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Penetration rate of grid-connected photovoltaic (PV) generation to the existing utility grid is rapidly increasing over the years. Since the power generated from PV systems fluctuates according to the weather condition, e.g., cloud passing, this can significantly disturb the stability of a weak utility grid. The integration of energy storage devices and its ramp-rate control technique are required to reduce the impact of PV systems output fluctuations and augment the stability of the utility grid. In this paper, ramp-rate control is applied to the direct connection of energy storage devices in PV generation system configuration. The direct connection of supercapacitors string and battery combination scheme is proposed to reduce the number of power converters so that the efficiency of the system is increased. In this work, the PV system output is controlled by directly controlling the energy storage system (ESS) to limit the changing rate of the PV output to a desired ramp-rate value, hence reducing the battery charge/discharge cycles and extending the expected lifetime of the ESS. The performance of the proposed direct connection scheme of the ESS and its ramp-rate control strategy is verified using a 1-kW PV system prototype.

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In recent years, electric unmanned areal vehicles (UAVs) are gaining popularity due to their capabilities in civil and military applications. The rationale is that UAVs can do more complex and risky tasks while maintaining mobility, safety, and cheap cost. UAVs are generally used for inspection and monitoring, search and rescue operation, delivery, precision agriculture, hobbyist applications, and many more. Due to a broad spectrum of applications, UAVs of various sizes and weights have evolved. UAVs have developed with the hybrid architecture of power supply incorporating batteries, fuel cells, solar photovoltaic systems, and supercapacitors for extended endurance and improved performance. The electric propulsion system is the central part of UAVs, which generates thrust to control and hover the UAVs in the air. The propulsion system includes an electric motor, electronic speed controller, power sources, and an energy management system for efficient operation. This paper presents an extensive and critical evaluation of state of the art on classifications, components of the electric propulsion system, the control strategy of electric motor, hybridization of power sources, and energy management system to identify research gaps and give insights exhortations for future research.

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