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What comes to mind when you think of solar power? Is it using solar panels on your home, or maybe something smaller scale, like powering your small outdoor lights? Here at Kea Aerospace, we’re currently developing a solar-powered aircraft that can fly in the stratosphere continuously for months at a time. However, solar power has a rich history demonstrating a variety of uses that spans thousands of years, from being used in Ancient Greece to rovers exploring the terrain of Mars.

Historians have unearthed evidence of civilisations making use of the sun’s power as early as the 7th century BCE, using glass to magnify the sun’s rays and light fires. Drawing on this knowledge, the Ancient Greeks and Romans of the 3rd century BCE lit ceremonial torches using mirrors to direct the sun’s beams. The Han Dynasty in China recorded the use of mirrors for similar purposes around 20 A.D.

The development of solar cells, the technology that allows us to capture and use the sun’s energy, can be traced back to a series of scientific discoveries throughout the nineteenth century. In 1839, French physicist Edmond Becquerel found that electricity generation could be increased by coating platinum electrodes with a conductive solution and exposing them to sunlight [i]

Becquerel’s discovery, called the photovoltaic effect, was used in further tests by Willoughby Smith in 1873, and William Grylls Adams and Richard Evan Day in 1876. These experiments revealed that the element selenium could act as a photoconductor and generate electricity when exposed to sunlight. Building on this research, Charles Fritts invented the world’s first selenium-based solar cell in 1883. Though it was only capable of powering small toys, Fritts’ invention showed that a solid material could convert light into electricity without the need for heat or machinery [ii]. This early breakthrough laid the groundwork for the development of modern solar cells.

Solar Power In Space

As early as 1946, solar power’s potential was on the radar of those in the aerospace sector. When the American Bureau of Aeronautics assigned contracts to aviation companies to build the first Earth-orbiting satellites, they also launched an investigation into how solar powered technology could recharge the satellite’s batteries [iii]. However, solar power didn’t become viable for the aerospace industry until 1954.

Gerald Pearson, a physicist at Bell Labs, unintentionally created a far more efficient version of the solar cell by using silicon instead of selenium. Pearson’s colleagues, Daryl Chapin and Calvin Fuller, developed this discovery further, creating the first solar cell that was efficient enough to power ordinary electrical appliances. The high cost of silicon solar cells stood in the way of its commercialisation, but the Army and Air Force still recognised it as the perfect energy source for a top-secret venture – the launch of Earth-orbiting satellites.

The Naval Research Laboratory, whose Vanguard program was selected as America’s first scientific satellite effort, initially refused to use solar powered components in their satellites. However, thanks to the innovative spirit of American space scientist, James Van Allen, Vanguard I ended up carrying six silicon-based solar cells. The Americans could finally claim a world’s first against the Soviets in the Space Race: the solar powered satellite.

Following closely after Vanguard I’s launch on 17 March 1958, two more solar powered satellites were successfully launched: the Soviet Union’s Sputnik 3, and the American Explorer 3. Within two months, there were now three orbiting satellites with solar cell components powering their radio transmitters. Vanguard I could continue transmitting data back to Earth until 1965 because of its solar components – its electrochemical batteries died within months of Vanguard’s launch.

Subsequently, both the Soviets and the Americans strived to use solar power on future satellites. The International Space Station currently has eight solar array wings that can generate approximately 240 kilowatts in direct sunlight.

Solar Powered Flight

In 1974, Astro Flight Inc., founded by brothers Roland and Robert (Bob) Boucher, received funding from the Advanced Research Project Agency (ARPA) for their Project Sunrise. Sunrise I was an uncrewed solar powered aircraft, the first of its kind, with solar cells covering the entire top of its 10-metre wingspan. Although it was hoped to reach 78,000ft (23.5 kilometres), Sunrise I managed to fly to only 8,000ft in its 28th flight before it was destroyed in turbulence. Despite its ill-fated ending, Sunrise I highlighted the possibility of using solar powered flight at greater altitudes. Crucially, it caught the attention of crewed-aircraft builders – how could they use solar power in their own endeavours?

Solar Challenger, a solar powered aircraft built by Paul MacCready’s company, AeroVironment, was the answer to that question. Challenger, equipped with over 16,000 silicon solar cells on its wings, made history in 1981 when it became the first crewed solar powered aircraft to complete a long-distance flight. The 262-kilometre demonstration flight from France to England was a success, demonstrating the potential of solar power for aviation.

Interestingly, MacCready was sceptical of his own success. In an interview, he was quoted as saying that “the least useful application of solar cells that I can think of is powering an airplane”[iv]. However, MacCready saw the value in using a solar powered aircraft to engage public interest and drive innovation in solar technology.

Solar Powered Unmanned Aerial Vehicles

Pathfinder, the first in its generation of solar powered Unmanned Aerial Vehicles (UAVs), lived a couple of different lives before landing in NASA’s hands in 1994. Originally funded by the CIA for military surveillance, the aircraft later served the Ballistic Missile Defence Organisation (BMDO). NASA then welcomed Pathfinder into the Environmental Research Aircraft and Sensor Technology (ERAST) program.

With its unique solar powered capabilities, Pathfinder monitored the impact of hurricanes on forest regrowth, measured algal concentrations in coastal waters, and assessed the health of coral reefs. It also set new altitude records for solar powered aircraft, flying at 50,500ft in 1995, and then 71,530ft in 1997. Although Pathfinder was unable to achieve perpetual flight, it illustrated the potential of solar powered high-altitude platforms.

Shooting forward a few decades, there’s now a broad range of companies working on solar powered UAV projects. In particular, there’s extensive interest in High Altitude Long Endurance (HALE) and High Altitude Platform Station (HAPS) aircraft for applications such as telecommunications and earth observation.

Breaking Endurance Records

Betrand Piccard and André Borschberg’s Solar Impulse aircraft made waves in the world of aviation when it was unveiled in 2009. Inspired by MacCready’s Solar Challenger, the plane was equipped with 12,000 silicon solar cells and a 63.4-meter wingspan, allowing it to set new endurance records for solar powered crewed flight. In 2010, it achieved the world’s first 26-hour solar powered flight, and two years later it completed the first intercontinental solar flight, travelling 19 hours from Madrid, Spain to Rabat, Morocco [v].

Not yet satisfied with their achievements, Betrand and Piccard unveiled the new and improved Solar Impulse 2 in 2014. The plane featured a massive 71.9-metre wingspan and over 1,000 additional solar cells, making it the first piloted solar powered aircraft to circumnavigate the globe. The epic journey took 16-and-a-half months and included 17 stopovers, traveling over 42,000 kilometres in total. In the process, Solar Impulse 2 set 19 official aviation records. Upon Impulse 2’s return to Abu Dhabi, Piccard commented that he felt a sense of disappointment upon returning to a world filled with pollution and noisy machinery as if he “had returned to the past” [vi]. Through their incredible feats, both Solar Impulse and Solar Impulse 2 proved that it is possible to travel around the world without producing fuel emissions.

Records are also being broken in the uncrewed aircraft domain. The Airbus Zephyr S flew for 64 days in 2022 and currently is the longest flying solar powered aircraft.

Solar Power On Land

Solar energy use has had a dramatic rise over the last 30 years in homes and in commerce. The average cost of a solar panel dropped by 90% from 2010 to 2020. We now regularly see solar panels on houses and used in businesses, massive solar farms have been built, and solar cars are being manufactured.

Bhadla Solar Park in India is currently the world’s largest solar farm. It covers 5,700 hectares and generates 2.25 Gigawatts of energy. Prototype solar powered cars such as the Lightyear 0, Sion and Aptera have been built and are progressing through development and manufacturing phases. We can expect to see a significant increase in solar energy use in our daily lives this decade.

To The Stratosphere And Beyond

In the 2020s, advanced aviation and aerospace technology is developing swiftly and solar power is at the vanguard of that change. Solar power is used in orbiting satellites, space probes and Mars rovers, but we can expect to see a multitude of new technologies develop.

There’s a convergence of emerging solar cell and battery technologies that will greatly expand the capability of solar powered flight. At Kea Aerospace, we envisage a future where thousands of solar powered aircraft are continuously flying in the stratosphere to acquire better-quality Earth observation data and providing telecommunications services. In the words of Bertrand Piccard, “The future is clean. The future is you. The future is now. Let’s take it further.”[vii]

References
[i] ‘What is the history of solar energy and when were solar panels invented?’
[ii] ‘The History of Solar Energy’  
[iii] Vanguard: A History, Constance McLaughlin Green and Milton Lomask, 1970, p.7.
[iv] ‘Solar Challenger’, Don Monroe, 1981
[v] ‘Solar Impulse HB-SIA Solar Airplane’
[vi] ‘Solar plane pioneer embarks on journey to help low-carbon innovations take flight’, RP Siegel, 27 June 2018
[vii] ‘Solar Impulse completes historic round-the-world trip’, BBC, 26 July 2016

Photo credits
[1] Mars rover – NASA
[2] International Space Station – NASA
[3] Pathfinder – NASA
[4] Solar array – Unsplash
[5] Kea Atmos Mk1 – Kea Aerospace