Electrical power is produced when the concentrated light is converted to heat which drives a heat engine (usually a steam turbine) connected to an electrical power generator.
Concentrated solar power
Concentrated solar power (CSP) are systems that use lenses or mirrors to concentrate a large area of sunlight, or solar thermal energy, onto a small area. Electrical power is produced when the concentrated light is converted to heat which drives a heat engine (usually a steam turbine) connected to an electrical power generator.
CSP should not be confused with photovoltaics, where solar power is directly converted to electricity without the use of steam turbines. The concentration of sunlight onto photovoltaic surfaces, similar to CSP, is known as concentrated photovoltaics (CPV).
Concentrated sunlight has been used to perform useful tasks from the time of ancient China. A legend has it that Archimedes used a "burning glass" to concentrate sunlight on the invading Roman fleet and repel them from Syracuse. In 1973 a Greek scientist, Dr. Ioannis Sakkas, curious about whether Archimedes could really have destroyed the Roman fleet in 212 BC lined up nearly 60 Greek sailors, each holding an oblong mirror tipped to catch the Sun’s rays and direct them at a tar-covered plywood silhouette 160 feet away. The ship caught fire after a few minutes; however, historians continue to doubt the Archimedes story.
In 1866, Auguste Mouchout used a parabolic trough to produce steam for the first solar steam engine. The first patent for a Solar Collector was obtained by the Italian Alessandro Battaglia in Genoa, Italy, in 1886. Over the following years, inventors such as John Ericsson and Frank Shuman developed concentrating solar-powered devices for irrigation, refrigeration, and locomotion. In 1913 Shuman finished a 55 HP parabolic solar thermal energy station in Meadi, Egypt for irrigation.
Another Genoese, Professor Giovanni Francia (1911–1980), designed and built the first solar concentrated plant which entered in operation in Sant’Ilario, near Genoa, Italy in 1968. This plant had the architecture of today’s solar concentrated plants with a solar receiver in the center of a field of solar collectors. The plant was able to produce 1 MW with superheated steam at 100 bar and 500 degrees celsius. The 10 MW Solar One power tower was developed in Southern California in 1981 but the parabolic trough technology of the nearby Solar Energy Generating Systems (SEGS), begun in 1984, was more workable. The 354 MW SEGS is still the largest solar power plant in the world.
CSP is used to produce renewable heat or cool or electricity (called solar thermoelectricity, usually generated through steam). CST systems use lenses or mirrors and tracking systems to focus a large area of sunlight onto a small area. The concentrated light is then used as heat or as a heat source for a conventional power plant (solar thermoelectricity).
Concentrating technologies exist in four common forms, namely parabolic trough, dish stirlings, concentrating linear fresnel reflector, and solar power tower. Each concentration method is capable of producing high temperatures and correspondingly high thermodynamic efficiencies, but they vary in the way that they track the Sun and focus light. Due to new innovations in the technology, concentrating solar thermal is becoming more and more cost-effective.
A design which requires water for condensation or cooling may conflict with location of solar thermal plants in desert areas with good solar radiation but limited water resources. The conflict is illustrated by plans of Solar Millennium, a German company, to build a plant in the Amargosa Valley of Nevada which would require 20% of the water available in the area. Some other projected plants by the same and other companies in the Mojave Desert of California may also be affected by difficulty in obtaining adequate and appropriate water rights. California water law currently prohibits use of potable water for cooling.
Other designs require less water. The proposed Ivanpah Solar Power Facility in south-eastern California will conserve scarce desert water by using air-cooling to convert the steam back into water. Compared to conventional wet-cooling, this results in a 90 percent reduction in water usage . The water is then returned to the boiler in a closed process which is environmentally friendly.
A parabolic trough is a type of solar thermal energy collector. It is constructed as a long parabolic mirror (usually coated silver or polished aluminum) with a Dewar tube running its length at the focal point. Sunlight is reflected by the mirror and concentrated on the Dewar tube. The trough is usually aligned on a north-south axis, and rotated to track the sun as it moves across the sky each day.
Alternatively the trough can be aligned on an east-west axis, this reduces the overall efficiency of the collector, due to cosine loss, but only requires the trough to be aligned with the change in seasons, avoiding the need for tracking motors. This tracking method works correctly at the spring and fall equinoxes with errors in the focusing of the light at other times during the year (the magnitude of this error varies throughout the day, taking a minimum value at solar noon). There is also an error introduced due to the daily motion of the sun across the sky, this error also reaches a minimum at solar noon. Due to these sources of error, seasonally adjusted parabolic troughs are generally designed with a lower solar concentration ratio. In order to increase the level of alignment, some measuring devices have also been invented.
Heat transfer fluid (usually oil) runs through the tube to absorb the concentrated sunlight. This increases the temperature of the fluid to some 400°C. The heat transfer fluid is then used to heat steam in a standard turbine generator. The process is economical and, for heating the pipe, thermal efficiency ranges from 60-80%. The overall efficiency from collector to grid, i.e. (Electrical Output Power)/(Total Impinging Solar Power) is about 15%, similar to PV (Photovoltaic Cells) but less than Stirling dish concentrators.
Current commercial plants utilizing parabolic troughs are hybrids; fossil fuels are used during night hours, but the amount of fossil fuel used is limited to a maximum 27% of electricity production, allowing the plant to qualify as a renewable energy source. Because they are hybrids and include cooling stations, condensers, accumulators and other things besides the actual solar collectors, the power generated per square meter of area varies enormously.
The solar power tower (also known as ‘Central Tower’ power plants or ‘Heliostat’ power plants or power towers) is a type of solar furnace using a tower to receive the focused sunlight. It uses an array of flat, movable mirrors (called heliostats) to focus the sun’s rays upon a collector tower (the target).
Early designs used these focused rays to heat water, and used the resulting steam to power a turbine. However, designs using liquid sodium in place of water have been demonstrated; this is a metal with high heat capacity, which can be used to store the energy before using it to boil water to drive turbines. These designs allow power to be generated when the sun is not shining.
The 10 MW Solar One and Solar Two heliostat demonstration projects in the Mojave Desert have now been decommissioned. The 15 MW Solar Tres Power Tower in Spain builds on these projects. In Spain, the 11 MW PS10 solar power tower and 20 MW PS20 solar power tower have been recently completed. In South Africa, a 100 MW solar power plant is planned with 4000 to 5000 heliostat mirrors, each having an area of 140 m². A site near Upington has been selected.
eSolar unveiled Sierra SunTower in the summer of 2009, a 5 MW plant located in Lancaster, California about 80 km (50 miles) northeast of Los Angeles. The project site occupies approximately 8 hectares (20 acres) in an arid valley in the western corner of the Mojave Desert at 35° north latitude. Sierra SunTower is interconnected to the Southern California Edison (SCE) grid and is the only CSP tower facility operating in North America.
BrightSource Energy entered into a series of power purchase agreements with Pacific Gas and Electric Company in March 2008 for up to 900 MW of electricity, the largest solar power commitment ever made by a utility. BrightSource is currently developing a number of solar power plants in Southern California, with construction of the first plant planned to start in 2009.
In June 2008, BrightSource Energy dedicated its Solar Energy Development Center (SEDC) in Israel’s Negev Desert. The site, located in the Rotem Industrial Park, features more than 1,600 heliostats that track the sun and reflect light onto a 60 meter-high tower. The concentrated energy is then used to heat a boiler atop the tower to 550 degrees Celsius, generating steam that is piped into a turbine, where electricity can be produced.
The US National Renewable Energy Laboratory (NREL) has estimated that by 2020 electricity could be produced from power towers for 5.47 cents per kWh. Google.org hopes to develop cheap, low maintenance, mass producible heliostat components to reduce this cost in the near future.
* Some Concentrating Solar Power Towers are air-cooled instead of water-cooled, to avoid using limited desert water
* Flat glass is used instead of the more expensive curved glass
* Some store the heat in molten salt containers to continue producing electricity while the sun is not shining
* Steam is heated to 500 C to drive turbines which generate electricity
Generally, installations uses from 150 hectares (1,500,000 m2) to 320 hectares (3,200,000 m2).
Recently, there has been a renewed interest in solar tower power technology, as is evident from the fact that there are several companies involved in planning, designing and building utility size power plants. This is an important step towards the ultimate goal of developing commercially viable plants. There are numerous example of case studies of applying innovative solution to solar power.
The Pit Power Tower combines a Solar Power Tower and an Aero-electric Power Tower in a decommissioned open pit mine. Traditional Solar Power Towers are constrained in size by the height of the tower and closer heliostats blocking the line of sight of outer heliostats to the receiver. The use of the pit mine’s "stadium seating" helps overcome the blocking constraint.
As Solar Power Towers commonly use steam to drive the turbines, and water tends to be scarce in regions with high solar energy, another advantage of open pits is that they tend to collect water, having been dug below the water table. The Pit Power Tower uses low heat steam to drive the Pneumatic Tubes in a co-generation system. A third benefit of re-purposing a pit mine for this kind of project is the possibility of reusing mine infrastructure such as roads, buildings and electricity.
A dish stirling or dish engine system consists of a stand-alone parabolic reflector that concentrates light onto a receiver positioned at the reflector’s focal point. The reflector tracks the Sun along two axes. The working fluid in the receiver is heated to 250–700 °C and then used by a Stirling engine to generate power.
Parabolic dish systems provide the highest solar-to-electric efficiency among CSP technologies, and their modular nature provides scalability. The Stirling Energy Systems (SES) and Science Applications International Corporation (SAIC) dishes at UNLV, and Australian National University’s Big Dish in Canberra, Australia are representative of this technology.
A dish 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 to 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.
The advantage of a dish system is that it can achieve much higher temperatures due to the higher concentration of light (as in tower designs). Higher temperatures leads to better conversion to electricity and the dish system is very efficient on this point. However, there are also some disadvantages. Heat to electricity conversion requires moving parts and that results in maintenance. In general, a centralized approach for this conversion is better than the dencentralized concept in the dish design. Second, the (heavy) engine is part of the moving structure, which requires a rigid frame and strong tracking system. Furthermore, parabolic mirrors are used instead of flat mirrors and tracking must be dual-axis.
In 2005 Southern California Edison announced an agreement to purchase solar powered Stirling engines from Stirling Energy Systems over a twenty year period and in quantities (20,000 units) sufficient to generate 500 megawatts of electricity. Stirling Energy Systems announced another agreement with San Diego Gas & Electric to provide between 300 and 900 megawatts of electricity. In January 2010, Stirling Energy Systems and Tessera Solar commissioned the first demonstration 1.5-megawatt power plant ("Maricopa Solar") using Stirling technology in Peoria, Arizona.
Fresnel solar reflectors
A Concentrating Linear Fresnel Reflector (CLFR) – also referred to as a Compact Linear Fresnel Reflector – is a specific type of Linear Fresnel Reflector (LFR) technology. Linear Fresnel Reflectors use long, thin segments of mirrors to focus sunlight onto a fixed absorber located at a common focal point of the reflectors. These mirrors are capable of concentrating the sun’s energy to approximately 30 times its normal intensity. This concentrated energy is transferred through the absorber into some thermal fluid (this is typically oil capable of maintaining liquid state at very high temperatures). The fluid then goes through a heat exchanger to power a steam generator. As opposed to traditional LFR’s, the CLFR utilizes multiple absorbers within the vicinity of the mirrors.
The first Linear Fresnel Reflector was developed in Italy in 1961 by Giorgio Francia of the University of Genoa. Francia demonstrated that such a system could create elevated temperatures capable of making a fluid do work. The technology was further investigated by companies such as the FMC Corporation during the 1973 oil crisis, but remained relatively untouched until the early 1990s. In 1993, the first CLFR was developed at the University of Sydney in 1993 and patented in 1995. In 1999, the CLFR design was enhanced by the introduction of the advanced absorber.
The reflectors are located at the base of the system and converge the sun’s rays into the absorber. A key component that makes all LFR’s more advantageous than traditional parabolic trough mirror systems is the use of Fresnel reflectors. These reflectors make use of the Fresnel lens effect, which allows for a concentrating mirror with a large aperture and short focal length while simultaneously reducing the volume of material required for the reflector. This greatly reduces the system’s cost since sagged-glass parabolic reflectors are typically very expensive. It should be noted, however, that in recent years thin-film nanotechnology has significantly reduced the cost of parabolic mirrors.
A major challenge that must be addressed in any solar concentrating technology is the changing intensity of the incident rays (the rays of sunlight striking the mirrors) as the sun progresses throughout the day. The reflectors of a CLFR are typically aligned in a north-south orientation and turn about a single axis using a computer controlled solar tracker system. This allows the system to maintain the proper angle of incidence between the sun’s rays and the mirrors, thereby optimizing energy transfer.
The absorber is located at the focal point of the mirrors. It runs parallel to and above the reflector segments to transport radiation into some working thermal fluid. The basic design of the absorber for the CLFR system is an inverted air cavity with a glass cover enclosing insulated steam tubes. This design has been demonstrated to be simple and cost effective with good optical and thermal performance.
For optimum performance of the CLFR, several design factors of the absorber must be optimized.
* First, heat transfer between the absorber and the thermal fluid must be maximized. This relies on the surface of the steam tubes being selective. A selective surface optimizes the ratio of energy absorbed to energy emitted. Acceptable surfaces generally absorb 96% of incident radiation while emitting only 7% through infra-red radiation. Electro-chemically deposited black chrome is generally used for its ample performance and ability to withstand high temperatures.
* Second, the absorber must be designed so that the temperature distribution across the selective surface is uniform. Non-uniform temperature distribution leads to accelerated degradation of the surface. Typically, a uniform temperature of 300 °C is desired. Uniform distributions are obtained by changing absorber parameters such as the thickness of insulation above the plate, the size of the aperture of the absorber and the shape and depth of the air cavity.
As opposed to the traditional LFR, the CLFR makes use of multiple absorbers within the vicinity of its mirrors. These additional absorbers allow the mirrors to alternate their inclination. This arrangement is advantageous for several reasons.
* First, alternating inclinations minimize the effect of reflectors blocking adjacent reflectors’ access to sunlight, thereby improving the systems efficiency.
* Second, multiple absorbers minimize the amount of ground space required for installation. This in turn reduces cost to procure and prepare the land.
* Finally, having the panels in close proximity reduces the length of absorber lines, which reduces both thermal losses through the absorber lines and overall cost for the system.
In March 2009, the German company Novatec Biosol constructed the Fresnel solar power plant known as PE 1. The solar thermal power plant is based on CLFR technology and has an electrical capacity of 1.4 MW. PE 1 comprises a solar boiler with mirror surface of approximately 18,000 m2. The steam is generated by concentrating sunlight directly onto a linear receiver, which is 7.40 metres above the ground. An absorber tube is positioned in the focal line of the mirror field where water is heated into 270 °C (543 K; 518 °F) saturated steam. This steam in turn powers a generator.
The commercial success of the PE 1 has led Novatec Biosol to design a 30 MW solar power plant known as PE 2. PE 2 will be constructed in Murcia, Spain in 2010. Novatec Biosol has also obtained permits for another 60 MW of related projects.
In April 2008, the solar thermal company Ausra opened a large factory in Las Vegas, Nevada that will produce linear Fresnel reflectors