Special Issue "Wind Energy"

Guest Editor
Dr. Cristina L. Archer
Department of Geological and Environmental Sciences, Physical Sciences Building, Room 115, California State University, Chico, CA 95929-0205; Consulting Assistant Professor, Department of Civil and Environmental Engineering, Stanford University, Stanford, CA 94305, USA
Website: http://www.stanford.edu/~lozej
E-Mail:
Interests: wind power research; on topics such as: evaluation of global wind power potential; development of methodologies for vertical wind speed extrapolations; offshore and high-altitude wind power; interconnecting wind farms

Guest Editor
Prof. Dr. Frede Blaabjerg
Institute of Energy Technology, Aalborg University, Niels Jernes Vej 10, DK-9220 Aalborg East, Denmark
Website: http://www.iet.aau.dk/~fbl
E-Mail:
Interests: wind power research; power electronics; control of wind turbines and wind farms; interconnection to grid; generators; power converters; ride-through operation

The "Wind energy" issue of "Energies" focuses on recent advances in the wind energy sector on a variety of topics, including: wind resource mapping, wind intermittency issues, wind harnessing technologies, effects of wind farms on local and global climate, effects of global climate change on wind resource.

Submission

All papers should be submitted to energies@mdpi.org with copy to the guest editor. To be published continuously until the deadline and papers will be listed together at the special websites.

Submitted papers should not have been previously published nor be currently under consideration for publication elsewhere. All papers are refereed through a peer review process. A guide for authors, sample copies and other relevant information for submitting papers are available on the Instructions for Authors page. Energies is an international peer-reviewed quarterly journal published by Molecular Diversity Preservation International.

Article Processing Charges (APC)

Article Processing Charges (APC) are 300 CHF per paper and additional English correction fees (250 CHF) will be added in certain cases (550 CHF per paper for those papers that require extensive additional formatting and/or English corrections) for paper submitted before 31 December 2009. Starting 1 January 2010, Article Processing Charges are of 800 CHF per accepted article.

• wind power
• wind energy
• meteorology
• numerical modeling
• intermittency
• renewable energy
• electricity

Title: Survey of the Economics of Wind Energy
Author: Bradley T. Ewing, Rawls College of Business and Wind Science & Engineering Research Center, Texas Tech University, Lubbock, TX 79409, USA; E-mail: bradley.ewing@ttu.edu
Abstract: This study provides an in-depth review of the current literature on the economics of wind energy. The survey focuses on the issues related to economic impacts and cost-benefit analysis of wind farms, trends in wind energy production, and wind speed forecasting.

Title: A Comprehensive Review and Life Cycle Assessment of Wind Energy Technologies
Authors: C. Sen, A. Labak, S. Hamidifar, F. Ghousia, K. Hafiz and Narayan C. Kar
Canada Research Chair in Hybrid Drivetrain Systems, Associate Professor (Cross-Appointed), Dept. of Electrical & Computer Engineering, Dept. of Mechanical, Automotive and Materials Engineering, 401 Sunset Ave., University of Windsor, Windsor, Ontario, N9B 3P4 Canada; Email: nkar@uwindsor.ca; www.uwindsor.ca/power; Tel: 519-253-3000 (ext.4796), Fax: 519-971-3695

Title: Global Offshore Wind Energy Assessment using QuikSCAT
Author: Qingfang Jiang
Affiliation: University Corporation for Atmospheric Research, Monterey, California
Abstract: The gridded (0.25º × 0.25º) 10-m winds derived from QuikSCAT data for the past ten years (i.e., 1999-2009) are used for assessing global offshore wind energy potential. In this study, we focus on the wind energy potential in low-level coastal (or island) jets that are within a few hundred kilometers of the coastline (islands), which may be achievable with today’s or near-future technology. The seasonal and intra-annual variations of wind energy in these coastal jets are examined.

Type of Paper: Review
Title: State of the art and trends in wind resource assessment
Authors: Oliver Probst and Diego Cárdenas
Affiliation: Physics Department, Instituto Tecnológico y de Estudios Superiores de Monterrey, Monterrey, N.L., CP64849, Mexico
Abstract: Given the significant rise of the utilization of wind energy, the accurate assessment of the wind potential is becoming more and more important. Direct applications of wind assessment techniques include the creation of wind maps on a local scale (typically 5-20km) and the micrositing of wind turbines, the estimation of vertical wind speed variations, prospecting on a regional scale (>100km), estimation of the long-term wind resource at a given site, and forecasting of the performance of wind farms on a time scale relevant for utility electricity dispatch. Local wind maps are based on the predicted modification of the regional wind flow pattern by the local atmospheric boundary layer which in turn depends on both topographic and roughness features and the measured wind rose obtained from one or several measurement towers within the boundaries of the planned development site. Initial models were based on linearized versions of the Navier-Stokes equations, whereas more recently full CFD models have been applied to wind farm micrositing. Linear models tend to perform well for terrain slopes lower than about 25% and have the advantage of short execution times, allowing for frequent updates of the expected wind farm performance during the measurement campaign. Long-term performance is frequently estimated from correlations with nearby reference stations with concurrent information and continuous time series for a period of at least 10 years. If the level of correlation is high enough, a long-term wind rose can be constructed for the site and a confidence level analysis can be conducted for the expected wind farm energy production. Simple methods consider only point-to-point linear correlations; more advanced methods take also the structure of the atmospheric boundary layer into account, make simultaneous use of several reference stations and account for the spatial distribution of the reference sites. Both for early prospecting in regions where only scarce or unreliable reference information is available, wind flow modeling on a larger scale (mesoscale) is becoming increasingly popular. These models combine meteorological flow models with (linearized) models for the flow in the atmospheric boundary layer to provide both long-term estimates of the wind potential of a prospective site and short-scale predictions for forecasts of the the expected output of wind farms. Wind resource evaluation and simulation is also important for the assessment of the structural performance of wind turbines. The short time scale structure of the stochastic component of the wind velocity is combined with the modal analysis of the combined rotor-tower system to determine resonant interactions which may lead to unacceptable stress levels at the rotor components. To obtain the true excitation spectrum seen by the wind turbine, both rotational sampling of the stochastic wind velocity components and spatial correlations have to be accounted for.

Type of Paper: Article
Title: Impact of Advanced Beam Modeling Techniques on Wind Turbine Blade Design
Authors: Wenbin Yu and Dewey H. Hodges
Affiliation: School of Aerospace Engineering, Georgia Institute of Technology, GA, USA
Abstract: The impact of asymptotically exact cross-section analysis coupled with a geometrically-exact beam theory on the analysis and design of wind-turbines is assessed. We first examine the differences of the structural and inertial properties obtained using the traditional approach versus those that come from asymptotically exact properties. Additional loss of accuracy can come from the use of traditional beam analyses such as those used in the wind industry. We then propose to show the differences in response predictions, including displacement, stress, natural frequencies and mode shapes, which will be contrasted with results obtained using codes that reflect current industry practice. Therefore, we expect these differences will be even larger.

Type of Paper: Article
Title: Grid Integration of Renewable Energy: the role of the STATCOM
Authors: Marta Molinas and Jon Are Suul
Affiliation: NTNU, Trondheim, Norway
Abstract: This paper provides background information on issues related to the compensation of reactive power with a STATCOM for grid integration of renewable energy sources. In this context, an overview of possible operating modes of the STATCOM is presented and the influence on voltage quality problems and on the general performance of the power system is discussed. The operation modes of the STATCOM are investigated under the scenario of large scale integration of fluctuating renewable energy sources to the grid, considering both normal and abnormal operating conditions. The operating principle of the STATCOM and the implemented control strategy are described in detail.
Wind and wave power integration are presented as the examples in which a STATCOM is connected to the power system at the point of common coupling (PCC) for several supporting functions. The cases selected for illustration and investigation of the STATCOM operation are wind and wave generation systems based on induction generators directly connected to the grid. Stationary, dynamic and transient modes of operation with special focus on the low voltage ride through (LVRT) capability are investigated analytically, by simulation and by measurements through different examples. An analytical method for approximate calculation of required rating of the STATCOM for LVRT fulfillment is provided and discussed.
In specific, the following operating modes are investigated and discussed in the chapter:
1.-Stationary and quasi-static modes of operation: voltage swell and sag for grid connected and off grid condition
2.-Dynamic operation: reactive power compensation with variable wind speed and flicker mitigation
3.-Transient operation: transient stability, low voltage ride through (LVRT) and torque transient alleviation
The measurement results provided are related to wind turbine/farm and wave energy converter emulation in reduced scale laboratory set-ups in which the fluctuating wind or wave was re-created from simulated data. Results presented give clear indications that the STATCOM is a good solution to several grid operational problems in the presence of fluctuating renewable energy sources.

Title: Nature’s limit to the wind energy extractable for human use
Authors: Sten T. Frandsen ¹, Søren E. Larsen ¹, Aksel Wallø ², Andrea N. Hahmann ¹
Affiliations: ¹ Risoe DTU;
² Copenhagen University
Abstract: The foremost explicit purpose of this note is to produce a new alternative estimate of the annual averaged kinetic energy generation rate in the atmosphere. In turn, we’ll also study the distribution of this energy generation with latitude and study the differences between sea and the continents.
Traditionally, this exercise has been done in General Circulation experiments based on global data for wind and temperature, and these previous efforts will be reviewed as part of the note. However, we believe that the present availability of e.g. the reanalysis data makes it possible to try to verify this type of estimates using explicit data for the kinetic energy dissipation rate obtained from the model used for the reanalysis.
The perspective of the study may be of interest when considering how much wind energy that can be extracted from future extended offshore areas.
The solar radiation hitting the earth is known with fair precision, 174,000 TWy/y of which 122,000 TWy/y (70%) reaches the atmosphere and earth’s surface. In comparison, the global human consumption of energy is 15 TWy/y, corresponding to approx. 0.01% of the incoming solar radiation. A small fraction of the radiation absorbed in the surface or in the atmosphere is directly converted into kinetic energy in the planet’s atmosphere (and ocean) through the action of pressure gradient forces. And only a fraction of the total atmospheric kinetic energy generation may be utilized by humanity, but that is another story.
We’ll also discuss shortly the possible feedback of extraction of wind energy on the atmospheric flow itself.
The analysis will be done for re-analysis data from ECMWF (interim reanalysis data).

Type of Paper: Article
Title: New Approaches for Very Short-term Steady-State Analysis of an Electrical Distribution System with Wind Farms
Authors: Antonio Bracale ¹, Daniela Proto ¹, Angela Russo ² and Pietro Varilone ³
Affiliations: ¹ Università degli Studi di Napoli Federico II, Italia
² Politecnico di Torino, Italia
³ Università degli Studi di Cassino, Italia; E-Mail: varilone@unicas.it
Abstract: In recent years, the characteristics of electrical distribution systems have been significantly modified due to the presence of more and more dispersed generation (DG) units. Distribution systems have become active, rather than passive, generating many new technical considerations that must be addressed, such as distribution network planning and operation.
In this paper, a comparison of some statistical approaches and artificial-neural-networks methods for VST wind-power forecasting (a few hours ahead) is provided. Both “point-forecast” and “pdf-forecast” methods are considered. The “point-forecast” methods are the Persistence Method, the Generalized Persistence Method, the Nielsen Method and the artificial-neural-networks methods. The “pdf-forecast” method is a Bayesian wind-speed prediction method. The paper is organized such that the statistical wind forecast methods (Persistence Method, Generalized Persistence Method, Nielsen Method, and Bayesian Method) and the artificial-neural-networks methods are presented first. Then, the deterministic and probabilistic load flows are discussed briefly. Finally, numerical applications and comparison of all the methods described are presented, using an actual, medium-voltage distribution system.

Title: Stochastic Turbulent Wind Profiles: A Closed Form Solution And Convergence Stability Analysis by Lyapunov Theory
Authors: M.T. de Vilhena, K.B. Mello, B.E.J. Bodmann and J.C. Carvalho
Affiliations: ¹Universidade Federal do Rio Grande do Sul, PROMEC & PPGMAp, Av. Osvaldo Aranha 99/4, 90046-900 Porto Alegre, RS, Brazil;
²Universidade de Caxias do Sul, Centro de Ciˆencias Exatas e Tecnologia, Rua Francisco Get´ulio Vargas 1130, 95070-560 Caxias do Sul, RS, Brazil;
³Universidade Federal de Pelotas, Faculdade de Meteorologia, Av. Eng. Ildefonso Sim˜oes Lopes 2751, 96060-290 Pelotas, RS, Brazil; E-mail: bejbodmann@gmail.com
Abstract: In this review we discuss stochastic turbulent wind profiles based on the three-dimensional stochastic Langevin equation for a selection of probability density functions. Its solution permits to simulate tracer dispersion in turbulent regimen, which is of interest in designing aeolian parks for wind energy conversion. We discuss the stochastic Langevin equation together with an analytical method for solving the three-dimensional and time dependent equation which is then applied to tracer dispersion for stochastic turbulence models. The solution is obtained using the Adomian Decomposition Method (ADM), which provides a direct scheme for solving the problem without the need for linearisation and any transformation. The results of the model are compared to case studies with measured data and compare to procedures and predictions from other approaches.

Type of Paper: Article
Title: Modeling and Simulation of Wind Generators and Wind Power Plants at Different Levels of Complexities
Authors: M. Singh¹, S. Santoso¹ and E. Muljadi²
Affiliations: ¹ The University of Texas, Austin, USA; E-Mail: ssantoso@mail.utexas.edu
² National Renewable Energy Laboratory, Golden, Colorado, USA
Abstract: The objective of the manuscript is to propose universal manufacturer-independent wind turbine and wind power plant models that can be shared, used, and improved without any proprietary restrictions. These models developed include fixed-speed, variable-speed with rotor resistance control principle and doubly-fed induction generator machines. These models are scalable, and can be used to represent a single turbine or an entire wind farm. Models with different levels of complexity have been developed. The base model represents the wind turbine generator(s) as an equivalent regulated current source. It includes representations of general turbine aerodynamics, the mechanical drive-train, and the electrical characteristics of the generator and converter. The more complex models include induction machine models as well as inverter and rectifier models with the power electronics explicitly modeled. The performance of the different models will be compared and validated with actual wind power data collected from wind power plants.

Type of Paper: Article
Title: Crowbar System in Doubly Fed Induction Wind Generators
Authors: M. B. C. Salles¹, K. Hameyer², J. R. Cardoso¹, A. P. Grilo³ and C. Rahmann⁴
Affiliations: ¹ Laboratory of Applied Electromagnetism - LMAG, University of São Paulo, Brazil
² Institute of Electrical Machines - IEM, RWTH Aachen University, Germany; E-Mail: kh@iem.rwth-aachen.de
³ Engineering, Modeling and Applied Social Science Center - Federal University of ABC, Brazil
⁴ Institute of Power Systems and Power Economics - IAEW, RWTH Aachen University, Germany
Abstract: In the last 15 years, the use of doubly fed induction machines in modern variable speed wind turbines has increased rapidly. This development has been driven by the cost reduction as well as the low-loss generation of Insulated Gate Bipolar Transistor (IGBT). According to new grid code requirements wind turbines must remain connected to the grid during grid disturbances and, moreover, they must also contribute to voltage support during and after grid faults. The crowbar system is essential to avoid the disconnection of the doubly fed induction wind generators from the network during faults. This insertion in the rotor circuits should be only for short period of time in order to reestablish rapidly the terminal voltage control. As a general rule, the activation and the deactivation of the crowbar system is based only on the DC-link voltage level of the back-to-back converters. In this context, the authors propose the inclusion of the critical speed analyze to certificate that the instability of the generator will be avoid.

Type of Paper: Article
Title: A Critical Examination of DC Power Transmission Approaches for Interconnecting Bulk Wind Generation with the Electric Grid
Authors: D.C. Ludois and G. Venkataramanan
Affiliation: College of Engineering, University of Wisconsin-Madison, 1415 Engineering Drive, Madison, WI 53706, USA; E-Mail: giri@engr.wisc.edu
Abstract: The application of high voltage DC (HVDC) transmission for integrating large scale and/or off-shore wind generation systems with the electric grid is attractive in comparison to extra high voltage (EHV) ac transmission due to a variety of reasons. While the technology of classical current source converters (CSC) using thyristors is well established for realization of large HVDC systems, the technology of voltage source converters (VSC) is emerging to be an alternative approach, particularly suitable for multi-terminal interconnections. More recently, a more modular scheme that may be termed ‘bridge of bridge’ converters (BoBC) has been introduced to realize HVDC systems. While it is clear that all these three approaches are functionally capable of realizing HVDC systems, the performance trade-offs between these alternatives are not readily apparent. This paper presents a critical examination of these topologies from the point of view of power semiconductor requirements, reactive component requirements, operating losses, waveform quality, fault tolerance, modularity, complexity, etc. Detailed analytical models will be used along with a benchmark application to develop a comparative evaluation of the alternatives that maybe used by wind energy/bulk transmission developers for performing engineering trade-off studies.

Type of Paper: Article
Title: CFD Analysis of Wind Turbine Configurations
Authors: Sugoi Gomez-Iradi, George N. Barakos and Xabier Munduate
Affiliation: Department of Engineering, The University of Liverpool, Room UG31, Harrison Hughes Bld., The Quadrangle, Liverpool L693GH, UK; E-Mail: g.barakos@liverpool.ac.uk
Abstract: The use of computational fluid dynamics (CFD) for the analysis of wind turbines is now popular amongst researchers and engineers in the wind energy sector. There are, however, known limitations in the ability of CFD to accurately predict the power and the aerodynamics of wind turbines. The only valid suggestion to resolve this issue is the detailed comparison between experiments and numerical predictions. Following this principle, this paper exploits the experimental data of NASA/NREL for a small full-size wind turbine and attempts to look at the details of the flow to identify model inaccuracies in numerical predictions. To this end, studies of the surface pressure on the blade are combined with investigations of the upstream flow using the state-of-the-art compressible flow solver Wind Multi-Block (WMB). The work suggests that the detailed modelling of the wind turbine including nacelle and tower is important for accurate predictions and also emphasis should be shifted to the study of the flow in the vicinity of the blade using CFD and flow velocity measurements. The combination of on- and off-blade data holds the key to good power predictions since any discrepancies in the surface pressure and blade torque must be related to flow changes around the aerofoil. The use of CFD is further demonstrated via comparison of the flow flow angle around the blades with established industrial methods. After validation and testing, CFD appears to hold the key for wind turbine performance predictions without the need for external data.

Type of Paper: Article
Title: Ancillary Services Provided by Modern Wind Turbine Technology - Modeling Considerations
Authors: Ioannis D. Margaris and Nikos Hatziargyriou
Affiliation: ¹ Department of Electrical and Computer Engineering, National Technical University of Athens, Athens 106 81, Greece; E-Mail: nh@power.ece.ntua.gr (N.H.)
² Public Power Corporation of Greece, Athens 106 81, Greece
Abstract: Increasing levels of wind penetration in modern power systems has set intensively high standards with respect to wind turbine technology during the last years. Security issues have become rather critical and operation of wind farms as conventional power plants is becoming a necessity as wind turbines replace conventional units in the production side. This paper includes modeling considerations for wind farms with three different wind turbine technologies – namely Doubly Fed Induction Generator (DFIG), Permanent Magnet Synchronous Generator (PMSG) and Active Stall Induction Generator (ASIG) based wind turbines. Aggregated models of the wind farms are being used and results for different load and wind profile cases are being analyzed and discussed.

Type of Paper: Article
Title: Cost-Effective Voltage Sag Ride-Through of HVDC-Connected Wind Farms
Authors: Torbjörn Thiringer¹ and Lennart Harnefors²
Affiliations: ¹ Chalmers University of Technology, Sweden; E-Mail: torbjorn.thiringer@chalmers.se
² ABB Power Technologies, Sweden; E-Mail: lennart.harnefors@se.abb.com
Abstract: In this paper, voltage-sag ride-through of offshore or remotely located wind farms connected to the main grid via voltage-source-converter (VSC) high-voltage dc (HVDC) transmission is considered. Presently, several sets of braking resistors are normally used: in the wind turbines as well as on the dc bus, and possibly also on the ac bus, of the HVDC transmission. Obviously, this is not a cost-effective solution. We here investigate the possibility of relying solely on the braking resistors which are installed in the wind turbines. It is shown that, by proper manipulation of the wind-farm ac-bus voltage by the VSC HVDC terminal, ride-through of main-grid faults is possible without overvoltage or overcurrent tripping.

Type of Paper: Article
Title: Why Offshore Wind Energy?
Authors: M. Dolores Esteban, J. Javier Diez, Jose S. López and Vicente Negro
Affiliation: Universidad Politécnica de Madrid, Spain; E-Mail: jjdiez@caminos.upm.es
Abstract: In the middle of 2009 there were only 1500 wind megawatts installed into the sea. Although the first offshore wind farm experiment took place in 1990, most of the facilities built up to now since then were pilot projects. These statements show the incipient of offshore wind power. At this moment, however, the boom of offshore wind energy finally seems to be happening. There are a few countries at the top of current offshore wind energy development (United Kingdom, Denmark, Holland, Sweden, Germany; followed at a distance by a few others, Spain, France, etc). This current situation, the row materials problem and the general commitments to reduce the emission of greenhouse gases are leading to predict a promising future for offshore wind power. This paper deal with a critical opinion about the causes of the recently boom of the offshore wind energy, and its comparison with other renewable energies (onshore wind energy, solar, hydraulic, wave, etc.). Also, a comparative between renewable and conventional energy is included in the text.

Type of Paper: Review
Title: Offshore Wind Farms: A Wide Ranging Review of the Environmental—Impacts (and Benefits) of Offshore Wind Energy
Authors: M. Elliott, J. Wilson, R. Perez-Dominguez, N. Cutts and L. Mander
Affiliation: Power and Process Europe, AMEC, St Matthews House, Haugh Lane, Hexham, Northumberland NE46 3PU, UK; E-Mail: Jennifer.Wilson@amec.com
Abstract: In recent years, as the development of offshore wind energy has become more rapid and demand for renewable energy increases, so too does the debate as to its environmental status and contribution to the surrounding habitat.
The aim of this review is to identify the environmental impacts (and benefits) of offshore wind farms, and, where possible, attempt to quantify these affects, thus providing a more accurate overview of how the installation of wind turbines can interact with their receiving habitat.

Title: Mass and Aerodynamic Imbalance Estimates of Wind Turbines
Authors: Ronny Ramlau, Jenny Niebsch and Nguyen Tuan Thien
Affiliation: ÖAW (Austrian Acdemy of Sciences), Altenbergerstr. 69, A-4040 Linz, Austria; E-Mail: jenny.niebsch@oeaw.ac.at
Abstract: Rotor imbalances of a wind turbine are an old problem but still receive signicant research because of its important eects on the operation time of wind turbines. The paper presents a method of reconstructing both inhomogeneous mass distributions of the rotor and deviations in pitch angles of the rotor blades from vibration measurements in the turbines nacelle. To describe the forward problem, the blade element momentum method was employed to calculate the loads on the blades. The vibration equation was formulated by the Finite Element Method and its solution was found analytically. The algorithms for ill-posed nonlinear inverse problems were used to nd the causes of imbalances from measurements. Numerical simulations of our model were performed with data from a wind turbine of the type V80-2MW
and articial vibration data. By applying our model, the causes of rotor imbalances are found. Applying the results to eliminate them considerably lowers the balancing costs and prevents early fatigue or even damages the many parts of wind turbine. Therefore, the life time of wind turbines is longer and they work more eective.

Type of Paper: Article
Title: Integrated Grounding System of Wind-Farms for Lightning and Fault Protection
Auhtor: Maria Lorentzou et al.
Affiliation: Hellenic Transmission System Operator, System Planning Department, 72 Kastoros Str., 18545, Piraeus, Greece; E-Mail: mlorentzou@desmie.gr
Abstract: Wind turbines and wind farm installations need to have a grounding system for the protection of human life and the installed equipment, in case of short-circuit incidents or lightning strikes. The probability of lightning striking a wind turbine is increased because high wind potential normally exists in places of high altitude. The objective of this paper is to present a methodology for the optimal design of individual wind turbine and wind farm grounding systems. In this design the minimization of the grounding resistance and the satisfaction of the safety criteria (step and touch voltage) are of major concern. A practical study case example is presented in order to underline the stages of the methodology.