Airborne wind energy systems use tethered flying devices to harvest wind energy beyond the height range accessible to tower-based wind turbines. Current commercial prototypes have reached power ratings of up to several hundred kilowatts, and companies are aiming at long-term operation in relevant environments. As consequence, system reliability, operational robustness, and safety have become crucially important aspects of system development. In this study, we analyze the reliability and safety of a 100-kW technology development platform with the objective of achieving continuous automatic operation. We first outline the different components of the kite power system and its operational modes. In the next step, we identify failure modes, their causes, and effects by means of failure mode and effects analysis (FMEA) and fault tree analysis (FTA). Potentially hazardous situations and mechanisms which can render the system nonoperational are identified, and mitigation measures are proposed. We find that the majority of these measures can be performed by a failure detection, isolation, and recovery (FDIR) system for which we present a hierarchical architecture adapted from space industry. KEYWORDS airborne wind energy, fault detection, fault isolation, fault recovery, FDIR, FMEA, FTA, health monitoring, kite power, reliability, safety INTRODUCTIONThe increasing need for renewable energy has led to a widespread deployment of wind turbines: initially, only on-shore, but for more than a decade, also off-shore. 1 The trend goes to ever larger turbines with increasing capacity factors because the wind power density generally increases with the distance from the ground, as a result of the wind shear. 2 On the other hand, the cost of larger structures scales unfavorably with a square-cube law and modern wind turbines are approaching an economically feasible size limit. 3 Airborne wind energy (AWE) systems, on the other hand, use tethered flying devices to harvest wind energy beyond the height range accessible to tower-based turbines. 4,5 The use of a tether allows the harvesting height to be adjusted continuously to optimize the availability of the wind resource. Compared with harvesting at the fixed hub height of wind turbines, the wind power that is available 95% of the time increases roughly by a factor of two. 6 Of particular interest are deep-sea applications because a tower is in principle not needed for the operation of the system. The tether attaches to the ground station at sea level, which substantially reduces the structural loads and thus also the required material. 7,8 The lower material effort, the increased capacity factor, and the access to a so far unused wind resource render AWE a potential cornerstone in a future low-carbon energy economy.However, the technology is operationally more complex than conventional wind turbines. Most implemented concepts rely on aerodynamic lift, and the tethered flying devices can thus not be stopped immediately when unexpected wind conditions or system failures occur. Exact...

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