Riso National Laboratory for Sustainable Energy

Roskilde, Denmark

Riso National Laboratory for Sustainable Energy

Roskilde, Denmark
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Toft H.S.,University of Aalborg | Sorensen J.D.,University of Aalborg | Sorensen J.D.,Riso National Laboratory for Sustainable Energy
Structural Safety | Year: 2011

Reliability-based design of wind turbine blades requires identification of the important failure modes/limit states along with stochastic models for the uncertainties and methods for estimating the reliability. In the present paper it is described how reliability-based design can be applied to wind turbine blades. For wind turbine blades, tests with the basic composite materials and a few full-scale blades are normally performed during the design process. By adopting a reliability-based design approach, information from these tests can be taken into account in a rational way during the design process. In the present paper, a probabilistic framework for design of wind turbine blades are presented and it is demonstrated how information from tests can be taken into account using the Maximum-Likelihood method and Bayesian statistics. In a numerical example, the reliability is estimated for a wind turbine blade in both ultimate and fatigue limit states. Information from tests is used to formulate the stochastic models used in the limit state equations. Partial safety factors for use in traditional deterministic design are estimated using the stochastic models. © 2011 Elsevier Ltd.

Mishin O.V.,Riso National Laboratory for Sustainable Energy | Mishin O.V.,Technical University of Denmark | Segal V.M.,Engineered Performance Materials Company, LLC | Ferrasse S.,Honeywell
Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science | Year: 2012

A detailed quantitative analysis of the microstructure has been performed in three orthogonal planes of 15-mm-thick aluminum plates heavily deformed via two equal channel angular extrusion (ECAE) routes. One route was a conventional route A with no rotation between passes. Another route involved sequential 90 deg rotations about the normal direction (ND) between passes. The microstructure in the center of these plates, and especially the extent of microstructural heterogeneity, has been characterized quantitatively and compared with that in bar samples extruded via either route A or route Bc with 90 deg rotations about the longitudinal axis. Statistically robust data were obtained in this work using gallium enhanced microscopy and EBSD mapping of large sample areas. For the plate processed using route A, the fraction of high-angle boundaries was found to strongly depend on the inspection plane, being smallest in the plane perpendicular to the ND (plane Z), where the largest subgrain size and most profound microstructural heterogeneities were also revealed. In comparison, the plate extruded with 90 deg rotations about the ND was less heterogeneous and contained smaller subgrains in plane Z. Comparing the plate and bar samples, the most refined and least heterogeneous microstructure was observed in the route Bc bar sample. The differences in the microstructure are reflected in the hardness data; the hardness is lowest after ECAE via route A and greatest in the bar sample processed using route Bc. © 2012 The Minerals, Metals & Materials Society and ASM International.

Sumer B.M.,Technical University of Denmark | Sen M.B.,Technical University of Denmark | Sen M.B.,Grontmij A S | Karagali I.,Technical University of Denmark | And 7 more authors.
Journal of Geophysical Research: Oceans | Year: 2011

Two parallel experiments involving the evolution and runup of plunging solitary waves on a sloping bed were conducted: (1) a rigid-bed experiment, allowing direct (hot film) measurements of bed shear stresses and (2) a sediment-bed experiment, allowing for the measurement of pore water pressures and for observation of the morphological changes. The two experimental conditions were kept as similar as possible. The experiments showed that the complete sequence of the plunging solitary wave involves the following processes: shoaling and wave breaking; runup; rundown and hydraulic jump; and trailing wave. The bed shear stress measurements showed that the mean bed shear stress increases tremendously (with respect to that in the approaching wave boundary layer), by as much as a factor of 8, in the runup and rundown stages, and that the RMS value of the fluctuating component of the bed shear stress is also affected, by as much as a factor of 2, in the runup and hydraulic jump stages. The pore water pressure measurements showed that the sediment at (or near) the surface of the bed experiences upward directed pressure gradient forces during the down-rush phase. The magnitude of this force can reach values as much as approximately 30% of the submerged weight of the sediment. The experiments further showed that the sediment transport occurs in the sheet flow regime for a substantial portion of the beach covering the area where the entire sequence of the wave breaking takes place. The bed morphology is explained qualitatively in terms of the measured bed shear stress and the pressure gradient forces. Copyright 2011 by the American Geophysical Union.

Hogberg P.,Swedish University of Agricultural Sciences | Johannisson C.,Swedish University of Agricultural Sciences | Yarwood S.,Oregon State University | Callesen I.,Riso National Laboratory for Sustainable Energy | And 3 more authors.
New Phytologist | Year: 2011

Trees reduce their carbon (C) allocation to roots and mycorrhizal fungi in response to high nitrogen (N) additions, which should reduce the N retention capacity of forests. The time needed for recovery of mycorrhizas after termination of N loading remains unknown. Here, we report the long-term impact of N loading and the recovery of ectomycorrhiza after high N loading on a Pinus sylvestris forest. We analysed the N% and abundance of the stable isotope 15N in tree needles and soil, soil microbial fatty acid biomarkers and fungal DNA. Needles in N-loaded plots became enriched in 15N, reflecting decreased N retention by mycorrhizal fungi and isotopic discrimination against 15N during loss of N. Meanwhile, needles in N-limited (control) plots became depleted in 15N, reflecting high retention of 15N by mycorrhizal fungi. N loading was terminated after 20 yr. The δ15N and N% of the needles decreased 6 yr after N loading had been terminated, and approached values in control plots after 15 yr. This decrease, and the larger contributions compared with N-loaded plots of a fungal fatty acid biomarker and ectomycorrhizal sequences, suggest recovery of ectomycorrhiza. High N loading rapidly decreased the functional role of ectomycorrhiza in the forest N cycle, but significant recovery occurred within 6-15 yr after termination of N loading. © The Authors (2010). Journal compilation © New Phytologist Trust (2010).

Boulet P.,Aix - Marseille University | Knofel C.,Aix - Marseille University | Knofel C.,Riso National Laboratory for Sustainable Energy | Kuchta B.,Aix - Marseille University | And 2 more authors.
Journal of Molecular Modeling | Year: 2012

A theoretical investigation of the adsorption of CO2 onto ZrO2 is presented. Various cluster models were used to mimic different basic and acidic sites on the surface. The method used was the density functional theory with the generalized gradient approximation and including Grimme's empirical model in order to properly describe the weak interactions that may occur between the adsorbate and the surface. We found that the adsorption at sites exhibiting two adjacent unsaturated zirconium atoms led to either the exothermic dissociation of CO2 or to a strongly physisorbed state. By contrast, on a single unsaturated zirconium, CO 2 was adsorbed in an apical manner. In this case, the molecule is highly polarized and the adsorption energy amounts to -64.6 kJ mol-1. Finally, the weakest adsorption of CO2 occurred on the basic OH sites on the surface. © Springer-Verlag 2012.

Jensen J.B.,Riso National Laboratory for Sustainable Energy | Engelbrecht K.,Riso National Laboratory for Sustainable Energy | Bahl C.R.H.,Riso National Laboratory for Sustainable Energy | Pryds N.,Riso National Laboratory for Sustainable Energy | And 3 more authors.
International Journal of Heat and Mass Transfer | Year: 2010

A two-dimensional finite element model describing the performance of parallel-plate regenerators with arbitrary channel width distributions has been developed in order to investigate the effect of non-uniform plate spacing on the performance of regenerators. Results for a series of hypothetical plate spacing distributions are presented in order to understand the impact of spacing non-uniformity. Simulations of more realistic distributions where the plate spacings follow normal distributions are then discussed in order to describe the deviation of the performance of a regenerator relative to one with uniform spacing as a function of the standard deviation of the plate distribution. It has been shown that the most significant reduction in performance occurs when a volume of fluid between 100% and 200% of the regenerator void volume is displaced in a single blow. © 2010 Elsevier Ltd. All rights reserved.

Jensen J.B.,Riso National Laboratory for Sustainable Energy | Bahl C.R.H.,Riso National Laboratory for Sustainable Energy | Engelbrecht K.,Riso National Laboratory for Sustainable Energy | Elmegaard B.,Technical University of Denmark | Pryds N.,Riso National Laboratory for Sustainable Energy
International Journal of Heat and Mass Transfer | Year: 2011

Non-uniform distributions of plate spacings in parallel plate regenerators have been found to induce loss of performance. In this paper, it has been investigated how variations of three geometric parameters (the aspect ratio, the porosity, and the standard deviation of the plate spacing) affects this loss in a single blow model of a parallel-plate regenerator. Simple analytical functions for the magnitude and the time scale of the reduction of performance are presented and compared to numerical results. © 2011 Elsevier Ltd. All rights reserved.

Borden Z.,University of California at Santa Barbara | Koblitz T.,University of California at Santa Barbara | Koblitz T.,Riso National Laboratory for Sustainable Energy | Meiburg E.,University of California at Santa Barbara
Physics of Fluids | Year: 2012

Bores, or hydraulic jumps, appear in many natural settings and are useful in many industrial applications. If the densities of the two fluids between which a bore propagates are very different (i.e., water and air), the less dense fluid can be neglected when modeling a bore analytically-a single-layer hydraulic model will accurately predict a bore's speed of propagation. A two-layer model is required, however, if the densities are more similar. Mass is conserved separately in each layer and momentum is conserved globally, but the model requires for closure an assumption about the loss of energy across a bore. In the Boussinesq limit, it is known that there is a decrease of the total energy flux across a bore, but in the expanding layer, turbulent mixing at the interface entrains high speed fluid from the contracting layer, resulting in an increase in the flux of kinetic energy across the expanding layer of a bore. But it is unclear if this finding will extend to non-Boussinesq bores. We directly examine the flux of energy within non-Boussinesq bores using two-dimensional direct numerical simulations and find that a gain of energy across the expanding layer only occurs for bores where the density ratio, defined as the ratio of the density of the lighter fluid to the heavier fluid, is greater than approximately one half. For smaller density ratios, undular waves generated at the bore's front dominate over the effects of turbulent mixing, and the expanding layer loses energy across the bore. Based on our results, we show that if one can predict the amount of energy radiated by bores through undular waves, it is possible to derive an accurate model for the propagation of non-Boussinesq bores. © 2012 American Institute of Physics.

Hahmann A.N.,Riso National Laboratory for Sustainable Energy | Pena A.,Riso National Laboratory for Sustainable Energy
European Wind Energy Conference and Exhibition 2010, EWEC 2010 | Year: 2010

A real-time weather forecasting system for Denmark based on the Advanced Research WRF (ARW-WRF; Weather, Research and Forecasting) mesoscale model is presented. Short-term wind forecasts (0-48 hours) for the period May to October 2009 are verified against hourly winds from 10-meter conventional METeorological Aerodrome Report (METAR) measurements and wind measurements from the 116/160 m meteorological mast/light tower located at the Risø National Test Station for Large Wind Turbines at Høvsøre, Denmark. The verification of the 10-m wind speed forecasts against METAR observations shows that winds are generally overestimated during the nighttime hours over land. During the daytime, mean errors are reduced over most stations to around 0.5 m s -1. When classifying the statistics according to the observed wind speeds, lower wind speeds are overestimated while underestimated at higher wind velocities. The model-simulated wind profiles at Høvsøre do not deviate strongly from the logarithmic wind profile which indicates neutral conditions, while observations show stable conditions, especially when the wind originates from the East quadrant. In addition, the local conditions surounding the observation and WRF point must be taken into account since the mesoscale model fails to correctly represent these conditions. For this reason, the use of a microscale model, is recommended.

Engelbrecht K.,Riso National Laboratory for Sustainable Energy | Bahl C.R.H.,Riso National Laboratory for Sustainable Energy
Journal of Applied Physics | Year: 2010

Active magnetic regenerator (AMR) refrigerators represent an alternative to vapor compression technology that relies on the magnetocaloric effect in a solid refrigerant. Magnetocaloric materials are in development and properties are reported regularly. Recently, there has been an emphasis on developing materials with a high entropy change with magnetization while placing lower emphasis on the adiabatic temperature change. This work uses model magnetocaloric materials and a numerical AMR model to predict how the temperature change and entropy change with magnetization interact and how they affect the performance of a practical system. The distribution of the magnetocaloric effect as a function of temperature was also studied. It was found that the adiabatic temperature change in a magnetocaloric material can be more important than the isothermal entropy change for certain conditions. A material that exhibits a sharp peak in isothermal entropy change was shown to produce a significantly lower cooling power than a material with a wide peak in a practical AMR system. © 2010 American Institute of Physics.

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