Solar balloons, in which solar radiation is used to heat directly the air in a hot-air flying balloon, can be used as a mixed power system that exploits both solar radiation and high-altitude winds. If the balloon is heated at ground level and then released, the upward drift due to solar energy and the lateral drift due to wind can both be used to generate energy. At high altitude, the balloon is completely deflated (except for a small Helium-filled chamber, which prevents the balloon from falling) and pulled to the ground: the aerodynamical drag is now strongly reduced with respect to the ascent. A production cycle is simulated for balloons of different sizes and in different working conditions, assessing the feasibility of such a system, calculating the energy output and the solar/wind proportion of the produced energy. Simulation results show that such a system can produce energy with non-negligible solar-to-electricity efficiency (up to 5%). Moreover, while this is primarily a solar-based system, wind can yield a significant amount of additional energy for wind velocities greater than 15. m/s. © 2012 Elsevier Ltd.
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