Real-Time Optimizing Control of an Experimental Crosswind Power Kite

The contribution of this article is to propose and experimentally validate an optimizing control strategy for power kites flying crosswind. The control strategy provides both path control (stability) and path optimization (efficiency). The path following part of the controller is capable of robustly following a reference path, despite significant time delays, using position measurements only. The path-optimization part adjusts the reference path in order to maximize line tension. It uses a real-time optimization algorithm that combines off-line modeling knowledge and on-line measurements. The algorithm has been tested comprehensively on a small-scale prototype, and this article focuses on experimental results

Crosswind Kite Control - A Benchmark Problem for Advanced Control and Dynamic Optimization

This article presents a kite control and optimization problem intended as a benchmark problem for advanced control and optimization. We provide an entry point to this exciting renewable energy system for researchers in control and optimization methods looking for a realistic test bench, and/or a useful application case for their theory. The benchmark problem in this paper can be studied in simulation, and a complete Simulink model is provided to facilitate this. The simulated scenario, which reproduces many of the challenges presented by a real system, is based on experimental studies from the literature, industrial data and the first author’s own experience in experimental kite control. In par- ticular, an experimentally validated wind turbulence model is included, which subjects the kite to realistic disturbances. The benchmark problem is that of controlling a kite such that the average line tension is maximized. Two different models are provided: A more comprehensive one is used to simulate the ’plant’, while a simpler ’model’ is used to design and implement control and optimization strategies. This way, uncertainty is present in the form of plant-model mismatch. The outputs of the plant are corrupted by measurement noise. The maximum achievable average line tension for the plant is calculated, which should facilitate the performance comparison of different algorithms. A simple control strategy is implemented on the plant and found to be quite suboptimal, even if the free parameters of the algorithm are well tuned. An open question is whether or not more advanced control algorithms could do better

Kite Generator System Modeling and Grid Integration

International audienceThis paper deals principally with the grid connection problem of a kite-based system, named the "Kite Generator System (KGS)." It presents a control scheme of a closed-orbit KGS, which is a wind power system with a relaxation cycle. Such a system consists of a kite with its orientation mechanism and a power transformation system that connects the previous part to the electric grid. Starting from a given closed orbit, the optimal tether's length rate variation (the kite's tether radial velocity) and the optimal orbit's period are found. The trajectory-tracking problem is not considered in this paper; only the kite's tether radial velocity is controlled via the electric machine rotation velocity. The power transformation system transforms the mechanical energy generated by the kite into electrical energy that can be transferred to the grid. A Matlab/simulink model of the KGS is employed to observe its behavior, and to insure the control of its mechanical and electrical variables. In order to improve the KGS's efficiency in case of slow changes of wind speed, a maximum power point tracking (MPPT) algorithm is proposed

Power Generation for a 2D Tethered Wing Model with a Variable Tether Length

Airborne wind energy systems consist of a lifting body and a tether. Several airborne wind energy systems have been created by others, but the most promising consists of a wing which translates through the air in a crosswind motion. Two computational models of a translating wing system were used to study the dynamics and performance of these systems. The rst model that was examined consists of a rigid connecting arm between a rotating base station and a wing. A study of this model showed that one can increase the power production of the system by changing the wing angle relative to the connecting arm during motion. Using a variable relative wing angle, an average power of 7:7W is generated which is an increase of 30% over the xed wing angle system. A second model was examined which used a exible tether that could change in length. For this system, power is generated as the tether is reeled o a drum at the base station when tether tension is high. The tether tension can be maximized by the optimal usage of the control parameters such as the reel-in and the bridle orientation of the kite-system. A study of this model showed that the system is capable of asymptotically stable periodic motions with a simple controller for tether length. In addition, this simple controller is able to achieve positive power production of 1:05kW in a 10m=s windspeed. The simple model demonstrates the concept that, for these types of systems, it may be possible to generate higher average cycle powersby strategically using energy to quickly accelerate the system at the ends of the stroke

Flight trajectory optimization of Fly-Gen airborne wind energy systems through a harmonic balance method

The optimal control problem for flight trajectories of Fly-Gen airborne wind energy systems (AWESs) is a crucial research topic for the field, as suboptimal paths can lead to a drastic reduction in power production. One of the novelties of the present work is the expression of the optimal control problem in the frequency domain through a harmonic balance formulation. This allows the potential reduction of the problem size by solving only for the main harmonics and allows the implicit imposition of periodicity of the solution. The trajectory is described by the Fourier coefficients of the dynamics (elevation and azimuth angles) and of the control inputs (onboard wind turbine thrust and AWES roll angle). To isolate the effects of each physical phenomenon, optimal trajectories are presented with an increasing level of physical representation from the most idealized case: (i) if the mean thrust power (mechanical power linked to the dynamics) is considered as the objective function, optimal trajectories are characterized by a constant AWES velocity over the loop and a circular shape. This is done by converting all the gravitational potential energy into electrical energy. At low wind speed, onboard wind turbines are then used as propellers in the ascendant part of the loop; (ii) if the mean shaft power (mechanical power after momentum losses) is the objective function, a part of the potential energy is converted into kinetic and the rest into electrical energy. Therefore, the AWES velocity fluctuates over the loop; (iii) if the mean electrical power is considered as the objective function, the onboard wind turbines are never used as propellers because of the power conversion efficiency. Optimal trajectories for case (ii) and (iii) have a circular shape squashed along the vertical direction. The optimal control inputs can be generally modeled with one harmonic for the onboard wind turbine thrust and two for AWES roll angle without a significant loss of power, demonstrating that the absence of high-frequency control is not detrimental to the power generated by Fly-Gen AWESs.The work by PoliMI had no external funding and was therefore self-funded by the research team. The work by ICF was carried out under the framework of the GreenKite-2 project (PID2019-110146RB-I00) funded by MCIN/AEI/10.13039/501100011033

Analysis of translating hydrofoil power generation systems (hydrokites)

The hydrokite is a novel hydro-power system that is based on emerging kite wind-energy systems which are currently being designed for use at high altitudes. The hydrokite system is comprised of a hydrofoil and a support system, and is designed to capture kinetic energy from the flow of a river while reducing negative impacts on the river ecology by minimally interfering with the rivers natural flow (i.e. no dams or river diversions are needed). This work presents some initial results which demonstrate the power performance capabilities of the hydrokite. Two different steady-state models for this system were studied to determine the effects of model parameters on power generation. A dynamic model was also developed and preliminary results are presented. These simplified initial models provide an upper bound for the power performance of an actual system as well as providing an understanding of the effects that parameter changes have on the system performance. This initial work shows that such a system could be a feasible, low impact method for generating renewable energy from low-head hydro sources