Converting the sun’s radiation directly into electricity is done by solar cells. These cells are made of semiconducting materials similar to those used in computer chips. When sunlight is absorbed by these materials, the solar energy knocks electrons loose from their atoms, allowing the electrons to flow through the material to produce electricity. This process of converting light (photons) to electricity (voltage) is called the photovoltaic effect. Photovoltaics (PV) are thus the field of technology and research related to the application of solar cells that convert sunlight directly into electricity.
Traditional solar cells are made from silicon, are usually flat-plate, and generally are the most efficient. Second-generation solar cells are called thin-film solar cells because they are made from amorphous silicon or nonsilicon materials such as cadmium telluride. Thin film solar cells use layers of semiconductor materials only a few micrometres thick. Because of their flexibility, thin film solar cells can double as rooftop shingles and tiles, building facades, or the glazing for skylights.
Third-generation solar cells are being made from a variety of new materials besides silicon, including solar inks using conventional printing press technologies, solar dyes, and conductive plastics. Some new solar cells use plastic lenses or mirrors to concentrate sunlight onto a very small piece of high efficiency PV material.
- Crystalline Silicon Technology
Crystalline silicon cells are made from thin slices (wafers) cut from a single crystal or a block of silicon. The type of crystalline cell produced depends on how the wafers are made. The main types of crystalline cells are:
- Single Crystalline (SC-Si)
- Multi Crystalline (mc-Si)
- Ribbon and sheet-defined film growth (ribbon/sheet c-Si)
The single crystal method provides higher efficiency, and therefore higher power generation. Crystalline silicon is the most common and mature technology representing about 80% of the market today. Cells turn between 14- 22% of the sunlight that reaches them into electricity. For c-Si modules, efficiency ranges between 12 -19%.
Individual solar cells range from 1 to 15 cm across (0.4 to 6 inches). However, the most common cells are 12.7 x 12.7 cm (5 x 5 inches) or 15 x 15 cm (6 x 6 inches) and produce 3 to 4.5 W – a very small amount of power. A standard c-Si module is made up of about 60 to 72 solar cells and has a nominal power ranging from 120 to 300 Wp depending on size and efficiency. The typical module size is 1.4 to 1.7 m² although larger modules are also manufactured (up to 2.5 m²). These are typically utilised for BIPV applications.
- Thin Film Thin Film modules are constructed by depositing extremely thin layers of photosensitive material on to a low-cost backing such as glass, stainless steel or plastic. Once the deposited material is attached to the backing, it is laser-cut into multiple thin cells. Thin Film modules are normally enclosed between two layers of glass and are frameless. If the photosensitive material has been deposited on a thin plastic film, the module is flexible. This creates opportunities to integrate solar power generation into the fabric of a building or end-consumer applications.
Types of Thin Film modules are commercially available:
- Amorphous (a-Si) and micro morph silicon (a-Si/µc-Si)
The semiconductor layer is only about 1 μm thick. Amorphous silicon can absorb more sunlight than c-Si structures. However, a lower flow of electrons is generated which leads to efficiencies that are currently in the range of 4 to 8%. With this technology the absorption material can be deposited onto very large substrates (up to 5.7 m² on glass), reducing manufacturing costs. An increasing number of companies are developing light, flexible a-Si modules perfectly suitable for flat and curved industrial roofs.
- Cadmium-Telluride (CdTe)
CdTe Thin Films cost less to manufacture and have a module efficiency of up to 11%. This makes it the most economical Thin Film technology currently available. The two main raw materials are cadmium and tellurium. Cadmium is a by-product of zinc mining. Tellurium is a by-product of copper processing. It is produced in far lower quantities than cadmium. Availability in the long-term may depend on whether the copper industry can optimise extraction, refining and recycling yields.
- Copper-Indium-Diselenide (CIS) and Copper-Indium-Gallium-Diselenide (CIGS)
CIGS and CIS offer the highest efficiencies of all Thin Film technologies. Efficiencies of 20% have been achieved in the laboratory, close to the levels achieved with c-Si cells. The manufacturing process is more complex and less standardised than for other types of cells. This tends to increase manufacturing costs. Current module efficiencies are in the range of 7-12%.
Solar thermal Technology
This type of systems uses lenses or mirrors and tracking systems to focus a large area of sunlight into a small beam. The concentrated heat is then used as a heat source for a conventional power plant. For example, with solar thermal energy, the only part of the sunlight that’s used is its heat. When you dry your washing on the line, or heat water with a solar hot water system, you’re using solar thermal energy. But it’s also possible to generate higher temperatures by using a lens or mirror to focus the sunlight onto a smaller area. With the right type of focusing system, the sunlight can be turned from something capable of merely drying clothes, to something hot enough to boil water to run a large steam turbine that powers a town or do many other useful things besides. A wide range of concentrating technologies exists: the most developed are the parabolic trough, the concentrating linear Fresnel reflector, the Stirling dish and the solar power tower. Various techniques are used to track the sun and focus light. In all of these systems a working fluid is heated by the concentrated sunlight, and is then used for power generation or energy storage. Thermal storage efficiently allows up to 24 hour electricity generation.
- Parabolic troughs
Parabola has the property of focusing the incoming radiation as its focus. Working on this principle, linear concentrators of parabolic shape are coated with highly reflective material and can be turned in angular movements towards the sun position and concentrate the incoming solar radiation onto a long-line receiving absorber tube. A working fluid is used to transfer the absorbed solar energy, which is then piped to an exchanger or a conventional conversion system. Parabolic trough systems cannot make use of diffused radiation as they use only direct-beam sunlight and require tracking systems to keep them focused toward the sun and are best suited to areas with high direct solar radiation. Most systems are oriented either east-west or north-south with single-axis tracking during the day.
- Solar Tower (Central Receiving System)
Central receiver systems use heliostats to track the sun by double axes mechanisms following the azimuth and elevation angles with the purpose to reflect the sunlight from many heliostats oriented around a tower and concentrate it towards a central receiver situated atop the tower. This technology has the advantage of transferring solar energy very efficiently by optical means and of delivering highly concentrated sunlight to one central receiver unit, serving as energy input to the power conversion system. In spite of the elegant design concept and in spite of the future prospects of high concentration and high efficiencies, the central receiver technology require more development for further up scaling plant performance. Its main attraction consists in the prospect of high process temperatures generated by highly concentrated solar radiation to supply energy to the topping cycle of any power conversion system and to feed effective energy storage systems able to cover the demand of modern power conversion systems.
Different receiver heat transfer media that have been successfully used are water/steam, liquid sodium, molten salt, ambient air, oil.
Solar Tower plants have the good long-term perspective for high conversion efficiencies and for use of very efficient energy storage systems by utilization of high temperatures in order to enlarge the solar capacity or solar share.
- Linear Fresnel reflector (LFR)
The Linear Fresnel technology uses long, flat or slightly curved mirrors to focus sunlight onto a linear receiver located at a common focal point of the reflectors. The receiver runs parallel to and above the reflectors and collects the heat to boil water in the tubes, generating high-pressure steam to power the steam turbine (water/direct steam generation, no need for heat exchangers). The reflectors make use of the Fresnel lens effect, which allows for a concentrating mirror with a large aperture and short focal length. This reduces the plant costs since sagged-glass parabolic reflectors are typically much more expensive. Since the optical efficiency as well as the working temperatures are considerably lower than with other CSP concepts, saturated steam conditions have to be considered for this technology. Development is now heading from demonstration plants to bigger, commercialized projects. The receiver is stationary and so fluid couplings are not required (as in troughs and dishes). The mirrors also do not need to support the receiver, so they are structurally simpler. When suitable aiming strategies are used (mirrors aimed at different receivers at different times of day), this can allow a denser packing of mirrors on available land area.
- Dish Stirling
A dish stirling system uses a large, reflective, parabolic dish (similar in shape to satellite television dish). It focuses all the sunlight that strikes the dish up onto a single point above the dish, where a receiver captures the heat and transforms it into a useful form. Typically the dish is coupled with a Stirling engine in a Dish-Stirling System, but also sometimes a steam engine is used. These create rotational kinetic energy that can be converted to electricity using an electric generator.
Concentrator photovoltaic’s (CPV) utilise lenses to focus sunlight on to solar cells. The cells are made from very small amounts of highly efficient, but expensive, semi-conducting PV material. The aim is to collect as much sunlight as possible. CPV cells can be based on silicon or III-V compounds (generally gallium arsenide or GaA). CPV systems use only direct irradiation. They are most efficient in very sunny areas which have high amounts of direct irradiation. The concentrating intensity ranges from a factor of 2 to 100 suns (low concentration) up to 1000 suns (high concentration). Commercial module efficiencies of 20 to 25% have been obtained for silicon based cells. Efficiencies of 25 to 30% have been achieved with GaAs, although cell efficiencies well above 40% have been achieved in the laboratory. The modules have precise and accurate sets of lenses which need to be permanently oriented towards the Sun. This is achieved through the use of a double-axis tracking system. Low concentration PV can be also used with one single-axis tracking system and a less complex set of lenses.
Third generation photovoltaic
After more than 20 years of research and development, third generation solar devices are beginning to emerge in the marketplace. Many of the new technologies are very promising.One exciting development is organic PV cells. These include both fully organic PV (OPV) solar cells and the hybrid dye-sensitised solar cells (DSSC).
Third generation technologies that are beginning to reach the market are called “emerging” and can be classified as:
- Advanced inorganic Thin Films such as spherical CIS and Thin-Film polycrystalline silicon solar cells.
- Organic solar cells which include both fully organic and hybrid dye-sensitised solar cells.
- Thermo-photovoltaic (TPV) low band-gap cells which can be used in combined heat and power (CHP) systems.
Third generation PV products have a significant competitive advantage in consumer applications because of the substrate flexibility and ability to perform in dim or variable lighting conditions. Possible application areas include low-power consumer electronics (such as mobile phone rechargers, lighting applications and self-powered displays), outdoor recreational applications, and BIPV.
In addition to the emerging third generation PV technologies mentioned, a number of novel technologies are also under development:
- Active layers can be created by introducing quantum dots or nanotechnology particles. This technology is likely to be used in concentrator devices.
- Tailoring the solar spectrum to wavelengths with maximum collection efficiency or increasing the absorption level of the solar cell. These adjustments can be applied to all existing solar cell technologies.
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