SER - Renewable Energy Systems
The demand for renewable energy grows exponentially throughout the world due to environmental pressures, primarily motivated by the global warming concern. In 2011, renewables accounted for about 20% of the electricity consumed in the world.
The renewable sources are, among other, hydro, biomass, biofuels, wind and solar. At first, hydroelectric projects were priority, which supplies around 15% of the energy in the world. In the first half of the last decade the wind power boomed. Today, the attention is turning towards solar.
In Brazil 75% of the consumed electricity comes from renewable sources. The hydroelectric power plants represent 95% of the total. In recent years, biomass, and especially wind power dynamically grew its shares in the national energy mix. Recently, solar energy began to be seriously considered as a viable source of exploitation and shows enormous potential for development.
In fact, the Solar Energy is indirectly found in almost every source of energy, hydropower, biomass, wind, fossil fuels and ocean energy. So, after all, what is solar energy? What is recognized as solar energy, in the strict sense, is the use of a portion of the energy radiation from the sun received by the earth to transform into heat or electricity.
Three major groups of technology exploitation of energy from the sun fit into this concept:
Of the three energies, thermal solar is the most widespread in Brazil. It consists of a mechanism to transfer heat from the sun to heat water and air through thermal collectors. Although it does not generate electricity, its use is important for being able to reduce electrical consumption, especially at peak load. It is estimated that 8% of the consumed electricity in the country are used for water heating. This accounts for between 18% and 25% of the demand during the peak load in the electrical system.
In an regular home, water heating should represent 30% of the energy bill. The technology is simple, primarily consisting of collector panels, insulated tank and piping. The flow of water is ensured by the "thermo-siphon effect" in which the hot water, having lower density, is pushed by the cold water injected into the base of the panels. The heated water is stored in an insulated reservoir until the consumption by the residence.
This technology uses mirrors to concentrate the sun's energy and convert it into high temperature heat to produce steam that drives turbines to generate electricity. CSP power plants consist of two parts:
a) collection and conversion solar energy into heat;
b) and heat conversion energy into electricity.
Types of CSP:
There are four types of concentrated solar systems:
Concentrated Solar Power Tower – CSPT: generate electricity by concentrating sunlight on top of a tower. This tower has a receiver heat exchanger which heats a conductor liquid (a mixture of salts in liquid state) that circulates in the system. The heat contained in the liquid conductor generates steam that passes through turbines and moves conventional generators to produce electricity.
Fresnel Reflectors: The principle of concentrating solar power is the same. This technology uses long mirrors which don´t focus a point but rather a line in which is positioned a tube containing the conductive liquid that is heated to high temperature.
Parabolic Trough: this technology is the most mature and deployed. Consists of long and concave mirrors concentrators (in the form of a semicircle) where the tube receptor is located in the focal axis.
The photovoltaic cell is the fundamental unit of the conversion process. These cells are optical-electrical components that convert sunlight directly into electricity. They are basically composed of a semiconductor materials and silicon is the material most frequently used.
When an electrode is heated, some of its electrons gain enough energy to escape. Thus an electron emitter is created, a cathode. Another electrode placed near this cathode, if it is cold enough, will receive the emitted electrons, becoming an anode. If the anode is connected to the cathode through a circuit containing an external load, a current will flow and action may be produced.
By focusing light onto the photovoltaic cell, the photons collide with the electrons of the P-type silicon structure giving them energy and transforming them into conductors. Due to the electrical field, the electrons are oriented and flow, thereby generating a flow of electrons (electric current) in the circuit connection. While the light continues to hit the cell, the electron flow will continue. The current intensity generated varies proportional to the intensity in proportion as the intensity of incident light.
Monocrystalline Silicon: These cells are obtained from cylindrical bars made of monocrystalline silicon in special ovens. This process achieves a purity of 98% to 99%, which is reasonably efficient from the energy and cost points of view. The silicon is melted together with a small amount of dopant which is typically boron p-type. After cutting and cleaning the impurities of the slices, N type impurities are added to form the junction. This process is done by controlled diffusion in which the silicon slices are exposed to phosphorus vapor in a furnace where the temperature varies between 800 to 1000oC.
Among the photovoltaic cells that use silicon base material, the monocrystaline ones are generally those with the highest efficiencies. The monocrystalline silicon cells are historically the most used and commercialized as direct solar energy electricity converter..
Polycristaline Silicon: These cells are produced from silicon blocks obtained by melting pure silicon in special molds. Once in the mold, the silicon slowly cools and solidifies. In this process, the athoms are not organized into a single crystal. A polycrystalline structure is formed with separation surfaces between crystals. Their efficiency in the conversion of sunlight into electricity is slightly smaller than the monocrystalline silicon.
Thin-Film or Amorphous Silicon: These cells are obtained by the deposition of thin layers of silicon or other semiconductor materials on glass or metal surfaces. An amorphous silicon cell differs from the other crystal structures due to its high degree of disorder in the structure of the atoms. The use of amorphous silicon in solar cells has shown great advantages in both electrical properties and the manufacturing process. As it absorbs the visible range of solar radiation and can be manufactured by deposition of several types of substrates, the amorphous silicon has been showing as a strong low cost photovoltaic technology.
Cadmium-Telluride (CdTe): represents 13% of solar cells market share. It is a technology that uses cadmium-telluride thin-film. Since the cadmium is highly toxic, it has a negative commercial appeal. Although produced by few manufacturers, it presents highly competitive prices, increasing its participation share on the market.
Copper Indium Gallium and Selenium - (CIGS): Trade name for thinf-film cells fabricated with Cu (In, Ga) SE2. Participation of 1% market share of solar cells and 13% efficiency yield. Currently suffer problems with the supply of indium for its production, since 75% of the material in the world is used to manufacture flat panel displays such as LCDs and plasma displays.
Gallium Arsenide (GaAs): is currently the most efficient technology used in solar cells, with 28% efficiency yield. However, the manufacturing cost is extremely high, making it prohibitive for commercial production, only being used in solar panels for satellites.