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.

European Solar Days

The First European Solar Days will be celebrated on the 16th and 17th May 2008 with more than 4000 events and will reach thousands of European citizens throughout Europe.

The 'Tag der Sonne' was first celebrated in Austria in 2002, and the idea has already been adopted by Germany, Switzerland and The Netherlands. In the case of Germany a whole week is dedicated to the campaign. This wonderful idea is now for the first time extended to more countries: France, Belgium, Portugal, Spain, Italy, Slovenia, and Norway. In 2009 other countries are expected to join the initiative and the event is planned to be ex-tended throughout Europe in the future.

The European Solar Days will bring together major players from the solar thermal and solar photovoltaic electricity sectors throughout Europe and will help in promoting the use of renewable energy.

The aim of the ESD is to raise awareness and promote the possibilities of solar power, Solar Thermal and Solar Photovoltaic, among both decision makers and the general public.

Comprehensive information on the European Solar Days is available at: www.solardays.eu

Photovoltaic Cells

Picture: DOE/EERE

Photovoltaic cells (PVs) (also known as "solar cells") work by transforming light that comes from the sun directly into electricity without an intermediate mechanical device or thermal process. The term photovoltaic is derived by combining the Greek word for light, "phos", with the word "voltaic". The term "volt" is a measure of electricity named for Alessandro Volta (1745-1827), a pioneer in the study of electricity. Photovoltaics literally means light-electricity.

The basic building unit of PV technology is the photovoltaic cell (PV cell). PV cells are made of a semiconductor material, typically silicon, which is treated chemically. When light hits the cell, a field of electricity is created within the layers causing the electricity to flow. This "photovoltaic effect" results in direct current (DC) electricity which is the same type of current produced by batteries.

In order to use this energy in most homes, an inverter is used to change the DC electricity to AC. Once electricity is generated, it can go to power anything in your house or be stored in batteries for later use. The greater the intensity of the light, the greater the flow across the layers and so the more electricity generated. But such a system does not necessary require direct sunlight to work.

Single PV cells are connected electrically to form PV modules, which are the building blocks of PV systems. Depending upon the application, the solar modules are typically wired together to form an array. Individual PV cells – averaging about 4 inches per side – typically converts 15% of the available solar radiation into about 1 or 2 watts of electrical power. Larger modules or arrays of modules are used to generate power for the grid.

Passive Solar Energy Designs

Here are a few video clips which show how to take maximum advantage of the sun's light and heat using passive solar energy designs:

Active Solar Heating

Diagram of a house with a combined solar domestic hot water and space heating system ("combisystem") (www.estif.org)

Active solar energy is more complex and requires mechanical devices to capture, store and convert the solar power into other useful forms of energy.

Active solar heating generates much more heat than passive systems do. Active solar heating relies strongly on three components: a solar collector to absorb the solar energy, a solar storage system, and a heat transfer system to disperse the heat to the appropriate places in your home. Liquid-based heating systems use a liquid to collect the energy in the solar collector; whereas air-based heating systems absorb the energy through the air.

Active solar technologies are used to convert solar energy into useable heat and cause air movement for ventilation or cooling.

Passive Solar Space Heating

Picture: westernwashingtonsolar.com

Space heating means heating the space inside a building. Passive solar space heating relies on incorporating building features that absorb heat and then release it slowly to maintain the temperature within the home. These building features, known as thermal mass, may include large windows, brick walls, and stone flooring. Passive solar heating techniques generally fall into one of three categories: direct gain, indirect gain, and isolated gain.

Direct gain is solar radiation that directly passes through the home's windows and is traped in the living space. Direct gain uses classic passive solar design strategy - the sunlight falls directly into the space and is absorbed by an abundance of thermal mass materials.

Indirect gain collects, stores, and distributes solar radiation using some thermal storage material (e.g., Trombé wall or a thermal storage wall). Conduction, radiation, or convection then transfers the energy indoors. Sunlight is absorbed by the wall, which heats up slowly during the day. Then as it cools gradually during the night, it releases its stored heat over a relatively long period of time indirectly into the space.

Isolated gain (e.g., sunspace) collect solar radiation in an area that can be selectively closed off or opened to the rest of the building. That heat than can be distributed into the living area in a variety of ways. The sunspace has the same characteristics as a direct-gain system - extensive south-facing glazing and thermal mass, and it should be well constructed, with low infiltration and high insulation levels.

In the case of passive solar space heating the whole house operates as a solar collector (passive solar home). A passive solar home is designed to let in as much sunlight as possible. It is like a big solar collector.

Sometimes for passive solar energy to be utilized effectively there must also be a means for the heated air to circulate throughout the home. Usually, the natural circulation of air is enough as long as doors are left open throughout the home, however, sometimes fans are also incorporated into the design to facilitate this.

Passive solar heating features can reduce heating bills by almost 50 percent and it requires little or no investment of external equipment. Building a passive solar home may even cost the same as building a conventional home, especially if you're working with a builder who is familiar with the processes of passive solar heating systems.

Solar Heating Systems

Solar heating can be used to heat the space in homes and buildings or to heat the water. There are two basic types of solar heating systems: passive solar heating and active solar heating.

The appropriate use of windows along with building design is called passive solar heating. The buildings can be designed to make the best use of the sun in winter while keeping the heat out in summer. South-facing, large windows, building materials that absorb and retain heat (such as stones and bricks), and efficient airflow are among the design features of a home that takes advantage of passive solar.

Active solar heating systems use mechanical equipment, such as pumps and fans, to increase the usable heat in a system. The heat is primarily used for heating water in homes, commercial buildings and industrial facilities. Active solar heating can be further divaded into liquid-based and air-based systems according to the kind of energy transfer fluid that is used.

Passive Solar, Active Solar and Photovoltaics

There are three different ways to harness the sun's energy: passive solar, using architectural design and natural materials to absorb the sun's energy; active solar, utilizing the sun's heat by means of solar collectors; and a third way in which solar energy can be harnessed is through the use of photovoltaic systems.

Passive solar is the capturing and storing the suns' energy - light and heat - without the use of any mechanical devices. As the solar radiation strikes windows, walls, floors, and other objects within the room it is converted to heat. A good example of a passive solar energy system is a greenhouse.

Active solar uses devices to collect, store, and circulate heat produced from solar energy. Active solar energy technologies convert sunlight into heat by using a particular energy transfer fluid. This is most often water or air but can also be a variety of other substances.

Photovoltaic systems directly convert sunlight into electricity using a semiconductor material such as silicon. The electrical energy from PVs can be stored in batteries for use when there is no sun (during cloudy days or at night).