Airborne Wind Energy
Aiming at making wind power cheaper and economically viable in a larger number of locations, the technology of Airborne Wind Energy (AWE) has been the subject of an increasing number of research projects in universities and enterprises of several countries. The idea behind AWE technology is to replace the tower of a conventional wind turbine by a tether whereas the blades are replaced by an airfoil (wing). The airfoil is kept airborne only by aerodynamic or aerostatic forces and the electric power can be generated either at the airfoil or at the ground, depending on the type of airfoil. The airfoil is attached to the ground with one or more cables responsible for transmitting the wind energy to the ground station. These AWE turbines are conceived to produce electricity at a lower cost than conventional technology, in addition to other important advantages. Firstly, the replacement of the tower by one or more cables of variable length allows the airfoil to operate at higher altitudes, where the wind is stronger and more consistent, allowing a greater wind power potential to be exploited. This, in turn, allows AWE technology to be used in larger number of sites, in particular those closer to the major consumption centers. In addition, AWE technology entails a substantial reduction in wind turbine costs, especially in material, transportation and installation, due to the absence of huge and heavy towers necessary to withstand the high mechanical stresses resulting from the turbine operation. The foundation for the anchorage point of the airfoil to the ground also becomes simpler and cheaper in AWE turbines.
In AWE turbines with electric energy generation at the airfoil, light and high-speed electric turbines are mounted on the airfoil, that in this case is a rigid wing. While the airfoil describes a trajectory similar to that of the blade tips of a conventional wind turbine, as illustrated in the figure, the power generated is characterized from the product between the wind apparent velocity at the turbines and the produced thrust. The electricity generated is then transmitted through the cable to the electrical infrastructure of conditioning and transmission on the ground. In the case of electric energy generation at the ground, the generated power is characterized from the pulling force at the cable and the speed at which it is unrolled while the airfoil flies in an lying eight figure to maximize the traction force (and consequently the power) without torsion accumulation in the cable. When the desired maximum length of the cable is reached, the airfoil is reconfigured to a condition of low aerodynamic efficiency and is quickly tracked back to the original position, spending a small amount of energy for reeling-in the cable, and the generation phase restarts. Because of this cyclic behavior, this type of AWE system is commonly referred to as pumping kite.
There are currently more than 55 AWE research groups around the world. Among them is the Brazilian group UFSCkite, pioneer and unique in Latin America, based at the Department of Systems and Automation of the Federal University of Santa Catarina. Established at the end of 2012, the goal of the UFSCkite project is to develop AWE technology, aligned with the country’s strategic science, technology and innovation interests. It is expected that, within a 10-year horizon, the technology developed will be ready to reach the market, benefiting Brazilian society through the generation of renewable electricity at a lower cost than the current one and in a larger number of places. The activities carried out by the group support the training of qualified professionals and foster research at undergraduate, master’s and doctoral levels. The UFSCkite also receives post-doctorates and exchange students from other countries, thus collaborating to strengthen cooperation ties between Brazilian and foreign universities.
Areas of activities
The UFSCkite project develops, in its own laboratory, prototypes of AWE turbines with ground generation unit. It is a very multidisciplinary project with an emphasis in the following areas:
- Control Systems: Different strategies are developed for flight control, cable traction control and (hierarchical) control of system operating modes.
- Filtering and estimation: it is necessary to filter the sensor signals and to estimate unknown system parameters, such as the aerodynamic coefficients of the airfoil.
- Surpevisory Systems: together with the development of the prototypes, the group is constantly improving a SCADA system for human operator interface with the wind turbine and data storage.
- Embedded Systems: ground and flight units have embedded systems responsible for tasks such as sensing, filtering, control and communication.
- Mechanic and Aerodynamic Design: design of the ground and flight units and mechanical structures for landing, take-off and operation of the tethered airfoil.
Schematic design of the flight control unit and ground unit.
- Power Electronics: In addition to the main power generation, a micro wind turbine (≈ 60W in figure below), is being developed and will be incorporated to the flight control unity for continuous energy supply of the actuators and electronics inside this unity.
- Measurements and Filtering: For the wind turbine operation it is necessary to measure and process information such as wind speed, airfoil position and cable traction.
- Wind Speed Profile Estimation: : A low-cost drone based wind measurement station for altitudes of up to 600 meters is being developed for prospecting of suitable sites for AWE technology deployment.
Schema of the wind measurement station.