Low Wind Situations Research Articles

Fabrication and Performance Analysis of the Aero-Leaf Savonius Wind Turbine Tree

Large wind turbines of the horizontal axis are commonly used to gather wind energy; however, their performance is found to be constrained in conditions of erratic and low-speed wind flow. In contrast, low wind conditions—which are typically present in dense urban areas—are found to favour vertical axis wind turbines (VAWT). These turbines have a simple design, are inexpensive and quiet, and are discovered to be better in low wind situations. In this research, we have chosen wind tree applications to absorb the most available wind energy. The new Aeroleaf Savonius Wind Turbine was developed numerically and a computational fluid dynamics simulation was performed on this new type of Savonius tree to predict its performance. The results indicated that the system could accept wind from any direction and could start rotating as soon as the site had a cut in wind speed of 3.3 m/s. The rotor speed increased by 10.4% from 5.5 to 6.3 m/s wind speed at 0.45 tip speed ratio. The tip speed ratio is 0.52 at the site’s high wind speed, and under these circumstances, the maximum Cp is 12.9%. The turbine was able to produce superior performance coefficients, according to the results.

The benefit of long-term high resolution wind data for electricity system analysis

Future energy systems rely increasingly on the wind power supply. Understanding its characteristics is essential for the functioning of future electricity systems. Critical low wind situations may endanger the security of supply. So far, historical observations of wind power production are limited to few recent historical years and may not suffice to quantify the expected overall wind contribution, its variability, and its regional balancing effects for future electricity systems. With a novel long-term high-resolution wind power production dataset (hourly on a 6 × 6 km grid for 20 years) we derive new insights. First, we find advantages of our high-resolution dataset compared to previous studies. Second, we find a strong variation in annual wind production (variation of up to 14% for Germany). And third, we find a potential benefit from electricity exchange with neighboring countries in low wind conditions (for Germany in 81% of the low wind situations). The results are highly relevant for further investigation on the level of secured capacity or to identify optimal power transmission capacities within energy market modeling.

Early-Morning Flow Transition in a Valley in Low-Mountain Terrain Under Clear-Sky Conditions

We investigate the evolution of the early-morning boundary layer in a low-mountain valley in south-western Germany during COPS (convective and orographically induced precipitation study) in summer 2007. The term low-mountain refers to a mountainous region with a relief of gentle slopes and with an absolute altitude that remains under a specified height (usually 1,500 m a.s.l.). A subset of 23 fair weather days from the campaign was selected to study the transition of the boundary-layer flow in the early morning. The typical valley atmosphere in the morning hours was characterized by a stable temperature stratification and a pronounced valley wind system. During the reversal period—called the low wind period—of the valley wind system (duration of 1–2 h), the horizontal flow was very weak and the conditions for free convection were fulfilled close to the ground. Ground-based sodar observations of the vertical wind show enhanced values of upward motion, and the corresponding statistical properties differ from those observed under windless convective conditions over flat terrain. Large-eddy simulations of the boundary-layer transition in the valley were conducted, and statistical properties of the simulated flow agree with the observed quantities. Spatially coherent turbulence structures are present in the temporal as well as in the ensemble mean analysis. Thus, the complex orography induces coherent convective structures at predictable, specific locations during the early-morning low wind situations. These coherent updrafts, found in both the sodar observations and the simulation, lead to a flux counter to the gradient of the stably stratified valley atmosphere and reach up to the heights of the surrounding ridges. Furthermore, the energy balance in the surface layer during the low wind periods is closed. However, it becomes unclosed after the onset of the valley wind. The partition into the sensible and the latent heat fluxes indicates that missing flux components of sensible heat are the main reason for the unclosed energy balance in the considered situations. This result supports previously published investigations on the energy balance closure.

A New Method to Estimate Diffusion in Stable, Low-Wind Conditions

Sonic anemometer observations were made 10 m above ground level for a period of 1 yr. From these data, Eulerian autocorrelation functions were computed for the horizontal and vertical wind velocity fluctuations for low wind speeds. Although the autocorrelation function for the vertical velocity component exhibited the well-known exponential form, the function for the horizontal components of the wind vector showed a negative loop for all stability classes at low wind speeds. This result might be an effect of low-frequency meandering of the flow. Observations of the standard deviations of the vertical wind component confirmed the proportionality with the friction velocity, though with a slightly lower constant of proportionality than has been found by other authors. A Lagrangian dispersion model (LDM) with random time steps and a negative intercorrelation parameter ρu,υ for the horizontal wind components was used to take the first of the above-mentioned findings into account. In a simple test case, it could be shown that using a negative tail in the autocorrelation function for the horizontal wind fluctuations in an LDM results in larger plume spreads as if the usual exponential law were used. This model characteristic is in agreement with enhanced dispersion in low-wind situations as found by different authors earlier. Because the model reduces to the Langevin equation for ρu,υ = 0.9, it has the advantage that it can be used for all wind speeds by simply adjusting the intercorrelation parameter. Last, the model was tested against field experiment data gathered by the Idaho National Engineering Laboratory during stable, low-wind conditions. The results with the new method for these experiments are very promising in comparison with methods used by other authors earlier.

A new mixed segment-puff approach for dispersion modeling

This paper presents a new mixed methodology for realistic and cost-effective simulation of shortterm air quality dispersion phenomena using the Gaussian formula. The method can be applied to shortrange, intermediate and, especially, long-range transport simulations. Pollutant dynamics are described by the temporal evolution of plume elements, treated as segments or puffs according to their size. While the segments provide a numerically fast simulation during transport conditions, the puffs allow a proper simulation of calm or low-wind situations. The methodology is incorporated into a computer package (AVACTA II, Release 3) that gives the user large flexibility in defining the computational domain, the three-dimensional meteorological and emission input, the receptor locations, and in selecting plume rise and sigma formulas. AVACTA II provides both pollutant concentration fields and dry/wet deposition patterns. The model uses linear chemistry and is applicable to any two-species reaction chain (e.g., SO 2 and SO 2− 4) where this approximation is reasonable and an appropriate reaction rate is available.

Turbulent transport near the ground as determined from measurements of the ozone flux and the ozone gradient

Simultaneous measurements of the vertical fluxes of ozone and heat were made near Albuquerque, New Mexico, in 1968, together with measurements of the vertical profiles of ozone and temperature in the first 16 meters above the surface. The eddy flux of ozone was determined by a new method using the decay of ozone in a box with open bottom, placed on the ground. The eddy flux of heat was determined as the difference between the net radiative flux and the soil heat flux. The vertical distributions of ozone, temperature, and wind were measured with sensors mounted on a platform that was raised and lowered electrically along a 16-meter tower. The profiles and the flux values are used to compute coefficients of eddy diffusivity of ozone and heat. The results show that measurements of the ozone flux and of the ozone gradient can be used for determinations of the eddy transport of atmospheric properties in low-wind situations, whenever measurements of temperature or wind profiles, and of the related fluxes, are not practical.