Photovoltaic cells

Photovoltaic cells convert the abundant, free, and clean energy of the sun directly into electricity. Already widely used in satellites, many consumer products, and residential or commercial electrical systems throughout the world, photovoltaic technology is one of the most promising alternative, renewable energy resources.

Background

Since ancient times, people have used energy from the sun. In the seventh century BCE, mirrors and glass were used to concentrate heat to light fires. Solar energy can also be converted into electricity. Photovoltaic (PV) cells, also called solar cells, convert sunlight directly into electricity at the atomic level through the process called photovoltaics.

A PV cell is made of a special semiconductor material, so that when photons, or small light particles, strike the cell, some of them are absorbed within the photoelectric material. The energy of the absorbed light loosens electrons (negatively charged components of an atom) and causes them to flow freely, producing an electric current.

French physicist Alexandre-Edmond Becquerel discovered the photovoltaic effect in 1839. He noticed that when exposed to light, certain metals or materials produced small quantities of electric current. In 1883, Charles Fritts built the first working solar cell by coating the semiconductor material selenium with a thin, almost transparent layer of gold. The early solar cells had low energy conversion efficiencies, transforming less than 1 percent of the absorbed solar energy into electricity.

In 1905, Albert Einstein published his theories about the nature of light and the PV effect, which laid the foundation for photovoltaic technology. The first silicon photovoltaic cell was developed by Daryl M. Chapin, Calvin Fuller, and Gerald Pearson at Bell Laboratories in 1954. With an efficiency of 6 percent, it was the first solar cell that could convert enough energy to power ordinary electrical equipment. After silicon was adopted for many kinds of electronic circuitry in the 1960s, silicon production increased exponentially, resulting in lower prices. Silicon became the standard semiconductor material for PV cells. At first, the crystalline form of silicon was more common, but the amorphous form eventually became widespread.

Applications

The first practical application of photovoltaics occurred in 1958, when the US satellite Vanguard 1 used a radio transmitter powered by solar cells. Unlike the battery-powered transmitter on board, which broadcast for less than one month, the solar battery sent signals for years. This breakthrough demonstrated the reliability of PV for electric power generation in space, and solar cells became indispensable in subsequent satellites. In 2000, solar panels were introduced at the International Space Station, which held the largest solar power array in space.

During the energy crisis in the 1970s, interest in PV technology for applications other than those for space and commerce grew. By 1978, the first commercial solar-powered calculators and wristwatches were introduced.

Stand-alone PV systems have become a major source of energy for remote areas far from conventional power lines. PV technology provides the necessary amount of reliable energy most economically. Applications of PV cells include ocean navigational buoys and lighthouses, remote scientific research and weather stations, telecommunications systems such as mountain-top radio transceivers, and emergency call boxes or road signs.

In industrialized nations, PV technology is used in grid-connected electrical systems to supplement conventional energy generation. Centralized PV power stations and PV systems in buildings are the two kinds of grid-connected installations. PV power stations, which send power instantaneously into the grid or distribution network through transformers and inverters, are especially cost-effective during hours of peak demand. A PV system in a building is a decentralized system with distributed generation in grid-connected PV arrays or in solar panels on the roofs of residential, commercial, or industrial buildings.

Approximately 17 percent of the people in the world do not have electricity. In developing countries and rural areas that do not have access to conventional electrical supplies, PV technology is playing an increasingly significant role. Domestic PV systems supply the power for lighting, refrigeration, and basic appliances in many villages and island communities. PV water pumps are also used worldwide for village water supplies and irrigation.

Advantages and Disadvantages

Photovoltaic technology has significant advantages over conventional and other alternative energy technologies. First, because PV systems make electricity directly from sunlight without gaseous or liquid fuel combustion, there is minimal impact on the environment. PV production is clean and quiet, producing no greenhouse gases or hazardous waste by-products. Ranging from microwatts to megawatts, PV energy is also flexible and can be used for a wide range of applications.

PV technology is also cost-effective over the life of the system. Sunlight is free and ubiquitous, so PV has a free, abundant fuel supply. PV systems are also inexpensive to construct and easy to operate and maintain for long periods of time, because there are no huge generators, complicated wiring, transmission lines, transformers, or moving parts that require frequent servicing or replacement. Because of this high reliability and ability to operate unattended, PV technology has been the choice for space satellites and remote areas, where power disruptions and repairs would be costly. Another significant advantage of PV systems is that they are modular, so the systems can be configured in a variety of sizes and moved as needed.

PV technology is more expensive than producing electricity from a grid, but it can provide energy during peak demand times, such as the hours when air conditioners are turned on during the summer. During these times, a grid-connected PV array can be used to meet the peak demand, rather than relying on extremely expensive peaking power plants or other limited energy resources. Thus, PV systems can prevent power outages such as brownouts and blackouts. Solar panels connected to a grid can also produce surplus electricity when the sun is shining, and this excess is credited against electricity used, resulting in an average 70 to 100 percent savings on electric bills.

Other limitations include efficiency and performance. Because PV technology depends on sunlight, weather conditions affect output. However, even on extremely cloudy days, a PV system can generate up to 80 percent of its maximum output.

The Future of Photovoltaics

Although sunlight is free, PV hardware manufacturing has been too expensive to compete with utilities. Hence, PV technology has been most cost-effective in remote or rural areas without conventional sources of electricity, rather than in urban areas with traditional grid power. However, as more research is done on less expensive materials, the technology improves, and costs decline, PV has the potential to become the leading alternative energy resource. It is estimated that installing PV systems in only 4 percent of the area of the world’s deserts would be enough to supply electricity for the whole world.

During the 1990s, research into other materials increased efficiency to more than 10 percent. In 1992, the University of South Florida developed a 15.89 percent thin-film cell. In 1994, the National Renewable Energy Laboratory (NREL) fabricated a solar cell made of gallium indium phosphide and gallium arsenide, which exceeded 30 percent efficiency. In 1999, the NREL and Spectrolab combined three layers of PV materials into a single 32.3 percent efficient solar cell.

In the twenty-first century, PV power generation has expanded to meet global energy needs. According to Our World in Data, in 2023, 1,419 gigawatts of solar PV were installed worldwide. China, the United States, and Japan were the top markets for growth during this year. Global revenues for the PV industry totaled $179 billion in 2023. World solar cell production was 612 gigawatts in 2023. China added 150 gigawatts in 2023 and planned to add another 170 gigawatts in 2025, aiming to hit 1 terawatt in 2026. Many governments and activist organizations encouraged both municipalities and individual consumers to begin utilizing solar power through photovoltaics.

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