A reliable and affordable supply of electricity is essential to protect public health and safety, and to sustain a vigorous economy in the West. Constraints in natural gas supplies, low hydropower generation due to drought, and increasing demand for electricity have led to enormous volatility in the West’s power market.
And as the population in the western states continues to grow, energy demand will continue to increase. With the addition of thermal storage, concentrating solar power technologies can provide power during periods when demand on the utilities is at its peak, even if the sun isn’t shining. This ability to provide dispatchable power during periods of peak demand establishes concentrating solar power technologies as a viable energy choice for the West.
Working in partnership with public and private organizations, NREL supports research and development that advances concentrating solar power technologies. Our goal is to make them competitive in U.S. and global power markets. We also provide critical analysis that identifies both opportunities and barriers to achieving this goal.
Many power plants today use fossil fuels as a heat source to boil water. The steam from the boiling water spins a large turbine, which drives a generator to produce electricity. However, a new generation of power plants with concentrating solar power systems uses the sun as a heat source. The three main types of concentrating solar power systems are: linear concentrator, dish/engine, and power tower systems.
Linear concentrator systems collect the sun’s energy using long rectangular, curved (U-shaped) mirrors. The mirrors are tilted toward the sun, focusing sunlight on tubes (or receivers) that run the length of the mirrors. The reflected sunlight heats a fluid flowing through the tubes. The hot fluid then is used to boil water in a conventional steam-turbine generator to produce electricity. There are two major types of linear concentrator systems: parabolic trough systems, where receiver tubes are positioned along the focal line of each parabolic mirror; and linear Fresnel reflector systems, where one receiver tube is positioned above several mirrors to allow the mirrors greater mobility in tracking the sun.
A dish/engine system uses a mirrored dish similar to a very large satellite dish. 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 to focus and concentrate sunlight onto a receiver on the top of a tower. A heat-transfer fluid heated in the receiver is used to generate steam, which, in turn, is used in a conventional turbine generator to produce electricity. Some power towers use water/steam as the heat-transfer fluid. Other advanced designs are experimenting with 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.
Concentrated sunlight has been used to perform useful tasks from the time of ancient China. A legend claims 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 55HP 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 354MW SEGS is still the largest solar power plant in the world.
Concentrated solar power systems are divided into
* concentrated solar thermal (CST)
* concentrated photovoltaics (CPV)
* concentrating photovoltaics and thermal (CPT)
Concentrated solar thermal (CST) is used to produce renewable heat or electricity (generally, in the latter case, 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).
A wide range of concentrating technologies exist, including the parabolic trough, Dish Stirling, Concentrating Linear Fresnel Reflector, Solar chimney 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 parabolic trough consists of a linear parabolic reflector that concentrates light onto a receiver positioned along the reflector’s focal line. The receiver is a tube positioned directly above the middle of the parabolic mirror and is filled with a working fluid. The reflector follows the Sun during the daylight hours by tracking along a single axis. A working fluid (eg molten salt) is heated to 150-350 °C as it flows through the receiver and is then used as a heat source for a power generation system.
Trough systems are the most developed CSP technology. The Solar Energy Generating Systems (SEGS) plants in California, Acciona’s Nevada Solar One near Boulder City, Nevada, and Plataforma Solar de Almería’s SSPS-DCS plant in Spain are representative of this technology.
Concentrating Linear Fresnel Reflectors are CSP-plants which use many thin mirror strips instead of parabolic mirrors to concentrate sunlight onto two tubes with working fluid. This has the advantage that flat mirrors can be used which are much cheaper than parabolic mirrors, and that more reflectors can be placed in the same amount of space, allowing more of the available sunlight to be used. Concentrating Linear Fresnel reflector can come in large plants or more compact plants.
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 the Big Dish in Canberra, Australia are representative of this technology.
A Solar chimney consists of a transparent large room (usually completely in glass) which is sloped gently up to a central hollow tower or chimney. The sun heats the air in this greenhouse-type structure which then rises up the chimney, hereby driving an air turbine as it rises. This air turbine hereby creates electricity. Solar chimneys are very simple in design and could therefore be a viable option for projects in the developing world.
A solar power tower consists of an array of dual-axis tracking reflectors (heliostats) that concentrate light on a central receiver atop a tower; the receiver contains a fluid deposit, which can consist of sea water. The working fluid in the receiver is heated to 500-1000 °C and then used as a heat source for a power generation or energy storage system.
Power tower development is less advanced than trough systems, but they offer higher efficiency and better energy storage capability. The Solar Two in Daggett, California and the Planta Solar 10 (PS10) in Sanlucar la Mayor, Spain are representative of this technology.
Concentrating solar power (CSP) is the main technology proposed for a cooperation to produce electricity and desalinated water in the arid regions of North Africa and Southern Europe by the Trans-Mediterranean Renewable Energy Cooperation DESERTEC.
 Concentrated photovoltaics
Concentrated photovoltaics (CPV) systems employ sunlight concentrated onto photovoltaic surfaces for the purpose of electrical power production. Solar concentrators of all varieties may be used, and these are often mounted on a solar tracker in order to keep the focal point upon the cell as the Sun moves across the sky.
Serious research and development work on concentrator PV systems has been conducted since the 1970s. For example, a linear-trough concentrator system was tested and installed at Sandia National Laboratories, and the first modern point focus photovoltaic concentrating system was developed in the Sandia, both late in that decade.
The latter system used a point focus acrylic Fresnel lens focusing on water-cooled Si cells and two axis tracking. A similar concept was used in other prototypes. Ramón Areces’ system, developed in the late 1970’s, used hybrid silicone-glass Fresnel lenses, while cooling of Si cells was achieved with a passive heat sink.
Luminescent solar concentrators (when combined with a PV-solar cell) can also be regarded as a Concentrating photovoltaics (CPV) system. Luminescent solar concentrators are useful as they can improve performance of PV-solar panels drastically.
Semiconductor properties allow solar cells to operate more efficiently in concentrated light, as long as the cell junction temperature is kept cool by suitable heat sinks. CPV operates most effectively in sunny weather since clouds and overcast conditions create diffuse light, which essentially cannot be concentrated.
Compared to conventional flat panel solar cells, CPV is advantageous because the solar collector is less expensive than an equivalent area of solar cells. CPV system hardware (solar collector and tracker) is targeted to be priced well under 3 USD/Watt, whereas silicon flat panels that are commonly sold are 3 to 5 USD/Watt (not including any associated power systems or installation charges).
CPV could reach grid parity in 2011. Low concentration CPV are systems with a solar concentration of 2-100 suns. For economic reasons, conventional or modified silicon solar cells are typically used, and, at these concentrations, the heat flux is low enough that the cells do not need to be actively cooled. The laws of optics dictate that a solar collector with a low concentration ratio can have a high acceptance angle and thus in some instances does not require active solar tracking.
Medium concentration CPV
From concentrations of 100 to 300 suns, the CPV systems require two-axes solar tracking and cooling (whether passive or active), which makes them more complex.
High concentration photovoltaics (HCPV)
High concentration photovoltaics (HCPV) systems employ concentrating optics consisting of dish reflectors or fresnel lenses that concentrate sunlight to intensities of 300 suns or more. The solar cells require high-capacity heat sinks to prevent thermal destruction and to manage temperature related performance losses.
Multijunction solar cells are currently favored over silicon as they are more efficient. The efficiency of both cell types rises with increased concentration; multijunction efficiency also rises faster. Multijunction solar cells, originally designed for non-concentrating space-based satellites, have been re-designed due to the high-current density encountered with CPV (typically 8 A/cm2 at 500 suns). Though the cost of multijunction solar cells is roughly 100x that of comparable silicon cells, the cell cost remains a small fraction of the cost of the overall concentrating PV system, so the system economics might still favor the multijunction cells.
Much of the original research into multijunction photovoltaics was sponsored by governments and the astronautics industry. More recently, the technical research and product development of CPV systems has grown due to investment in terrestrial electric generating systems. Recent technological advances in triple-junction solar cells by Fraunhofer Institute ISE have yielded 41.1% conversion efficiency. In May 2008, IBM demonstrated a prototype CPV using computer chip cooling techniques to achieve an energy density of 2300 suns.
Recently, Concentrix (Germany) and Amonix (USA) have announced operating AC efficiencies of 23% and 25%, respectively. These numbers point to significantly higher annual energy generation per receiver area unit with HCPV than with competing technologies.
In November 2009, NREL released a technical report presenting the oppotunities and challenges of CPV technology, from a state of the art review.
Concentrated Photovoltaics and Thermal
Concentrating Photovoltaics and Thermal (CPVT) technology produces both electricity and thermal heat in the same module. Thermal heat that can be employed for hot tap water, heating and heat-powered air conditioning (solar cooling), desalination or solar process heat.
CPVT systems can be used in private homes and increase total energy output to 40-50%, as compared with normal PV panels with 10-20% efficiency, and they produce more thermal heat in wintertime compared with normal thermal collectors. Also, thermal systems do not overheat.
Australian, American, and Chinese researchers are exploring the potential for Combined Heat and Power Solar (CHAPS), while Europeans are now producing CHAPS systems.
Currently, the cost of building a CSP station is about $2.5 to $4 per watt, while the fuel (the sun’s radiation) is free. Therefore a 250 MW CSP station today will cost $600–1000 million to build.
Future of Concentrated Solar Power
A study done by Greenpeace International, the European Solar Thermal Electricity Association, and the International Energy Agency’s SolarPACES group investigated the potential and future of concentrated solar power. The study found that concentrated solar power could account for up to 25% of the world’s energy needs by 2050. Also, with this expansion of concentrated solar power, thousands of new jobs would be created and millions of tonnes of carbon dioxide would be prevented from being released. The increase in investment would be from 2 billion euros worldwide to 92.5 billion euros in that time period.
Spain is the leader in concentrated solar power technology, with more than 50 projects approved by the government in the works. Also, it exports its technology, further increasing the technology’s stake in energy worldwide. Because of the nature of the technology needing a desert like area, experts predicted the biggest growth in places like Africa, Mexico, the southwest United States. The study examined three different outcomes for this technology: no increases in CSP technology, investment continuing as it has been in Spain and the US, and finally the true potential of CSP without any barriers on its growth.
Finally, the study acknowledged how technology for CSP was improving and how this would result in a drastic price decrease by 2050. It predicted a drop from the current range of .23 to .15 euros per kilowatt, down to .14 to .10 euros a kilowatt. Recently the EU has begun to look into developing a €400 billion ($774 billion) solar power plant based in the Sahara region using CSP technology known as Desertec.
It is part of a wider plan to create "a new carbon-free network linking Europe, the Middle East and North Africa". The plan is backed mainly by German industrialists and predicts production of 15% of Europe’s power by 2050. Morocco is a major partner in Desertec and as it has barely 1% of the electricity consumption of the EU, it will produce more than enough energy for the entire country with a large energy surplus to deliver to Europe.
Other organizations expect CSP to cost $0.06(US)/kWh by 2015 due to efficiency improvements and mass production of equipment. That would make CSP as cheap as conventional power. Investors such as venture capitalist Vinod Khosla expect CSP to continuously reduce costs and actually be cheaper than coal power after 2015.
In September 2009, Bill Weihl, Google.org’s green energy czar said that the firm was conducting research on the heliostat mirrors and gas turbine technology, which he expects will drop the cost of solar thermal electric power to less than $0.05/kWh in 2 or 3 years.
In 2009, scientists at the National Renewable Energy Laboratory (NREL) and SkyFuel teamed to develop large curved sheets of metal that have the potential to be 30% less expensive than today’s best collectors of concentrated solar power by replacing glass-based models with a silver polymer sheet that has the same performance as the heavy glass mirrors, but at a much lower cost and much lower weight. It also is much easier to deploy and install. The glossy film uses several layers of polymers, with an inner layer of pure silver.
The solar thermal power industry is growing rapidly with 1.2 GW under construction and another 13.9 GW announced globally through 2014. Spain is the epicenter of solar thermal power development with 22 projects for 1,037 MW under construction, all of which are projected to come online by the end of 2010.
In the United States, 5,600 MW of solar thermal power projects have been announced. In developing countries, three World Bank projects for integrated solar thermal/combined-cycle gas-turbine power plants in Egypt, Mexico, and Morocco have been approved.