Airborne Wind Turbines Research Articles

Phase-averaged dynamics of a periodically surging wind turbine

The unsteady power generation of a wind turbine translating in the streamwise direction is relevant to floating offshore wind turbines, kite-mounted airborne wind turbines, and other nontraditional wind-energy systems. To study this problem experimentally, measurements of torque, rotor speed, and power were acquired for a horizontal-axis wind turbine actuated in periodic surge motions in a fan-array wind tunnel at the Caltech Center for Autonomous Systems and Technologies. Experiments were conducted at a diameter-based Reynolds number of ReD=6.1×105 and at tip-speed ratios between 5.2 and 8.8. Sinusoidal and trapezoidal surge-velocity waveforms with maximum surge velocities up to 23% of the free-stream velocity were tested. A model in the form of a linear ordinary differential equation (first-order in time) was derived to capture the time-resolved dynamics of the surging turbine. Its coefficients were obtained using torque measurements from a stationary turbine, without the need for unsteady calibrations. Its predictions compared favorably with the measured amplitude- and phase-response data. Furthermore, increases in the period-averaged power of up to 6.4% above the steady reference case were observed in the experiments at high tip-speed ratios and surge velocities, potentially due to unsteady or nonlinear aerodynamic effects. Conversely, decreases in mean power with increased surge velocity at low tip-speed ratios were likely a result of the onset of stall on the turbine blades. These results inform the development of strategies to optimize and control the unsteady power generation of periodically surging wind turbines, and motivate further investigations into the unsteady aerodynamics of wind-energy systems.

Jet Streams as Power Generating Electrical Energy in Libya

The supreme wind sources are extremely huge, and according to estimations, these winds can supply Libya with great quantity of electrical energy. Among the examples of contemporary engineering technologies in this field, is to create a new generation of Airborne Wind Turbines. Scientists realized that winds near the Earth’s surface are too weak to provide a regular source of energy due to the presence of aerobic swirls and obstacles, which represent a source of ground friction being the cause of weakening wind power. Some consider that the Earth’s surface is a totally inappropriate place for investing wind energy. As an alternative solution, we start to think about the establishment of wind farms in another place away from the Earth’s surface by developing a new type that can run within the upper-air layers, precisely at jet streams areas. In comparison with fluctuating winds blowing gently near the Earth’s surface, scientists estimate that the energy of jet streams increases a thousand times than that can be gathered from the most powerful winds on high hills. To be able to provide a clear picture of the possibility of energy investment of jet streams, we shall present, across the pages of this paper, an explanation of the topic through the following aspects: How do Airborne Wind Turbines’ trip start, their advantages and difficulties faced, benefits and economic feasibility, General Atmospheric Circulation and jet streams. Since Libya is among the fortunate countries in the world, through which subtropical jet streams pass, we made an analysis and follow-up of daily synoptic charts, which show jet winds’ speed, direction and their altitudes for a period of 60 consecutive months starting from January 1, 2003 until December 31, 2007. Also, an analysis was made of daily observational data of jet winds recorded by Tripoli Upper-air Station during the period from the beginning of March 1987 until the end of February 1989. The paper’s results summarized that jet streams will be in the future one of the important clean energy strategic sources, and can provide a capacity, whose expected intensity is up to 19 kilowatts per square meter. It recommends giving attention to jet streams at the research, experimental and technological levels because research, experimental and technological proficiencies can provide more exciting solutions for energy problems in Libya.

New Energy Technologies: Microalgae, Photolysis and Airborne Wind Turbines

Because of the near-term risk of extreme weather events and other adverse consequences from climate change and, at least in the longer term, global fossil fuel depletion, there is worldwide interest in shifting to noncarbon energy sources, especially renewable energy (RE). Because of possible limitations on conventional renewable energy sources, researchers have looked for ways of overcoming these shortcomings by introducing radically new energy technologies. The largest RE source today is bioenergy, while solar energy and wind energy are regarded as having by far the largest technical potential. This paper reviews the literature on proposed new technologies for each of these three RE sources: microalgae for bioenergy, photolysis and airborne wind turbines. The main finding is that their proponents have often underestimated the difficulties they face and the time taken for their introduction on a very large scale.

Airborne Wind Turbine to Produce More Power from High Altitude Winds

This examination identifies a structure for the higher height Airborne Wind Turbines. Winds at high height are quicker than winds close to the world's surface and contain multiple occasions the power giving an amazing undiscovered vitality asset. Also, high height winds are accessible all the more reliably contrasted with close surface breezes. Henceforth, Airborne Wind Turbines produce a progressively unsurprising and uniform power yield bringing about twofold the vitality generation when contrasted with a traditional turbines. The higher elevation Airborne Wind turbines would at first fly at a tallness multiple times higher than ordinary breeze turbines by vertical take-off. The lift required for the Airborne Wind Turbine's vertical take-off is provided by engine generators associated with the turbines, at long last the all-out frameworks conveyed by helium expand. The breezes at high elevation moving through the turbines turn the generators at rapid. The power created by the turbines is transmitted to the ground by means of the strengthened composite tie that stays the Airborne Wind Turbine to the ground.

Drag-mode airborne wind energy vs. wind turbines: An analysis of power production, variability and geography

Airborne wind energy (AWE) is a wind power technology that harvests energy at high altitudes. The performance of AWE systems relative to traditional wind power turbines (WT) is of key relevance to any future commercialization. In particular, the power generation as well as its consistency over time will be key performance indicators. This study aims at identifying crucial factors that will influence the competitiveness of drag-mode AWE systems against WTs. To that end, the hourly power production of several drag-mode AWE designs is investigated using realistic wind data, and compared to the hourly power production of classical WTs. These results are then analyzed through three performance indicators: total annual power production, Gini coefficient, and correlation coefficient. The results show that AWE systems with multiple smaller wings have the highest annual production. The AWE power production of all AWE systems correlates in time at all sites with the production of WTs, and the Gini coefficients are similar. This observation challenges the belief that AWE systems will outcompete WTs thanks to a more consistent power output than WTs. However, the wing design as well as the local wind shear have a significant impact on the AWE performance.

New Energy Technologies: Microalgae, Photolysis and Airborne Wind Turbines

Because of the near-term risk of extreme weather events and other adverse consequences from climate change, and, at least in the longer term, global fossil fuel depletion, there is world-wide interest in shifting to noncarbon energy sources, especially renewable energy (RE). Because of possible limitations on conventional renewable energy sources, researchers have looked for ways of overcoming these shortcomings by introducing radically new energy technologies. The largest RE source today is bioenergy, while solar energy and wind energy are regarded as having the largest technical potential. This paper reviews the literature on proposed new technologies for each of these three RE sources: microalgae for bioenergy, photolysis and airborne wind turbines. The main finding is that their proponents have underestimated the difficulties facing their introduction on a very large scale.

Control of a Drag Power Kite over the Entire Wind Speed Range

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 nonlinearities 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 powertrain 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.

Weight optimisation of coreless axial‐flux permanent magnet machines

This study explores the upper limits in power-to-weight and torque-to-weight ratios of coreless axial-flux machines with permanent magnets. Moreover, it provides a comprehensive multifunctional optimisation procedure that is utilised for obtaining these limits. The procedure encompasses analytical analysis of electro-magnetic, thermal and structural (mechanical) aspects of axial-flux machines. Obtaining global minima is ensured by considering the whole machine design space, and mapping it into the performance space, where a Pareto front can be easily identified. From it, an optimal motor/generator for airborne wind turbines is identified. The design has a power-to-weight ratio of 6.4 kW/kg (19 Nm/kg at 3200 rpm) including structural (purely mechanical) parts, at an efficiency of 95%. This is a significantly higher ratio than the one in modern commercial machines or designs reported in the literature. Therefore, the resulting machine is manufactured and experimentally tested in order to verify the claimed limits and the optimisation methodology.

Design space exploration of gyrocopter‐type airborne wind turbines

AbstractThis paper presents a multidisciplinary framework for the design and analysis of gyrocopter‐type airborne wind turbines. In this concept, four rotary wings provide lift to a flying vehicle, and excess power is extracted using gearboxes and generators before being transferred to the ground through electrical conductors embedded in a structural tether. A physical breakdown of the system was performed, and five models were constructed: wind model, rotor aerodynamics, structural mass, electrical system, and tether (structures and aerodynamics). A stochastic optimizer in the framework enforces interdisciplinary compatibility and maximizes electrical power transmitted to the ground under various operating conditions. The framework is then used to explore the design space of this advanced concept in numerous flight conditions. The effect of implementing new technologies was also studied in order to evaluate their effect on the overall performance of the system. It is shown through a 1.3MW design that a gyrocopter‐type airborne generator could provide more power than a ground‐based wind turbine for a given blade radius, although only a fraction of the available wind power can be harvested using off‐the‐shelf technologies and components. The work presented in this study demonstrates the challenges of designing a high altitude wind generator and shows that performance is affected by complex interactions between each subsystem. Copyright © 2015 John Wiley & Sons, Ltd.

Conceptualization and Multiobjective Optimization of the Electric System of an Airborne Wind Turbine

Airborne wind turbines (AWTs) represent a radically new and fascinating concept for future harnessing of wind power. This concept consists of realizing only the blades of a conventional wind turbine (CWT) in the form of a power kite flying at high speed perpendicular to the wind. On the kite are mounted a turbine, an electrical generator, and a power electronics converter. The electric power generated is transmitted via a medium voltage cable to the ground. Because of the high flight speed of the power kite, several times the actual wind speed, only a very small swept area of the turbine is required according to Betz's Law and/or a turbine of low weight for the generation of a given electric power. Moreover, because of the high turbine rotational speed, no gear transmission is necessary and the size of the generator is also reduced. For takeoff and landing of the power kite, the turbines act as propellers and the generators as motors, i.e., electric power is supplied so that the system can be maneuvered like a helicopter. In the present work, the configuration of power electronics converters for the implementation of a 100 kW AWT is considered. The major aspect here is the trade-off between power-to-weight ratio (W/kg) and efficiency. The dependence of cable weight and cable losses on the voltage level of power transmission is investigated, and a comparison is made between low voltage (LV) and medium voltage (MV) versions of generators. Furthermore, the interdependence of the weight and efficiency of a bidirectional dual active bridge dc-dc converter for coupling the rectified output voltage of a LV generator to the MV cable is discussed. On the basis of this discussion, the concept offering the best possible compromise of weight and efficiency in the power electronics system is selected and a model of the control behavior is derived for both the power flow directions. A control structure is then proposed and dimensioned. Furthermore, questions of electromagnetic compatibility and electrical safety are treated. In conclusion, the essential results of this paper are summarized, and an outlook on future research is given. To enable the reader to make simplified calculations and a comparison of a CWT with an AWT, the aerodynamic fundamentals of both the systems are summarized in highly simplified form in an Appendix, and numerical values are given for the 100 kW system discussed in this paper.