History and Future of Solar Energy
At age 19, in 1839, French physicist Edmond Becquerel realized that certain materials produced small quantities of electric current when they were exposed to light. In the 1870s, scientists studied the photovoltaic effect (the ability of the sun to produce electricity) with selenium. Selenium PV cells were developed that converted light into electricity at 1-2% efficiency. The efficiency of a solar cell is the amount of available sunlight it converts to electricity.
Early in the 20th century, the Polish scientist Czochralski developed a method for growing single-crystal silicon.
In the early 1950s, scientists at Bell Laboratories used the Czochralski process to develop the first crystal silicon PV cell, with an efficiency rate of 4%, which grew to 6% a few months later. The first public service trial of the Bell Solar battery was with a telephone carrier system in Americus, GA.
By the late 1950s, solar cells reached 10% efficiency. In 1958, Vanguard I, the first PV-powered satellite, was launched. Solar panels are used on most space vessels that orbit Earth and Mars.
Although spacecraft are launched by chemical rockets that act as propellants, solar power can provide enduring acceleration as the craft move through space over time. Solar arrays are used to power spacecraft because they are very hardy compared to other power sources, and wear out slowly.
Spacecraft use PV concentrator solar arrays that intensify sunlight on the solar cells using Fresnel lenses to direct the sunlight to a specific spot. Think of starting a fire with a magnifying glass and you’ve got the basic principle. Concentrating the sunlight allows for fewer solar arrays, which can be a very expensive component of a spacecraft.
Solar panels on spacecraft are built to pivot as the vessel moves, so they can always be pointed at the sun even as the rest of the craft moves around. A tracking mechanism is often put into the solar arrays to keep them pointed toward the sun. However, scientists sometimes purposefully orient solar panels out of direct alignment with the sun. Putting solar arrays “off point” happens when the batteries are completely charged and less electricity is needed; the extra power is vented into space as heat.
The space missions Magellan, Mars Global Surveyor, Mars Observer and the Hubble Space Telescope have all used solar power. European and Japanese space agencies are researching the development of solar power satellites - satellites with large PV arrays that would beam power back to Earth with microwaves or lasers.
Throughout the decades, researchers, corporations and the government have continued to explore new innovations in and uses for solar technology, both in space and here on Earth.
What’s the Future for Solar Energy?
Because cost is an issue in producing solar cells, researchers are investigating advanced approaches to developing them. Dye-impregnated solar cells using titanium dioxide to generate voltage offer an alternative to solar cells using crystalline silicon. Other potential advances include polymer (plastic) solar cells and photoelectrochemical cells that produce hydrogen directly from water in the presence of sunlight. Hydrogen is a means for transporting electricity.
Group III-V technologies, based on elements in Group III and V of the Periodic Table, alloy gallium arsenide with elements such as indium, phosphorus and aluminum to create semiconductors with high conversion efficiencies either under normal sunlight or concentrated sunlight.
Researchers at the National Center for Photovoltaics recently devised a cadmium telluride PV cell with 16.4% efficiency. Cadmium telluride is a promising technology for thin-film solar cells. In the thin-film manufacturing process, layers of differing electricity-producing materials are applied sequentially to a glass, plastic or steel backing. Such cells require less semiconductor material than earlier solar panels.
Through the use of concentrators that focus more sunlight onto a solar cell, NCPV researchers reached a peak efficiency of 21.5 percent from a thin-film copper indium gallium diselenide (CIGS) cell. Such technologies are in the early stages and the future will tell which of them emerge as alternatives to current solar-cell technology.