# Concentrated solar power

(Redirected from Total spectrum solar concentrator)
2014 December - Crescent Dunes completed site.
The three towers of the Ivanpah Solar Power Facility.
Part of the 354 MW SEGS solar complex in northern San Bernardino County, California.
Bird's eye view of Khi Solar One, South Africa

Concentrated solar power (also called concentrating solar power, concentrated solar thermal, and CSP) systems generate solar power by using mirrors or lenses to concentrate a large area of sunlight, or solar thermal energy, onto a small area. Electricity is generated when the concentrated light is converted to heat, which drives a heat engine (usually a steam turbine) connected to an electrical power generator[1][2][3] or powers a thermochemical reaction (experimental as of 2013).[4][5][6] Heat storage in molten salts allows solar thermal plants to continue to generate after sunset and adds value to such systems when compared to photovoltaic panels.

CSP is being commercialized and the CSP had a capacity of 4,815 MW in 2016 up from 354 MW in 2005. As of 2017, Spain had a total capacity of 2,300 MW making this country the world leader in CSP. United States follows with 1,740 MW. Interest is also notable in North Africa and the Middle East, as well as India and China. The global market has been dominated by parabolic-trough plants, which accounted for 90% of CSP plants at one point.[7] The largest CSP projects in the world are Ivanpah Solar Power Facility (392 MW) in the United States (which uses solar power tower technology), Mojave Solar Project (354 MW) in the United States (which uses parabolic troughs) and Dhirubhai Ambani Solar Park (125 MW) in India (which uses Fresnel reflectors)[8]

In most cases, CSP technologies currently cannot compete on price with photovoltaics (solar panels), which have experienced huge growth in recent years due to falling prices and much smaller operating costs.[9] CSP generally needs large amount of direct solar radiation, and its energy generation falls dramatically with cloud cover. This is in contrast with photovoltaics, which can produce some electricity also from diffuse radiation. For example, base load purpose CSP tariff in the extremely dry Atacama region of Chile reached below ¢5.0/kWh.[10][11] In 2017, CSP represented less than 2% of worldwide installed capacity of solar electricity plants.[12] In 2017, drastically falling prices of CSP plants are turning clean CSP cheaper compared to other base load power plants using gas/coal/nuclear fuel even in high moisture and dusty atmosphere at sea level.[13][14]

CSP is not to be confused with concentrator photovoltaics (CPV). In CPV, the concentrated sunlight is converted directly to electricity via the photovoltaic effect.

## History

Solar steam engine for water pumping, near Los Angeles circa 1901

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 49 m (160 ft) away. The ship caught fire after a few minutes; however, historians continue to doubt the Archimedes story.[15]

In 1866, Auguste Mouchout used a parabolic trough to producе 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, invеntors such as John Ericsson and Frank Shuman developed concentrating solar-powered dеvices for irrigation, refrigеration, and locomоtion. In 1913 Shuman finished a 55 HP parabolic solar thermal energy station in Maadi, Egypt for irrigation.[16][17][18][19] The first solar-power system using a mirror dish was built by Dr. R.H. Goddard, who was already well known for his research on liquid-fueled rockets and wrote an article in 1929 in which he asserted that all the previous obstacles had been addressed.[20]

Professor Giovanni Francia (1911–1980) designed and built the first concentrated-solar plant, which entered into operation in Sant'Ilario, near Genoa, Italy in 1968. This plant had the architecture of today's concentrated-solar 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 °C.[21] 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 was the largest solar power plant in the world, until the 390 MW Ivanpah power tower project reached full power.

A pilot 5 MW CSP power tower, Solar One, was converted to a 10 MW CSP power tower, Solar Two, decommissioned in 1999. Due to the success of Solar Two, a commercial power plant, called Solar Tres Power Tower, was built in Spain, renamed Gemasolar Thermosolar Plant. Gemasolar's results have paved the way for the Crescent Dunes project. Ivanpah difficulties arise also from not having considered the lessons about the benefits of thermal storage. Solana in Arizona is at 25% below projected numbers, Ivanpah in California, is at 40% below projected numbers. A slightly bigger photovoltaic power station, like the 290 MW Agua Caliente Solar Project peaked at most to 741 GWh in 2014, comparing with the 280 MW Solana growing 719 GWh. Another operator, that of the 280 MW Genesis Solar, projected only 580 GWh production and instead made 621 GWh in 2015.

CSP was originally treated as a competitor to photovoltaics, and was built without energy storage. By 2015 PV commercial power was selling for 13 recent CSP contracts.[22][23] However, increasingly, by 2015 CSP was being bid with 3 to 12 hours of thermal energy storage, making CSP the dispatchable form of solar energy [24]. As such, it is increasingly seen as competing with natural gas for flexible, dispatchable power. With storage included, CSP is the cheapest form of dispatchable solar at utility-scale, about ten times cheaper than combining PV with battery for storage.[citation needed]

## Current technology

CSP is used to produce electricity (sometimes called solar thermoelectricity, usually generated through steam). Concentrated-solar technology systems use mirrors or lenses with 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). The solar concentrators used in CSP systems can often also be used to provide industrial process heating or cooling, such as in solar air conditioning.

Concentrating technologies exist in four optical types, namely parabolic trough, dish, concentrating linear Fresnel reflector, and solar power tower.[25] Although simple, these solar concentrators are quite far from the theoretical maximum concentration.[26][27] For example, the parabolic-trough concentration gives about 13 of the theoretical maximum for the design acceptance angle, that is, for the same overall tolerances for the system. Approaching the theoretical maximum may be achieved by using more elaborate concentrators based on nonimaging optics.[26][27][28]

Different types of concentrators produce different peak temperatures and correspondingly varying thermodynamic efficiencies, due to differences in the way that they track the sun and focus light. New innovations in CSP technology are leading systems to become more and more cost-effective.[29][30]

### Parabolic trough

Parabolic trough at a plant near Harper Lake, California

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 filled with a working fluid. The reflector follows the sun during the daylight hours by tracking along a single axis. A working fluid (e.g. molten salt[31]) is heated to 150–350 °C (302–662 °F) as it flows through the receiver and is then used as a heat source for a power generation system.[32] Trough systems are the most developed CSP technology. The Solar Energy Generating Systems (SEGS) plants in California, the world's first commercial parabolic trough plants, Acciona's Nevada Solar One near Boulder City, Nevada, and Andasol, Europe's first commercial parabolic trough plant are representative, along with Plataforma Solar de Almería's SSPS-DCS test facilities in Spain.[33]

### Enclosed trough

Inside an enclosed trough system

The design encapsulates the solar thermal system within a greenhouse-like glasshouse. The glasshouse creates a protected environment to withstand the elements that can negatively impact reliability and efficiency of the solar thermal system.[34] Lightweight curved solar-reflecting mirrors are suspended from the ceiling of the glasshouse by wires. A single-axis tracking system positions the mirrors to retrieve the optimal amount of sunlight. The mirrors concentrate the sunlight and focus it on a network of stationary steel pipes, also suspended from the glasshouse structure.[35] Water is carried throughout the length of the pipe, which is boiled to generate steam when intense solar radiation is applied. Sheltering the mirrors from the wind allows them to achieve higher temperature rates and prevents dust from building up on the mirrors.[34]

GlassPoint Solar, the company that created the Enclosed Trough design, states its technology can produce heat for Enhanced Oil Recovery (EOR) for about $5 per million British thermal units in sunny regions, compared to between$10 and $12 for other conventional solar thermal technologies.[36] ### Solar power tower The PS10 solar power plant in Andalucía, Spain, concentrates sunlight from a field of heliostats onto a central solar power tower. A solar power tower consists of an array of dual-axis tracking reflectors (heliostats) that concentrate sunlight on a central receiver atop a tower; the receiver contains a fluid deposit, which can consist of molten salt. Optically a solar power tower is the same as a circular Fresnel reflector. The working fluid in the receiver is heated to 500–1000 °C (773–1,273 K or 932–1,832 °F) and then used as a heat source for a power generation or energy storage system.[32] An advantage of the solar tower is the reflectors can be adjusted instead of the whole tower. 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 CESA-1 in Plataforma Solar de Almeria Almeria, Spain, are the most representative demonstration plants. The Planta Solar 10 (PS10) in Sanlucar la Mayor, Spain, is the first commercial utility-scale solar power tower in the world. The 377 MW Ivanpah Solar Power Facility, located in the Mojave Desert, is the largest CSP facility in the world, and uses three power towers.[37] Ivanpah generated only 0.652 TWh (63%) of its energy from solar means, and the other 0.388 TWh (37%) was generated by burning Natural Gas. [38][39][40] The National Solar Thermal Test Facility, NSTTF located in Albuquerque, New Mexico, is an experimental solar thermal test facility with a heliostat field capable of producing 6 MW.[41] ### Fresnel reflectors Fresnel reflectors are made of many thin, flat mirror strips to concentrate sunlight onto tubes through which working fluid is pumped. Flat mirrors allow more reflective surface in the same amount of space than a parabolic reflector, thus capturing more of the available sunlight, and they are much cheaper than parabolic reflectors. Fresnel reflectors can be used in various size CSPs.[42][43] Fresnel reflectors are sometimes regarded as a technology with a worse output than other methods. The cost efficiency of this model is what causes some to use this instead of others with higher output ratings. Some new models of Fresnel Reflectors with Ray Tracing capabilities have begun to be tested and have initially proved to yield higher output than the standard version.[44] ### Dish Stirling 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 (482–1,292 °F) and then used by a Stirling engine to generate power.[32] Parabolic-dish systems provide high solar-to-electric efficiency (between 31% and 32%), and their modular nature provides scalability. The Stirling Energy Systems (SES), United Sun Systems (USS) 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 world record for solar to electric efficiency was set at 31.25% by SES dishes at the National Solar Thermal Test Facility (NSTTF) in New Mexico on January 31, 2008, a cold, bright day.[45] According to its developer, Ripasso Energy, a Swedish firm, in 2015 its Dish Sterling system being tested in the Kalahari Desert in South Africa showed 34% efficiency.[46] The SES installation in Maricopa, Phoenix was the largest Stirling Dish power installation in the world until it was sold to United Sun Systems. Subsequently, larger parts of the installation have been moved to China as part of the huge energy demand. ## Solar thermal enhanced oil recovery Heat from the sun can be used to provide steam used to make heavy oil less viscous and easier to pump. Solar power tower and parabolic troughs can be used to provide the steam which is used directly so no generators are required and no electricity is produced. Solar thermal enhanced oil recovery can extend the life of oilfields with very thick oil which would not otherwise be economical to pump.[47] ## CSP with thermal energy storage In a CSP plant that includes storage, the solar energy is first used to heat the molten salt or synthetic oil to store thermal/heat energy at high temperature in insulated tanks[48].[49] Later hot molten salt is used for steam production to generate electricity by steam turbo generator as per requirement.[50] Thus solar energy which is available in daylight only is used to generate electricity round the clock on demand as a Load following power plant.[51] The thermal storage capacity is indicated in hours of power generation at nameplate capacity. Unlike solar PV or CSP without storage, the power generation from solar thermal storage plants is dispatchable and self-sustainable similar to coal/gas-fired power plants.[52] ## Deployment around the world 1,000 2,000 3,000 4,000 5,000 1984 1990 1995 2000 2005 2010 2015 Worldwide CSP capacity since 1984 in MWp National CSP capacities in 2016 (MWp) Country Total Added Spain 2,300 0 United States 1,738 0 India 225 0 South Africa 200 100 Morocco 180 0 United Arab Emirates 100 0 Algeria 25 0 Egypt 20 0 Australia 12 0 China 10 10 Thailand 5 0 Source: REN21 Global Status Report, June, 2017[53] The commercial deployment of CSP plants started by 1984 in the US with the SEGS plants. The last SEGS plant was completed in 1990. From 1991 to 2005, no CSP plants were built anywhere in the world. Global installed CSP-capacity increased nearly tenfold between 2004 and 2013 and grew at an average of 50 percent per year during the last five of those years.[54]:51 In 2013, worldwide installed capacity increased by 36% or nearly 0.9 gigawatt (GW) to more than 3.4 GW. Spain and the United States remained the global leaders, while the number of countries with installed CSP were growing but the rapid decrease in price of PV solar, policy changes and the global financial crisis stopped most development in these countries. 2014 was the best year for CSP but was followed by a rapid decline with only one major plant completed in the world in 2016. There is a notable trend towards developing countries and regions with high solar radiation with several large plants under construction in 2017. CSP is also increasingly competing with the cheaper photovoltaic solar power and with concentrator photovoltaics (CPV), a fast-growing technology that just like CSP is suited best for regions of high solar insolation.[55][56] In addition, a novel solar CPV/CSP hybrid system has been proposed recently.[57] Worldwide Concentrated Solar Power (MWp) Year 1984 1985 1989 1990 ... 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 Installed 14 60 200 80 0 1 74 55 179 307 629 803 872 925 420 110 Cumulative 14 74 274 354 354 355 429 484 663 969 1,598 2,553 3,425 4,335 4,705 4,815 Sources: REN21[53][58]:146 [54]:51 · CSP-world.com[59] · IRENA[60] ## Efficiency The conversion efficiency ${\displaystyle \eta }$ of the incident solar radiation into mechanical work − without considering the ultimate conversion step into electricity by a power generator − depends on the thermal radiation properties of the solar receiver and on the heat engine (e.g. steam turbine). Solar irradiation is first converted into heat by the solar receiver with the efficiency ${\displaystyle \eta _{Receiver}}$ and subsequently the heat is converted into work by the heat engine with the efficiency ${\displaystyle \eta _{Carnot}}$, using Carnot's principle.[61][62] For a solar receiver providing a heat source at temperature ${\displaystyle T_{H}}$ and a heat sink at room temperature ${\displaystyle T^{0}}$, the overall conversion efficiency can be calculated as follows: ${\displaystyle \eta =\eta _{\mathrm {Receiver} }\cdot \eta _{\mathrm {Carnot} }}$ with ${\displaystyle \eta _{\mathrm {Carnot} }=1-{\frac {T^{0}}{T_{H}}}}$ and ${\displaystyle \eta _{\mathrm {Receiver} }={\frac {Q_{\mathrm {absorbed} }-Q_{\mathrm {lost} }}{Q_{\mathrm {solar} }}}}$ where ${\displaystyle Q_{\mathrm {solar} }}$, ${\displaystyle Q_{\mathrm {absorbed} }}$, ${\displaystyle Q_{\mathrm {lost} }}$ are respectively the incoming solar flux and the fluxes absorbed and lost by the system solar receiver. For a solar flux ${\displaystyle I}$ (e.g. ${\displaystyle I=1000\,\mathrm {W/m^{2}} }$) concentrated ${\displaystyle C}$ times with an efficiency ${\displaystyle \eta _{Optics}}$ on the system solar receiver with a collecting area ${\displaystyle A}$ and an absorptivity ${\displaystyle \alpha }$: ${\displaystyle Q_{\mathrm {solar} }=\eta _{\mathrm {Optics} }ICA}$, ${\displaystyle Q_{\mathrm {absorbed} }=\alpha Q_{\mathrm {solar} }}$, For simplicity's sake, one can assume that the losses are only radiative ones (a fair assumption for high temperatures), thus for a reradiating area A and an emissivity ${\displaystyle \epsilon }$ applying the Stefan-Boltzmann law yields: ${\displaystyle Q_{\mathrm {lost} }=A\epsilon \sigma T_{H}^{4}}$ Simplifying these equations by considering perfect optics (${\displaystyle \eta _{\mathrm {Optics} }}$ = 1), collecting and reradiating areas equal and maximum absorptivity and emissivity (${\displaystyle \alpha }$ = 1, ${\displaystyle \epsilon }$ = 1) then substituting in the first equation gives ${\displaystyle \eta =\left(1-{\frac {\sigma T_{H}^{4}}{IC}}\right)\cdot \left(1-{\frac {T^{0}}{T_{H}}}\right)}$ The graph shows that the overall efficiency does not increase steadily with the receiver's temperature. Although the heat engine's efficiency (Carnot) increases with higher temperature, the receiver's efficiency does not. On the contrary, the receiver's efficiency is decreasing, as the amount of energy it cannot absorb (Qlost) grows by the fourth power as a function of temperature. Hence, there is a maximum reachable temperature. When the receiver efficiency is null (blue curve on the figure below), Tmax is: ${\displaystyle T_{\mathrm {max} }=\left({\frac {IC}{\sigma }}\right)^{0.25}}$ There is a temperature Topt for which the efficiency is maximum, i.e. when the efficiency derivative relative to the receiver temperature is null: ${\displaystyle {\frac {d\eta }{dT_{H}}}(T_{\mathrm {opt} })=0}$ Consequently, this leads us to the following equation: ${\displaystyle T_{opt}^{5}-(0.75T^{0})T_{\mathrm {opt} }^{4}-{\frac {T^{0}IC}{4\sigma }}=0}$ Solving this equation numerically allows us to obtain the optimum process temperature according to the solar concentration ratio ${\displaystyle C}$ (red curve on the figure below)  C Tmax Topt 500 1000 5000 10000 45000 (max. for Earth) 1720 2050 3060 3640 5300 970 1100 1500 1720 2310 Theoretical efficiencies aside, real-world experience of CSP reveals a 25%–60% shortfall in projected production. ## Costs As of 2017, new CSP power plants are economically competitive with fossil fuels in certain regions, such as Chile, Australia[63], and the Middle east and North Africa Region (MENA)[64]. Nathaniel Bullard, a solar analyst at Bloomberg New Energy Finance, calculated that the cost of electricity at the Ivanpah Solar Power Facility, a project contracted in 2009 and completed in 2014 in Southern California, would be lower than that from photovoltaic power and about the same as that from natural gas.[65] However, due to the rapid price decline of photovoltaics, in November 2011, Google announced that they would not invest further in CSP projects Google had invested US$168 million on BrightSource.[66][67] IRENA has published on June 2012 a series of studies titled: "Renewable Energy Cost Analysis". The CSP study shows the cost of both building and operation of CSP plants. Costs are expected to decrease, but there are insufficient installations to clearly establish the learning curve.

By 2012, there was 1.9 GW of CSP installed, with 1.8 GW of that being parabolic trough.[68] The US Department of Energy publishes the up to date list of CSP power plants at the National Renewable Energy Laboratory (NREL) under a contract from SolarPACES, the international network of CSP researchers and industry experts. As of 2017, there is 5 GW of CSP installed globally, with most of that in Spain at 2.3 GW, and the US at 1.3 GW.

Other organizations had predicted CSP to cost $0.06(US)/kWh by 2015 due to efficiency improvements and mass production of equipment.[79] That would have made 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 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 much lower cost and 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. Telescope designer Roger Angel (Univ. of Arizona) has turned his attention to CPV, and is a partner in a company called Rehnu. Angel utilizes a spherical concentrating lens with large-telescope technologies, but much cheaper materials and mechanisms, to create efficient systems.[80] Recent experience with CSP technology in 2014-2015 at Solana in Arizona, and Ivanpah in Nevada indicate large production shortfalls in electricity generation between 25% and 40%. Producers blame clouds and stormy weather, but critics seem to think there are technological issues. These problems are causing utilities to pay inflated prices for wholesale electricity, and threaten the long-term viability of the technology. As photovoltaic costs continue to plummet, many think CSP has a limited future in utility-scale electricity production.[81] ### Very large scale solar power plants There have been several proposals for gigawatt size, very-large-scale solar power plants. They include the Euro-Mediterranean Desertec proposal and Project Helios in Greece (10 GW), both now canceled. A 2003 study concluded that the world could generate 2,357,840 TWh each year from very large scale solar power plants using 1% of each of the world's deserts. Total consumption worldwide was 15,223 TWh/year[82] (in 2003). The gigawatt size projects would have been arrays of standard-sized single plants. The largest single plant in operation is the 370 MW Ivanpah Solar. In 2012, the BLM made available 97,921,069 acres (39,627,251 hectares) of land in the southwestern United States for solar projects, enough for between 10,000 and 20,000 GW.[83] ### Suitable sites Deserts at high altitude located in tropics is more suitable for CSP where the normal direct irradiance is higher. Abandoned opencast mines, moderate hill slopes and crater depressions may be advantageous in the case of power tower CSP as the power tower can be located on the ground integral with the molten salt storage tank.[84][85] ## Effect on wildlife Dead warbler burned in mid-air by solar thermal power plant Insects can be attracted to the bright light caused by concentrated solar technology, and as a result birds that hunt them can be killed (burned) if the birds fly near the point where light is being focused. This can also affect raptors who hunt the birds.[86][87][88][89] Federal wildlife officials have begun calling these power towers "mega traps" for wildlife.[90][91][92] According to rigorous reporting, in over six months, actually only 133 singed birds were counted.[93] By focusing no more than four mirrors on any one place in the air during standby, at Crescent Dunes Solar Energy Project, in three months, the death rate dropped to zero.[94] ## See also ## References 1. ^ Boerema, Nicholas; Morrison, Graham; Taylor, Robert; Rosengarten, Gary (2013-11-01). "High temperature solar thermal central-receiver billboard design". Solar Energy. 97: 356–368. doi:10.1016/j.solener.2013.09.008. 2. ^ Law, Edward W.; Prasad, Abhnil A.; Kay, Merlinde; Taylor, Robert A. (2014-10-01). "Direct normal irradiance forecasting and its application to concentrated solar thermal output forecasting – A review". 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