Solar-Powered Unmanned Aerial Vehicles
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Solar-powered unmanned aerial vehicles (UAVs) are uninhabited aircraft that leverage sun radiation to partially or completely power their onboard systems. The reduced load on batteries or other energy storage systems increases both flight endurance and range. When properly designed, the aircraft’s solar panels can generate excess power that recharges the battery over the day. In suitable environmental conditions, the batteries are recharged enough to keep the aircraft airborne throughout the night. This continuous discharge-recharge cycle allows solar-powered multi-day flight and, from an energy perspective, leads to a perpetual flight capability. Robotic technologies optimally complement this long-endurance flight capacity. By providing the aircraft with accurate and robust automatic flight control and guidance, the workload of the operators is reduced significantly.
History and State of the Art
Small-scale solar UAVs designed for Low-Altitude Long-Endurance (LALE) applications have therefore recently gained more attention. Though faced with the more challenging meteorological phenomena of the lower atmosphere (clouds, rain, wind gusts, and thermals), low-altitude UAVs provide the advantages of higher-resolution imaging with reduced cloud obstruction, lower complexity and cost, and simplified handling, e.g., through hand-launchability. As a result, solar-powered LALE UAVs aiming to reach flight times up to 14 h have been studied in academia (Weider et al., 2007; Malaver et al., 2015) and industry (AeroVironment, 2013; ByeAerospace, 2015). Research targeting perpetual endurance in these small-scale robotic aircraft has been relatively sparse. Solar-powered perpetual flight has been demonstrated by SoLong (Cocconi, 2005) (Fig. 1c), which performed a continuous 48-h flight while actively seeking out thermal updrafts, and SkySailor (Noth, 2008a), which demonstrated a 27-h flight without the use of thermals. In addition, AtlantikSolar has demonstrated a continuous 81-h flight (Oettershagen et al., 2017) and a fully autonomous 26-h flight with a day-/night-capable payload in a mock-up search and rescue mission (Oettershagen et al., 2018b). Nevertheless, these UAVs were mainly developed for demonstrating the feasibility of perpetual flight, but not for performing actual applications.
Design and Performance Metrics
Solar-powered UAV development emphasizes flight efficiency considerations. Lightweight structures, aerodynamically efficient airframes, and an optimal combination of solar modules and onboard energy storage are required to realize flight endurance and range advantages. Noth (2008a) provides a comprehensive reference on solar UAV design with a focus on low-altitude flight, while Colella and Wenneker (1996), Romeo et al. (2007), and Nickol et al. (2007) cover high-altitude solar UAV design. Zhu et al. (2014) provide a more recent overview over solar aircraft and performance-driving technologies such as solar module and battery types.
Key Research Findings
Given the similarity of their flight operations, non-solar UAVs and solar UAVs that are not perpetual flight capable face similar challenges and can share most hardware (airframe, sensors, autopilot) and software (estimation, flight control, path planning) (AeroVironment, 2013). In contrast, perpetual flight-capable solar UAVs face specific challenges such as low energetic safety margins, low payload capacity, high operational complexity, and limited autonomy without environment awareness. The research performed to solve these issues is outlined below.
System Design Approaches for Higher Energetic Margins and Payload Capacity
Issue: Today’s solar-powered UAV designs have limited energetic safety margins and payload capacity. For example, SoLong (Cocconi, 2005) had only 2 h of excess time, and SkySailor (Noth, 2008a) only had 5.8 % of battery energy remaining at the end of the night. As a result, the robustness against deteriorated weather (e.g., clouds) and the payload capacity are low.
Current research: First, many authors focus on model-based conceptual design approaches for solar UAVs (Brandt and Gilliam, 1995; Shiau et al., 2010; Leutenegger et al., 2010; Morton et al., 2015; Morton and Papanikolopoulos, 2016). Some recent methodologies (Morton et al., 2013; Oettershagen et al., 2017) emphasize perpetual flight robustness, i.e., high energetic safety margins under deteriorated weather conditions. Second, less authors present detailed system design studies and flight tests. For HALE UAVs, a number of publications (Romeo et al., 2007; Nickol et al., 2007) exist, but only the initial work by NASA (Flittie and Curtin, 1998) has led to published flight results. Important design studies for LALE UAVs are presented by Morton et al. (2015), Noth (2008a), Cocconi (2005), Weider et al. (2007), and Oettershagen et al. (2017, 2018b). Overall, recent demonstrations indicate significant progress: whereas the Zephyr HALE UAV performed both a continuous 14-day summer flight (Federation Aeronautique Internationale, 2010) and an 11-day winter flight (Sion Power, 2014), the AtlantikSolar LALE UAV achieved an 81-h-long summer flight that increased the minimum state of charge from the ≤10% of SoLong and SkySailor to about 39% (Oettershagen et al., 2017). In addition, AtlantikSolar achieved the first multi-day flight with a meaningful, i.e., day-/night-capable, sensing payload at low altitude (Oettershagen et al., 2018b). Third, battery and solar module research is performed outside of the UAV community. Especially battery improvements (Zu and Li, 2011), which are motivated by the use of mobile devices and electric vehicles, were shown to automatically lead to significant solar UAV performance improvements (Oettershagen et al., 2017).
Flight Automation for Less Operational Complexity
Issue: The central design differences of high-performance solar UAVs to standard UAVs are (a) a lightweight yet fragile structure, (b) a large wingspan for low induced drag, (c) a large wing area for low cruise speed but large solar cell area, and (d) the distribution of battery mass in a span-loader concept along the wing. As a result, these UAVs fly slowly and tend to have slow and undamped dynamics especially around the yaw axis. Together with their structural fragility (represented by vertical load factor limits of only 2–5 g’s), these characteristics make them very hard to fly. For example, SoLong’s 48-h flight required six highly trained pilots.
Current research: Flight control for fixed-wing UAVs has made significant progress in the past years. In addition to linear control techniques, approaches such as robust control, model-based control, model predictive control, and adaptive control have been presented (How, 2014). However, the challenging flight dynamics of solar UAVs require specific adaptations that are not comprehensively discussed in the literature. An exception is Oettershagen et al. (2017), which integrates a simple yet robust flight control system incorporating airspeed-dependent gain scheduling and trims, coordinated turn control, overspeed protection, and both static and dynamic structural overload protection. In Oettershagen et al. (2018b) this functionality is extended to allow a fully autonomous multi-day solar-powered flight that did not require a single pilot intervention. Large HALE UAVs in addition have to deal with aeroelastic effects which led to the NASA Helios crash (Noll et al., 2004). Aeroelastic models and flight controllers (Patil et al., 2001; Frulla, 2004; Mardanpour and Hodges, 2015) have been developed to manage those effects.
Increased Flight Autonomy Through Fully Environment-Aware Navigation
Issue: For fully autonomous operation, UAVs need to be aware of terrain. In addition, solar UAVs in particular are susceptible to weather: wind gusts, precipitation, clouds, or thunderstorms can damage the aircraft or reduce its performance and thus have to be avoided. However, the current literature does not provide navigation methods that properly consider weather. As a result, today, UAVs and solar UAVs can only be operated in closely controlled environments.
Current research: Literature on weather-aware UAV path planning is sparse. The necessary elements (e.g., wind dependence, sun dependence) are only considered separately in relatively unrelated fields of research. On the one hand, wind-aware planning using forecasted wind data and 3D wind fields exists (Rubio and Kragelund, 2003; Chakrabarty and Langelaan, 2013). Rubio et al. (2004) extend temperature and icing effects. All these planners however do not include solar models. On the other hand, research that incorporates solar models often neglects wind, assumes a perfectly clear sky or constant sun position, and thus solves a very simplified problem (Klesh and Kabamba, 2007, 2009; Spangelo et al., 2009; Hosseini et al., 2013; Dai, 2013). Recently, a planning approach that considers all safety and performance-relevant aspects (terrain, system state, and up-to-date meteorological weather data including thunderstorms, precipitation, humidity, winds, gusts, sun radiation, and clouds) was released, and the first-ever real-world large-scale mission with a solar UAV above the Arctic Ocean was shown (Oettershagen et al., 2018a).
Examples of Application
All applications that require data to be collected or distributed consistently or on a large scale can benefit from the long-endurance flight capabilities of solar-powered UAVs. In today’s data-driven society, many such applications exist (Colella and Wenneker, 1996; Zhu et al., 2014). Solar-powered UAVs could therefore be of pivotal societal and commercial use in large-scale disaster relief support, e.g., after earthquakes or floods (Rudin et al., 2017), meteorological surveys in remote and otherwise inaccessible areas (Runge et al., 2007), continuous border or maritime patrol (Romeo et al., 2007), telecommunications relay (Tozer and Grace, 2001), or large-scale agricultural or industrial aerial inspection (Herwitz et al., 2004).
Future Directions for Research
Solar-powered UAVs have made significant progress since the first perpetual flight by SoLong (Cocconi, 2005). Nevertheless, their widespread use by non-experts in complex real-life applications requires further research. Flight control, perception, and path planning research results can be shared with non-solar UAVs. The future solar-UAV-specific research closely aligns with section “Key Research Findings”. First, a remaining system design challenge is to increase the UAV’s operational and handling robustness while reducing the complexity and preserving the energetic performance. The steady advances in battery and solar module technology (Zu and Li, 2011; National Renewable Energy Laboratory, 2016), combined with research on novel concepts such as a 2-m-wingspan perpetual-flight-capable solar UAV (Morton and Papanikolopoulos, 2016) or vertical takeoff and landing (VTOL) solar UAVs (D’Sa et al., 2016), can be leveraged to achieve this goal. Second, the robotics research challenge is to implement reliable real-time weather-aware navigation for fully autonomous operation in cluttered terrain or adverse weather.
- Ackerman E (2013) Giant solar-powered UAVs are atmospheric satellites. IEEE spectrum. http://spectrum. ieee.org/automaton/robotics/aerial-robots/giant-solar- powered-uavs-are-atmospheric-satellites
- AeroVironment (2013) AeroVironment solar-powered puma AE small unmanned aircraft achieves continuous flight for more than nine hours. Press release. http://www.avinc.com/resources/press_release/aeroviro nment-solar-powered-puma-ae-small-unmanned-aircraft- achieves-contin
- Boucher RJ (1984) History of solar flight. In: AIAA/SAE/ASME 20th Joint Propulsion ConferenceGoogle Scholar
- British Broadcasting Corporation (BBC) (2016) Solar impulse lands in California after pacific crossing. http:// www.bbc.com/news/science-environment-36122618. Retrieved 10 Dec 2017
- ByeAerospace (2015) Industry first: solar-electric silent falcon prepares for initial customer orders. Press release. http://www.byeaerospace.com/news/2016/3/2/ industry-first-solar-electric-silent-falcon-prepares-for- initial-customer-orders
- Chakrabarty A, Langelaan J (2013) UAV flight path planning in time varying complex wind-fields. In: American Control Conference (ACC)Google Scholar
- Cocconi A (2005) AC propulsion’s solar electric powered SoLong UAV. Technical Report, AC propulsion. Retrieved from https://archive.org/details/ACPropulsionSolongUAV2005 Google Scholar
- Dai R (2013) Path planning of solar-powered unmanned aerial vehicles at low altitude. In: Circuits and Systems (MWSCAS), 2013 IEEE 56th international midwest symposium on, IEEE, pp 693–696Google Scholar
- D’Sa R, Jenson D, Henderson T, Kilian J, Schulz B, Calvert M, Heller T, Papanikolopoulos N (2016) SUAV:Q – an improved design for a transformable solar-powered UAV. In: Intelligent Robots and Systems (IROS), 2016 IEEE/RSJ international conference on, IEEE, pp 1609–1615Google Scholar
- Federation Aeronautique Internationale (2010) QinetiQ Hale Team (GBR) record. Official record. www.fai.org/record/qinetiq-hale-team-gbr-16052
- Flittie K, Curtin B (1998) Pathfinder solar-powered aircraft flight performance. In: 23rd Atmospheric Flight Mechanics ConferenceGoogle Scholar
- Frulla G (2004) Aeroelastic behaviour of a solar-powered high-altitude long endurance unmanned air vehicle (HALE-UAV) slender wing. In: Proceedings of the Institution of Mechanical Engineers, part G. J Aerosp Eng 218(3):179–188Google Scholar
- Hosseini S, Dai R, Mesbahi M (2013) Optimal path planning and power allocation for a long endurance solar-powered UAV. In: American Control Conference (ACC), IEEE, pp 2588–2593Google Scholar
- How JP (2014) UAV control. In: Valavanis KP, Vachtsevanos GJ (eds) Handbook of Unmanned Aerial Vehicles, chap 26–30. Springer Publishing Company, Incorporated., pp 527–710Google Scholar
- Kucinski W (2018) Airbus Zephyr S breaks world flight endurance record during maiden flight. SAE international. https://www.sae.org/news/2018/08/airbus- zephyr-s-breaks-world-flight-endurance-record-during- maiden-flight
- Malaver AJR, Gonzalez LF, Motta N, Villa TF (2015) Design and flight testing of an integrated solar powered UAV and WSN for remote gas sensing. In: IEEE Aerospace ConferenceGoogle Scholar
- Morton S, Papanikolopoulos N (2016) Two meter solar UAV: design approach and performance prediction for autonomous sensing applications. In: Intelligent Robots and Systems (IROS), 2016 IEEE/RSJ international conference on. IEEE, pp 1695–1701Google Scholar
- Morton S, Scharber L, Papanikolopoulos N (2013) Solar powered unmanned aerial vehicle for continuous flight: conceptual overview and optimization. In: IEEE International Conference on Robotics and Automation (ICRA)Google Scholar
- Morton S, D’Sa R, Papanikolopoulos N (2015) Solar powered UAV: design and experiments. In: IEEE International Conference on Intelligent Robots and Systems (IROS)Google Scholar
- National Renewable Energy Laboratory (2016) Best research-cell efficiencies. Retrieved from http://www.nrel.gov/pv/ on 10 Apr 2018
- Nickol C, Guynn M, Kohout L, Ozoroski T (2007) High altitude long endurance UAV analysis of alternatives and technology requirements development. Technical report, NASA/TP-2007-214861Google Scholar
- Noll TE, Brown JM, Perez-Davis ME, Ishmael SD, Tiffany GC, Gaier M (2004) Investigation of the Helios prototype aircraft mishap. Volume I: mishap report. Technical report, National Aeronautics and Space Administration (NASA)Google Scholar
- Noth A (2008a) Design of solar powered airplanes for continuous flight. Ph.D. thesis, ETH ZurichGoogle Scholar
- Noth A (2008b) History of solar flight. Autonomous systems lab, ETH Zurich. http://www.sky-sailor.ethz.ch/docs/HistoryofSolarFlightv2.0-A.Noth2008.pdf Google Scholar
- Oettershagen P, Förster J, Wirth L, Ambühl J, Siegwart R (2018a) Meteorology-aware multi-Goal path planning for large-scale inspection missions with solar-powered aircraft. J Aerosp Inf Syst. https://doi.org/10.2514/1.I010635
- Oettershagen P, Stastny T, Hinzmann T, Rudin K, Mantel T, Melzer A, Wawrzacz B, Hitz G, Siegwart R (2018b) Robotic technologies for solar-powered UAVs: fully-autonomous updraft-aware aerial sensing for multi-Day search-and-rescue missions. J Field Robot (JFR)Google Scholar
- Osborne T (2016) U.K. will buy two Zephyr 8 UAVs. Aviationweek aerospace daily. http://aviationweek.com/awindefense/uk-will-buy-two-zephyr-8-uavs
- Romeo G, Frulla G, Cestino E (2007) Design of a high-altitude long-endurance solar-powered unmanned air vehicle for multi-payload and operations. In: Proceedings of the Institution of Mechanical Engineers, part G: J Aerosp Eng 221(2):199–216Google Scholar
- Ross H (2008) Fly around the world with a solar powered airplane. In: The 26th congress of international council of the aeronautical sciences (ICAS), American Institute of Aeronautics and AstronauticsGoogle Scholar
- Rubio JC, Kragelund S (2003) The trans-pacific crossing: long range adaptive path planning for UAVs through variable wind fields. In: Digital Avionics Systems Conference. DASC’03. The 22nd, IEEE, vol 2, pp 8–BGoogle Scholar
- Rubio JC, Vagners J, Rysdyk R (2004) Adaptive path planning for autonomous UAV oceanic search missions. In: AIAA 1st Intelligent Systems Technical Conference, pp 20–22Google Scholar
- Runge H, Rack W, Ruiz-Leon A, Hepperle M (2007) A solar powered HALE-UAV for arctic research. In: 1st CEAS European air and space conferenceGoogle Scholar
- Sion Power (2014) Sion power’s lithium-sulfur batteries power high altitude pseudo-satellite flight. Press release. http://sionpower.com/media-center.php?code=sion-powers-lithiumsulfur-batteries-power-high-alt Google Scholar
- Weider A, Levy H, Regev I, Ankri L, Goldenberg T, Ehrlich Y, Vladimirsky A, Yosef Z, Cohen M (2007) SunSailor: solar powered UAV. Technical report, Aerospace engineering faculty, technion. http://webee. technion.ac.il/people/maxcohen/SunSailorArt19nov06. pdf