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Biplane wind turbine blades have been shown to have improved structural performance, aerodynamic performance, and reduced aerodynamic loads compared to conventional blade designs. Here, the impact of these factors on blade mass is quantified for the first time. The objectives of this work are to quantify the mass of biplane wind turbine blades which have been designed for realistic loads, and to understand the mass-driving constraints for such blades. A numerical optimization approach is used to design the internal structure of biplane wind turbine blades, minimizing blade mass subject to a number of design requirements which are imposed as constraints. The mass reductions are significant, showing that the optimal biplane blades are more than 45% lighter than a similarly-optimized monoplane blade. This is primarily due to the improved resistance to flapwise deflection when compared to the monoplane blades, which allows for considerably less spar cap material to be used in the biplanes. Biplane blades are also shown to have improved resistance to edgewise fatigue damage, requiring less trailing edge reinforcement. Given such large mass reductions, some criticality is required, and the limitations of the present approach are discussed. The results of the optimization present strong evidence that biplane wind turbine blades may be an enabling concept for the next generation of lighter, larger, and more cost-effective wind turbine blades.

Elemental sulfur is a promising medium for moderate to high-temperature thermal energy storage (TES) systems due to its low cost and excellent chemical stability up to very high temperatures (1200 °C). Previous studies show that vertically-oriented tubes of isochorically contained thermal storage media (i.e., supercritical CO2) can exhibit higher heat transfer rates than horizontal tubes. Storing thermal storage media in vertical tubes in a TES system also has some potential system-level advantages related to exergy capacity, operation and maintenance, and cost. This paper investigates the heat transfer behavior and performance of sulfur contained in vertically-oriented tubes between room temperature (25 °C) and 600 °C. Experimental and computational analyses show that the natural convection heat transfer behavior for sulfur in a vertically-oriented tube is strongly dependent on the sulfur viscosity, which varies greatly over the range of temperatures used in this study. Validated Nusselt number correlations for vertical tubes of lengths between 0.5 and 3 m and diameters between 5.5 and 21.2 cm are developed for use in parametric studies and designs. In comparison to the horizontally-oriented tube, the vertical tube can have better heat transfer performance with some ranges of tube length and diameter. Therefore, the selection of the tube orientation strongly depends on the tube dimensions and application needs. The results from the current study provide important quantitative and qualitative design bases for sulfur-based TES (SulfurTES) systems.

Elemental sulfur provides a low-cost, high-performance thermal storage option for a wide range of applications and over an exceptionally wide range of temperatures (50 °C to over 600 °C). In previous efforts we have shown impressive performance for 200–600 °C, while in this study we examine the low-temperature (50–200 °C) thermal charge and discharge behavior of isochoric sulfur-based storage using a detailed computational model solving for the conjugate heat transfer and solid-liquid phase change dynamics. The model provides excellent agreement with experimental results. We show that sulfur exhibits lower viscosity because of reduction in the chain-length of polymeric sulfur caused by trace amounts of organic substances resulting in attractive charge and discharge performance. The results from the parametric analysis of pipe diameter on the charge and discharge heat transfer characteristics are used to develop a simple, generalized correlation that relates the transient sulfur temperature and liquid fraction evolution as a function of dynamically evolving buoyancy-Fourier number due to the solid-liquid phase change. This solid-liquid buoyancy-Fourier, BFs-l, correlation can be used for effectively designing sulfur-based thermal energy storage systems for transient operation in low temperature applications.

Elemental sulfur is a low-cost energy storage media suitable for many medium to high temperature applications, including trough and tower concentrated solar power (CSP) and combined heat and power (CHP) systems. In this study, we have demonstrated the viability of an elemental sulfur thermal energy storage (SulfurTES) system using a laboratory-scale thermal battery. The SulfurTES battery design uses a shell-and-tube thermal battery configuration, wherein stationary elemental sulfur is isochorically stored in multiple stainless steel tubes and a heat transfer fluid (air) is passed over them through the surrounding shell. The safe and reliable operation was demonstrated for twelve thermal charge–discharge cycles in the temperature range of 200–600 °C, during which the SulfurTES battery stored up to 7.6 kW h of thermal energy with volumetric energy density range up to 255 kW h/m3. Furthermore, the SulfurTES battery is operated in a hybrid thermal charging mode to demonstrate its ability to store surplus electrical energy. The present study establishes the feasibility of SulfurTES as a concept that could provide attractive system cost and volumetric energy density for a wide range of thermal energy storage applications.

We present an experimental examination of the relationship between the surface morphology of Mo and its instantaneous sputtering rate as function of low-energy plasma ion fluence. We quantify the dynamic evolution of nano/micro features of surfaces with built-in architecture, and the corresponding variation in the sputtering yield. Ballistic deposition of sputtered atoms as a result of geometric re-trapping is observed, and re-growth of surface layers is confirmed. This provides a self-healing mechanism of micro-architectured surfaces during plasma exposure. A variety of material characterization techniques are used to show that the sputtering yield is not a fundamental property, but that it is quantitatively related to the initial surface architecture and to its subsequent evolution. The sputtering yield of textured molybdenum samples exposed to 300 eV Ar plasma is roughly 1/2 of the corresponding value for flat samples, and increases with ion fluence. Mo samples exhibited a sputtering yield initially as low as 0.22 ± 5%, converging to 0.4 ± 5% at high fluence. The sputtering yield exhibits a transient behavior as function of the integrated ion fluence, reaching a steady-state value that is independent of initial surface conditions. A phenomenological model is proposed to explain the observed transient sputtering phenomenon, and to show that the saturation fluence is solely determined by the initial surface roughness.

The thermomechanical damage and residual stresses in plasma-facing materials operating at high heat flux are experimentally investigated. Materials with micro-surfaces are found to be more resilient, when exposed to cyclic high heat flux generated by an arc-jet plasma. An experimental facility, dedicated to High Energy Flux Testing (HEFTY), is developed for testing cyclic heat flux in excess of 10 MW/m2. We show that plastic deformation and subsequent fracture of the surface can be controlled by sample cooling. We demonstrate that W surfaces with micro-pillar type surface architecture have significantly reduced residual thermal stresses after plasma exposure, as compared to those with flat surfaces. X-ray diffraction (XRD) spectra of the W-(110) peak reveal that broadening of the Full Width at Half Maximum (FWHM) for micro-engineered samples is substantially smaller than corresponding flat surfaces. Spectral shifts of XRD signals indicate that residual stresses due to plasma exposure of micro-engineered surfaces build up in the first few cycles of exposure. Subsequent cyclic plasma heat loading is shown to anneal out most of the built-up residual stresses in micro-engineered surfaces. These findings are consistent with relaxation of residual thermal stresses in surfaces with micro-engineered features. The initial residual stress state of highly polished flat W samples is compressive ( ≈ -1.3 GPa). After exposure to 50 plasma cycles, the surface stress relaxes to −1.0 GPa. Micro-engineered samples exposed to the same thermal cycling show that the initial residual stress state is compressive at (- 250 MPa), and remains largely unchanged after plasma exposure.

Elemental sulfur is a low-cost, chemically stable thermal storage medium suitable for many medium to high temperature applications. In this study, we investigate the heat transfer behavior of sulfur, isochorically stored in a horizontally-oriented thermal storage element (steel tube) using experimental, analytical, and computational methods. The sulfur container was uniformly and non-uniformly heated along its axis from 50 to 600 °C to simulate the potential operating conditions for the full-scale thermal energy storage systems. The results of the study reveal distinct sulfur heat transfer mechanisms based on the temperature range and mode of thermal charging. For temperatures from 50 to 200 °C, the sulfur heat transfer behavior is governed by two primary mechanisms; 1) solid–liquid phase change, and 2) sulfur viscosity that varies strongly with temperature. From 200 to 600 °C, the buoyancy-driven natural convection is the dominant heat transfer mechanism and facilitates significantly high thermal charge rates. For axially non-uniform thermal charging, the axial temperature gradient induces natural convection along the axis that rapidly redistributes the thermal energy within the sulfur mass. Such axial convection has a strong impact on the thermal characteristics, including thermal charge/discharge rate and exergetic efficiency of the thermal storage systems. These observations and the high-fidelity computational model used in this study provide important means to identify the design parameters and operating conditions for which sulfur-based thermal energy storage (SulfurTES) systems will provide desirable thermal performance at a low thermal storage cost.

This study outlines a methodology for modeling and optimizing multi-phase thermal energy storage systems for solar thermal power plant (STPP) operation by incorporating energy and exergy analyses to a TES system employing a storage medium that can undergo multi-phase operation during the charging and discharging period. First, a numerical model is developed to investigate the transient thermodynamic and heat transfer characteristics of the storage system by coupling conservation of energy with an equation of state to model the spatial and temporal variations in fluid properties during the entire working cycle of the TES tank. Second, parametric studies are conducted to determine the impact of key design parameters on both energy and exergy efficiencies. The optimal values must balance exergy destroyed due to heat transfer and exergy destroyed due to pressure losses, which have competing effects. Optimization is utilized to determine parameter values within a feasible design window, which leads to a maximum exergetic efficiency of 87%.

Elemental sulfur is a promising storage material for low to high temperature thermal energy storage (TES) applications due to its high chemical stability, high heat transfer rate, and low cost. In this study, we investigate the performance of sulfur-based TES systems (SulfurTES) in a single-tank thermal battery configuration. In general, the results show that a moderate shell aspect ratio and standard tube diameters can be used to provide a range of high performance. An experimentally validated 2D numerical model is used here. The model predicts system-level performance based on the energetic and exergetic efficiencies for a range of geometric parameters and operating mass flow rates. This analysis shows the competing effects of the design and operating conditions on the performance parameters, and reveals governing parameter spaces unique to the specified performance targets. We have proposed a strategy to identify this parameter space, for which, the SulfurTES system will achieve required thermal performance, and a design procedure to incorporate such parameter space in system design. This work provides a systematic approach in TES performance investigation, and establishes an important framework to design industrial-scale SulfurTES systems that will offer high thermal performance using low-cost materials.

Elemental sulfur thermal energy storage (SulfurTES) is a promising low-cost solution for many medium to high temperature (300–1200 °C) TES applications. Demonstrations of SulfurTES have shown that the heat transfer behavior of sulfur in isochoric tubes is critical to system thermal performance. Previous studies have elucidated and quantified the sulfur heat transfer rate for idealized uniform charge and discharge; however, nonuniform conditions are more likely to be encountered in practice and need to be understood. This paper uses experimental and computational efforts to investigate sulfur heat transfer as well as exergy and energy performance in vertically-oriented tubes for two nonuniform thermal charge scenarios: top-heating and bottom-heating. In comparison with uniform thermal charge, the top-heating causes significant thermal stratification of sulfur that helps the SulfurTES system achieve superior exergetic performance. In contrast, the bottom-heating causes rapid mixing between hot and cold sulfur resulting in high charge rates. Both nonuniform charge strategies could be utilized during the operation of the SulfurTES system to improve system performance as well as provide operational flexibility. Using the computational results, this article originally develops two simplified analytical procedures to estimate the energy and exergy performance of sulfur in tubes of different sizes under top- and bottom-heating. The current study provides significant qualitative and quantitative heat transfer descriptions and design bases for SulfurTES systems and encourages further investigations into the complicated thermal performance for other thermal storage applications.

Large wind turbine blades are being developed at lengths of 75–100 m, in order to improve energy capture and reduce the cost of wind energy. Bending loads in the inboard region of the blade make large blade development challenging. The “biplane blade” design was proposed to use a biplane inboard region to improve the design of the inboard region and improve overall performance of large blades. This paper focuses on the design of the internal “biplane spar” structure for 100-m biplane blades. Several spars were designed to approximate the Sandia SNL100-00 blade (“monoplane spar”) and the biplane blade (“biplane spar”). Analytical and computational models are developed to analyze these spars. The analytical model used the method of minimum total potential energy; the computational model used beam finite elements with cross-sectional analysis. Simple load cases were applied to each spar and their deflections, bending moments, axial forces, and stresses were compared. Similar performance trends are identified with both the analytical and computational models. An approximate buckling analysis shows that compressive loads in the inboard biplane region do not exceed buckling loads. A parametric analysis shows biplane spar configurations have 25–35% smaller tip deflections and 75% smaller maximum root bending moments than monoplane spars of the same length and mass per unit span. Root bending moments in the biplane spar are largely relieved by axial forces in the biplane region, which are not significant in the monoplane spar. The benefits for the inboard region could lead to weight reductions in wind turbine blades. Innovations that create lighter blades can make large blades a reality, suggesting that the biplane blade may be an attractive design for large (100-m) blades.

Standard area weighting methods for particle-in-cell simulations result in systematic errors on particle densities for a non-uniform mesh in cylindrical coordinates. These errors can be significantly reduced by using weighted cell volumes for density calculations. A detailed description on the corrected volume calculations and cell-centered weighting algorithm in a non-uniform mesh is provided. The simple formulas for the corrected volume can be used for any type of quadrilateral and/or triangular mesh in cylindrical coordinates. Density errors arising from the cell-centered weighting algorithm are computed for radial density profiles of uniform, linearly decreasing, and Bessel function in an adaptive Cartesian mesh and an unstructured mesh. For all the density profiles, it is shown that the weighting algorithm provides a significant improvement for density calculations. However, relatively large density errors may persist at outermost cells for monotonically decreasing density profiles. A further analysis has been performed to investigate the effect of the density errors in potential calculations, and it is shown that the error at the outermost cell does not propagate into the potential solution for the density profiles investigated.

This paper describes a thermodynamic model that simulates the discharge cycle of a single-tank thermal energy storage (TES) system that can operate from the two-phase (liquid–vapor) to supercritical regimes for storage fluid temperatures typical of concentrating solar power plants. State-of-the-art TES design utilizes a two-tank system with molten nitrate salts; one major problem is the high capital cost of the salts ( International Renewable Energy Agency, 2012 ). The alternate approach explored here opens up the use of low-cost fluids by considering operation at higher pressures associated with the two-phase and supercritical regimes. The main challenge to such a system is its high pressures and temperatures which necessitate a relatively high-cost containment vessel that represents a large fraction of the system capital cost. To mitigate this cost, the proposed design utilizes a single-tank TES system, effectively halving the required wall material. A single-tank approach also significantly reduces the complexity of the system in comparison to the two-tank systems, which require expensive pumps and external heat exchangers. A thermodynamic model is used to evaluate system performance; in particular it predicts the volume of tank wall material needed to encapsulate the storage fluid. The transient temperature of the tank is observed to remain hottest at the storage tank exit, which is beneficial to system operation. It is also shown that there is an optimum storage fluid loading that generates a given turbine energy output while minimizing the required tank wall material. Overall, this study explores opportunities to further improve current solar thermal technologies. The proposed single-tank system shows promise for decreasing the cost of thermal energy storage.

Low-thrust electric propulsion is increasingly being used for spacecraft missions primarily due to its high propellant efficiency. As a result, a simple and fast method for low-thrust trajectory optimization is of great value for preliminary mission planning. However, few low-thrust trajectory tools are appropriate for preliminary mission design studies. The method presented in this paper provides quick and accurate solutions for a wide range of transfers by using numerical orbital averaging to improve solution convergence and include orbital perturbations. Thus, preliminary trajectories can be obtained for transfers which involve many revolutions about the primary body. This method considers minimum fuel transfers using first-order averaging to obtain the fuel optimum rates of change of the equinoctial orbital elements in terms of each other and the Lagrange multipliers. Constraints on thrust and power, as well as minimum periapsis, are implemented and the equations are averaged numerically using a Gausian quadrature. The use of numerical averaging allows for more complex orbital perturbations to be added in the future without great difficulty. The effects of zonal gravity harmonics, solar radiation pressure, and thrust limitations due to shadowing are included in this study. The solution to a transfer which minimizes the square of the thrust magnitude is used as a preliminary guess for the minimum fuel problem, thus allowing for faster convergence to a wider range of problems. Results from this model are shown to provide a reduction in propellant mass required over previous minimum fuel solutions.