Susan W. Stewart

Site Specific Optimization of Rotor/Generator Sizing of Wind Turbines

Economics, including all incentives, is the primary factor that drives the development of wind farms. Optimizing the wind turbine generator size-to-rotor size design based on an economic figure of merit shows that maximum wind turbine capacity factor does not yield the best economics for a given wind resource. A large rotor on a small generator will have a high capacity factor but a low annual output of electrical energy. For the same capital investment a different configuration would produce more electricity making the project more economically sound. This study varied rotor-to-generator size at a fixed capital cost and used a modified blade element momentum model to predict annual electrical energy production for each design at a given wind resource. Optimal design was the design that resulted in the highest annual electrical energy production. This was done at a series of fixed costs and a series of wind resources defined by the Weibull distribution parameters. The results indicated the following: At larger turbine sizes, (higher capital cost per turbine), the economics shifted toward a larger generator and smaller rotor (relatively). This exact relationship is dependent on the wind resource. At large turbine sizes, greater flexibility is shown in optimum generator sizing vs. rotor sizing. Having multiple generator size options for the same rotor size allows developers to more closely match and capitalize on the characteristics of their wind resource. The end result of the research is a set of diagrams developers can use to select the best turbine based on economics for their wind resource. This provides an additional tool they can use to make their projects more cost effective.Copyright

Wind/Wave Misalignment in the Loads Analysis of a Floating Offshore Wind Turbine

Wind resources far from the shore and in deeper seas have encouraged the offshore wind industry to look into floating platforms. As a result, the International Electrotechnical Commission is developing a new technical specification for the design of floating offshore wind turbines that extends existing design standards for land-based and fixed-bottom offshore wind turbines. The work summarized in this paper supports the development of best practices and simulation requirements in the loads analysis of floating offshore wind turbines by examining the impact of wind/wave misalignment on the system loads under normal operation. We conducted simulations of a spar-type floating offshore wind turbine system under a wide range of wind speeds, significant wave heights, peak-spectral periods, and wind/wave misalignments using the aero-servo-hydro-elastic tool FAST. The extreme and fatigue loads were calculated for all of the simulations. The extreme and fatigue loading as a function of wind/wave misalignment are represented as load roses and a directional binning sensitivity study is performed. This study focused on identifying the number and type of wind/wave misalignment simulations needed to accurately capture the extreme and fatigue loads of the system in all possible meteorological and ocean conditions considered, and for a downselected set of conditions identified as the generic U.S. East Coast site. For this axisymmetric platform (except for the mooring lines), perpendicular wind and waves play an important role in the loading of the support structure. Therefore, including these conditions in the design loads analysis can improve the estimation of extreme and fatigue loads. However, most support-structure locations experience their highest extreme and fatigue loads when the wind and waves are aligned. These findings are specific to the spar-type platform, but we expect that the results presented here will be similar to other floating platforms.

The Usefulness of Entropic Average Temperatures

The second law Carnot efficiency, entropy balances, and many other principles of the second law are stated with assumed constant temperature heat sinks and reservoirs; i.e., assuming heat transfer across a boundary at a constant temperature. However, real world heat exchangers reject and receive heat transfer to a flowing fluid with a varying temperature making the application of many aspects of the second law inappropriate or complex. For such varying temperature cases, an entropic average temperature can be defined and easily calculated that can be substituted for the varying temperature heat sink or source temperature. The constant temperature restricted second law statements can then be used with this entropic average temperature. This entropic average temperature concept is simple to understand and is very useful in the presentation of thermodynamic concepts to new students, making it seem less abstract and more applicable to real world processes with which they are familiar. This paper serves to develop the concept and details of the entropic average temperature and show its usefulness while emphasizing its benefit for inclusion in engineering thermodynamics syllabi.Copyright

Wind Energy Workforce Development: Engineering, Science, & Technology

Broadly, this project involved the development and delivery of a new curriculum in wind energy engineering at the Pennsylvania State University; this includes enhancement of the Renewable Energy program at the Pennsylvania College of Technology. The new curricula at Penn State includes addition of wind energy-focused material in more than five existing courses in aerospace engineering, mechanical engineering, engineering science and mechanics and energy engineering, as well as three new online graduate courses. The online graduate courses represent a stand-alone Graduate Certificate in Wind Energy, and provide the core of a Wind Energy Option in an online intercollege professional Masters degree in Renewable Energy and Sustainability Systems. The Pennsylvania College of Technology erected a 10 kilowatt Xzeres wind turbine that is dedicated to educating the renewable energy workforce. The entire construction process was incorporated into the Renewable Energy A.A.S. degree program, the Building Science and Sustainable Design B.S. program, and other construction-related coursework throughout the School of Construction and Design Technologies. Follow-on outcomes include additional non-credit opportunities as well as secondary school career readiness events, community outreach activities, and public awareness postings.

Design Study Comparison of Plain Finned Versus Louvered Finned-Tube Condenser Heat Exchangers

Enhanced fins are widely used in residential air conditioning system finned-tube condenser designs. While this heat transfer augmentation technique increases the heat transfer coefficient in the heat exchanger, it also increases the air side frictional pressure drop. These two effects compete with each other, making it difficult to determine the relative goodness between plain fin versus enhanced fin designs with realistic constraints. In the past, this design tradeoff has been largely determined by experimental trial and error or heuristic figures of merit. No studies are available showing the effect of fin augmentation on overall system performance under consistent cost and frontal area constraints. The residential air conditioning system model calculates all component and system performance parameters. The condenser design requires the specification of approximately ten design parameters. A search method is used to vary these ten parameters and reach an optimum design based on a COP (efficiency) figure-of-merit with condenser cost and other appropriate constraints. It was found that when optimized, louvered fin designs always show better system performance than the optimum plain fin design for the cases studied. However a decrease in system efficiency can result if louvers are merely added to a plain fin optimum design.© 2003 ASME

A Turbulence Intensity Similarity Distribution for Evaluating the Performance of a Small Wind Turbine in Turbulent Wind Regimes

Field tests on a Skystream wind turbine in a turbulent environment were used to study the impacts of turbulence intensity on power performance. The objective of the study was to examine the influence of turbulence intensity on the deviations in power output from their published power curves, commonly experienced by small wind turbines, such that more accurate predictions in performance can be made for future small wind projects. One-minute averaged data was used to explore the distribution of turbulence intensities experienced in several wind speed bins. It was found that the resulting turbulence intensity distributions have a similar distribution across these wind speeds and thus it was compared to four common statistical distributions. A goodness of fit study was conducted finding a gamma distribution was the best fit. This finding constitutes a more elegant approach to empirically adjusting power curves for turbulence intensity than has been suggested by previous studies.

Evaluating the Aerodynamic Performance of Small Horizontal Axis Wind Turbines

A power performance field test campaign has been run on a 2.4 kW rated small horizontal axis wind turbine. The wind-electric system has been instrumented with power and meteorological sensors with the goal of developing a field-test facility with the capability of performing comprehensive power production and rotor aerodynamics studies. A power curve has been resolved for the wind turbine using the method of bins and filtering data based on the environmental characteristics of the test site. Field test results agree well with similar data campaigns available through the Small Wind Certification Council and the turbine manufacturer. Comparisons are made between various averaging intervals (30-sec and 60-sec) along with an assessment of how differing averaging intervals change correlation between power and meteorological metrics. Some insight into how obstructions at the test site affect data correlation and the “bin method” analysis are given. WT_Perf was used to produce computational performance predictions for the wind turbine rotor. Comparisons between WT_Perf and field test data show reasonable agreement between the data sets when analyzing the operational wind speed and rotational speed envelopes of the wind turbine.

Assessing Wind Resource Potential in the Built Environment

This study addresses the issue of siting wind turbines on existing structures in the built environment for optimal performance. Annually averaged wind power maps were produced over the surface of two different building types using a Detached Eddy Simulation (DES) model in order to assess the feasibility of building integrated wind under various wind resource conditions. The modeling approach was first applied to a cubical geometry for which validation of the CFD results was possible with existing field measurements. A pitched roof building was also modeled to study the power density over top of typical residential shaped structures. The average annual power density for twenty-seven locations over the top of the modeled structures was analyzed under varying wind direction distributions (wind roses). The overall results of this study have the potential to inform the wind energy and built environment communities on best practices for siting wind turbines on or near buildings.Copyright

Adaptive Geometry Wind Turbine Blades for Increasing Performance

With wind turbines working to capture energy at different wind speeds rotor morphing could potentially increase energy capture over wind speeds up to the rated speed. This study examines what the optimal geometry might look like at different wind speeds, how it might differ from one speed to another, and how much increase in power and annual energy production could be realized with the optimal geometry at each wind speed. Using a blade-element theory based analysis and conducting simulations on the 1.5 MW WindPACT turbine and the 5MW NREL concept turbine, variations in blade twist and collective pitch, chord, radius, and airfoil characteristics were considered. The results indicate that there are negligible benefits to changing blade collective pitch, twist, chord, and airfoil characteristics. Only radius increase has a dominant effect, with 20% increase in radius resulting in power increase of over 45% at 8 and 10 m/s and much higher percentage increases at lower speeds, for both turbines. The increase in annual energy production is in the range of 20%. However, a larger radius increases rotor thrust.Copyright

Economical Design of Wind-Solar Hybrid Renewable Energy Systems

Every location on Earth has its own unique set of natural resources to draw upon for sustainable energy production. As these resources are generally of an intermittent nature, hybrid systems will be necessary in many situations to achieve economical energy independence while meeting our inconsistent demands for electricity with minimal or no energy storage. Wind and solar resources often have complimentary attributes that combined can more closely match energy load requirements. This match can be customized for optimum economy by adjusting the orientation and design of the PV system as well as the rotor length and generator size of the wind turbine system. Different load requirements and electricity rate structures require a different design approach in order to achieve optimum cost savings. Using the Penn State SURFRAD wind speed and solar radiation data set the design process for solar-wind hybrid renewable energy systems is explored for the case of a grid-tied residential scale application with a time of use electricity rate structure.Copyright