Up! How to Launch and Retrieve a Tethered Aircraft

Abstract

The basic model of the principle of crosswind power as described by Miles M. Loyd in 1980 (Loyd, 1980), promises a high energy revenue from airborne wind energy. With a growing awareness of the need of sustainable energy resources and the current technological possibilities, different initiatives are undertaken to harvest the energy of wind at higher altitudes. One of these initiatives is the Powerplane, which makes use of a glider type aircraft with a wing span of 36 meters for producing 1 MW of electric power. The aircraft flies patterns in the sky such that it produces a high tension in the connecting tether. The system generates power as the tether is unwound from a drum located at the ground under this high tension. When the tether is fully unwound, the aircraft performs a dive while the tether is quickly retrieved with a minimum tension. The difference in tension during retrieval and unwinding allows net energy to be generated. Before the 1 MW Powerplane can be put on the market as a working system, several technological hurdles must be taken. A conventional wind turbine is always remotely situated at its operational altitude as soon as it is installed. On the contrary, the Powerplane needs a method to get there. Therefore one of the main design challenges is the step between an idle state on the ground, and generating energy at operational altitude. This challenge is the main objective of the research done in this thesis: to find out how the aircraft can be launched and retrieved such that the Powerplane will be a competitive system for wind energy generation. There are many different ways for doing this, but which way is most feasible? To answer this question, first of all the main requirements a launch and retrieval system has to comply with are outlined. A functional analysis lead to different design options, of which the most interesting options where analysed more extensively by various methods. Combining promising design options gave three concepts. One concept based on buoyancy, a concept making use of propeller thrust and one which uses a rotational platform to launch the aircraft. Analysing these concepts in a trade-off resulted in abandoning the buoyancy concept, but the other two required more research. This resulted in an improvement of the propeller thrust concept by combining it with a circular runway. The operating principle of the rotating platform concept is analysed by means of computer simulations.Even though the results of the simulations where promising, they where not convincing. Before a definite decision on the concepts can be made, more research is required. The effect of the added weight of the propeller thrust concept should be determined by studying the Powerplane concept in general. The concept of the rotational platform needs to be investigated more to make sure that the operational altitude can be reached without additional equipment. Nevertheless, the research outlined in this thesis and the recommendations made, provide a firm basis to find the optimum solution for launching and retrieving the aircraft. I hope the content of this research will help to make the concept of Airborne Wind Energy a step closer to becoming a world wide success.