Mechanical, Pneumatic and Thermal Energy Storages for Micro-Air Vehicles

Abstract

The growing demand for high-power propulsion systems with high energy density for micro-air vehicles (MAVs) has motivated a worldwide search for alternatives to the present conventional propulsion units which are mainly internal combustion engines (ICEs) and electric motors. Here, we evaluate the advantages of alternative mechanical energy-based storage system having high energy density and high power density characteristics. Additional key requirements for such an alternative propulsion system are high power for taking off and maneuverability, efficient energy conversion during cruise flights, easy speed control, allowing high-altitude operation, low noise signature, low thermal signature, low pollutants emission and easy restarting when needed. The various excellent features of both the internal combustion engine and the electric motor have placed them as the leading means for propulsion systems. However, the breathing difficulties of the ICEs at high-altitude conditions, their relatively low reliability and their high noise signature encouraged the search for alternative solutions. Although electric motors overcome these major drawbacks, the energy density of their energy source is much lower than that of the hydrocarbon fuels (more than an order of a magnitude) and their ability to function in missions requiring long flight durations is significantly inferior. Here, we examine alternative means of energy storage systems. Alternative mechanical energy storages including elastic (different types of springs), compressed air, thermal (phase-change materials and cryogenic reservoirs) and kinetic (flywheels) were analyzed and examined. Promising emerging technologies which are based on exceptional materials like specially designed elastomers and carbon nanotubes (CNTs) have been critically reviewed here. The carbon nanotubes are known for their very high energy density, where the energy can be delivered directly to the aircraft propeller with a remarkably high conversion efficiency.