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Rüzgar enerjisi dönüşüm sistemlerinin aerodinamik kapsamı ve güç belirlenmesi analizlerinde potansiyel akım yöntemleri

Aerodynamic aspects of wind energy conversion systems and potential flow methods in performance prediction analysis

  1. Tez No: 66829
  2. Yazar: ALİ ALPER AKYÜZ
  3. Danışmanlar: DOÇ. DR. M. ADİL YÜKSELEN
  4. Tez Türü: Yüksek Lisans
  5. Konular: Uçak Mühendisliği, Aircraft Engineering
  6. Anahtar Kelimeler: Belirtilmemiş.
  7. Yıl: 1997
  8. Dil: Türkçe
  9. Üniversite: İstanbul Teknik Üniversitesi
  10. Enstitü: Fen Bilimleri Enstitüsü
  11. Ana Bilim Dalı: Uçak Mühendisliği Ana Bilim Dalı
  12. Bilim Dalı: Belirtilmemiş.
  13. Sayfa Sayısı: 58

Özet

ÖZET Günümüzde dünya çapında kullanımı en hızlı artan enerji kaynağı ve teknolojisi rüzgar enerjisi dönüşüm sistemleridir. Rüzgar gücü, çoğunlukla, yenilenebilir bir kaynak olması ve çevreye verdiği zararın diğer teknolojilere göre çok düşük olması nedeniyle gündeme gelmiştir. Ancak, rüzgar enerjisinden elektrik elde edilmesinin yaygınlaşmaya başlamasının başlıca nedeni, dönüşüm sistemlerinin ve elektrik enerjisi üretim maliyetlerinin yeni fosil-yakıtlı güç santralleriyle rekabet edebilecek düzeye inmiş olmasıdır. Bunu sağlayan etken de, özellikle 1970'lerdeki petrol şoku sırasında yeni ve yenilenebilir enerji kaynak ve teknolojileri üzerinde başlatılıp yoğunlaştırılan araştırma-geliştirme çalışmaları olmuştur. Rüzgar enerji sistemlerinin geliştirilmesi üzerindeki araştırmalar türbin sistemlerinin aerodinamik ve mekanik performanslarının artırılması, dayanıklılıklarının ve yorulma ömürlerinin geliştirilmesi, rüzgar alanlarının modellenmesi ve simule edilmesi ve açık denizde kurulması düşünülen türbinler üzerinde yoğunlaşmıştır. Aerodinamik araştırmalar ise, kesit profillerinin karakteristiklerinin performans üzerindeki etkileri, dinamik akım ayrılması ve taşıma kaybı mekanizmaları, iz bölgesi araştırmaları ve potansiyel akım metodlarıyla türbin çevresindeki akım özelliklerinin ve performans etkilerinin hesaplanması üzerine yapılmaktadır. Performans analizi, pal ve rotor üzerine etkiyen aerodinamik yüklerin ve ortalama güç çıktısının belirlenmesini amaçlamaktadır. Rotor tasarımında en öncelikli ve kritik aşamalardan biridir. Basit pal elemanı/momentum yöntemi anlama ve hesaplama kolaylığı sağlamasına karşın, eksenden sapmış (yanal) akım ve değişken aerodinamik etkiler gibi bazı koşullarda bu yöntemin yeterli geçerlilik sağlamadığı bilinmektedir. Bu yüzden kullanılması önerilen potansiyel akım yöntemleri, bilgisayar sistemlerinin gelişmesiyle bütüncül analiz olanakları sunmaktadırlar. Klasik potansiyel akım yöntemleri olan taşıma çizgisi, taşıma yüzeyi ve üç boyutlu panel yöntemleri uygun sonuçlar verebilmektedir. Aerodinamik araştırmaların asıl yaran, rüzgar türbinlerinin tasarımlarında bütüncül yaklaşımlara elde edilen bilgilerin eklenmesiyle görülecektir. Bu tasarım programlarının geliştirilmesi, yeni ürünlerin içerdiği risklerin ve maliyetlerin azalmasıyla sonuçlanacaktır. ix

Özet (Çeviri)

SUMMARY This study searches the opportunity of use for potential flow methods in a flow analysis around a wind turbine blade and performance prediction of wind turbine rotors after a brief explanation about wind energy conversion systems, commercial use of these systems and aerodynamic research and development studies over them. Sorts of significant and accurate method for the understanding of general characteristics of flow around the turbine are potential flow methods such as lifting line, lifting surface, asymptotic acceleration potential and panel methods. They are analytical and numerical methods that can be translated into computer codes. The fastest growing energy resource today is wind power. Worldwide electricity generation capacity from wind has risen from 3700 megawatts (MW) in 1994 to 4900 MW by the end of 1995. The rate of increase in installed capacity has been %150 from 1990 to the beginning of 1996 and this corresponds to an annual average increase of %20. This rapid growth in utility and continuous technological development in conversion systems promises a significant alternative of an energy resource for several countries and regions. Wind power used to be interesting as it was a renewable resource and the environmental impact of the conversion process has been minimum comparing with the other technologies. But the main reason for widespread use of this technology today is the decrease of capital and generation costs of the conversion systems, becoming competitive with conventional technologies. Also implementation of the full social cost concept by means of CO2 taxes or supports has led the commercialization of this clean technology, especially in industrial countries. European wind power industry has achieved a great improvement. 860 MW of installed capacity in 1992 has exceeded 2500 MW at the end of 1995. Major wind power companies that produce greater turbines and cost-effective models are Europe- based. Among the developing countries that have a great potential, India has led the way for the implementation of a commercial market for wind power.Though technology is mature, economical obstacles in front of the wind power utility are market stimulation for conventional technologies, financing of capital costs, site selection and some technological obstacles. Among reasons supporting the evolution and development process of wind power on the way leading to commercialization are financial facilities and high purchasing prices for renewable energy and resources especially in the European Union. But beyond these measures, the greatest force behind commercial use of wind power is the technological development process these systems faced during recent years; and the main factor providing this development is the R&D studies started by the oil shock of the seventies and concentrated on renewable resources as alternatives to fossil fuels. As wind turbines possess lighter and more aerodynamic blades, better aerodynamic and electronic control systems, and as they're produced in masses, costs have dramatically reduced; and this trend still haven't come to an end. Efficient energy extraction from the wind is dependent on meteorological characteristics of the region selected and more significantly on the turbine capacity and design. Research for a suitable site should consider wind characteristics and continuity for the commercial sustainability of the region. Aerodynamic and structural design aims more performance for less wind velocity. Though wind turbines are classified according to their rotation axis as horizontal axis and vertical axis wind turbines, both use similar aerodynamic principles. They use lifting force or dragging force for rotation. A, great majority of turbines used or produced today are horizontal axis wind turbines driyen by lifting forces. Rotor blades have airfoil sections similar to those of airplanes and design phase resembles to those of propellers; in fact they're modified from those methods. Research on development of wind energy systems are concentrated on the improvement of aerodynamic and mechanical performances of turbines, improvement of their strength-to-weight ratio and fatigue life, modeling and simulation of wind fields and development of offshore turbines. Large scale" offshore turbines offer great promise both for land use and planning problems and social acceptance, and capacity increase because of undisturbed wind field. By means of aer-odynamic research, technological improvement and a full understanding opportunity for the flow around a turbine exists. The subjects related with aerodynamic research today consists of : 1. Rotor Configuration, Blade Properties, Number of Blades 2. Aerodynamic Control 3. Profile Characteristics, Selection Criteria, Design and Impacts on Stall. 4. Blade element momentum method and potential flow methods for Performance Analysis. XI5. Dynamic Loads Analysis 6. Unsteady Aerodynamics, Dynamic Stall Experiments and models, Wake Effects 7. Turbulence Impacts on Rotor Blades Horizontal axis wind turbines are also evaluated according to their rotor configurations like wind orientation (direction of the air flow from front or behind the tower), blade properties (rigid or teetering), number of blades (two or three). Turbines most widely used for electricity generation currently are two or three- bladed, receiving the wind from in front of the tower. Recent research heavily emphasize the development of large two bladed turbines with rated powers more than 1MW. Aerodynamic control is used for limiting and optimizing power output. Methods to provide this is the utilization of constant, passive stall control or active pitch control. A recent system that has begun to be used in turbines commercially is variable speed rotors that tops the efficiency while reducing the dynamic loads. Stall delay met by the rotating machinery is a problem. Experiments state that there is a spanwise velocity distribution due to three dimensional flow separation. For this reason, special profile designs were needed to avoid generator damage by excessive rotation that was considered not to occur because of stall, likely to happen in forward flight. Special designs of profile families have used two major methods: aerodynamic shape optimization or potential flow methods-boundary layer analysis couplings. Performance prediction and analysis aims at the availability of aerodynamic loads and mean power output. It's a prior and critical position. The simplest and most widely used approach for performance is blade element/momentum method. Blade element/momentum method is based on the principle that the rotor can be considered as the spanwise sum of thin strips, called blade elements. Thus, loads on the blade can be obtained by the integration of the characteristics of these elements. Usually experimental profile characteristics are used for this purpose. Potential flow methods are used for a better analysis as BEM method is unable to take wake effects and stall conditions into account. In contrary, they require more of computation hardware and time. Toughest issue in front of the wind turbine model users is the determination of dynamic loads that play the major role on fatigue life of a rotor. Stochastic character of the wind and the flexible structure öf turbine causes an increasing nurnjber of parameters for the calculation of flow. Dynamic stall and dynamic inflow are fhg two majc* investigation areas for wind turbine flow characteristics. Potential xumethods offer an even greater promise for the understanding of the mechanisms of such flow conditions. A series of models were developed in order to simulate the stochastic loads on the rotor. Best models to simulate the wind are almost the most difficult ones to be solved, though fast computation methods were developed recently. All currently used methods use the longitudinal component of atmospheric turbulence. Some of the rotor reactions were, in contrast, reported to be sensitive against horizontal and vertical components of turbulence. Though blade element/momentum method provides an ease of understanding and computing, it's known that it is not accurate for some flow conditions like yawed flow, unsteady aerodynamics or stall conditions. They are also unable to consider wake effects on the turbine performance. For the detailed analysis of effects of these complex flow conditions on performance and three-dimensional flow field, analytical methods are sought. Vortex methods widely used by helicopter and propeller designers are suitable to be adapted for wind turbine rotors. Main obstacle for the use of these methods is the computation burden. However the progress taken by computer systems recently has led the optimism about the faster solution in the near future. The problem of potential flow is solved by the representation of rotor blades and vortex sheet by singularity elements like source, doublet and vortice elements; determination of the distribution of these singularities according to the boundary conditions and finally solution of the velocity field. Methods used for solution are lifting line, lifting surface and panel methods as well as others like free wake methods and asymptotic acceleration method. The main equation for the flow field is known as Laplace equation for flow perturbation potential and this equation is solved for the boundary condition of zero normal velocity over the surface of the body. Another aerodynamic condition is Kutta condition; wake shouldn't include any force, or, in another saying, there shouldn't be any pressure difference between lower and upper sides of the vortex sheet representing the wake. Wake shape for wind turbines is helical, so wake shape should be prescribed or, by means of iteration, a free wake method can be used. An expanding wake is reported not to affect the performance significantly, comparing with a cylindrical one. Lifting line method was first developed and modified for propellers by Prandtl. It reduces the lifting wing to a bound vortex and discretization is made by horseshoe vortexes which can at the same time represent the wake. Lifting surface method does not take the thickness into account but the chord. Singularities effect on the camber line. Discretization is made by doublet or equivalent vortex elements. The xmrepresentation of leading edge in lifting surface method causes a singularity. By the solution of Laplace equation under the boundary conditions, both methods lead to the determination of singularity distribution, velocity and pressure distributions and thus blade loads and performance coefficients. In panel method, aerodynamic surface is discreticized by a number of quadrilateral elements chordwise and spanwise. Constant, but initially unknown singularity distributions are placed on each panel. Main purpose of the method is the determination of the strengths of these singularities. Hence, velocity, pressure and loads distribution can be found resulting in a total power and thrust coefficient for the rotor. The leading edge singularity problem no longer exists in panel methods. These methods can be extended for the use of yawed flows and they can be coupled with three dimensional boundary layer analysis in order to take into account the viscosity and three dimensional flow separation effects. Wake shape can be either assumed beforehand or a relaxation method can be used. The prescription of an expanding wake is reported to be in good agreement with experiments, but not leading to sifnificant changes relative to a non-expanding one. An integral analysis of flow field around the turbine is possible by panel methods translated into computer codes. In this approach, codes generated for use in aircraft and ship propellers can be adapted for use in wind turbines. The choice of method depends on the balance between accuracy of the method and computation cost and time. Panel methods are expected to be more widely used by further improvements in computing facilities. A new method is asymptotic acceleration potential method developed in Delft Technology University. Accelerations in the flow field are represented by modal pressure differences on the blade. Integration of the acceleration of air particles lead to the velocities in rotor plane; thus aerodynamic loads can be determined. Though it's reported to have a significantly less computation time, the method is not used for different flow conditions yet. Aerodynamic research of any kind will contribute into the integrated design schemes developed recently. Development of these programs in which aerodynamic, structural and financial constraints are integrated will result in reduced risks and costs for new wind energy conversion systems. The constraint for the use is the financial one; optimum program should be selected according to the hardware and computation time it requires for execution. xiv

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