In recent years, we have realized that the world's supplies of coal, gas and oil are limited. Nuclear power has been used as an alternative solution to fossil fuels. However, the use of nuclear power and fossil fuels incurred environmental problems so there is widespread public antipathy. As a result, the popularity of renewable energy has grown during the past twenty years. The World Energy Council estimated that renewable energy sources, such as solar, wind, hydro, wave and bio-mass, met 18% of the world's energy needs in 1990 (World Energy Council, 1993). Their scenario is that the contribution from renewable energy could increase 30% by 2020.
One of the most promising of the renewable energy sources is the direct conversion of solar energy into electricity by photovoltaic generation. There are many reasons for growing popularity:
1. Photovoltaic generators do not pollute the air and do not leave waste products.
2. Photovoltaic generators are silent during operation.
3. They work effectively even in cloudy weather. They are more efficient at low temperatures.
4. As there are no moving parts, they work reliably for 20-30 years with little maintenance.
5. Solar energy is available everywhere so power can be generated anywhere it is needed. This makes photovoltaic generators attractive in the many places where there is no mains supply.
6. Photovoltaic generators can be planned and installed within a few months in contrast to conventional power stations which take at least five years to become operational.
7. Finally, photovoltaic generators can be located anywhere, such as in the roof or walls of an existing or already planned building, therefore they do no need to use up extra land.
The photovoltaic effect was first observed by Edmund Becquerel in 1839. Much later, in the 1930s, solid state researches developed the first photocells which were used in photographic exposure meters. In 1954, the Bell Telephone Laboratories made crystalline silicon solar cells with a conversion efficiency of 6% which was used in space programs. The market for photovoltaic modules has been growing steadily since; in 1991 it had reached about 50 MW per annum.
The total radiant power from the sun falling on one square meter of a surface area can be as high as 1000W/m2 on a clear summer's day and it can fall to 100W/m2 in cloudy conditions. In northern Europe, it seldom exceeds 850W/m2 (Treble F.C., 1993).
The inactive energy, solar energy, can be converted into electrical energy by solar cells. The absorption of light in semiconductors creates additional electrical charge carriers, both electrons and holes equally. If an electric field exists within the semiconductor, the negative electrons and positive holes move in opposite directions and this electrical charge separation results in the creation of a voltage. The movement of the electrical charges creates an electrical current and voltage so both current and voltage are generated simultaneously. This is the photovoltaic effect, the creation of a voltage by the action of light.
The basic way to establish an electric field in a semiconductor is to make a p-n junction. The electric field at the junction attracts electrons from the p-side and forces them to the n-side making it negatively charged. Similarly holes from the n-side are forced to the p-side, making this positively charged. Thus holes are creating a voltage. Figure 1 shows the basic features of a solar cell. The front contact grid is a thin metallic grid on the front surface and the back contact usually covers the whole of the back. This is called an n-on-p cell. Silicon is one of the popular semiconductor in the electronics industry so it is used for solar cells. Most commercial cells have a probable 20% efficiency which is the ratio of the maximum output power to the input power from the sun, but over 25% efficiency has been achieved in the laboratory. The theoretical limit for crystalline silicon cells is about 30% under 1000W/m2 irradiance and 25 Â°C operating temperature (Hill B., 1995). Solar cells which were made from gallium arsenate have achieved 34.2% efficiency.
Solar cells are fine objects which must be protected from any possible damage. The cells are usually connected in series, in parallel or a combination of both in order to produce necessary power and voltage. A photovoltaic module which is a collection of solar cells was bought about US$4/Wp (US$ per peak watt) in 1995. Modules must be capable of reliable operation for many years. The current target is a lifetimes of 30 years.
In 1994 the total world sales of photovoltaic modules reached 70 MWp per year. In recent years, photovoltaic modules have found many applications in various sectors. The main applications are given below:
1. Space applications: solar cells were first used to produce electricity for satellites in 1958. Since then, photovoltaic power generation has become an essential energy source in space. Solar cells can operate near or far from sun.
2. Telecommunication: transmitters and repeater stations are often located in distant places such us mountains, islands or deserts. Solar power has proved the cheapest and most reliable power for transmitters and repeater stations.
3. Electricity in villages: the majority of the population of the developing countries, approximately two billion people, live in small villages without electricity. As almost developing countries will find extending the mains grid to a few customers far removed from the mains supply lines too expensive, photovoltaic systems are the obvious, cheaper alternative. A small photovoltaic module with a battery can provide enough power for basic lighting, TV and a small refrigerator for a house. By 1993 more than 10,000 home systems had been installed in Indonesia. In addition, solar home systems had been installed in the Philippines, the Dominican Republic, Columbia, India, Kenya, Mexico, Morocco, Sri Lanka and Zimbabwe by 1993. The average price of a 50 Wp solar home system was about US$500 in 1993. Assume that a 50 Wp solar house system in future will cost about US$250, then 400 million solar home systems will be
installed in the world. The other applications of solar modules in villages are water pumping, irrigation, water purification, street lighting and TV receivers (Lysen E.H., 1994).
4. Grid connected buildings: the solar modules can be fixed on roofs or walls so no additional land is required. The most sensible use of photovoltaic cladding would be on commercial buildings because they need energy during working hours rather than at night. Photovoltaic cladding presently costs about 800m-2 in comparison with marble cladding cost around 1000m-2, granite cladding 800m-2. Photovoltaic cladding gives high-tech images for office blocks at lower cost than marble.
5. Central power stations: photovoltaic power stations have, so far, only been installed for purposes of research. Today, Austria, Germany, Italy, Spain and USA have small stations of this type.
The other applications of photovoltaic systems are pocket calculators, watches, clocks, torches, garden lights, portable radios, battery chargers for boats, caravans, electric cars, toys, railway signals, traffic warning lights, alarm systems, automatic weather stations, military equipment and so on.
The photovoltaic system cannot at present compete with mains electricity. However, early in the next century, when economies of scale are expected to bring about a reduction in manufacturing costs, solar power will be an important energy source.