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This paper presents an aeroservoelastic modeling approach to investigate dynamic load alleviation in large wind turbines with composite blades and trailingedge aerodynamic surfaces. The tower and rotating blades are modeled using geometrically-nonlinear composite beams, and linearized about reference rotating conditions with potentially arbitrarily-large structural displacements. The aerodynamics of the rotor are represented using a linearized unsteady vortexlattice method and the resulting aeroelastic system is written in a state-space description that is both convenient for model reductions and control design. A linear model of a single blade is then used to design an H ∞ regulator, capable of providing load reductions of up to 13% in closed-loop on the full wind turbine nonlinear aeroelastic model. When combined with passive load alleviation through aeroelastic tailoring, dynamic loads can be further reduced to 35%. While the separate use of active flap controls and passive mechanisms for load alleviation have been well-studied, an integrated approach involving the two mechanisms has yet to be fully explored and is the focus of this paper. Finally, the possibility of exploiting torsional stiffness for active load alleviation on turbine blades is also considered. IntroductionHorizontal-Axis Wind Turbines (HAWT) have been steadily increasing in size since they were first considered for large-scale energy production, both in terms of tower height and rotor diameter [Barlas and van Kuik(2010)]. At the time of writing, the largest wind turbines in operation have rotors measuring above 120 m in diameter, but rotors of up to 160 m are already being developed. Larger blades are necessarily more flexible and, as a result, aeroelastic effects previously not seen in smaller rotors are beginning to surface [Hansen et al.(2006)Hansen, Sørensen, Voutsinas, Sørensen, This has brought about new needs in terms of modeling requirements and poses new technological challenges, in particular, with respect to an increased need for methods of load alleviation to prolong fatigue life [Bottasso et al.(2013)Bottasso, Campagnolo, Croce, and Tib Passive load alleviation through aeroelastic tailoring is attractive in its simplicity, design and is now well understood [Shirk et al.(1986)Shirk, Hertz, and Weisshaar]. It relies on a blade structure designed with bend-twist coupling (twist-towardsfeather) to reduce the angle of attack as the blade bends upwards. This mod-1

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An aeroservoelastic model, capturing the structural response and the unsteady aerodynamics of turbine rotors, will be used to demonstrate the potential of active load alleviation using aerodynamic control surfaces. The structural model is a geometrically non‐linear composite beam, which is linearized around equilibrium rotating conditions and coupled with time‐domain aerodynamics given by a linearized 3D unsteady vortex lattice method. With much of the existing work relying on blade element momentum theory with various corrections, the use of the unsteady vortex lattice method in this paper seeks to complement and provide a direct higher fidelity solution for the unsteady rotor dynamics in attached flow conditions. The resulting aeroelastic model is in a state‐space formulation suitable for control synthesis. Flaps are modeled directly in the vortex lattice description and using a reduced‐order model of the coupled aeroelastic formulation, a linear‐quadratic‐Gaussian controller is synthesized and shown to reduce root mean square values of the root‐bending moment and tip deflection in the presence of continuous turbulence. Similar trend is obtained when the controller is applied to the original non‐linear model of the turbine. Trade‐offs between reducing root‐bending moment and suppressing the negative impacts on torsion due to flap deployment will also be investigated. Copyright © 2014 John Wiley & Sons, Ltd.

The ability to achieve dual-mode thermal regulation for switchable heating and cooling on a single platform has thus far been challenged by the availability of suitable materials. The materials need to possess both high solar reflectance and high transmittance, necessitating large and small thicknesses in the same coating layer, respectively (i.e., the thickness constraint). Herein, for the first time, a single-layer coating made in a facile one-step process is reported, which exhibits rapid switch between high solar reflection (≈96.6%) and high solar transmission (≈86.6%). In the dry state, high solar reflectance and infrared (IR) emittance (>96% from 8 to 13 µm) enable passive radiative cooling, resulting in all-day near/sub-ambient temperatures in the demanding weather conditions of the tropical climate. Upon wetting, high transparency in the broadband range (0.3-2.5 µm) allows solar heating, leading to switchable thermal regulation. Such unprecedented performances are achieved through a unique hierarchical porous structure comprising of vertically aligned microscale pores in nanoscale pore matrix. This structure breaks the thickness constraint and broadens its applicability, in particular for seasonal areas with large temperature variation throughout the day.

In this article, we report a new type of lignin-based wood-like aerogel filters composed of aligned micrometer-sized pores and cross-linked lignin-based cell walls, as well as their air filtration-related properties. The aerogel filters were prepared via facile unidirectional ice-crystal-induced self-assembly from an aqueous solution followed by annealing at 300 C. The cross-linking of lignin and reinforcement with a very small amount of graphene significantly enhance the mechanical stiffness, thermal stability and humidity/water resistance of the aerogels. Simultaneously, abundant functional groups retained from lignin and the aligned pore channels lead to high filtration efficiency for ultrafine particles accompanied with fairly low pressure drop.Moreover, these low-cost and renewable biomass-based filters also exhibit outstanding long-term filtration efficiency. Through filtration tests with particles of various sizes, it is revealed that the air filtration by this type of aerogel filters is dominated by diffusion, rather than impaction or interception mechanism, which offers a new avenue for design of novel high-performance air filters.