Large-scale Solar Technology
Large-scale solar power generation got its start in the Mojave Desert of California more than 20 years ago. The Solar Electric Generating System (SEGS) was built by Luz Industries, delivering a total of 354 megawatts (MW) of power, which it still generates to this day. SEGS is a parabolic trough solar installation that uses concave mirrors to focus the sun's rays on oil-filled pipes. The fluid in these pipes is then pumped to make high-pressure steam, driving turbine generators and creating electricity. Since then, solar power at a utility-scale level has evolved in a number of exciting and revolutionary technologies. The promise of this market pressure and innovation holds great things to come in competitive renewable energy.
Central Station Solar Technologies
- Solar Thermal:
- Solar Photovoltaics:
Solar thermal energy is designed to harness solar energy for heat, which is then used to generate electricity. Solar thermal, also referred to as Concentrated Solar Power (CSP) differs significantly from photovoltaics. Photovoltaic technology generates electricity directly from sunlight, whereas solar thermal energy uses lenses and reflectors to concentrate solar heat to generate power. This heat is then used to generate electricity which can be stored or released directly onto the grid. In recent years the solar thermal market has experienced renewed growth and a number of technologies have emerged that include solar dishes, solar troughs, solar towers and linear fresnel reflectors.
Trough technology is mature and clean, with a long track record demonstrating viability in large-scale application. The technology has been in use since the 1980s. Today, more than 300MW of solar troughs are in operation, with more than 6GW currently in development.
A parabolic trough is a solar concentrator that tracks the sun around a single, rotational axis. Sunlight is reflected from parabolic-shaped mirrors and is concentrated onto the receiver tube at the focal point of the parabola. For CSP applications, synthetic heat transfer oil pumped through the receiver tube and is heated to approximately 752 F (400 C). The oil transports the heat from the solar field to the power block where the energy is converted to high-pressure steam in a series of heat exchangers. This steam is converted into electrical energy using a conventional steam turbine.
The main components of parabolic trough technology include the trough reflector, a receiver tube or heat collection element, and the sun tracking system and support structure.
The cylindrical parabolic reflector reflects incident sunlight from its surface onto the receiver at the focal point. Typically, the reflector is made of thick glass silver mirrors formed into the shape of a parabola. Mirrors can be made from thin glass, plastic films or polished metals. The receiver tube or heat collection element consists of a metal absorber surrounded by a glass envelope. The absorber is coated with a selective coating to maximize energy collection and to minimize heat loss. This glass envelope is used to insulate the absorber from heat loss, and is typically coated with an anti-reflective surface to increase the transmittance of light through the glass to the absorber. For high temperature CSP applications, the space between the absorber and glass tube is evacuated to form a vacuum. The sun tracking system is an electronic control system and associated mechanical drive system used to focus the reflector onto the sun. Usually made of metal, the collector support structure holds the mirrors in accurate alignment while resisting the effects of the wind.
Compact Linear Fresnel Reflector
The Compact Linear Fresnel Reflector (CLFR) is a solar collector and steam generation system conceived in the early 1990s in Australia. Solar concentrators boil water with focused sunlight, creating high-pressure steam that drives a conventional turbine to generate electricity on a utility-scale. This high-pressure steam can also be used to augment power at existing fossil-fired plants, increasing a facility’s energy output and reducing its emissions. A third use of CLFR is for industrial applications that require large amounts of high-temperature steam, such as enhanced oil recovery and food processing.
A CLFR system, which consists of multiple solar collector lines, gathers energy by reflecting and concentrating sunlight to roughly 30 times the intensity of sunshine at the Earth’s surface. Mirrors focus the sunlight on an elevated absorber to heat and boil water, resulting in high-temperature steam that then drives a conventional turbine housed in a power block. Computer systems typically manage the mirror positions, tracking the motion of the sun throughout the day to maintain the focus point on the absorber.
CLFR systems are both environmentally sound and durable. CLFR does not burn any fuels nor produce any pollution. The steam generated by CLFR collectors is recondensed to water and reused, minimizing the overall water consumption. At night and during stormy weather, the reflector units invert, exposing steel to the sky for reduced exposure to weather events such as ice, hail and high winds.
Parabolic Dish-Stirling Engine
Originally developed by Robert Stirling in 1816, the Stirling cycle uses a working fluid (typically Helium, Nitrogen or Hydrogen gas) in a closed cylinder containing a piston. A Dish-Stirling system is composed of a solar concentrator with high reflectivity, a cavity solar receiver, and a Stirling engine, or microturbine that is attached to an alternator. The operation consists of heating a fluid located in the receiver until reaching a temperature approximate to 1382ºF (750ºC). This energy is used to generate power by the engine or microturbine. Typically, the system uses solar tracking to maximize the exposure to the sun’s rays.
A dish/engine system uses a mirrored dish (similar to a very large satellite dish). The dish-shaped surface collects and concentrates the sun's heat onto a receiver, which absorbs the heat and transfers it to fluid within the engine. The heat causes the fluid to expand against a piston or turbine to produce mechanical power. The mechanical power is then used to run a generator or alternator to produce electricity.
Tower systems are made up of a heliostat field comprised of movable mirrors, which are oriented according to the solar position, in order to reflect the solar radiation concentrating it up to 600 times on a receptor located on the upper part of a tall tower. This heat is transferred to a fluid, generating steam that in turn expands on a turbine that is coupled to a generator to produce electricity.
The main components of tower technology include a heliostat, the receiver and the tower. The heliostats capture solar radiation and direct it to the receiver. They are composed of a reflective surface, a supporting structure and mechanisms used to orientate them, following the sun’s movement. The most commonly-used reflective surfaces are glass mirrors. The receiver sits atop the tower, and transfers received heat to an operating fluid (e.g., water, molten salts). This fluid is then transported to other parts of the plant to generate high temperature steam which then produces electricity through a turbine. Rounding out these main components is the tower, which holds a boiler on the top of the structure, and is built to a height above the heliostat field to receive the solar radiation from the heliostats.
Photovoltaics (PV) allow for direct conversion of light into electricity, hence its name: photo=light, and voltaic=electricity. Photovoltaic technology uses a conducting material which performs this process, such as silicon. Advances in technology continue to bring to the market different material applications which have both provided thinner modules of silicon, and also the usage of other semiconducting materials to achieve thinner applications and improved efficiency in converting light to electricity. Since the development of the first solar cell in 1954, its usage has continued to grow steadily along with its efficiency.
Flat Panel Photovoltaic
Flat panel photovoltaic (PV) cells are usually made of silicon, which is a semiconductor that can absorb and insulate the photons present in sunlight, freeing the electrons to be used as electric current. In flat panel PV, silicon is placed under non-reflective glass where it collects the electromagnetic energy of the sun, and conducts it as electric current. Each flat panel is made up of a number of solar cells, and through metal connections, the panel is able to conduct all the electricity generated in direct current to a converter which turns it into alternating current. That alternating current is then transmitted to the grid where it is carried to its end use.
Many commercial solar cells use what is called a “thin film” of material to convert sunlight into electricity. Thin film solar cells use layers of semiconductor materials only a few micrometers thick. Among the materials used in thin film are amorphous silicon and cadmium telluride. Amorphous silicon is a special form of crystalline silicon which is deposited as an extremely thin film (far thinner than the thickness of a human hair) and requires only about 1% of the silicon used in crystalline systems. Cadmium telluride uses both cadmium and telluride to produce a thin film of electricity-generating panels at a cost-effective rate, and with a capability of maintaining substantial generation at higher temperatures.
Concentrating sunlight onto PV offers the ability to improve the efficiency of the cell itself, and to increase the supply of photons to the cells. Some of the technologies being developed and used include Fresnel point focus, Fresnel line focus and low concentration. Fresentl point focus (high concentration-GaAs) lenses concentrate direct solar radiation onto a focal point, providing concentration ratios of 500 and reducing the needed surface area of the PV cell. With this capability, higher quality and more expensive materials like Gallium Arsenide are used for the semiconductors. By comparison, Fresnel line focus (medium concentration-Si) lenses are flat cylindrical lenses that condense or diffuse light in a linear direction. This technology has lower concentration ratios than Fresnel point lenses, so high efficiency silicon semiconductors are used instead of expensive GaAs semiconductors. Also used is low concentration technology which uses mirrors instead of lenses to concentrate solar radiation. Since the solar radiation is much less condensed, conventional silicon semiconductors are often used because of their affordability.