Photovoltaic simply means converting sunlight into electricity. The process of converting light (photons) to electricity (voltage) is called the PV effect and hence the name. Utility companies are using PV technology for large power stations, while homes use about 10 – 20 solar panels mounted at a fixed angle facing south or on a tracking device that follow the sun’s rays and capture the most sunlight. The solar panels combined create a solar array – in large-scale utility or industrial applications hundreds of these are interconnected to form a PV system.
Solar cells are traditionally made from silicon and are flat-plated. Second-generation cells known as thin-film solar cells are made from amorphous silicon or non-silicon materials such as cadmium telluride. These types of solar cells use layered semiconductor materials that are a few micrometers thick and provide greater flexibility. They are also used in the building industry for rooftop shingles, tiles, facades and skylights.
Third-generation solar cells using plastic lenses or mirrors help concentrate sunlight onto a very small piece of high efficiency PV material. These systems are increasingly becoming cost effective for utilities and industry as a whole, however, are confined to locations and countries that have an abundance of sunlight.
A solar panel allows photons, or particles of light, to free electrons from atoms thereby generating the flow of electricity. PV cells establish an electric field which occurs when opposite charges or poles are separated giving it a positive or negative electrical charge. Each cell is basically a sandwich made up of two slices of semi-conducting silicon material – phosphorous is placed into the top layer which adds extra electrons and creates a negative charge. The bottom layer consists of boron resulting in fewer electrons, or a positive charge. This creates an electric field between the silicon layers junction – a photon of sunlight knocks an electron free which causes the electric field to push the electron out of the silicon junction. The metal conductive plates of the cell collect the electrons and transfer them to the wires, ready for use. At this stage, the electrons flow like any other source of electricity.
Types of PV Solar Panels
- Mono-crystalline: the highest durability and efficiency that can range from 16-18% and considered to be significant by industry standards.
- Poly-crystalline: The efficiency of this module ranges between 12-14% and has a high aesthetic value with blue and black crystalline cells.
- Thin-film: Thin-film (amorphous) modules are the latest innovation, however, with lesser efficiency and expected life. They offer good pricing and flexibility and are preferred for smaller off grid installations. Another variation is the flexible solar panel often used in replenishing batteries. Due to the flexibility, these panels are used in boats, recreational vehicles, and other automobiles as they assume the shape of the base and come in ratings between 3 – 18W.
Solar PV panels have been seen as the most attractive technology in the UAE due to cost and resource availability, while CSP with thermal energy storage remains attractive for its potential to provide base load power.
The main factor fueling the sharp rise in solar projects in the Middle East region is the drop in the cost of utility-scale solar PV power plants from $7.00/watt in 2008 to less than $1.50/watt in 2014. Given the almost 75% reduction in cost, a plant five times larger can now be built at the same cost due to its cost competitiveness. As solar prices are falling, the cost of generating electricity from natural gas is going up – the staple fuel for much of the region’s power generation infrastructure.
Concentrated Solar Power
A new generation of power plants with concentrating solar power (CSP) systems uses the sun as a heat source. The 3 main types of concentrating solar power systems are:
- linear concentrator
- power tower systems
Linear concentrator systems are collectors of the sun’s energy using long rectangular, curved (U-shaped) mirrors tilted toward the sun. The sunlight focuses on the tubes (or receivers) that run the length of the mirrors which when reflected heats the fluid that flows through the tubes. This hot fluid boils the water in a conventional steam-turbine generator to produce electricity. There are two major types of linear concentrator systems: parabolic trough systems and compact linear Fresnel reflector systems (CLFR) – the receiving tube is positioned above the mirrors to allow for greater mobility in tracking the sun.
Parabolic trough systems use curved mirrors to focus the sun’s energy onto a receiver tube that runs down the center of a trough. A high-temperature heat transfer fluid (such as a synthetic oil) absorbs the sun’s energy at temperatures of 750°F or higher, and passes through a heat exchanger to heat water and produce steam which in turn then drives a conventional steam turbine power system to generate electricity. A typical solar field contains hundreds of parallel rows of troughs connected as a series of loops and placed on a north-south axis so the troughs can track the sun from east to west. Individual collector modules are typically 15-20 feet in height and 300-450 feet in length.
CLFR uses the principles of curved-mirror trough systems, but with long parallel rows of lower-cost flat mirrors. These modular reflectors focus the sun’s energy onto elevated receivers containing a system of tubes through which water flows. The concentrated sunlight boils the water, generating high-pressure steam for direct use in power generation and industrial steam applications.
A dish/engine system uses a mirrored dish similar to a very large satellite dish, although to minimize costs, the mirrored dish is usually composed of many smaller flat mirrors formed into a dish shape. The dish-shaped surface directs and concentrates sunlight onto a thermal receiver, which absorbs and collects the heat and transfers it to the engine generator. The most common type of heat engine used today in dish/engine systems is the Stirling engine. This system uses the fluid heated by the receiver to move pistons and create mechanical power. The mechanical power is then used to run a generator or alternator to produce electricity.
A power tower system uses a large field of flat, sun-tracking mirrors known as heliostats that focus and concentrate sunlight onto a receiver atop the tower. A heat-transfer fluid heated in the receiver is used to generate steam, which is then used in a conventional turbine generator to produce electricity. Some power towers use water/steam as the heat-transfer fluid. Other advanced designs include molten nitrate salt because of its superior heat-transfer and energy-storage capabilities. The energy-storage capability, or thermal storage, allows the system to continue to dispatch electricity during cloudy weather or at night and is used as backup power.
The next article of the renewable energy series will cover wind energy! Stay tuned for that.
Until we rendezvous again,