladed concepts are not yet a commercial success due to complex dynamics, high tip speed (noise; they tend to spin faster than 3-bladed) and aesthetics. Now they are installed for onshore applications, but the tendency lies on offshore applications (more wind). • Downwind concepts have a lower yaw demand and are built to withstand typhoon wind storms (e.g., Hitachi/Fuji 2MW). But there is fatigue due to tower wake crossing. • Vertical axis turbines typically have a lower efficiency than horizontal axis turbines (not very popular). On the other side they have no yaw/pitch and a low C.G.12 (renewed interest for offshore applications). Looking at the role of wind energy in today’s energy mix there are many resources available on the web. The energy mix is changing dramatically. The trend is to move as quick as possible to a larger penetration of renewables. For example, in Germany the German “Energiewende” (Energy Transition) tries to achieve a low carbon, environmentally sound, reliable and affordable mix with a very large share of renewables (60%) by 2050. Goal is to completely phasing-out nuclear energy by 2022. This is a challenging goal, because nuclear power still represents a significant share of the energy mix. Renewables are really playing an important role in the energy transition. For the sake of environment and the future, it is necessary to provide electricity at affordable prices in order to secure, that even energy-hungry countries (e.g. Germany with its big industries) are still be able to be competitive and buy electricity at affordable prices (big challenge!). Both, wind and solar PV are on a dramatically increasing path and playing a bigger role in the changing EU power mix. At the same time, other resources are reducing. For example, in 2016 wind has overtaken coal as the 2nd largest form of power generation capacity13. Total installed wind power capacity in Europe shows a steady increase of wind (today’s capacity factor, which is fraction of time a wind plant operates at full power, is at an average of 26%). Mainly onshore, but also offshore is growing fast and Antriebsablauf Maschinenhaus 10 Windnachführung 11 Welle 12 C.G.= central gravity (generator is located low on the turbine and not on the tower 13 Information is regarding installed nameplate power capacity, which is different from the actual power output (main difference: typically, renewable energy sources are not always operating at full power 8 9

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there will be more of it in the future 14. Total installed wind power capacity by country shows also a difference of wind availability in terms of nature availability and governmental policies by different countries. Wind penetration rates are very important numbers, because they tell about what the actual role of wind is in the electricity mix. So much energy is coming from wind (average in Europe is 15%, which is already a significant number) and the number of installations is increasing very fast. Penetration differs depending on the country, countries with excellent wind resources have a very large penetration. Europe with its ~200 GW, represents 1/3 of the world wide’s installed wind capacity (~650 GW). While the number of annual installations is determined by fluctuations, depending on changes in policies and nature of big projects coming online. But still, there is a sustained growth of wind. Again, this technology is relatively new, playing a much bigger role and is growing faster (compared to the last 20 years). Costs of wind energy are very important to evaluate the technology from many points of view (how renewable it is, emissions etc.), to provide energy at reasonable prices in the end. One way of looking at costs is LCOE (Levelized Cost of Energy). The graphic shows that onshore-wind costs less than offshore. At the same time wind seems to be competitive with many other technologies, particularly with solar (which is extremely encouraging). Reason for the falling costs of the technology is due technological improvement, economies of scale, competition (of companies and players) and experience (gathered through the years). The other way is looking at the average awarded auction prices by commissioning date (i.e. when the plant will come online). It is different from LCOE, because it might not reflect full costs, aggressive bidding etc. Wind is not present everywhere in the same way. There are better places for wind and places, where it is not a good idea to install wind power plants (bad wind conditions and high installation costs). Not only a matter of where wind is available, but also how large the population density is. Installing wind turbines close to people is going to generate always some kind of friction 15 (wind cannot be made invisible and will always make some noise). Like all RETs, wind cannot play the same role in different countries. Therefore, it is necessary to optimize the mix depending on the local conditions and it is also important to have an efficient distribution grid (to generate efficiently with low impacts and good prices). By looking at the numbers, wind is growing very fast and it is playing a very important role especially in Europe, which is certainly the leader in wind technology nowadays. But of course, there are issues affecting the growth. Issues are either beyond reach of wind technology (e.g., policy uncertainties, permitting & approval time/delays of projects, grid capacity and transmission limits and skilled industrial/labor availability) or affected by technological innovation (e.g., cost of energy from wind, environmental impacts - wildlife, noise, visual, radar etc. , social acceptance, availability of suitable onshore sites and current technological limits – for exploitation of deep-water offshore resources). Most social, economic and environmental impacts of wind energy are positive, but there are also some negative impacts, which can in part be mitigated16 by technological innovation. Negative impacts are air-borne vibrations (noise – completely “quite” turbines don’t exist but slowing down the rotor during the nighttime is an option), ground-borne vibrations (turbines transmit loads to the ground, which generate shakings) and water-borne vibrations (especially during installation). Wind energy affects also local climate, due to that large wind farms might slightly alter humidity, which might influence crops as well. Other issues are social acceptance (visual impact, flickering caused by the blades and radar interference) and the manufacturing (energy, emissions), end of life decommissioning & disposal and

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24% avrg. onshore capacity vs 38% avrg. Offshore capacity Spannung/Unstimmigkeit Lindern/mindern

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recycling. It is impossible to do something which then has zero negative impacts! It is always a matter of balancing and understanding of what the priorities are. The idea of using wind to generate mechanical work goes back many centuries and the idea to generate electricity from wind started already at the end of the 1800s. But, the modern history of wind energy can be traced back by the early 1970s. Specifically towards the oil crisis, where the oil prices increased rapidly. This event had a number of social and economic consequences including the spread of interest for alternative ways of generating electricity. This led to many prototypes17 and first attempts were conducted by large (mostly aerospace) companies. The experiments were extremely useful, but the design was killed by several issues like fatigue or noise. There is slowly but steady growth of new technologies in the field of wind power. The blade is the most important component of a wind turbine, but as well all other components have undergone similar improvements in terms of materials, solidity, airfoils, shape and add-ons. Again, in the early 70’s there was the idea of transferring knowledge from similar disciplines (aeronautical engineering, rotor-craft application…). There was a lot of knowledge transfer. However, there was also the need for a lot of dedicated knowledge about a wide range of topics and disciplines, including better understanding of wind, meteorology, materials, safety, design, … This led to a highlyfocused group of industry, designers, researchers, institutes, …The importance of a specifically dedicated scientific community18 , the focus on the relevant science19 and a tight integration between academia and industry20 became impulsive! The areas of technical innovation can be divided into: • Wind turbine level: o Aerodynamics and electromechanical conversion: making them efficient as much as possible and light-weighted at the same time. o Active smart blades see improvement at all levels from aerodynamical design. Today they are still complex and monolithic (massive) structured. They require heavy maintenance and inspections. o Construction technology and offshore substructure technology tries to find the best way of getting out the most of these technologies. Needs a lot of research and costs. They should have sensing and advanced controls (a possible game changer; what is happening around the turbine?). o Materials: good mechanical properties and recyclable, which are cheap as well. • Wind plant level: o Aerodynamics: Understand/affect wakes. Improved mixing/faster recovery. o Wind plant layout optimization (condition monitoring, wind sensing, sea state sensing and wind forecasting). o Substructures and interconnection equipment. o Coordinated control: Improvement of plant output and reduction of fatigue damage. Wind technology has unique aspects, which are distinguished form similar applications. E.g., significant differences with respect to aeronautical applications are: • Dimensions: need relatively low-cost materials, large volumes • Reliability/maintenance: performance with simplicity and robustness • Very different key design objectives (low cost of energy/electricity (CoE) for wind energy) Rotating components also distinguishes it from civil applications. Performance should always be ensured with simplicity and robustness, in order to operate independently always with low supervision. Trends of improvement indicate an increase of size. But: Larger machines cannot be designed by simple upscaling of smaller ones. The increase of the size leads to a square increase of energy and a cubic

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Pioneer wind turbine comes from Denmark (developed in a garage on a low-budget) Worldwide community, with many experts dedicated to all relevant fields All fields interacting together (there are many!) Important to know, where the effort should be put

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increase of weight (costs are correlated to the weight). To avoid cubic law of growth there is a strong need for R&D and technological innovation. Trends of improvement also show interest for offshore wind turbines. Reasons for going offshore, which have potential for reduced LCOE, are: • Huge available resources • Improved social acceptability • Lower environmental impacts (e.g. noise) • Scale / logistics (less limited to the size, less complicated travel roads with components) • More capacity for less cash thanks to: Cost reductions and increased competition Offshore wind has great potential resources and offers great opportunities. But at the same time, it has great technological problems, e.g. special logistics, maintenance or the harsh environment. Various possible offshore foundations are either fixed (majority of installations, but only in shallow waters) or floating (installed in deeper waters).

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Concluding Remarks: Wind energy by itself does not solve the energy problem However, it can greatly contribute to the energy mix, especially in some countries From the technological point of view, there are many fascinating problems that need to be solved (research) and opportunities for improvement

Wind Energy – Wind turbine aerodynamics and regulation

Aerodynamics can be explained by the Stream Tube Theory (STT), where the flow travels along a one-dimensional tube (simple assumption). The flow enters at a higher speed and exits the tube at a lower one, because kinetic energy is extracted from the flow. The theory is refined by including the wake swirl: The wake rotates, because since the flow of air interacts with the rotor and applies a torque to the rotor (making the rotor spin), then by the principle of action and reaction, the rotor applies to air and equals an opposite torque, that makes the wake spin (swirl is the rotation of the wake in the opposite direction of the rotor’s rotation.). The following can be assumed to simplify the theory: There is a stationary flow, a constant mass flow rate along stream tube (no interaction among annuli and no mixing at stream tube boundary) and an incompressible and inviscid flow. The rotor is a disk (actuator disk infinite number of blades). Air enters at a speed V the tube’s inlet at ambient conditions and extits at the tube’s outlet at a lower speed VD. At the rotor disk, flow losses are measured (V - 𝑣𝑖). Induced velocity can be introduced as 𝑣𝑖 = a⋅V (a = axial induction factor, is used when talking about non-dimensional quantities; a ~ 1/3). 5

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The rotor disk’s velocity is then ~2/3 of the incoming ambient wind speed (very significant reduction of the wind flow). There is also a change in pressure: Behind the rotor disk, p0 (p0>p) increases towards the rotor disk. Behind the rotor disk, there is a lower pressure p1 than the ambient pressure p. Moving downstreams towards the outlet, p1 recovers and equals to p. If the speed has changed, this means that a force has been applied. This force can ba computed either by using the axial thrust [𝑑𝑇 = 𝑑ṁ (𝑉 − 𝑉𝐷) = 𝑑𝐴 (𝑝0 − 𝑝 1)] or the Bernoulli’s theorem21. Because the turbine is extracting kinetic energy and converting it into electricity, the tube un the Brnoulli’s Thm. can be divided into two energy preserving areas (before and after the turbine). Before22 the turbine there is no dissipation and mixing leads to a constant energy [ 𝑝 + ½ 𝜌𝑉2 = 𝑝0 + ½ (𝜌𝑉 – 𝑣𝑖)2 ]. After23 the turbine there is no dissipation and mixing leads to a constant energy [ 𝑝1 + ½ (𝜌𝑉 – 𝑣𝑖)2 =)𝑝 + ½ 𝜌𝑉D 2 ]. Taking both together,the thrust force can ...