The Flight of Solar-Powered Aircraft Helios

Here is a video showing compilation of test flights of the solar-powered, remotely piloted NASA's Helios aircraft.

Helios set out from Kauai in the Hawaiian Islands before 9:00 AM on Monday, August 13, 2001. Just over seven hours later, it reached 96,500 feet. Flying at about 25 miles an hour, the mission lasted nearly 17 hours, landing at 1:43 a.m. August 14, 2001

This was the first time a non-rocket powered aircraft has maintained flight this far above the earth. At this altitude above ground the sky is almost black, stars shine in the daytime, and the horizon looks curved.

Helios was an ultralight flying wing built by AeroVironment Inc, as a part of NASA's Environmental Research Aircraft and Sensor Technology (ERAST) project. Covered with solar cells, Helios' impressive 247 foot wide wing exceeds the wing span and even overall length of a Boeing 747 jet airliner.

Helios had about 62,000 solar cells across the wing. The solar cells collect energy from the Sun and convert it to electricity, which runs the 14 small motors. The motors turn the 14 propellers, which are specially designed to pull the aircraft aloft even in the very thin air that's 18 miles high.

Helios was intended as a technology demonstrator, but in the extremely thin air 100,000 feet above Earth's surface, the flight of Helios also approached conditions for winged flight in the atmosphere of Mars, since the atmosphere at that height above earth replicates the atmosphere near the Martian surface.

More Key Steps in Photovoltaic History

The world’s most advanced solar-powered airplane Icare Credit: University of Stuttgart, Germany

In 1955, the first solar-powered automobile was publicly demonstrated in Chicago. It was a 15-inch Sunmobile built by William G. Cobb of the General Motors Corporation. Light energy falling on 12 selenium photoelectric cells created electric current to power a tiny electric motor. The solar automobile was one of 253 free exhibits of the General Motors Powerama, Chicago, Illinois.

In 1976 NASA’s Lewis Research Center starts the installation of 83 PV power systems on every continent except Australia.

In 1978 was installed the world's first village PV system at Papago Indian Reservation, Schuchuli, Arizona.

In 1980 ARCO Solar is the first company to produce more than 1 MW of PV modules in one year.

In 1981 Paul MacCready builds the first PV-powered aircraft known as the Solar Challenger, which flies from France to England across the English Channel. The aircraft had over 16,000 solar cells mounted on its wings, which produced 3,000 watts of power.

Volkswagen began testing PV arrays mounted on the roofs of vehicles in 1982.

In 1982 a Danish-born Australian eco adventurer Hans Tholstrup drives the first solar-powered car - the Quiet Achiever - almost 2,800 miles between Sydney and Perth in 20 days. The vehicle had photovoltaic system of 1 kW and the average speed was 23 km/h. Tholstrup is the founder of the World Solar Challenge in Australia, considered the world championship of solar car racing.

In 1984, a 1 megawatt photovoltaic electricity plant began to operate in Sacramento, California.

ARCO Solar introduced a G-4000, the first commercial thin film photovoltaic module in 1986.

The world’s most advanced solar-powered airplane, the Icare, flew over Germany in 1996. The wings and tail surfaces of the Icare are covered by 3,000super-efficient solar cells, with a total area of 21 m2.

The 11th tallest skyscraper in New York City, built in the 1990s - the Condé Nast Building, officially 4 Times Square - includes building integrated photovoltaic (BIPV) panels on the 37th through the 43rd floors on the south and west-facing facades to produce a portion of the building’s power.

In 1999 Germany launches a 100,000 solar roofs scheme to promote the on-site generation of clean electricity.

On August 13, 2001 after long research and trial, the solar powered wing aircraft Helios set an unofficial world record by a winged aircraft of flying at a sustained altitude above 96,000 feet (29,250 m) for 40 minutes. Helios is a remotely operated (by two controllers using computers on the ground), solar powered aircraft developed by NASA and AeroVironment Inc.

In 2002 Japan installed 25,000 solar rooftops.

Bringing PV Cells from Space back down to Earth

During the 1960s and early 1970s the use of solar cells in space flourished but down on Earth electricity from the sun seemed not very perspective option. The high costs of the PV cells made them uneconomical for use on the earth where low price is the main factor.

In the early 1970's a way to lower to cost of solar cells was discovered. Dr. Elliot Berman, with financial help from Exxon Corporation, designed a significantly less costly solar cell by using a poorer grade of silicon and packaging the cells with cheaper materials. This brought the price down from $100 per watt to around $20 per watt.

The energy crises of the 1970s led to a worldwide push for alternative renewable sources of energy, and photovoltaic were seen as a possible solution. Major research activities in the field took place and the main objective of photovoltaic research has been to reduce costs in order to bring solar power to people.

Significant efforts were made to develop PV power systems for residential and commercial uses, both for stand-alone, remote power as well as for utility-connected applications. The photovoltaic industry attracted the interest of large energy companies and government agencies. With their investment of capital, tremendous improvements in manufacturing, performance and quality of PV modules were possible.

In the 1980s, photovoltaics became a popular power source for consumer electronic devices. PV cells were incorporated in watches, radios, lanterns and other small battery-charging applications. During the same period, international applications for PV systems to power water pumping, refrigeration, telecommunications, rural health clinics, and off-grid households increased dramatically, and remain a major portion of the present world market for PV products.

Photovoltaics and the Space Industry

The International Space Station in December, 2001. Credit: the crew of STS-108, NASA

Starting in the 1950s and 60s, the space industry was the first market for photovoltaics. Photovoltaics were light and the "fuel" is both weightless and free for the taking. The high costs were never an issue since money was never a problem with the space industry.

In 1954 Bell Laboratories built the first photovoltaic module. It was billed as a solar battery and was mostly just a curiosity as it was too expensive to gain widespread use. Bell Labs used a new process called the Czochralski process to develop the first crystalline silicon photovoltaic cell with an efficiency of about 4 percent. The new technology got the first major commercial push when NASA integrated it into its new space program.

In 1955, the preparation on satellite energy supply by solar cells began. Western Electric put for sale commercial licenses for solar cells production. Hoffman Electronics - Semiconductor Division introduced a commercial photovoltaic product with 2 % efficiency for US$ 25 per cell with 14 mW peak power. The energy cost was US$ 1,785 per W.

In 1957, Hoffman Electronics introduced a solar cell with 8 % efficiency. A year later, in 1958, the same company introduced a solar cell with 9 % efficiency. The first radiation proof silicon solar cell was produced for the purposes of space technology. On 17th March 1958, the first satellite powered by solar cells, Vanguard I, was launched. The system ran continuously for 8 years. Two other satellites, Explorer III and Vanguard II, were launched by Americans, and Sputnik III by Russians.

In 1959, Hoffman Electronics introduced commercially available solar cells with 10 % efficiency. Americans launched the satellites Explorer VI with photovoltaic field of 9,600 cells and Explorer VII.

In 1962, Bell solar cells powered Telstar, the world's first communications satellite.

1964 - NASA launches the first Nimbus spacecraft - a satellite powered by a 470-watt photovoltaic array.

In 1965, the Japanese scientific programme for Japanese satellite launch commenced. The following year, in 1966, NASA launches the first Orbiting Astronomical Observatory, powered by a 1-kilowatt photovoltaic array, to provide astronomical data in the ultraviolet and X-ray wavelengths filtered out by the earth’s atmosphere.

Today, the space industry is still a significant user of photovoltaics since they play an important role in space, providing electrical power to satellites in an orbit around the Earth. Solar cells power virtually all satellites, including those used for communications, defence, and scientific research. More than 600,000 flight-proven solar cells are powering over 60 satellites.

The International Space Station uses multiple solar arrays to power all the equipment on board. The success of the space and planetary exploration missions often depends on their on-board PV power sources — providing power for experiments and for getting the data back to Earth. The three Mars rovers - Pathfinder rover Sojourner, Spirit and Opportunity, completed their missions successfully, powered by PV.

See also: http://www.aerospaceweb.org/question/spacecraft/q0298b.shtml and http://mars.jpl.nasa.gov/MPF/roverpwr/power.html)

Photovoltaic History - Key Milestones in the 1900s (Timeline)

Bell Labs engineer testing solar battery in 1954 Credit: Bell Labs website

1904 - Albert Einstein published his paper on the photoelectric effect (along with a paper on his theory of relativity). Wilhelm Hallwachs makes a semiconductor-junction solar cell (copper and copper oxide).

1914 - The existence of a barrier layer in photovoltaic devices was noted.

1916 - Robert Millikan provided experimental proof of the photoelectric effect.

1918 - Polish scientist Jan Czochralski developed a way to grow single-crystal

silicon. 

1921 - Albert Einstein received the Nobel Prize for his theories explaining the photoelectric effect.

1932 - Audobert and Stora discover the photovoltaic effect in Cadmium selenide (CdSe), a photovoltaic material still used today.

1954 - Bell Labs announces the invention of the first modern silicon solar cell. The scientists Gerald Pearson, Daryl Chapin, and Calvin Fuller develop the silicon photovoltaic (PV) cell at Bell Labs—the first solar cell capable of converting enough of the sun’s energy into power to run everyday electrical equipment. Bell Telephone Laboratories produced a silicon solar cell with 4% efficiency and later achieved 11% efficiency. Reporting the Bell discovery, The New York Times praised it as "the beginning of a new era, leading eventually to the realization of harnessing the almost limitless energy of the sun for the uses of civilization".

1959 - Hoffman Electronics creates a 10% efficient commercial solar cell, and introduces the use of a grid contact, reducing the cell's resistance.

1962 - Bell Telephone Laboratories launches the first telecommunications satellite, the Telstar (initial power 14 watts).

1963 - Sharp Corporation succeeds in producing practical silicon PV modules. Japan installed a 242-W PV array on a lighthouse, the world's largest array at that time.

1965 - Peter Glaser conceives the idea of the satellite solar power station. 

1973 - The University of Delaware builds “Solar One,” one of the world’s first photovoltaic PV) powered residences. The system is a PV/thermal hybrid. 

1980 - At the University of Delaware, the first thin-film solar cell exceeds 10% efficiency using copper sulfide/cadmium sulfide.

1982 - The first, photovoltaic megawatt-scale power station goes on-line in Hisperia, California. It has a 1-megawatt capacity system, developed by ARCO Solar, with modules on 108 dual-axis trackers.

1983 - ARCO Solar dedicates a 6-megawatt photovoltaic substation in central

California. The 120-acre, unmanned facility supplies the Pacific Gas & Electric Company’s utility grid with enough power for 2,000-2,500 homes.

1985 - 20% efficient silicon cell are created by the Centre for Photovoltaic Engineering at the University of New South Wales.

1993 - Pacific Gas & Electric completes installation of the first grid-supported photovoltaic system in Kerman, California. The 500-kilowatt system was the first “distributed power” effort.

1998 - Subhendu Guha, a noted scientist for his pioneering work in amorphous silicon, led the invention of flexible solar shingles, a roofing material and state-of-the-art technology for converting sunlight to electricity.

1999 - Total worldwide installed photovoltaic power reached 1000 megawatts.

Photovoltaic (PV) History - the Beginning

The effect of light on the electric properties of certain materials was observed way back even before electricity became generally available.

In 1839 the nineteen-year-old French physicist Alexandre Edmond Becquerel observed the photovoltaic effect for the first time. Experimenting with metal electrodes in a weak electrolyte or conducting solution (such as salt water) exposed to sunlight, he discovered the appearance of small amounts of electric current.

However, Becquerel's discovery couldn't find any practical use and was limited being tagged as an observed phenomenon. The photo conductivity of an element, selenium, was noted by the English electrical engineer Willoughby Smith in 1873 while he was working with Selenium.

In 1876 William Grylls Adams* and his student Richard Day, discovered that illuminating a junction between selenium and platinum can have a photovoltaic effect. This effect is the basis for the modern solar cell. An electricity expert, Werner von Siemens, stated that the discovery was "scientifically of the most far-reaching importance". The selenium cells were not efficient, but it was proved that light, without heat or moving parts, could be converted into electricity.

William G. Adams published also a paper on the selenium cell 'The action of light on selenium,' in "Proceedings of the Royal Society, A25, 113.

In 1883 Charles Fritts, an American inventor, built what many regard as the first true photovoltaic cell. He developed from selenium wafers a solar cell that had less than 1-2% a conversion rate but represents the beginning of solar technology as we know it today.

In 1887 Heinrich Hertz noticed the photoelectric effect, and published his paper entitled “On an Effect of Ultraviolet Light upon the Electric Discharge.” He noticed that the spark created at a receiving electric circuit increased when ultraviolet light hit the negative terminal.

* William Grylls Adams, an English professor of Natural Philosophy at King's College, London was the brother of John Couch Adams, the astronomer who discovered Neptune.

Solar Energy and the Ancient World

Energy from the sun has been used by people for centuries. As early as the 7th century B.C., ancient people used simple magnifying glasses to concentrate the light of the sun into beams so hot they would cause wood to catch fire. The Greeks and Romans use magnifying glasses to burn the sails of enemy ships. It was first applied to use in 212 B.C., by the Greek scientist Archimedes. Solar energy was used to defend the harbour of Syracuse (Sicily) against the Roman fleet. Archimedes used a mirror or "burning mirror" as they had called it, to set fire to Rome's wooden ships while standing on shore.

From ancient time people have also used solar energy for drying, cooking, heating baths and warming their homes. Without electricity, mankind learned to orient their buildings to capture the heat of the sun during the day.

The Greeks were the first to use solar architecture, over 2,000 years ago. They learned to build their houses to take maximum advantage of the solar energy. The sun's rays entered their homes during the winter, but weren't able to enter during the summer. There were entire cities built this way. The Romans, Egyptians, Chinese, and Native Americans also warmed their homes through passive solar energy designs. Anasazi Indians build cliff dwellings with southern exposures, providing passive solar heating and cooling.

The Romans first of all put glass in windows, which allowed the sun's light to pass through but trapped its heat. They even built glass greenhouses so they could have fresh fruits and vegetables all winter.