Airborne wind energy (AWE) is a fascinating technology to convert wind power into electricity with an autonomous tethered aircraft. Deemed a potentially game-changing solution, AWE is attracting the attention of policy makers and stakeholders with the promise of producing large amounts of cost-competitive electricity with wide applicability worldwide. Since the pioneering experimental endeavors in the years 2000–2010, there has been a clear technology convergence trend and steady progress in the field. Today, AWE systems can operate automatically with minimal supervision in all operational phases. A first product is also being commercialized. However, all-around fully autonomous operation still presents important fundamental challenges that are conceptually similar to those of other systems that promise to change our lives, such as fully autonomous passenger cars or service drones. At the same time, autonomous operation is necessary to enable large-scale AWE, thus combining challenging fundamental problems with high potential impact on society and the economy. This article describes the state of the art of this technology from a system perspective and witha critical view on some fundamental aspects, presents the latest automatic control results by prominent industrial players, and finally points out the most important challenges on the road to fully autonomous AWE systems.

A control scheme for drag power kites, also known as airborne wind turbines, for the entire wind speed range is proposed, including 1) a temperature controller allowing for temporary overloading of the powertrain; 2) a limitation controller ensuring that power, force, speed, and actuator constraints are satisfied; 3) a tangential flight speed controller; and 4) a tangential force control allocation, which inverts the nonline arities of the plant and allocates the flight speed controller’s tangential force demand to the available actuators. The drag power kite plant model is based on a point-mass model and a simple aerodynamics model with various drag contributions. Simulations are conducted with the parameters of the 20 kW Wing 7 developed by Makani Power, Inc. The proper working of the control scheme is indicated by the good match of the simulation results with independent simulation results and measurements published by Makani. A temporary overloading of the power train with about twice the nominal power can be concluded as a requirement; otherwise the mean power would be significantly lower. Because of the reduction of the lift and thus reduction of the centripetal force at high wind speeds, the inside-down figure eight can be concluded as the best pattern.

This paper represents an expert view from Europe of future emerging technologies within the wind energy sector considering their potential, challenges, applications and technology readiness and how they might evolve in the coming years. These technologies were identified as originating primarily from the academic sector, some startup companies and a few larger industrial entities. The following areas were considered: airborne wind energy, offshore floating concepts, smart rotors, wind-induced energy harvesting devices, blade tip-mounted rotors, unconventional power transmission systems, multi-rotor turbines, alternative support structures, modular high voltage direct current generators, innovative blade manufacturing techniques, diffuser-augmented turbines and small turbine technologies. The future role of advanced multiscale modelling and data availability is also considered. This expert review has highlighted that more research will be required to realise many of these emerging technologies. However, there is a need to identify synergies between fundamental and industrial research by correctly targeting public and private funding in these emerging technology areas as industrial development may outpace more fundamental research faster than anticipated.

During pumping mode, airborne wind energy systems are operated in two phases: A power generating reel-out phase and a power dissipating reel-in phase. The ground winch is connected via a DC-link voltage source converter to the grid. The control of its DC-link voltage is a challenging task due to the bidirectional power flow over the DC-link. Two PI controller designs are discussed: the classical PI controller with constant parameters and a nonlinear PI controller with online parameter adjustment. Based on a worst-case analysis of the physical properties, bounds on the constant parameters of the classical PI controller are derived leading to a conservative design to assure a stable operation also during the reel-in phase where the system dynamics are non-minimum phase. To overcome these limitations in the closed-loop bandwidth, a nonlinear PI controller is proposed which adjusts its parameters online. For controller design, the linearized system model is used and the controller parameters are computed via "online pole placement". Simulation results illustrate robustness, stability and improved control performance of the proposed nonlinear PI controller in comparison to the classical PI controller.

Crosswind kite power replaces the tower and the support structure of a conventional wind turbine by a lightweight tether leading to a potentially lower levelized cost of electricity. However, in this chapter it is shown that tethering the kite to the top of a tower instead of to the ground can have advantages: Most notably, the “cosine loss” is reduced, i.e. the misalignment of the wind velocity vector and the direction of the traction power transfer. Hence, a tower can increase the power and energy yield up to about the double. Even for small tower heights compared to the kite’s operation altitude, a significant efficiency increase can be obtained. Further advantages of a tower are highlighted e.g. for the autonomous start and landing and for the wind velocity measurement. Possible tower concepts are illustrated.

As an alternative to conventional wind turbines, this study considered kites with onboard wind turbines driven by a high airspeed due to crosswind flight ("drag power"). The hypothesis of this study was, that if the kite's lift coefficient is maximized, then the power, energy yield, allowed costs and profit margin are also maximized. This hypothesis was confirmed based on a kite power system model extended from Loyd's model. The performance of small-scale and utility-scale kites in monoplane and biplane configurations were examined for increasing lift coefficients. Moreover, several parameters of the utility-scale system were optimized with a genetic algorithm. With an optimal lift coefficient of 4.5, the biplane outperformed the monoplane. A 40 m wing span kite was expected to achieve a rated power of about 4.1 MW with a power density of about 52 kW/M-2. A parameter sensitivity analysis of the optimized design was performed. Moreover, to demonstrate the feasibility of very high lift coefficients and the validity of a utilized simplified airfoil polar model, CFD5 of a proposed high-lift multi-element airfoil were performed and the airfoil polars were recorded. Finally, a planform design of a biplane kite was proposed. (C) 2017 Elsevier Ltd. All rights reserved.

An approach for a fault-tolerant power electronic system for drag power kites is proposed. The key idea is to use a high number of electrical cables in the tether and leave rotor drivetrains in groups isolated on the kite, in the tether, and on the ground. The power flow is paralleled behind an uninterruptible power supply of each drivetrain group on the ground. It is shown that this approach hardly affects the overall system performance, for example, due to the anyways high optimal number of electrical cables in the tether. With this approach, a fault in one drivetrain group does only affect the same drivetrain group and a fault between two drivetrain groups affects only the same two drivetrain groups. A fuse system and a fault shut-off control for the power converters are proposed, with which also faults between two drivetrain groups lead at most to unavailability of only one drivetrain group. In particular also open circuit faults and short circuit faults in the tether are mitigated and are overall not harmful for the system and a usual multicopter-like landing is still possible. Therefore, the proposed power electronic system design has no single point of failure. A generalization and a number of variants are discussed. In detail, a number of power electronic topologies and both tether power transmission types, DC and AC, are possible. The proposed approach was verified by a high number of circuit simulations and by a proof-of-principle demonstrator. In all cases it was confirmed that any open circuit fault and any short circuit fault lead at the most to unavailability of only one rotor drivetrain group.

Crosswind kite power is a promising alternative wind power technology. However, unlike the rotor blades of a conventional wind turbine, a kite needs to be launched prior to power generation and needs to be landed during low-wind conditions or for maintenance. This study proposes multicopter-based concepts for an autonomous solution. Basic system components and different system configurations are discussed. Static and dynamic feasibility analyses are carried out. Results show that such systems are feasible and have advantages compared to other launching and landing concepts. However, also the weaknesses of such systems become apparent e.g. the increased airborne mass.

Purpose: The free osteomyocutaneous fibular flap has become one of the primary options for mandibular reconstruction, because of the later introduction and development of virtual surgical planning (VSP). However, VSP is associated with high additional pre-operative effort and costs. Therefore, the purpose of the study was to develop a new individual cost-effective pre-operative planning concept for free fibula mandible reconstruction and to compare it with VSP regarding clinical parameters and post-operative outcome.Materials and methods: 31 patients undergoing mandibular reconstruction with a microvascular free fibular flap were divided into two groups and retrospectively reviewed. For the first group A (18 of 31 patients), an individual method with stererolithographic (STL) models, silicon templates and hand-made cutting guides was used (about 250 V planning costs/patient). For the second group B (13 of 31 patients), VSP including pre-fabricated cutting guides was used (about 2500 V planning costs/patient).Results: We found no statistically significant differences with respect to intra-operative time of mandibular reconstruction, duration of hospitalisation or post-operative complications between the two groups (p >= 0.05).Conclusion: The surgical outcomes and operative efficiency of this individual and cost-effective planning concept are comparable with the much more expensive complete VSP concept. (C) 2017 European Association for Cranio-Maxillo-Facial Surgery. Published by Elsevier Ltd. All rights reserved.

This study considered a tethered multicopter, powered from ground by electrical cables integrated in the tether. For a low tether mass and diameter, but high-efficient and simple propeller drives, a higher tether voltage than the rated voltage of the propeller drives was considered. To avoid additional power converters to step-down the tether voltage, the propeller drives were proposed to be connected partly in series. A control method was developed to stabilize the voltages and simultaneously achieve the requested propeller forces and moments. The feasibility of the proposed concept and the effectiveness of the developed control method were demonstrated by experiments. The proof-of-principle demonstrator was a tethered octocopter with 48 V tether voltage, wherein two or four voltage levels were created by a series connection. The demonstrator flew stably for several minutes until a test flight was completed successfully. During flight, the voltage control errors were always less than 0.75 V. A possible application of such a system with a scaled-up voltage is crosswind kite power, where the kite is equipped with small onboard turbines and the electricity is transmitted to the ground via electrical cables. During launching and landing, the turbines are used as propellers, i.e., the kite is a tethered multicopter and hovers from ground into a launching position, or vice versa.

Major parts of a crosswind kite power system based on the ground generation principle can be tested without risks to the kite in a laboratory or workshop by coupling the winch(es) to loads, which emulate the kite's forces (power-hardware-in-the-loop, PHIL). As such tests can be costly and time consuming, this paper proposes an intermediate step: Instead of emulating the computed kite loads on a coupled real load, they are emulated on the existent electrical drives of the winches. With less effort, the winches turn similarly as if they where loaded by a real kite. As the loads are virtually present only, but hardware is involved, one may call this principle “virtual”-PHIL. Measurements with a small-scale prototype are presented.

In order to deal with a large cosmological constant a relaxation mechanism based on modified gravity has been proposed recently. By virtue of this mechanism the effect of the vacuum energy density of a given quantum field/string theory (no matter how big is its initial value in the early universe) can be neutralized dynamically, i.e. without fine tuning, and hence a Big Bang-like evolution of the cosmos becomes possible. Remarkably, a large class {F-m(n)} of models of this kind, namely capable of dynamically adjusting the vacuum energy irrespective of its value and size, has been identified. In this paper, we carefully put them to the experimental test. By performing a joint likelihood analysis we confront these models with the most recent observational data on type Ia supernovae (SNIa), the Cosmic Microwave Background (CMB), the Baryonic Acoustic Oscillations (BAO) and the high redshift data on the expansion rate, so as to determine which ones are the most favored by observations. We compare the optimal relaxation models F-m(n) found by this method with the standard or concordance Lambda CDM model, and find that some of these models may appear as almost indistinguishable from it. Interestingly enough, this shows that it is possible to construct viable solutions to the tough cosmological fine tuning problem with models that display the same basic phenomenological features as the concordance model.

In the recently proposed cosmological constant (CC) filter mechanism based on modified gravity in the Palatini formalism, gravity in the radiation, matter and late-time de Sitter eras is insensitive to energy sources with the equation of state -1. This implies that finite vacuum energy shifts from phase transitions are filtered out, too. In this work we investigate the CC filter model at very early times. We find that the initial big bang singularity is replaced by a cosmic bounce, where the matter energy density and the curvature are finite. In a certain case this finiteness can be observed already on the algebraic level.

We study the problem of relaxing a large cosmological constant in the astrophysical domain through a dynamical mechanism based on a modified action of gravity previously considered by us at the cosmological level. We solve the model in the Schwarzschild–de Sitter metric for large and small astrophysical scales, and address its physical interpretation by separately studying the Jordan's frame and Einstein's frame formulations of it. In particular, we determine the extremely weak strength of fifth forces in our model and show that they are virtually unobservable. Finally, we estimate the influence that the relaxation mechanism may have on pulling apart the values of the two gravitational potentials Ψ(r) and Φ(r) of the metric, as this implies a departure of the model from General Relativity and could eventually provide an observational test of the new framework at large astrophysical scales, e.g. through gravitational lensing.

According to theoretical physics the cosmological constant (CC) is expected to be much larger in magnitude than other energy densities in the universe, which is in stark contrast to the observed Big Bang evolution. We address this old CC problem not by introducing an extremely fine-tuned counterterm, but in the context of modified gravity in the Palatini formalism. In our model the large CC term is filtered out, and it does not prevent a standard cosmological evolution. We discuss the filter effect in the epochs of radiation and matter domination as well as in the asymptotic de Sitter future. The final expansion rate can be much lower than inferred from the large CC without using a fine-tuned counterterm. Finally, we show that the CC filter works also in the Kottler (Schwarzschild-de Sitter) metric describing a black hole environment with a CC compatible to the future de Sitter cosmos.

In the Higgs inflation scenario the Higgs field is strongly coupled to the Ricci scalar in order to drive primordial inflation. However, in its original form in pure metric formulation of gravity, the ultraviolet (UV) cutoff of the Higgs interactions and the Hubble rate are of the same magnitude, and this makes the whole inflationary evolution dependent of the unknown UV completion of the Higgs sector. This problem, the unitarity violation, plagues the Higgs inflation scenario. In this letter we show that, in the Palatini formulation of gravitation, Higgs inflation does not suffer from unitarity violation since the UV cutoff lies parametrically much higher than the Hubble rate so that unknown UV physics does not disrupt the inflationary dynamics. Higgs-Palatini inflation, as we call it, is, therefore, UV-safe, minimal and endowed with predictive power.

We study the role of the cosmological constant (CC) as a component of dark energy (DE). It is argued that the cosmological term is in general unavoidable and it should not be ignored even when dynamical DE sources are considered. From the theoretical point of view quantum zero-point energy and phase transitions suggest a CC of large magnitude in contrast to its tiny observed value. Simply relieving this disaccord with a counterterm requires extreme fine-tuning which is referred to as the old CC problem. To avoid it, we discuss some recent approaches for neutralising a large CC dynamically without adding a fine-tuned counterterm. This can be realised by an effective DE component which relaxes the cosmic expansion by counteracting the effect of the large CC. Alternatively, a CC filter is constructed by modifying gravity to make it insensitive to vacuum energy.

We present an unconventional approach for addressing the old cosmological constant (CC) problem in a class of F(R,G) models of modified gravity. For a CC of arbitrary size and sign the corresponding cosmological evolution follows an expansion history which strikingly resembles that of our real universe. The effects of the large CC are relaxed dynamically and there is no fine-tuning at any stage. In addition, the relaxation mechanism alleviates the coincidence problem. The upshot is that a large cosmological constant and the observed cosmic expansion history coexist peacefully in the Relaxed Universe. This model universe can be thought of as an interesting preliminary solution of the cosmological constant problem, in the sense that it provides a successful dynamical mechanism able to completely avoid the fine-tuning problem (the toughest aspect of the CC problem). However, since the Relaxed Universe is formulated within the context of modified gravity, it may still suffer of some of the known issues associated with these theories, and therefore it can be viewed only as a toy-model proposal towards a final solution of the CC problem.

We demonstrate that there exists a large class of action functionals of the scalar curvature and of the Gauss-Bonnet invariant which are able to relax dynamically a large cosmological constant (CC), whatever it be its starting value in the early universe. Hence, it is possible to understand, without fine-tuning, the very small current value of the CC as compared to its theoretically expected large value in quantum field theory and string theory. In our framework, this relaxation appears as a pure gravitational effect, where no ad hoc scalar fields are needed. The action involves a positive power of a characteristic mass parameter, M, whose value can be, interestingly enough, of the order of a typical particle physics mass of the Standard Model of the strong and electroweak interactions or extensions thereof, including the neutrino mass. The model universe emerging from this scenario (the Relaxed Universe) falls within the class of the so-called LXCDM models of the cosmic evolution. Therefore, there is a cosmon entity X (represented by an effective object, not a field), which in this case is generated by the effective functional and is responsible for the dynamical adjustment of the cosmological constant. This model universe successfully mimics the essential past epochs of the standard (or concordance) cosmological model (LCDM). Furthermore, it provides interesting clues to the coincidence problem and it may even connect naturally with primordial inflation.