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Concentrating solar power


The PS10 solar power tower near Seville concentrates sunlight from a field of heliostats on a central tower.
A parabolic trough is the most widely deployed type of solar thermal power plant
Fresnel reflectors are not as efficient as parabolic mirrors in Nevada Solar One but these ones from the company Ausra are much cheaper to build.
Dish engine systems eliminate the need to transfer heat to a boiler by placing a Stirling engine at the focal point.

Concentrating solar power (CSP) systems use lenses or mirrors and tracking systems to focus a large area of sunlight into a small beam. The concentrated light is then used as a heat source for a conventional power plant or is concentrated onto photovoltaic surfaces.

Contents

History

Concentrated sunlight has been used to perform useful tasks from the time of ancient China. A legend claims that Archimedes used polished shields to concentrate sunlight on the invading Roman fleet and repel them from Syracuse. As legends go, this was probably not possible. In 1866, Auguste Mouchout used a parabolic trough to produce steam for the first solar steam engine.[1] Over the following 50 years, inventors such as John Ericsson and Frank Shuman developed concentrating solar-powered devices for irrigation, refrigeration, and locomotion.[2]

Division

Concentrating solar power systems are divided into

  • concentrating solar thermal (CST)
  • concentrating photovoltaics (CPV)
  • and Concentrating Photovoltaics and Thermal (CPT)

Concentrating solar thermal

Concentrating 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 into a small beam. 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.[3] 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 being more and more cost-effective.[4]

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 right 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[5]) 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.[6] 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.[7]

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. [8][9]

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.[6] 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.[6] 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 Thermal 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.

Concentrating photovoltaics

Concentrating 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. [10]

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. New additions are the Concentrix Concentrating Photovoltaics and Stellaris.[3]

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.[11]

Efficiency

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.

Expected future efficiencies are nearly 50%.

Grid Parity

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

Low concentration CPV are systems with a solar concentration of 2-100 suns[12]. 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

Useful Reference Encyclopedia concentrations of 100 to 300 suns, the CPV systems require solar tracking and cooling, 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 200 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.[13]

In May 2008, IBM demonstrated a prototype CPV using computer chip cooling techniques to achieve an energy density of 2300 suns.[14]

Recently, Concentrix (Germany) and Amonix (USA) have announced operating AC efficiencies of 23%[15] and 25%[16], respectively. These numbers point to significantly higher annual energy generation with HCPV than with competing technologies.

Concentrating 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[17].

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.[18]

Australian, American, and Chinese researchers are exploring the potential for Combined Heat and Power Solar (CHAPS), while Europeans are now producing CHAPS systems.[19]

The 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 available 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. [20] 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 Mexico and 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. The findings of the third part are shown in the table below:

Time Investment Capacity
2015 21 billion euros a year 420 megawatts
2050 174 billion euros a year 1500 gigawatts

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 .15 to .23 euros currently per kilowatt, to .10 to .14 euros a kilowatt. [20]. 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[21].

Other organizations expect CSP to cost 6c(US)/kwh by 2015 due to efficiency improvements and mass production of equipment. [22] 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. [23]

Related

  • Clean Technology Fund
  • Geothermal heat pump
  • National Renewable Energy Laboratory
  • Salt evaporation pond
  • Sandia National Laboratory
  • Solar air conditioning
  • Solar thermal collector
  • Solar hot water
  • Thermoelectricity
  • Total Spectrum Solar Concentrator
  • Desertec

References

  1. ^ Butti and Perlin (1981), p.68
  2. ^ Butti and Perlin (1981), p.60–100
  3. ^ a b Types of solar thermal CSP plants
  4. ^ New innovations in solar thermal
  5. ^ Molten salt as CSP plant working fluid
  6. ^ a b c Martin and Goswami (2005), p.45
  7. ^ "Linear-focusing Concentrator Facilities: DCS, DISS, EUROTROUGH and LS3". Plataforma Solar de Almería. http://www.psa.es/webeng/instalaciones/parabolicos.html. Retrieved 2007-09-29. 
  8. ^ Compact CLFR
  9. ^ Ausra compact CLFR introducing cost-saving solar rotation features
  10. ^ MSU-CSET Participation Archive with notation in the Murray Ledger & Times
  11. ^ What is a luminescent solar concentrator?
  12. ^ A Strategic Research Agenda for Photovoltaic Solar Energy Technology Photovoltaic technology platform
  13. ^ World Record: 41.1% efficiency reached for multi-junction solar cells Fraunhofer ISE
  14. ^ IBM Research Unveils Breakthrough In Solar Farm Technology Physorg
  15. ^ Concentrix plant maintains 23% efficiency compoundsemiconductor
  16. ^ Amonix Introduces CPV System renewableenergyworld
  17. ^ Photovoltaic Thermal Applications absolicon
  18. ^ "Solar Concentrator Parabolic Trough combining Photovoltaic and Thermal Panels". Absolicon. http://www.absolicon.com. Retrieved 2009-01-23. 
  19. ^ "Researchers Explore Hybrid Concentrated Solar Energy System". Renewable Energy World. http://www.renewableenergyworld.com/rea/news/story?id=53981. Retrieved 2009-01-23. 
  20. ^ a b Concentrated solar power could generate 'quarter of world's energy' Guardian
  21. ^ Europe's Saharan power plan: miracle or mirage? Reuters
  22. ^ CSP and photovoltaic solar power Reuters
  23. ^ Concentrating Solar Power desertec-australia

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