Energy transfer from airborne high altitude wind turbines: Part I, A feasibility study of an autogirogenerator from an existing helicopter

Current designs of ground based, wind energy extraction systems have limitations of wind instability and high cost of installations. The efficiency of these systems is optimal for a nominal speed and decrease sharply for higher or lower winds. This paper introduces large air rotors at high altitude for powerful and relatively stable air stream out of ground effect. This first part deals with an existing helicopter that is transformed to an autogiro for power generation. The helicopter is linked to the ground with a cable that connects the airborne generator to the ground power grid. In our case the air rotor system flies at an altitude of about 4 km that is statistically the best compromise between power available and altitude. Two versions of the helicopter are considered: electrical (motors-generators) and hybrid (turboshafts + generators). In the electrical version, the electric motors power the helicopter that climbs up the required altitude and lift the cables. At this point, the motors are switched to generator-mode and the helicopter keeps altitude as an autogiro and generates energy. The hybrid solution adds the generator(s) to the helicopter. The hybrid-helicopter climbs with the turboshafts and then trip-off the engines to work as an autogiro-generator. We used is the largest available helicopter: the CH47 Chinook. This choice is because it is economically convenient to use the largest wind generator possible. Both fully electric and hybrid solutions proved to be technically feasible. However, the pure electric solution requires a huge amount of power from the power grid (7 MW), therefore has relatively high installation costs. For this reason, the hybrid solution is more practical. The average power produced is more than 0.8 MW. Unfortunately, the cost per kWh is two order of magnitude higher than the carbon produced one. Therefore, this solution is convenient only when you have problems to take the fuel or the grid to the place where energy is needed. Helicopter/autogiro stability and control systems enable to change altitude and to deal with emergencies. This airborne system provides the following main advantages: power production capacity higher than conventional ground-based small rotor designs; the installation is environmentally friendly also for the propeller noise.