Browsing by Subject "Wind Turbine"

Now showing 1 - 5 of 5
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    Development of a Hydraulic Energy Storage System for Hybrid Wind Turbine Transmissions
    (2021-05) Mohr, Eric
    Mid-size wind turbines are an under-recognized means to help prevent irreversible climate damage caused by unprecedented human-made carbon emissions. A high-power hydraulic energy storage system can be added to turbine transmissions to capture energy in high wind speeds, and release energy in low wind speeds. The hybrid system stabilizes the output power of the transmission, and increases reliability while offering ancillary benefits such as fault-ride through and pitch and yaw control in severe weather. The hybrid system was constructed. Experimental characterization of hybrid parameters determined that a modified heat transfer model of the accumulator is realistic, and that the thermal time constant of the accumulator is around 80 seconds. A high fidelity simulation is produced which is experimentally validated. The simulation is then used to find the additional annual energy production compared to the non-hybrid system is 3.5%. This value can only be attained with the addition of a clutch and directional valve.
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    Modeling, Control and Optimization of a Novel Compressed Air Energy Storage System for Off-Shore Wind Turbines
    (2016-08) Saadat, Mohsen
    Integrating wind and solar energy into the electric power grid is challenging due to variations in wind speed and solar intensity. Moreover, to maintain the stability of electric power grid, there must be always a balance between the energy production and consumption which is not easy since both of them undergo drastic variation over time. Large scale energy storage systems can solve these issues by storing the extra energy when supply exceeds demand power, and regenerating energy and send it to the electric grid when demand power surpasses the supply. This dissertation focuses on the optimal design and control of a new type of Com- pressed Air Energy Storage (CAES) system that is especially applicable to off-shore wind turbines. The system is designed such that it addresses the need for a compact and energy dense storage system with high roundtrip efficiency for large-scale energy storage applications. The contributions of this work are also beneficial for designing power dense gas compressors/expanders with high thermal efficiency. The material of this thesis can be divided into two parts: In the first part, different approaches and techniques are studied to increase the power density of a liquid piston air compressor/expander system without sacrificing its efficiency. These methods are then combined and optimized in the form of a single design to maximize the performance of an air compressor/expander unit which is the most critical component of the energy storage system under investigation. In the second part, component-level and supervisory-level controllers are designed and developed for the combined wind turbine and energy storage system such that both short-term and long-term objectives are achieved. Improvement of thermal efficiency of an air compressor/expander is achievable by increasing heat transfer between air under compression/expansion and its surrounding solid material in the compression/expansion chamber. This will prevent heat loss by reducing air temperature rise/drop during the compression/expansion phase. Here, liquid piston (instead of conventional solid piston) is used in the compression/expansion chamber where the chamber volume is filled with porous material that increases heat transfer area by an order of magnitude, and therefore improves the thermal efficiency. Since the liquid piston is driven by a variable displacement pump/motor, optimal compression/expansion trajectories are calculated and applied to further increase heat transfer and improve the performance of the system. This improvement is verified both analytically and experimentally. Based on numerical results, utilizing porous material in the compression/expansion chamber with optimized distribution, combined with the corresponding optimal compression/expansion trajectory has the potential to increase the power density by more than 20 folds, without reducing its thermal efficiency. An alternative method to increase heat transfer is to introduce micro-size water droplets (through water spray) in the chamber during air compression/expansion process. Since water has a high heat capacity, the generated heat during compression can be absorbed by water and therefore reduce the temperature rise of air during compression. The same phenomenon but in opposite direction happens in expansion case (heat transfer from water droplets to air) that prevents air from getting very cold which causes poor efficiency. A numerical model is developed and used to study the effect of water spray amount and timing on the thermal performance of air compressor/expander. The opti- mal timing of water spray is calculated to maximize the effectiveness of a given amount of water that is sprayed into the air. The optimally designed liquid piston air compressor/expander unit is then combined with the other components of energy storage system, as well as a wind turbine. Nonlinear techniques are used to design plant-level controllers in order to coordinate different parts in the system, and to achieve both short term objectives (maintaining the frequency of electric generator while capturing maximum wind power) and long term objectives (tracking the power demanded from electric grid and regulating the pressure in the storage vessel). Finally, the combined wind turbine and energy storage system is studied for maximizing the total achievable revenue by optimizing the storage/regeneration sequence according to varying electricity price and available wind power (given storage size and its nominal power). According to the results, an increase of up to 137% in total revenue is achievable by equipping a conventional wind turbine with a CAES system while tracking the calculated optimal storage/regeneration sequence. Additionally, by incorporating the price of different components of energy storage system, a study is conducted to find the effect of system size on maximum achievable revenue that can lead to the economical size selection of the energy storage system.
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    Near-wake studies of a utility-scale wind turbine using natural snowfall-based visualization and Particle Image Velocimetry
    (2018-12) Dasari, Teja
    Wind turbine technology has been extensively researched in the past century. A major part of this research effort was to improve the efficiencies of energy extraction and the lifespan of the turbines to make the technology economically viable. One important barrier in this regard is the insufficient knowledge on the wakes of the turbines which directly affect the turbine performance and also cause premature failure of the turbines (Frandsen et al. 2006; Barthelmie et al. 2007). Only when the complex behavior of wakes is understood can they be successfully controlled/modified/mitigated to improve overall wind turbine/farm efficiencies. In order to achieve this objective, a clear need exists for a complete understanding of wind turbine loading, subsequent vortex system, role of ambient turbulence and coherent turbulent structures within the wake (Sørensen 2011). Furthermore, the overall wake development and behavior is strongly dependent on the near-wake (1 – 4D) characteristics. The current research work employs natural snowfall based visualization techniques demonstrated by Toloui et al. 2014 and Hong et al. 2014 to further probe the near-wake behavior of the 2.5 MW EOLOS wind turbine located in Rosemount, MN. The broad objectives of the study are to examine the near-wake (within 0.2 to 1 D) dynamics under different regions of turbine operation, ambient conditions and turbine loading. The specific objectives are to (i) better visualize the wind turbine blade generated coherent structures; (ii) study the characteristic behaviors of these coherent structures and explore correlations with the turbine operation and response characteristics; (iii) provide unique and realistic near-wake visualization data to the wind energy research community; (iv) explore the characteristics of the extreme near-wake in a holistic fashion with unprecedented spatiotemporal resolutions that can provide critical insights on utility-scale wake behavior. Accordingly, visualization data are collected with varying regions of interest ranging from ~ 20 m to ~120 m. The typical whole-wake measurements include visualization and super-large-scale Particle Image Velocimetry (SLPIV) measurements in the near wake of the turbine in a field of view (FOV) of the size of a football field (~ 120 m vertical × 70 m streamwise). The SLPIV measurements provide velocity deficit and turbulent kinetic energy assessments over the entire rotor span. The instantaneous velocity fields from SLPIV indicate the presence of intermittent wake contraction states which are in clear contrast with the expansion states typically associated with wind turbine wakes. These contraction states feature a pronounced upsurge of velocity in the central portion of the wake. The wake velocity ratio R_w, defined as the ratio of the spatially-averaged velocity of the inner wake to that of the outer wake, is introduced to categorize instantaneous near wake into expansion (R_w<1) and contraction states (R_w>1). Based on R_w criterion, the wake contraction occurs 25% of the time during the 30-minute time duration of SLPIV measurements. The contraction states are found to be correlated with the rate of change of blade pitch by examining the distribution and samples of time sequences of wake states with different turbine operation parameters. Moreover, blade pitch change is shown to be strongly correlated to the tower and blade strains measured on the turbine, and the result suggests that the flexing of the turbine tower and the blades could indeed lead to the interaction of rotor with the turbine wake, causing wake contraction. Similar visualization data collected along the wake symmetry plane, along the tower axis, revealed an accelerating flow field behind the nacelle of the turbine. This region is also characterized by relatively higher turbulence characteristics due to the shear production of TKE. This region of TKE with relatively high values (or peak in TKE) is found to waver about the hub elevation which might be an effect of yaw error on the turbine. The smaller field of view studies representing visualization of tip vortex behavior, near the elevation corresponding to the bottom blade tip, over a broad range of turbine operational conditions, demonstrate the presence of the state of consistent vortex formation as well as various types of disturbed vortex states. The histograms corresponding to the consistent and disturbed states are examined over a number of turbine operation/response parameters, including turbine power and tower strain as well as the fluctuation of these quantities, with different conditional sampling restrictions. This analysis establishes a clear statistical correspondence between these turbine parameters and tip vortex behaviours under different turbine operation conditions, which is further substantiated by examining samples of time series of these turbine parameters and tip vortex patterns. This study not only offers benchmark datasets for comparison with the-state-of-the-art numerical simulation, laboratory and field measurements but also sheds light on understanding wake characteristics and its downstream development, turbine performance and regulation, as well as developing novel turbine or wind farm control strategies.
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    On the Effects of a Vortex Breaker on the Wake Meandering Characteristics of a Miniature Wind Turbine
    (2017-12-14) Storm, Noah J.
    This investigation sought to understand how the addition of a vortex breaker to the nacelle of a miniature wind turbine might disrupt, enhance, mitigate, or otherwise alter the meandering characteristics of the wake flow. Velocity measurements were taken in the wake of the turbine in a wind tunnel experiment, and analyzed with MATLAB and Microsoft Excel. Three vortex breakers were designed, and each of these cases was compared to the wind turbine with no nacelle additions. A decrease in mean streamwise wake velocity and appreciable shifts in the peak meandering frequency and intensity were observed. Peak spanwise frequencies were normalized by the mean wake velocity to compensate for effects due to increased drag on the turbine when the vortex breakers were added. This normalization is appropriate, as previous experiments have shown that wake meandering scales with the Strouhal Number (St = fD/U) for utility and small-scale turbines [1]. Meandering frequency variations between vortex breaker cases and the bare nacelle case were partially attenuated when analyzing Strouhal percent differences. This suggests that the alterations in the meandering characteristics were a combinatorial result of both increased nacelle drag and vortex interactions between the nacelle vortex and blade tip vortices.
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    Rich Data for Wind Turbine Power Performance Analysis
    (2019-07-31) Davison, Brian; B.Davison@napier.ac.uk; Davison, Brian; University of Minnesota St. Anthony Falls Laboratory
    The post-installation verification of wind turbine performance is an essential part of a wind energy project. Data collected from meteorological instruments and from the turbine is analysed to produce an estimate of the annual energy production (AEP) which is compared against expectations. However, turbine warranties can impose very strict data filtering criteria which can lead to high rates of data loss. As a consequence, measurement campaigns may last longer than expected and incur additional costs for the development. This project aims to investigate the extent of the problem and the potential of alternative data filtering strategies with respect to data loss, AEP estimates and the dispersion of points in the power curve scatter plot. In doing so, it targets a wide range of meteorological parameters with theoretical relationships to wind turbine power production with particular interest in those not accounted for in the current standard. The identification of viable filtering strategies with lower data loss would provide significant benefits to wind energy development projects in terms of greater control over timescales and reduced costs. Data from a sample of power performance tests is analysed to explore the range and severity of the problem of data loss. It confirms the wide variation in warranty conditions, demonstrates the extent and likelihood of data losses and quantifies the financial implications within the limits of commercial sensitivity. When indirect costs are taken into consideration, the impact of extended measurement campaigns can theoretically reach tens of millions of pounds. A new, high-fidelity dataset is then compiled so that the effects of alternative filtering strategies can be examined. The dataset covers the whole of 2017 and consists of over 700 parameters of which 74 are selected for investigation here. The eFAST method of global sensitivity analysis is used in combination with correlation analysis to reduce this number to 11 parameters which are then used to define alternative filtering criteria. Similar AEP estimates are obtained by application of conventional and experimental criteria to the research dataset. In the case of the experimental filters, however, the data loss was 11% compared to 63% data loss with conventional filters. Conventional filters were also shown to increase the dispersion in the power curve scatter plot by over 10%, while dispersion did not increase significantly with the experimental filters.