Low Wind Speed Conditions Research Articles

Comparison of Self Tuning P and PI Voltage Control of DFIG in Wind Power Generation Considering Two Mass Shaft Model

As wind energy is a low cost and renewable energy and wind turbines are also simple and reliable, wind power plants are vastly penetrated in power grids and Since power control is significantly important for them, doubly fed induction generators as a variable speed induction generators are applied instead of other electric machines in wind power plants to be connected to the grid with flexible controllers. In this paper a doubly-fed induction generator with external rotor resistance and power converters are modeled as a voltage source which has an adjustable phase and amplitude with an ordinary turbine connected to a practical two mass shaft model which is used and controlled by a PI in comparison to P self tuning voltage controller to control the output active and reactive powers for typical high and low wind speed conditions. Simulation results show that the P and PI self tuning voltage controllers perform successfully in power controlling but P controller is simpler and domain of output oscillations are less than PI controller’s

Propagation of uncertainty analysis of CO2transfer velocities derived from the COARE gas transfer model using satellite inputs

A propagation of uncertainty method is developed to describe gas transfer uncertainties for carbon dioxide (CO2) using the National Oceanic and Atmospheric Administration-Coupled Ocean Atmosphere Response Experiment (NOAA-COARE) gas transfer model (COAREG). The uncertainty model assesses the impact uncertainties in satellite inputs to COAREG have on the resulting carbon dioxide gas transfer velocities. The model impact of 10 m temperature, 10 m specific humidity, 10 m wind speed, sea surface temperature, longwave and shortwave downward surface flux are investigated. Nine years of gas transfer velocities and their uncertainties were constructed and analyzed. The climatological mean transfer velocities were found to have standard errors of less than 15%. Spatial and temporal variations of the uncertainties from these inputs are presented and the percentage variance explained for each input was dominated by uncertainties in wind speed. Uncertainty due to the covariance between air temperature and wind speed and the uncertainties in the 10 m air temperature at low wind speed conditions were found to be significant. Additionally, the likelihood air temperature uncertainties affect the gas transfer uncertainty is greatest in regions where air temperature exceeds sea surface temperature. Buoyancy-driven transfer in COAREG increases gas transfer velocities uncertainties under these conditions. These conditions most typically occur in regions along the equatorial cold tongue region east of South America and in the North Pacific and North Atlantic.

Experimental investigation of surface-layer parameters in low wind-speed conditions in a suburban area

Low wind-speed conditions in a suburban area were investigated through the analysis of sonic anemometer observations gathered at three vertical levels, 5, 9 and 25 m, during an experimental campaign carried out in the city of Turin in northern Italy. The data analysis allowed assessing how the surface-layer variables are affected by the specific heterogeneous conditions characterized by a calm regime. The applicability of the known theories for the surface layer and their limitations in such an environment were investigated. The roughness and the stability parameters, the friction-velocity and the wind-velocity standard deviations were analysed accounting for the variability of the wind speed and direction and of the urban structure. Formulations for some of these quantities, to be used in air pollution modelling, were considered, discussed and verified with the observations. The low wind-speed regime proved to be more effective in influencing the characteristics of the surface-layer structure and of the turbulence than the urban geometry itself, with the latter mainly affecting the surface-roughness parameters.

A method for sea surface wind field retrieval from SAR image mode data

To retrieve wind field from SAR images, the development for surface wind field retrieval from SAR images based on the improvement of new inversion model is present. Geophysical Model Functions (GMFs) have been widely applied for wind field retrieval from SAR images. Among them CMOD4 has a good performance under low and moderate wind conditions. Although CMOD5 is developed recently with a more fundamental basis, it has ambiguity of wind speed and a shape gradient of normalized radar cross section under low wind speed condition. This study proposes a method of wind field retrieval from SAR image by combining CMOD5 and CMOD4 Five VV-polarisation RADARSAT2 SAR images are implemented for validation and the retrieval results by a combination method (CMOD5 and CMOD4) together with CMOD4 GMF are compared with QuikSCAT wind data. The root-mean-square error (RMSE) of wind speed is 0.75 m s−1 with correlation coefficient 0.84 using the combination method and the RMSE of wind speed is 1.01 m s−1 with correlation coefficient 0.72 using CMOD4 GMF alone for those cases. The proposed method can be applied to SAR image for avoiding the internal defect in CMOD5 under low wind speed condition.

Azimuth Ambiguities Removal for Ship Detection Using Full Polarimetric X-Band SAR Data

Synthetic aperture radar (SAR) ship detection is an important application in the field of maritime security. Azimuth ambiguities caused by the aliasing of the Doppler phase history of each point are often visible in SAR images particularly in ocean areas of low wind speed condition, e.g., in coastal areas, in harbors, etc. The main sources of azimuth ambiguities are man-made metallic structures over the ocean, e.g., ships, oil platforms etc., and over land near the coast, e.g., big tanks, bridges' pylons etc., that have a high SAR backscatter responses. Although the ambiguities' backscatter is generally low, in many situations, it is above the surrounding ocean clutter and are mistaken by classic detection techniques, like constant false alarm rate, as real targets causing false positives. This paper addresses both the discrimination of real targets from non-trivial false positives, namely those due to azimuth ambiguities and the detection itself using a Generalized- K distribution approach. The methodology is firstly proposed and demonstrated over a significant data set of full polarimetric X-band SAR data, which have been acquired by the German satellite TerraSAR-X during the experimental dual receive antenna campaign in April and May 2010. It is based on the intrinsic configuration of monostatic two-channel PolSAR systems and relies on the different signature of azimuth ambiguities in cross-polarized channels. Automatic Identification System messages collected and collocated with the data set analyzed are used as ground truth to evaluate and validate the proposed methodology.

Atmospheric CO2 Variations in Two Contrasting Environmental Sites Over India

We analyzed the influence of environmental parameters on the temporal variation of atmospheric carbon dioxide (CO2) mixing ratios in two environmentally contrasting Indian sites, Dehradun (30.1°N, 77.4°E, humid subtropical station) and Gadanki (13.5°N, 79.18°E, dry tropical station), from October 2010 to September 2011. The annual range of mixing ratios is low in Gadanki as compared to those of Dehradun because of relatively less monthly variation in temperature and relative humidity (RH) at Gadanki. At both the stations, the minimum mixing ratios are present during the high ecosystem productivity seasons in the afternoon hours. The maximum values are in the early morning hours. However, low wind speed conditions control the unexpected afternoon high mixing ratios in Gadanki during the pre-monsoon season. The early morning maximum is high during monsoon and post-monsoon seasons in Dehradun and Gadanki, respectively, whereas morning inflexion occurred earlier in Gadanki compared with Dehradun. The effect of cloudiness on the CO2 uptake depends on the canopy cover.

Stable Atmospheric Boundary Layers and Diurnal Cycles: Challenges for Weather and Climate Models

The representation of the atmospheric boundary layer is an important part of weather and climate models and impacts many applications such as air quality and wind energy. Over the years, the performance in modeling 2-m temperature and 10-m wind speed has improved but errors are still significant. This is in particular the case under clear skies and low wind speed conditions at night as well as during winter in stably stratified conditions over land and ice. In this paper, the authors review these issues and provide an overview of the current understanding and model performance. Results from weather forecast and climate models are used to illustrate the state of the art as well as findings and recommendations from three intercomparison studies held within the Global Energy and Water Exchanges (GEWEX) Atmospheric Boundary Layer Study (GABLS). Within GABLS, the focus has been on the examination of the representation of the stable boundary layer and the diurnal cycle over land in clear-sky conditions. For this purpose, single-column versions of weather and climate models have been compared with observations, research models, and large-eddy simulations. The intercomparison cases are based on observations taken in the Arctic, Kansas, and Cabauw in the Netherlands. From these studies, we find that even for the noncloudy boundary layer important parameterization challenges remain.

AERMOD performance evaluation for three coal-fired electrical generating units in Southwest Indiana

An evaluation of the steady-state dispersion model AERMOD was conducted to determine its accuracy at predicting hourly ground-level concentrations of sulfur dioxide (SO2) by comparing model-predicted concentrations to a full year of monitored SO2 data. The two study sites are comprised of three coal-fired electrical generating units (EGUs) located in southwest Indiana. The sites are characterized by tall, buoyant stacks, flat terrain, multiple SO2 monitors, and relatively isolated locations. AERMOD v12060 and AERMOD v12345 with BETA options were evaluated at each study site. For the six monitor–receptor pairs evaluated, AERMOD showed generally good agreement with monitor values for the hourly 99th percentile SO2 design value, with design value ratios that ranged from 0.92 to 1.99. AERMOD was within acceptable performance limits for the Robust Highest Concentration (RHC) statistic (RHC ratios ranged from 0.54 to 1.71) at all six monitors. Analysis of the top 5% of hourly concentrations at the six monitor–receptor sites, paired in time and space, indicated poor model performance in the upper concentration range. The amount of hourly model predicted data that was within a factor of 2 of observations at these higher concentrations ranged from 14 to 43% over the six sites. Analysis of subsets of data showed consistent overprediction during low wind speed and unstable meteorological conditions, and underprediction during stable, low wind conditions. Hourly paired comparisons represent a stringent measure of model performance; however, given the potential for application of hourly model predictions to the SO2 NAAQS design value, this may be appropriate. At these two sites, AERMOD v12345 BETA options do not improve model performance. Implications: A regulatory evaluation of AERMOD utilizing quantile-quantile (Q–Q) plots, the RHC statistic, and 99th percentile design value concentrations indicates that model performance is acceptable according to widely accepted regulatory performance limits. However, a scientific evaluation examining hourly paired monitor and model values at concentrations of interest indicates overprediction and underprediction bias that is outside of acceptable model performance measures. Overprediction of 1-hr SO2 concentrations by AERMOD presents major ramifications for state and local permitting authorities when establishing emission limits.

Feedback Mechanisms Between Cyanobacterial Blooms, Transient Hypoxia, and Benthic Phosphorus Regeneration in Shallow Coastal Environments

We investigated the dissolved oxygen metabolism of the Curonian Lagoon (Baltic Sea) to assess the relative contributions of pelagic and benthic processes to the development of transient hypoxic conditions in shallow water habitats. Metabolism measurements along with the remote sensing-derived estimates of spatial variability in chlorophyll a were used to evaluate the risk of hypoxia at the whole lagoon level. Our data demonstrate that cyanobacterial blooms strongly inhibit light penetration, resulting in net heterotrophic conditions in which pelagic oxygen demand exceeds benthic oxygen demand by an order of magnitude. The combination of bloom conditions and reduced vertical mixing during calm periods resulted in oxygen depletion of bottom waters and greater sediment nutrient release. The peak of reactive P regeneration (nearly 30 μmol m−2 h−1) coincided with oxygen depletion in the water column, and resulted in a marked drop of the inorganic N:P ratio (from >40 to <5, as molar). Our results suggest a strong link between cyanobacterial blooms, pelagic respiration, hypoxia, and P regeneration, which acts as a feedback in sustaining algal blooms through internal nutrient cycling. Meteorological data and satellite-derived maps of chlorophyll a were used to show that nearly 70 % of the lagoon surface (approximately 1,000 km2) is prone to transient hypoxia development when blooms coincide with low wind speed conditions.

One Year of Surface-Based Temperature Inversions at Dome C, Antarctica

In 2005 the Study of Stable Boundary Layer Environment at Dome C (STABLEDC) experimental campaign was conducted at the plateau station of Concordia at Dome C, Antarctica. Temperature profiles measured with a microwave radiometer were used to study the characteristics of surface-based temperature inversions over the course of a year. Statistics of temperature profiles for every month are discussed; the difference between daytime and nocturnal cases observed during the summer months disappears during winter. Surface-based temperature inversions occurred in 70 % of the time during summer, and almost all of the time during winter. During winter the occurrence of warming events leads to a decrease in the temperature difference between the top and the base of the inversion (i.e. the inversion strength). The inversion strength maxima ranged between \(3\,^{\circ }\mathrm{C}\) (December) and \(35\,^{\circ }\mathrm{C}\) (August) corresponding to gradients of 0.1 and \(0.3\,^{\circ }\mathrm{C}\, \mathrm{m}^{-1}\) , respectively. The average surface-based inversion height presents a daily cycle during the summer months with values up to 200 m in the morning hours, while it affects a layer always deeper than 100 m during the winter months. The relationships between inversion strength and the downward longwave radiative flux, absolute temperature, and wind speed are examined. The inversion strength decreases as the longwave radiation increases. A clear anti-correlation between inversion strength and near-surface temperature is evident throughout the year. During the winter, the largest inversion strength values were observed under low wind-speed conditions; in contrast, a clear dependence was not found during the summer.

Impacts of solidity and hybrid system in small wind turbines performance

Wind energy represents a very important source of energy for many countries nowadays. Wind energy provides an efficient and an effective solution to reduce fusel fuel consumption as well as pollutant emissions. VAWTs (vertical axis wind turbines) were originally considered as very promising, before being subrogated by the present, horizontal axis turbines. There is now a resurgence of interests for VAWTs, in particular Darrieus turbines. VAWTs (vertical axis wind turbines) like the Darrieus turbine appear to be particularly promising for the conditions of low wind speed, but suffer from a low efficiency compared to horizontal axis turbines. Additionally, VAWTs are not always self-starting, which is a major drawback. The present paper introduces the main problem of the self-stating capability of Darrieus turbine and investigates some techniques to improve this drawback. The effect of the turbine solidity and the usage of hybrid system between drag and lift types have been investigated in this paper numerically using CFD (Computational Fluid Dynamics) technique and experimentally. A considerable improvement of the H-rotor Darrieus turbine self-starting capability can be obtained by these techniques.

Thermal enhancement of gas transfer velocity of CO2 in an Amazon floodplain lake revealed by eddy covariance measurements

In November 2011, the partial pressures of carbon dioxide (pCO2) in water and air in a floodplain lake of the Amazon River in Brazil were 800 ± 75 and 387 ± 8 ppmv, respectively. Turbulent CO2 fluxes from the lake measured with eddy covariance ranged from 0.05 to 2.2 µmol m−2 s−1. The corresponding gas transfer velocities k600 ranged from 1.3 to 31.6 cm h−1, averaging 12.2 ± 6.7 cm h−1. At moderate to high wind speed, k600 increased with wind speed, with values above parameterizations for other lake ecosystems. During the prevailing tropical low wind speed (below 2.7 m s−1) and high insolation conditions, unexpected high k600 values (up to 20 cm h−1) were obtained and correlated with latent heat and sensible heat fluxes. In Amazonian open lakes, owing to long quiescent periods of low wind speed but extremely high daytime insolation and heat fluxes, thermal enhancement makes time‐integrated gas transfer velocities four to five times higher than those computed from classic wind parameterization.

Distribution and production of reactive mercury and dissolved gaseous mercury in surface waters and water/air mercury flux in reservoirs on Wujiang River, Southwest China

Transformation and distribution of mercury (Hg) species play an important role in the biogeochemical cycling of mercury in aquatic systems. Measurements of water/air exchange fluxes of Hg, reactive mercury (RHg), and dissolved gaseous mercury (DGM) concentrations were conducted at 14 sites in five reservoirs on the Wujiang River, Guizhou, Southwest China. Clear spatial and temporal variations in Hg fluxes, RHg, and DGM concentrations were observed in the study area. Hg fluxes and RHg concentrations exhibited a consistent diurnal variation in the study area, with maximum fluxes and concentrations during daytime. A typical diurnal trend of DGM with elevated concentration at night was observed in a eutrophic reservoir with elevated bacteria abundance, suggesting a bacteria‐induced production of DGM in this reservoir. For other reservoirs, a combination of sunlight‐stimulated production and loss via photo‐induced oxidation and evaporation regulated the diurnal trends of DGM. Seasonal variations with elevated Hg fluxes and RHg concentrations in warm season were noticeable in the study area, which highlighted the combined effect of interrelationships between Hg species in water and environmental parameters. Hg fluxes exhibited much more significant correlations with RHg and THg concentrations and air temperature compared to DGM concentrations and solar radiation. The measured fluxes were significantly higher than those simulated using the water/air thin film Hg0 gradient model. Aside from the potential limitations of dynamic flux chamber method, this may also suggest the thin film gas exchange model is not capable of predicting water/air Hg flux under low wind speed conditions. Additionally, it is speculated that DGM concentrations might vary significantly in surface waters with depth, and measurements of DGM at a depth of 2–4 cm below the water surface probably underestimated the DGM concentration that should be taken into account in simulations of water/air flux using the thin film gas exchange model. An empirical model of water/air Hg flux was developed, and the simulated fluxes were compared well with measurements. The model yields a mean annual Hg emission of 3.2 ± 1.0 kg in the study area.

Seasonal and diurnal analysis of NO2 concentrations from a long-duration study conducted in Las Vegas, Nevada

A study, conducted in Las Vegas, NV from mid-December 2008 to mid-December 2009 along an interstate highway, collected continuous and integrated samples for a wide variety of air pollutant species including NO2 and NOX associated with roadway traffic. This study examined long-term trends of NO2 and NOX in a near-road environment compared with previous near-road studies typically lasting only a few days to months. Study results revealed concentration gradients for NO2 and NOX with highest absolute and average concentrations at distances closest to the roadway throughout the year. Diurnal ambient temperature changes also influenced concentrations due to atmospheric chemistry activity as well as concentration changes due to seasonal effects. These concentration gradients were observed for all wind conditions; however under downwind conditions (winds from highway), the concentration gradients are more pronounced. Higher pollutant concentrations are generally observed during low wind speed conditions especially when those winds were from the highway. Understanding long-term, seasonal variability and levels of pollutant concentrations in the near-road environment is important to researchers and decision-makers evaluating exposures and risks for near-road populations; identifying locations for future near-road monitoring sites; and determining the viability and effectiveness of mitigation strategies. Implications: Population exposures to traffic emissions near roads have led to heightened public health concerns and awareness of the long-term levels and variability of these air pollutants. Epidemiological studies have lead to improved understanding of the associated risks and health effects of near road air pollutant emissions. While short-term studies provide insights on near-road air quality, longer term trends need to be understood, especially for reactive pollutants such as NO2. Supplemental Materials: Supplemental materials are available for this paper. Go to the publisher's online edition of the Journal of the Air & Waste Management Association.

A General Lagrangian Approach to Simulate Pollutant Dispersion in Atmosphere for Low-wind Condition

In this work we present a semi-analytical Lagrangian particle model to simu late the pollutant dispersion during low wind speed conditions. The model is based on a methodology, which solves the Langevin equation through the assumption that coefficient of the integrating factor is a complex function. The method leads to a non-linear stochastic integral equation, which is solved by the Method of Successive Approximations or Picard's Iterat ive Method. Taking into account the isomorphis m between the co mp lex and real p lane by writing down the low wind fo rmulat ion in polar form, the procedure allow to determine a formu la for the lo w wind direction. Furthermo re, an exp ression analogous to the Eulerian autocorrelation function suggested by Frenkiel(1) appears in the real co mponent solution. The model results present an improvement in relat ion to the other models and are shown to agree very well with the field tracer data collected during stable conditions at Idaho National Engineering Laboratory (INEL).

Connecting Turbulence and Meandering Parameterization to Describe Passive Scalars Dispersion in Low Wind Speed Conditions

The following study deals with meandering of the horizontal mean wind. The main motivation for such investigation came from the difficulty in describing contaminant dispersion in meandering conditions. Observational field measurements point out that the autocorrelation function of the horizontal wind components, obtained for the meandering cases, displays an oscillating behavior with the presence of large negative lobes. Such negative lobes are described by an equation containing functions that represent patterns of movement associated to meandering and turbulence. As a consequence, this mathematical formulation connects the turbulence and meandering phenomenon establishing the employment of hybrid parameters in models that describe the meandering dispersion. Therefore, considering this dualistic aspect between meandering and turbulence manifestations, a new set of relations for the turbulence parameterization joined with the meandering of the wind have been developed and are available. This new turbulence parameterization for a stable shear forcing planetary boundary layer, united with a meandering mean time scale is able to describe contaminant meandering enhanced spread in a low wind speed stable planetary boundary layer.

Comparison of Energy Yield of Small Wind Turbines in Low Wind Speed Areas

The objective of this paper is to compare the energy yield and generated electricity cost of several commercially available small wind turbines under low wind speed condition. Many small wind turbines were not able to generate the amount of electricity promised by the manufacturers. The predicted annual energy yield simulated using manufacturers' power curve were sometimes higher than actual measured values by up to a factor of two. Furthermore, above a rotor size of 3 m, large diameter turbines performed better, having both low generated electricity cost ( € /kWh) and high annual energy yield per swept area (kWh/m 2).

High Altitude Cycloidal Wind Turbine System Design

As a novel high altitude wind power system, this paper describes a cycloidal wind turbine mounted on a tethered balloon or parafoil. The rotor system was designed conceptually and the performance was estimated. The rotor blade span length is 10 meters and its diameter is 10 meters. When the rotor operates at 1,500 meters above ground, it produces 4.6 times more energy by the balloon tethered model. The parafoil tethered rotor power generation becomes 4.1 times at the same condition, which is a little smaller comparing with the balloon tethered model. The reason is its energy consumption at low wind speed condition to produce supplementary lifting force through mode conversion.

Time Domain Modeling and Analysis of Dynamic Gear Contact Force in a Wind Turbine Gearbox with Respect to Fatigue Assessment

The gearbox is one of the most expensive components of the wind turbine system. In order to refine the design and hence increase the long-term reliability, there has been increasing interest in utilizing time domain simulations in the prediction of gearbox design loads. In this study, three problems in time domain based gear contact fatigue analysis under dynamic conditions are discussed: (1) the torque reversal problem under low wind speed conditions, (2) statistical uncertainty effects due to time domain simulations and (3) simplified long term contact fatigue analysis of the gear tooth under dynamic conditions. Several recommendations to deal with these issues are proposed based on analyses of the National Renewable Energy Laboratory’s 750 kW land-based Gearbox Reliability Collaborative wind turbine.

Short-term observation of wind energy potentiality in the Wol-Ryong wind site

Accurate analysis of the atmospheric wind profiles and climate characteristics in a certain area is a prerequisite for providing reliable information on a wind energy site. Two 2-D ultrasonic anemometers and one cup anemometer, arranged perpendicularly to the prevailing wind direction, were used to measure the atmospheric wind environment at a height of 4.5 m in the coastal region of Wol-Ryong, Jeju, South Korea. This study aims to estimate future wind resources with various estimation methods. The wind energy is theoretically estimated at 75 m, the hub height of 800 kW and 1500-kW-class wind turbines, at the Wol-Ryong site. Methods using three equations, a logarithmic profile, a modified logarithmic profile, and a power law, are applied for accurate prediction of the atmospheric wind profile. In addition, yearly wind power can be calculated by using the probability distribution from Weibull and Rayleigh profiles. It is found that the predicted wind speed is strongly affected by surface atmospheric conditions such as the friction velocity, atmospheric stability, and averaged roughness length. The Rayleigh profile gives more power generation than the Weibull distribution, especially under low-windspeed conditions. The logarithmic profile method seems to be the proper method for estimating the energy production at the Wol-Ryong site for neutral atmospheric conditions. On the other hand, the other two methods — the modified logarithmic profile method and the power law method — seem to be improper for neutral conditions.