drones Article Development of a Solar-Powered Unmanned Aerial Vehicle for Extended Flight Endurance Yauhei Chu †, Chunleung Ho †, Yoonjo Lee † and Boyang Li * Department of Aeronautical and Aviation Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, China; [email protected] (Y.C.); [email protected] (C.H.); [email protected] (Y.L.) * Correspondence: [email protected]; Tel.: +852-340-082-31 † Authors have contributed equally. Abstract: Having an exciting array of applications, the scope of unmanned aerial vehicle (UAV) application could be far wider one if its flight endurance can be prolonged. Solar-powered UAV, promising notable prolongation in flight endurance, is drawing increasing attention in the industries’ recent research and development. This work arose from a Bachelor’s degree capstone project at Hong Kong Polytechnic University. The project aims to modify a 2-metre wingspan remote-controlled (RC) UAV available in the consumer market to be powered by a combination of solar and battery-stored power. The major objective is to greatly increase the flight endurance of the UAV by the power generated from the solar panels. The power system is first designed by selecting the suitable system architecture and then by selecting suitable components related to solar power. The flight control system is configured to conduct flight tests and validate the power system performance. Under fair experimental conditions with desirable weather conditions, the solar power system on the aircraft results in 22.5% savings in the use of battery-stored capacity. The decrease rate of battery voltage Citation: Chu, Y.; Ho, C.; Lee, Y.; Li, during the stable level flight of the solar-powered UAV built is also much slower than the same B. Development of a Solar-Powered configuration without a solar-power system. Unmanned Aerial Vehicle for Extended Flight Endurance. Drones Keywords: solar-powered; UAV; endurance extension; flight experiments 2021, 5, 44. https://doi.org/ 10.3390/drones5020044 Academic Editor: 1. Introduction Abdessattar Abdelkefi With widening the application scope of unmanned aerial vehicle (UAV) as the driving force, the development of solar-powered UAV recently has attracted more attention in Received: 30 April 2021 academia and commercial industries. A critical factor limiting the scope of application Accepted: 21 May 2021 Published: 24 May 2021 of conventional battery-powered electric UAVs is their energy storage capability. The conventional UAV is powered by the energy stored in batteries on board to maintain the Publisher’s Note: MDPI stays neutral propulsion and functioning of flight control electronics. The amount of carried electric with regard to jurisdictional claims in power limits its flight range before takeoff. Although increasing the size of the battery published maps and institutional affil- or installing more batteries can increase the energy storage capacity, the weight of the iations. aircraft also increases. In turn, more power is consumed to carry the extra weight, resulting in the flight range being not necessarily prolonged. A possible approach to overcome this physical limit is the use of solar power as an energy source on UAV. Solar-powered UAV, using solar cells installed onboard, captures solar energy reaching the aircraft surface during daylight. Such generated power is supplied to the motor to propel the aircraft and Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. other electronics or to recharge the battery on board. The battery supplies power when in This article is an open access article darkness or under clouds. distributed under the terms and This project is aimed at the development of a small fixed-wing hand-launched solar- conditions of the Creative Commons powered UAV. A remote-controlled (RC) model glider for leisure purpose available on Attribution (CC BY) license (https:// the consumer market, a 759-2 Phoenix 2000 RC plane, is modified to be powered by a creativecommons.org/licenses/by/ hybrid of solar power and battery-stored power. This project suggested key points for 4.0/). the modification of an existing conventional RC plane to be solar-powered. The most Drones 2021, 5, 44. https://doi.org/10.3390/drones5020044 https://www.mdpi.com/journal/drones Drones 2021, 5, 44 2 of 19 Drones 2021, 5, 44 2 of 19 modification of an existing conventional RC plane to be solar-powered. The most signifi- significantcant advantage advantage of modifying of modifying an existing an existing plan planee over over building building a new a new one one is the is thesaving saving on ondevelopment development time time and and cost cost on the on airframe, the airframe, making making solar-powered solar-powered UAV UAV available available to more to moreusers userswith witha more a more straightforward straightforward design design and andbuilding building process. process. Numerous Numerous adaptive adaptive de- designssigns are are required required for forsuch such modification, modification, for instance, for instance, modifying modifying the power the power system system archi- architecturetecture to include to include solar solar cells. cells. TheThe primaryprimary objectiveobjective ofof thisthis workwork isis to increase the flightflight duration of the UAV. FlightFlight teststests areare conductedconducted toto comparecompare thethe powerpower systemsystem performanceperformance ofof thethe RCRC planeplane betweenbetween thethe manufacturer’smanufacturer’s defaultdefault versionversion andand thethe solar-powersolar-power modifiedmodified version.version. ObjectivesObjectives areare alsoalso setset forfor thethe flightflight controlcontrol system.system. FlightFlight teststests forfor determiningdetermining thethe flightflight rangerange wouldwould lastlast forfor an extended period. period. To To ensure ensure a a fair fair comparison, comparison, the the flight flight control control system system shall shall be becapable capable of flying of flying the the aircraft aircraft autonomously autonomously without without human human intervention. intervention. The The finished finished so- solar-poweredlar-powered UAV UAV is is shown shown in in Figure Figure 1.1 . FigureFigure 1.1. TheThe solar-poweredsolar-powered UAVUAV builtbuilt inin this this project project named named ‘Sun’. ‘Sun’. 2.2. LiteratureLiterature ReviewReview 2.1.2.1. ExamplesExamples ofof Solar-PoweredSolar-Powered UAVUAV ProjectProject TheThe firstfirst reviewed reviewed solar-powered solar-powered UAV UAV project project is AtlantikSolar, is AtlantikSolar, which which was conducted was con- byducted Oettershagen by Oettershagen et al. [ 1et] ofal. the[1] of Autonomous the Autonomous Systems Systems Lab, Lab, Swiss Swiss Federal Federal Institute Institute of Technologyof Technology Zurich. Zurich. Having Having completed completed an 81-hour an 81-hour continuous continuous flight flight that coveredthat covered a distance a dis- oftance 2338 of km, 2,338 the km, UAV the established UAV established a new world a new record. world Itrecord. has a conventionalIt has a conventional glider configu- glider ration,configuration, having ahaving wingspan a wingspan of 5.69 m of and 5.69 a m total and mass a total of mass 6.93 kg. of 6.93 A new kg. methodology A new methodol- was developedogy was developed as a result as of a the result project of forthe theprojec designt for of the solar-powered design of solar-powered UAVs for energetically UAVs for robustenergetically perpetual robust flight perpetual in sub-optimal flight in meteorological sub-optimal meteorological conditions. The conditions. project started The project with energeticstarted with system energetic modelling system to modelling consider theto consider variations the invariations operating in conditionsoperating conditions and local meteorologicaland local meteorological conditions. conditions. The airframe The and airframe energy and generation energy generation and storage and system storage was sys- de- signedtem was according designed to according the parameters to the obtainedparameters from obtained the energetic from the system energetic modelling. system Power mod- systemelling. Power components, system suchcomponents, as the Maximum such as the Power Maximum Point Trackers Power Point (MPPT) Trackers and a (MPPT) battery managementand a battery system, management were custom-made system, were and custom-made are capable and of regulating are capable the of energy regulating flow and the providing detailed energy flow information. The flight control system was designed with energy flow and providing detailed energy flow information. The flight control system state-space models. With the design of aircraft systems completed, the design of the whole was designed with state-space models. With the design of aircraft systems completed, the UAV was preliminarily verified with lab tests and short flight tests. design of the whole UAV was preliminarily verified with lab tests and short flight tests. The second solar-powered UAV project reviewed is that conducted by Morton et al. [2] The second solar-powered UAV project reviewed is that conducted by Morton et al. at The Centre for Distributed Robotics, University of Minnesota. A 4-metre wingspan [2] at The Centre for Distributed Robotics, University of Minnesota. A 4-metre wingspan solar UAV for the objective of low altitude aerial sensing applications was developed. solar UAV for the objective of low altitude aerial sensing applications was developed. The