As an emerging type of aircraft exploring near-space, solar-powered unmanned aerial vehicles (SUAVs) exhibit new characteristics distinct from traditional aircraft in terms of performance, technology, and mission flight time. Among these, the specific energy and specific power of the solar-powered UAV energy system are core factors affecting the overall performance of the aircraft. Therefore, this paper first describes the current development status of solar cells and energy storage batteries for solar-powered UAVs, and then looks forward to the future development trends of solar-powered UAV energy systems in terms of diversified energy acquisition, efficient energy system management, and integrated energy payload.
Some people treat it as a children's flying toy, some see it as a powerful tool for long-range aerial photography, and some regard it as an auxiliary tool for developing "smart agriculture" and improving lifestyles. This is the drone.
In recent years, with the rapid development of flight control, navigation, and communication technologies, drones have not only formed their own industry but have also experienced exceptionally strong growth. With continuous market growth, expansion into new markets, and increasingly diverse application scenarios, drones are becoming more varied and specialized, including the currently popular cutting-edge technology – solar-powered drones.
Solar-powered drones, as the name suggests, are drones that use solar energy as their power source for flight and operations. They are characterized by ultra-long flight time, persistent loiter time, ultra-high altitude flight, and wide operational range, making them comparable in value to "quasi-satellites," while also offering advantages such as flexible deployment and high economic efficiency. In recent years, for purposes such as early warning of major natural disasters, high-altitude maritime patrol and monitoring, emergency rescue and disaster relief, counter-terrorism and stability maintenance, and mobile communication, countries around the world have been investing heavily in and accelerating the research and development of solar-powered drones, hoping to seize a new high ground in the future drone industry.
The United States and the United Kingdom are the two pioneers and leaders in the development of solar-powered drones. The former, years ago, had Aurora Flight Sciences and General Atomics respectively develop solar-powered drones such as the "Perseus" and "Altas," and later companies like Facebook, Google, and Airbus led the development of a new batch of solar-powered drones. The latter, aiming to verify key technologies for solar-powered flight, developed the "Breeze" and "Westwind" series of solar-powered drones early on, which have been effectively used in the defense sector.
Following closely behind the US and UK, my country has also made rapid progress and achieved remarkable results in the development of solar-powered drones in recent years.
In 2016, Northwestern Polytechnical University's "Phantom Solar-Powered WiFi Drone" emerged, demonstrating my country's strength in the research and development of solar-powered WiFi drones. In 2017, the "Flying Cloud" project, which builds an aerial local area network based on near-space solar-powered drones, was launched, and the "Rainbow" solar-powered drone successfully completed its near-space flight test, marking my country as the third country in the world to master near-space solar-powered drone technology. Meanwhile, recently, my country's independently developed medium-to-large solar-powered unmanned aerial vehicle, the "Mozi II," successfully completed its maiden flight, indicating that the gap between my country and the UK and the US is narrowing.
The successful maiden flight of China's solar-powered large unmanned aerial vehicle (UAV), the "Qimingxing-50," in early September has attracted widespread attention, with numerous media outlets questioning the current level of China's solar-powered large UAV development. It is understood that several solar-powered large UAVs developed in China have already completed their maiden flights. Chinese experts believe that from the 45-meter-class "Rainbow" T4 solar-powered large UAV, which first flew in 2017, to the 50-meter-class "Qimingxing-50," which also first flew this September, China is poised to produce domestically made solar-powered large UAVs with a wingspan reaching 60 meters (click for details).
Currently, China has at least the following models under development: the "Rainbow" T4 solar-powered large UAV, the "Qimingxing-50" solar-powered large UAV, and the "Mozi II" solar-powered aircraft. Among them, the "Rainbow" T4, China's first solar-powered large UAV to complete its maiden flight, completed its first test flight in 2017, flying continuously for approximately 15 hours before landing smoothly, successfully completing a flight test at an altitude of over 20,000 meters. This also placed my country among the world's top three in the field of solar-powered large UAVs, following the UK and the US. According to public information, the "Rainbow" T4 UAV has a wingspan of 45 meters. It is said that this wingspan provides a very high starting point for the development of domestically produced solar-powered large UAVs. A 45-meter wingspan is roughly equivalent to the width of 25 cars lined up side-by-side, and a flight altitude of 20,000 meters is equivalent to the total height of 48 Tianjin TV Towers stacked on top of each other. This type of solar-powered large UAV is equipped with eight engines, and its wings are almost entirely covered with solar panels; its core technologies and equipment are all domestically produced.
The "Qimingxing-50" solar-powered large unmanned aerial vehicle (UAV), which made its maiden flight in early September this year, has a wingspan of 50 meters and a flight altitude of up to 20,000 meters. This aircraft is the first ultra-large aspect ratio high-altitude low-speed large UAV developed by China's aviation industry, the first large UAV with a twin-fuselage layout, and the first all-electric large UAV powered solely by solar energy. Industry insiders believe that the successful maiden flight of the "Qimingxing-50" solar-powered large UAV not only boosts confidence in the development of the solar-powered large UAV field but will also further promote the development of key technologies in my country's new energy, composite materials, and flight control fields. Comparing publicly available information, the "Rainbow" T4 UAV and the "Qimingxing-50" UAV are similar in size and have roughly the same number of engines, suggesting that their capabilities are essentially equivalent.
Are there any domestically produced large solar-powered drones with even larger wingspans?
At previous Zhuhai Airshows, Rainbow Company has showcased a scaled-down model of a large solar-powered drone. According to promotional materials released by Rainbow Company at the 2021 Zhuhai Airshow, this large solar-powered drone has a wingspan of 60 meters, a service ceiling of over 20,000 meters, a maximum endurance of over 30 days, and a payload capacity of 30 kilograms.
According to aviation experts, based on the development pattern of large solar-powered UAVs and the development cycle of related models, my country's new large solar-powered UAV with a larger wingspan and larger payload is expected to make its debut at this year's Zhuhai Airshow.
Large solar-powered drones are constantly evolving, with increasingly larger wingspans, aiming to carry and operate heavier, more power-consuming payloads during long-endurance missions lasting a month or even several months. To achieve long endurance, especially uninterrupted day-and-night flight, large solar-powered drones need larger wingspans, more photovoltaic cells, and multiple motors to provide sufficient power for high-altitude cruising. If the electricity stored during the day is insufficient for nighttime use, the drone's loiter time will be severely limited.
If a drone carries a small payload, it may be difficult to perform its missions. Therefore, large solar-powered drones need to maximize their power generation capacity by increasing their wingspan. Additionally, high-aspect-ratio aircraft generally have a higher lift-to-drag ratio, which typically results in longer loiter times and greater cruising distances. Considering these factors, the wingspan of large solar-powered drones will continue to increase. However, during drone launch and ascent, longer, thinner wings are more susceptible to external airflow interference. If structural strength issues are not resolved, the wings may break before the drone reaches its mission altitude. Therefore, increasing wingspan requires addressing structural strength challenges.
Airbus recently completed a new round of test flights for its Zephyr solar-powered unmanned aerial vehicle (UAV). During the test flights, the Zephyr flew in the stratosphere at an average altitude of 21,000 meters, with a wingspan of 25 meters and a weight of less than 75 kilograms. It uses solar energy during the day and rechargeable batteries for propulsion at night, demonstrating the latest achievements in the development of solar-powered UAVs.
In recent years, the rapid development of the drone industry has accelerated the expansion and deepening of drone applications. To meet the needs of different fields, drone types have become increasingly diverse and abundant, and solar-powered drones have emerged. Solar-powered drones are those that rely on solar energy as their power source, and these drones feature ultra-long flight time, persistent loiter time, ultra-high altitude flight, and wide range of operational capabilities.
Leveraging their significant advantages, solar-powered drones have wide applications in both military and civilian sectors, and their value is comparable to that of quasi-satellites. In reality, many countries use solar-powered drones for atmospheric research, weather forecasting, environmental and disaster monitoring, traffic control monitoring, internet services, television services, and military applications, bringing numerous conveniences to people's production and lives.
However, the best application scenario for solar-powered drones is undoubtedly in communications. Solar-powered drones have four value-added benefits for communications development: first, they enhance existing communication services by acting as aerial base stations; second, they fill communication gaps in remote areas; third, they provide emergency communications during sudden disasters; and fourth, they meet the needs of long-range relay communications for both military and civilian sectors.
Based on the aforementioned communication value, many countries around the world are currently developing solar-powered drones. Among them, the United States and the United Kingdom are the two earliest players in this field. The former developed the "Passius" and "Altas" solar-powered drones many years ago, while the latter also developed the "Breeze" and "Westwind" series of solar-powered drones early on.
Following these two countries, my country's solar-powered drone development has also gradually begun. In 2016, the "Phantom Solar-Powered Drone" developed by Northwestern Polytechnical University in my country emerged, marking the beginning of the development of solar-powered drones in China. Subsequently, the "Flying Cloud" project was successfully launched, and the "Mozi II" successfully completed its maiden flight, all of which signify that my country has achieved remarkable results in the development of solar-powered drones.
