The achievement of a solar powered aircraft capable of continuous flight was still a dream some years ago, but this great challenge has become feasible today. In fact, significant progresses have been realized recently in the domains of flexible solar cells, high energy density batteries, miniaturized MEMS and CMOS sensors, and powerful processors.
The concept is quite simple; equipped with solar cells covering its wing, it retrieves energy from the sun in order to supply power to the propulsion system and the control electronics, and charge the battery with the surplus of energy. During the night, the only energy available comes from the battery, which discharges slowly until the next morning when a new cycle starts.
Nevertheless, major interdisciplinary effort is necessary to optimize and integrate concepts and technologies to a fully functional system. As a matter of fact, the major issue is the combination and sizing of the different parts in order to maximize a certain criterion, for example the endurance, one parameter being the embedded payload.
In 2004, the Autonomous Systems Lab of EPFL/ETHZ launched the Sky-Sailor project under a contract with the European Space Agency. The objectives are the study and realization of a solar aircraft fully autonomous in navigation and power generation flying on Earth and thus validate the feasibility of a Mars dedicated version.
This lecture presents the methodology used for the global design of solar powered airplanes that are intended to achieve continuous flight on Earth. It was applied to the first prototype of
Sky-Sailor but it is rather general so that it can be used as much for small airplane weighing some hundreds of gram as for solar high altitude long endurance (HALE) platforms with a
wingspan of several tens of meters.
Brief description of the principle
Solar panels, composed by solar cells connected in a certain configuration, cover a certain surface of wing or other part of the airplane (tail, fuselage,…). During the day, depending on the sun irradiance and the inclination of the rays, the convert light into electrical energy. A converter, called Maximum Power Point Tracker, ensures that the maximum amount of power is obtained from the solar panels. This power is used firstly to power the propulsion group and the onboard electronics, and secondly to charge the battery with surplus of energy.
During the night, as no more power comes from the solar panels, only the battery supplies the various elements. This is schematically represented on the figure below.
Conceptual Design Methodology
Aircraft design is the name given to the activities that span the creation on paper of a new flight vehicle. The design process is usually divided into three phases or levels of design
Conceptual Design Preliminary Design Detail Design.
Design of Solar Powered Airplanes for Continuous Flight December 2006
This methodology will only focus on conceptual design where the general configuration and size is determined. Parametric trade studies are conducted using preliminary estimates of aerodynamics and weight to converge on the best final configuration. The feasibility of the design to accomplish a given mission is established but the details of the configuration are not defined.
One will also consider only level flight. Whether it is intended to achieve surveillance at low altitude or serve as a high altitude communication platform, a solar aircraft capable of continuous flight needs to fly at constant altitude. In fact, the first one would be useless for ground surveillance at high altitude and the second one wouldn’t cover a sufficient area at low altitude.
In this case, the energy and mass balances are the starting point of the design. In fact, the energy collected during the day by the solar panels has to be sufficient to power the motor, the onboard electronics and also charge the battery that provides enough power to fly from dusk to the next morning when a new cycle starts. Likewise, the lift force has to balance exactly the airplane weight so that the altitude is maintained.
This leads finally to an hen and egg problem: the required power consumption allows dimensioning the various parts, like motor, solar cells, battery, etc. but at the same time these parts determine the airplane gross weight used for the calculation of the required power.