Feature Report: Solar, an Energy of the Future

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The Two Types of Solar Energy, Photovoltaic and Thermal

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.

The Olivenza solar power plant in Spain. © SUNPOWER CORP

Two Forms of Solar Energy for Different Uses

Photovoltaic solar power was first used industrially in France in 1954

Like water and air, the Sun is one of the Earth’s life support systems, providing heatIn the field of statistical thermodynamics today, heat refers to the transfer of the thermal agitation of the particles making up matter... 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:

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 industry using large solar plants while at the same time offering a stand-alone capability in the most isolated rural regions.

The Photovoltaic Effect

The photovoltaic effectCreation of electric current when a semiconductor material is struck by light photons. (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 siliconSilicon crystals come from silica, the main compound in quartz and sand. Silicon is a semi-conducting material.. A photovoltaic panel is made up of several cells producing direct current, which is then converted into alternating currentA flow of electric charge that changes direction twice per period... by an inverter.

Panels can be used in small systems or large plants.

Low-Temperature Solar Thermal Power

The heat emitted by the Sun can be used directly, for residential, commercial and industrial applications via devices operating at temperatures under 100°C). This technology accounts for the lion’s share of solar power generated worldwide (See Close-Up: "Solar in the Global Energy Mix").

50 minutes, the time it takes the Sun to provide the equivalent of the world’s annual energy consumption.

Solar thermal collectors are used to absorb the heat from the Sun’s rays and transfer it to a fluid, which in turn transports it to the areas to be heated. There are different types of collector, ranging from simple models for home use to more sophisticated versions for industrial facilities.

Concentrated Solar Power

The heat collected from sunlight can also be used to generate electricity, using vast ground-mounted arrays called concentrated solar power plants.

Solar radiation is concentrated and converted to very-high-temperature heat using mirror arrays. This heat is used to generate into mechanical energy and then electricity, using a similar process as nuclear plants.

 

Solar Thermal CollectorsOne of the most common collectors used is the flat-plate collector, which consists of an insulated box with a glass or plastic cover. Inside, a dark metal plate absorbs the heat from the sun trapped inside the box. This heat is transferred to air, water or any other heat transfer fluid that does not freeze. A circulation system transports it to its point of use. The temperature difference compared to ambient air can be as high as 70°C.

Unglazed collectors also exist in rubber or plastic and are used to heat pools, for example.

Vacuum tube collectors are suited to industrial applications requiring high temperatures, such as cleaning slaughterhouses and pasteurizing canned food. They are made of glass vacuum tubes for optimal thermal insulation. Inside, an absorber collects solar energy and transfers it to a fluid.