A novel energy storage system incorporating electrically rechargeable liquid fuels as the storage medium.

We propose a novel concept of energy storage that incorporates electrically rechargeable liquid fuels made of electroactive species, known as e-fuels, as the storage medium. This e-fuel energy storage system comprises an e-fuel charger and an e-fuel cell. The e-fuel charger electrically charges e-fuels, while the e-fuel cell subsequently generates electricity using charged e-fuels whenever and wherever on demand. The e-fuel energy storage system possesses all the advantages of conventional hydrogen storage systems, but unlike hydrogen, liquid e-fuels are as easy and safe to store and transport as gasoline. The potential e-fuel candidates have been identified to include inorganic electroactive materials, organic electroactive materials, and suspension of solid electroactive materials. In this work, we demonstrate an example e-fuel energy storage system for large-scale energy storage using inorganic e-fuels composed of V2+/V3+ and VO2+/VO2+ redox couples, and compare the performance of the e-fuel energy storage system with that of existing technologies. Results show that our e-fuel charger achieves a charge efficiency of as high as ∼94%, while the e-fuel cell is capable of delivering a peak power density of 3.4 W cm-2, which is 1.7 times higher than that of hydrogen fuel cells. More excitingly, the e-fuel energy storage system exhibits a round-trip efficiency of 80.0% and an electrolyte utilization of 83.0% at an ultra-high discharge current density of 1,000 mA cm-2, which are 19.9% and 67.3% higher than those of conventional vanadium redox flow batteries. This unprecedented performance allows a 27.0% reduction in the capital cost of the e-fuel energy storage system compared with that of vanadium redox flow batteries.

Aerodynamic characteristics along the wing span of a dragonfly Pantala flavescens.

We investigated the characteristics of interwing aerodynamic interactions across the span of the high aspect ratio, flexible wings of dragonflies under tethered and free-flying conditions. This revealed that the effects of the interactions on the hindwings vary across four spanwise regions. (i) Close to the wing root, a trailing-edge vortex (TEV) is formed by each stroke, while the formation of a leading-edge vortex (LEV) is limited by the short translational distance of the hindwing and suppressed by the forewing-induced flow. (ii) In the region away from the wing root but not quite up to midspan, the formation of the hindwing LEV is influenced by that of the forewing LEV. This vortex synergy can increase the circulation of the hindwing LEV in the corresponding cross-section by 22% versus that of the hindwing in isolation. (iii) In the region about half-way between the wing root and wing tip there is a transition dominated by downwash from the forewing resulting in flow attached to the hindwing. (iv) A LEV is developed in the remaining, outer region of the wing at the end of a stroke when the hindwing captures the vortex shed by the forewing. The interaction effects depend not only on the wing phasing but also on the flapping offset and flight direction. The aerodynamics of the hindwings vary substantially from the wing root to the wing tip. For a given phasing, this spanwise variation in the aerodynamics can be exploited in the design of artificial wings to achieve greater agility and higher efficiency.

Effects of flexibility and aspect ratio on the aerodynamic performance of flapping wings.

In the current study, we experimentally investigated the flexibility effects on the aerodynamic performance of flapping wings and the correlation with aspect ratio at angle of attack α = 45°. The Reynolds number based on the chord length and the wing tip velocity is maintained at Re = 5.3 × 103. Our result for compliant wings with an aspect ratio of 4 shows that wing flexibility can offer improved aerodynamic performance compared to that of a rigid wing. Flexible wings are found to offer higher lift-to-drag ratios; in particular, there is significant reduction in drag with little compromise in lift. The mechanism of the flexibility effects on the aerodynamic performance is addressed by quantifying the aerodynamic lift and drag forces, the transverse displacement on the wings and the flow field around the wings. The regime of the effective stiffness that offers improved aerodynamic performance is quantified in a range of about 0.5-10 and it matches the stiffness of insect wings with similar aspect ratios. Furthermore, we find that the aspect ratio of the wing is the predominant parameter determining the flexibility effects of compliant wings. Compliant wings with an aspect ratio of two do not demonstrate improved performance compared to their rigid counterparts throughout the entire stiffness regime investigated. The correlation between wing flexibility effects and the aspect ratio is supported by the stiffness of real insect wings.

Aerodynamics, sensing and control of insect-scale flapping-wing flight.

There are nearly a million known species of flying insects and 13 000 species of flying warm-blooded vertebrates, including mammals, birds and bats. While in flight, their wings not only move forward relative to the air, they also flap up and down, plunge and sweep, so that both lift and thrust can be generated and balanced, accommodate uncertain surrounding environment, with superior flight stability and dynamics with highly varied speeds and missions. As the size of a flyer is reduced, the wing-to-body mass ratio tends to decrease as well. Furthermore, these flyers use integrated system consisting of wings to generate aerodynamic forces, muscles to move the wings, and sensing and control systems to guide and manoeuvre. In this article, recent advances in insect-scale flapping-wing aerodynamics, flexible wing structures, unsteady flight environment, sensing, stability and control are reviewed with perspective offered. In particular, the special features of the low Reynolds number flyers associated with small sizes, thin and light structures, slow flight with comparable wind gust speeds, bioinspired fabrication of wing structures, neuron-based sensing and adaptive control are highlighted.

Correction to 'Aerodynamics, sensing and control of insect-scale flapping-wing flight'.

[This corrects the article DOI: 10.1098/rspa.2015.0712.].

Analytical model for instantaneous lift and shape deformation of an insect-scale flapping wing in hover.

In the analysis of flexible flapping wings of insects, the aerodynamic outcome depends on the combined structural dynamics and unsteady fluid physics. Because the wing shape and hence the resulting effective angle of attack are a priori unknown, predicting aerodynamic performance is challenging. Here, we show that a coupled aerodynamics/structural dynamics model can be established for hovering, based on a linear beam equation with the Morison equation to account for both added mass and aerodynamic damping effects. Lift strongly depends on the instantaneous angle of attack, resulting from passive pitch associated with wing deformation. We show that both instantaneous wing deformation and lift can be predicted in a much simplified framework. Moreover, our analysis suggests that resulting wing kinematics can be explained by the interplay between acceleration-related and aerodynamic damping forces. Interestingly, while both forces combine to create a high angle of attack resulting in high lift around the midstroke, they offset each other for phase control at the end of the stroke.

Scaling law and enhancement of lift generation of an insect-size hovering flexible wing.

We report a comprehensive scaling law and novel lift generation mechanisms relevant to the aerodynamic functions of structural flexibility in insect flight. Using a Navier-Stokes equation solver, fully coupled to a structural dynamics solver, we consider the hovering motion of a wing of insect size, in which the dynamics of fluid-structure interaction leads to passive wing rotation. Lift generated on the flexible wing scales with the relative shape deformation parameter, whereas the optimal lift is obtained when the wing deformation synchronizes with the imposed translation, consistent with previously reported observations for fruit flies and honeybees. Systematic comparisons with rigid wings illustrate that the nonlinear response in wing motion results in a greater peak angle compared with a simple harmonic motion, yielding higher lift. Moreover, the compliant wing streamlines its shape via camber deformation to mitigate the nonlinear lift-degrading wing-wake interaction to further enhance lift. These bioinspired aeroelastic mechanisms can be used in the development of flapping wing micro-robots.

Force evaluation in the lattice Boltzmann method involving curved geometry.

The present work investigates two approaches for force evaluation in the lattice Boltzmann equation: the momentum-exchange method and the stress-integration method on the surface of a body. The boundary condition for the particle distribution functions on curved geometries is handled with second-order accuracy based on our recent works [Mei et al., J. Comput. Phys. 155, 307 (1999); ibid. 161, 680 (2000)]. The stress-integration method is computationally laborious for two-dimensional flows and in general difficult to implement for three-dimensional flows, while the momentum-exchange method is reliable, accurate, and easy to implement for both two-dimensional and three-dimensional flows. Several test cases are selected to evaluate the present methods, including: (i) two-dimensional pressure-driven channel flow; (ii) two-dimensional uniform flow past a column of cylinders; (iii) two-dimensional flow past a cylinder asymmetrically placed in a channel (with vortex shedding); (iv) three-dimensional pressure-driven flow in a circular pipe; and (v) three-dimensional flow past a sphere. The drag evaluated by using the momentum-exchange method agrees well with the exact or other published results.