Solar energy offers solar-powered unmanned aerial vehicle (UAV) the possibility of unlimited endurance. Some researchers have developed techniques to achieve perpetual flight by maximizing the power from the sun and by flying in accordance with its azimuth angles. However, flying in a path that follows the sun consumes more energy to sustain level flight. This study optimizes the overall power ratio by adopting the mission profile configuration of optimal solar energy exploitation. Extensive simulation is conducted to optimize and restructure the mission profile phases of UAV and to determine the optimal phase definition of the start, ascent, and descent periods, thereby maximizing the energy from the sun. In addition, a vertical cylindrical flight trajectory instead of maximizing the solar inclination angle has been adopted. This approach improves the net power ratio by 30.84% compared with other techniques. As a result, the battery weight may be massively reduced by 75.23%. In conclusion, the proposed mission profile configuration with the optimal power ratio of the trajectory of the path planning effectively prolongs UAV operation.
The current high endurance of UAV has been achieved by harvesting solar energy through photovoltaic cells [
Some studies focus on UAV kinematics to enhance the performance of UAV. The bank angle of UAV is crucial for determining the incidence angle of the sun on the solar panel for optimal solar harvesting [
Another path planning method, also known as energy management strategy (EMS) [
The maximum sun inclination angle was determined by changing the UAV’s heading angle, which resulted in higher energy consumption because of the need for establishing a path following the sun [
There are also researches done that suggested that the power net gain can be maximized by referring to local solar spectral density from weather forecasts [
Moreover, several works have ignored the upper and lower boundary limits of the battery’s state of charge (SOC). These limits must be between 0.25 and 0.90 to prolong the battery’s shelf life and fully utilize the battery pack installed in any UAV [
Framed by these contexts, this study adopts the optimal mission phase configuration of solar-powered UAV to optimize energy harvesting with minimal power consumption, to maximize the overall net power ratio, and to reduce the battery weight. Here, an alternative approach is presented by adopting an optimal mission phase’s configuration with vertical cylindrical trajectory to increase the ratio of power gained to power consumed.
The sun elevation angle has also been considered to optimize the phases of the UAV flight, which include cruise, ascent, and descent, and to maximize the energy from the sun. This optimization has also minimized the battery capacity required and enables the reduction of the battery weight to maintain perpetual flight. As a result, the overall power consumption is reduced as the overall weight of the UAV is reduced. In addition, the proposed strategy enables the UAV to fly at a low altitude trajectory to avoid the highly volatile wind region at higher altitudes [
In this section, a recently developed aircraft trajectory model is elaborated [
Cartesian coordinate system [
The aircraft is assumed to fly in still air with a constrained radius
The flight path angle
The optimum flight velocity is the velocity at the minimum required power
According to the Cartesian coordinate system, the equations of motions are derived from three axes, as shown in [
The thrust vector is assumed to be aligned with the free-stream velocity, which satisfies the small angle of attack.
This section explains the energy gained and lost during a flight, where the energy required, gained, and lost during the flight is discussed in terms of charged and discharged battery capacity. The standard electric propulsion model is used to derive the power to sustain a flight, as shown in (
In addition, the UAV is assumed to be fully mounted with solar cells on the wings, and the effective area of solar energy collection is assumed to be
The solar inclination
The battery power output and capacity are based on a lithium-sulfur rechargeable battery [
The performance of the battery is affected by temperature. However, the working temperature is in the range of low altitude temperature. Battery discharge and charge temperature’s lowest boundary is −20°C. This temperature allows the battery to operate efficiently until the altitude of 5500 m, which is lower than the assumed ceiling altitude.
A solar-powered aircraft mission profile is generally divided into three main phases: cruise, ascent, and descent. During the cruise flight, the UAV employs energy from the battery to sustain a level cruise flight. The ascent flight begins when the UAV starts to gain its gravitational potential energy and stores the excess energy fed into the battery. The descent flight of the UAV occurs when the aircraft starts to glide without using any power source. The power required to control the surface operation may also be disregarded. The UAV continues to cruise when it reaches an initial altitude of 100 m. The mission phases are illustrated in Figure
Mission phase.
At cruise, the aircraft begins to discharge the battery with the minimum power required to sustain a level flight. In this simulation, the altitude is set to clearance of 100 m to avoid geographical obstruction. The UAV is also set to fly at the minimum power to maximize the UAV’s endurance in this phase. In addition, the final
In the ascent phase, the UAV climbs steadily with the available excess power at a speed that can be calculated using (
In this phase, the UAV starts to employ the gravitational potential energy collected during the ascent phase for unpowered glide. Thus, it is assumed that the UAV does not consume any power to sustain its level flight during the descent phase. The minimal descent rate
The proposed power management strategy for the mission profile configuration aims to optimize the time management of the mission phase with respect to the elevation angles of the sun to maximize solar energy. Flying according to the sun’s inclination angle does not significantly affect energy efficiency because this consumes more power than the amount of energy generated from the sun.
The adaptation of a vertical cylindrical trajectory minimizes the battery mass. Hence an optimal net power is obtained during the flight. Moreover, this study adopts an additional element by defining the optimal time of mission transition in relation to the elevation angle of the sun to improve the overall power ratio gained. This structure of the mission phases, in which the start-end-time of the ascent, cruise, and descent phases of a flight are defined, leads to better energy savings and therefore improves aircraft endurance.
The three phases are looped in the time function for 24 hours, starting from 0:15 hours until 00:00 hours at the interval of 15 minutes to obtain the optimal time to start each phase. Although the elevation angle and intensity of the sun differ at each hour, the UAV still flies according to the planned path without following the sun’s angle. Thus, no extra energy is used in the performance of the sun-following maneuver, but changes in solar intensity over time are utilized to improve the final performance.
The power ratio and
UAV model data.
Parameter | Value |
---|---|
| 0.011 |
| 0.7 |
| 0.22 |
| 0.711 m |
| 0.1566 |
| 0.992 |
Aircraft weight | 4.201 N |
| 0.317 N |
| 7.65 Ah |
| 12.6 V |
| 0.0125 |
The proposed power management strategy also sustains the perpetual flight capabilities of the solar-powered UAV. However, the results in Sections
The trend of the power ratio with the start hours of the cruise phase is illustrated in Figure
Trend of the power ratio at the initialization of the cruise phase.
Trend of the power ratio at the initialization of the ascent phase.
The trend of the power ratio at the initialization of the descent phase is shown in Figure
Trend of the power ratio at the initialization of the descent phase.
Trend of the power ratio with respect to the start time of the cruise and descent phases.
Figure
Trend of the power ratio with respect to the start times of the cruise and ascent phases.
Trend of the power ratio with respect to the start times of the ascent and descent phases.
Based on these simulation results, the optimum region for UAV to cruise is from the 0.25th hour to the 8.5th hour and from the 14th hour to the 24th hour. The best point for the UAV to ascend is at the interval of the 8th hour to the 14th hour. The optimum region for the descent phase is the region of the 13th hour to the 19th hour. The region of the optimal mission profile phases over time is illustrated in Figure
Recommended mission profile phase region.
The recommended mission profile trajectory obtained based on the extensive simulation is plotted in Figure
Recommended mission trajectory.
A comparison of the
Battery SOC for a 24-hour flight.
A detailed comparison of the parameters of the proposed and previous models is given in Table
24-hour flight mission performance data.
Parameters | Original model | Proposed model | Improvement (%) |
---|---|---|---|
Power ratio | 1.3070 | 1.7101 | 30.84 |
Ceiling altitude (m) | | | 55.04 |
A comparison of the power ratio performance of the proposed power management strategy and that of various approaches [
Comparison of the proposed model with different approaches.
Approaches | Power ratio | Improvement (%) |
---|---|---|
Flight performance parameters [ | 1.0300 | 66.03 |
Aircraft parameters optimization [ | 1.1125 | 53.71 |
Energy management system [ | 1.3070 | 30.84 |
Proposed power management strategy | 1.7101 | — |
A case study for a perpetual flight mission was simulated using the proposed power management strategy. This step is crucial for determining whether or not the battery pack contains sufficient
Battery SOC for 72 hours.
The simulation of a 72-hour perpetual mission achieved a power ratio of 1.7100, which is slightly lower than that of a 24-hour flight mission. However, this difference is almost negligible. The
In addition, the minimal battery
Battery weight comparison.
In conclusion, this study proposes adopting vertical cylindrical trajectory with an additional element by defining the optimal time of mission transition in relation to the elevation angle of the sun, which improves the overall gained power ratio. Structuring the mission phases to define the start-end-time of the ascent, cruise, and descent phases of a flight increases the saved energy and improves aircraft endurance. This approach improves the net power ratio by 30.84% compared with other techniques. As a result, the battery weight may be massively reduced by 75.23%. In addition, the proposed strategy also enables a lower-altitude flight, which protects the UAV from highly volatile winds in high-altitude regions. Therefore, the proposed power management strategy confirms that perpetual solar-powered UAV operations are feasible and sustainable.
Radius
Displacement in
Displacement in
Displacement in
Velocity in
Acceleration in
Heading angle
Heading angle rate
Heading angle acceleration
Minimal power velocity
Minimum sink rate
Ceiling altitude
Weight of UAV
Air density
Wing’s surface area
Angle of attack
Gravitational acceleration
Solar elevation angle
Solar azimuth angle
Solar inclination angle
Mass of battery
Battery capacity
Battery resistance
Battery open circuit voltage
State of charge of battery
Rate of change of
Efficiency of battery pack
Efficiency of propeller
Efficiency of solar panel
Unmanned aerial vehicle
Magnitude of velocity
Tangential velocity
Normal velocity
Ascending velocity
Acceleration
Tangential acceleration
Normal acceleration
Ascending acceleration
Flight path angle
Oswald efficiency factor
Aspect ratio
Zero-lift drag coefficient
Zero angle of attack lift coefficient
Lift curve slope
Lift coefficient
Drag coefficient
Drag
Magnitude of lift
Tangential lift
Normal lift
Vertical lift
Thrust required
Solar spectral density
Power consumption
Power derived from sun
Power of battery
Energy consumption
Energy derived from sun
Aerodynamic coefficient
Energy management strategy.
The authors declare that they have no competing interests.
This study was supported by the Universiti Sains Malaysia (Grant no. 304/PAERO/60312047).