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close this book Boiling Point No. 15 - April 1991
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The History of Photovoltaic Solar Cells

by P Spinks (Editorial summary of article 'Plug into the Sun' which first appeared in the New Scientist Magazine, London, the weekly review of sience and technology - 22 September 1990)

Two British researchers were the first to convert sunlight into electricity. In 1876, W G Adams and A E Day made the first photovoltaic cell. They used selenium. But 60 years later the cell's efficiency was still only a paltry 1%. Not until the early 1950s did Bell Laboratories, the American electronics company, achieve a conversion efficiency of 6%. Bell used a cell made of monocrystalline silicon, the first of which, launched in 1958, spurred large-scale research and boosted efficiencies to between 15 and 29% (depending on the size of the cell or panel, and whether trials were conducted in the field or in the laboratory).

Oil companies and electronics groups are at the forefront of developments in photovoltaics. This has eclipsed the other main branch of using solar energy directly, solar thermal, in which radiation from the sun is converted into heat instead of electricity. But even now, photovoltaic cells cost between $4 and $6 (ú2. 10 and ú3.20) per peak watt, which is the maximum power available during peak sunshine; costs need to fall by a factor of at least six to compete with conventional means of generating electricity.

RCA Laboratories, the American record company, produced the first amorphous silicon cells in 1974. The performance of amorphous silicon cells, however, degrades after their initial exposure to sunlight. This degradation is known as the Staebler-Wronski effect. The Chronar Corporation, the biggest American manufacturer of amorphous silicon cells, produced the world's biggest amorphous silicon panel. It is 13 square feet (1.21 square metres) in area and delivers 74 peak watts with an efficiency of 6.6%.

Other companies have made progress with alternative thin-film materials that do not degrade. The British company, BP-Solar, works with cadmium telluride, which has demonstrated an efficiency of 9%. Siemens Solar, part of the West German electronics concern, works with copper indium diselenide, which has an efficiency of more than 14%.

The US Department of Energy (DOE) aims to produce a cell with an efficiency of 35%, though 40% is considered the theoretical limit. Boeing, the American aircraft manufacturer, claims it recorded anefficiency of 37% for a tandem cell last year, using lenses to concentrate sunlight onto its layers of gallium arsenide and gallium antimonide. Though gallium arsenide is brittle, which makes the material difficult to work with, it is most efficient at high temperatures, especially when the radiation is 400 to 700 times the usual intensity of the sun. This makes it suitable for solar concentrators. These use sun-tracking mirrors and lenses to intensify and focus sunlight on panels, which means fewer panels are needed.

The costs of p.v. solar panels are crucial to the future of photovoltaic power. On average, they have tumbled from around $1,000 per square foot in early 1970s to $500 per square foot today. During this period, the power produced by a square foot panel surged from 6 to 13 watts. The falling costs and rising efficiencies of panels have cut the average cost of power from large plants, some of which are connected to national grids, from $1.50 per kilowatt hour in 1980 to around $0.5 today. Coal-fired power currently costs up to $0.06 (about 3 pence) per kilowatt hour depending on the type of coal and the conversion process used.

Though still uneconomic compared with fossil or nuclear power stations, there are pilot photovoltaic power plants in several countries. Most plants are found in the US, Japan and Norway; there are a few experimental plants in Saudi Arabia and Italy. The feasibility of photovoltaic power in temperate climates is being assessed by plants in West Germany, Switzerland, Finland, Austria and Britain. Their capacities range from 15 kilowatts to the world's biggest photovoltaic plant of 6.5 peak megawatts in California.

In general, the pilot plants show that photovoltaic power is initially expensive but largely pays its way within 2 to 10 years: maintenance is minimal because most plants have few or no moving parts, and sunshine is free. Grid-connected plants accounted for 4% of the total photovoltaic market in 1989. BP Solar expects them to account for 23% of this market by the turn of the century, when the company predicts photovoltaic sales of between $10 and $20 billion, which is equivalent to 1200 peak megawatts. Yet this represents only 0.01% of the projected global energy demand by 2000; costs must fall further and faster for photovoltaic power to make a more significant impact. This year American funding rose to $43 million. Japan spends about $50 million each year on solar energy research. Only West Germany spends asimilar amount of money in this field.

The future lies not in supplying mains power but in storing what lime energy we can eke out of photovoltaic power" says Anthony Derrick of IT Power, a British energy consultancy. Lead-acid and nickel-cadmium batteries usually store photovoltaic energy. Unlike ordinary car batteries, they must deliver small currents over long periods. But there is a need for lighter, more efficient and maintenance-free batteries.

Other advocates of solar energy however, are more sceptical of the future of photovoltaic energy. They say that the conversion of solar energy into heat, instead of electricity, deserves more consideration. "More than twice as much time and effort is spent worldwide on photovoltaic as on solar thermal energy," notes Jurgen Schmid of West Germany's Fraunhofer Institute for Solar Energy Systems, one of the world's leading centres of solar thermal research. He says that electricity accounts for about 10% of global energy consumption while thermal energy accounts for more than 30%.

Two types of systems harness solar thermal energy. Active systems for small domestic water heaters use mirrors to focus sunlight onto pipes blackened to promote the abosorption of solar radiation. Water flowing through the pipes is thus heated by the intensified sunlight. Parabolic mirrors are also used to focus sunlight onto cooking pots. Passive systems based on insulation never received much funding until energy-efficiency became important and novel systems for heating buildings and eliminating draughts were introduced.

A wall is covered with a transparent plastic barrier which allows the sun's rays to pass through and heat the wall but does not allow the heat to be radiated out. The wall heat can then heat the inside of the room directly or be carried by air circulation systems to other rooms. Passive heating is traditional in some high altitude countries such as Nepal which receive much sunlight during the day but are very cold at night - see Boiling Point 7 - Trompf walls.

The bias now appears to be towards conventional fossil and nuclear fuels and non-competing renewables such as photovoltaics. "Although developments in solar thermal systems promise light at the end of the tunnel, the electricity and oil companies still prefer the glamour and high-tech of photovoltaic power - perhaps because it does not compete directly with their vested interests in conventional fuels".

Ed note: Unfortunately, there is little bias towards the problem of domestic cooking in the less developed countries. They have the sun and there have been hundreds of designs of solar heaters/cookers "invented" and promoted but none has been widely accepted. They all fail on the 3 basic points - they can only cook when the sun is shining - they require major changes in social habits or they are much too expensive. Even I % of the money now being spent on research and development for the rich countries might lead to an improvement on the three stone fire now used by hundreds of millions of people and might also reduce the production of greenhouse gases but who will pay for it? .