The Two Types of Solar Energy.
Photovoltaic technology directly converts sunlight into electricity. Solar thermal technology harnesses its heat. These different technologies both tap the Sun’s energy, locally and in large-scale solar farms.
Like water and air, the Sun is one of the Earth’s life support systems, providing heat and light. Solar energy, which is renewable, widely available and clean, provides enough energy to meet the world’s annual consumption needs every 50 minutes. The challenge is to collect a share – however small – of this heat and radiant energy.
Two major technologies have been developed to harness it:
- Photovoltaic solar technology, which directly converts sunlight into electricity using panels made of semiconductor cells.
- Solar thermal technology, which captures the sun’s heat. This heat is used directly or converted into mechanical energy and in turn electricity, known as concentrated solar power . This heat is used directly (low‑temperature solar thermal) or converted into mechanical energy and in turn electricity (concentrated solar power – CSP).
Two different types of installations are used:
- Individual systems for homes or small communities. Photovoltaic panels can power electrical devices, while solar thermal collectors can heat homes or hot water (See Close-Up: “Solar, a Boundless, Universally Accessible Energy Source”).
- Photovoltaic or concentrated solar power plants that cover hundreds of acres produce electricity on a large scale, which can be fed into power grids.
Solar energy is one of the most attractive renewable energies because of its flexibility – its capacity to power cities and industries using large solar plants while at the same time offering a stand-alone capability in the most isolated rural regions.
Photovoltaic Solar Power
The photovoltaic effect (or photoelectric effect) converts light into electricity. It was discovered by French physicist Edmond Becquerel in 1839 and was first used in industrial applications in 1954. The principle: an electric current occurs when electrons are displaced. For this to happen, photons (light particles) excite the outermost electrons of the atoms of certain semiconductor elements.
In practice, light hitting a photovoltaic cell is converted into electricity by a semiconductor, generally silicon. A photovoltaic panel is made up of several cells producing direct current, which is then converted into alternating current by an inverter.
Panels can be used in small systems or large plants.
Low-Temperature Solar Thermal Power
Low-temperature solar thermal power technology produces heat from the Sun’s rays and uses it directly. Operating at temperatures below 100°C, the installations cater to residential and commercial applications (hot water and heating) as well as to a wide variety of industrial needs. This technology accounts for the lion’s share of solar power generated worldwide.
Solar thermal collectors are used to absorb the heat from the Sun’s rays and transfer it to a heat transfer fluid, such as air, water or antifreeze, which in turn transports it to the areas to be heated.
Most widespread are flat-plate collectors, which are made up of a dark surface area that absorbs the Sun’s rays and a layer of thermal insulation topped with a sheet of glass that generates a greenhouse effect . Collectors operate at up to 70°C above ambient temperature. There are also solar thermal air collectors, used for agricultural crop drying, for example, and unglazed systems made using rubber or plastic, rather than a glass covering, which are used primarily to heat swimming pools. Vacuum tube collectors are best suited to high-temperature industrial uses, such as cleaning slaughterhouses or pasteurizing canned goods. They are made up of glass vacuum tubes for optimal thermal insulation. Inside, an absorber captures the solar energy and transfers it to a fluid.
Concentrated Solar Power
This second type of thermal solar power technology concentrates the warmth of the Sun’s rays using collectors to heat a transfer fluid (gas, oil or molten salt, for example) to a high temperature. The fluid heats a network of water, which produces steam and drives a turbine (mechanical energy), thereby generating electricity.
Heat from the Sun’s rays is collected in large power plants where flat or curved mirrors are installed over vast areas. The technology is best suited to countries where the sunlight is intense, for example in desert regions.