# Concentrated solar power

The PS10 Solar Power Plant concentrates sunlight from a field of heliostats onto a central solar power tower.
Part of the 354 MW SEGS solar complex in northern San Bernardino County, California.

Concentrated solar power (also called concentrating solar power, concentrated solar thermal, and CSP) systems use mirrors or lenses 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 or powers a thermochemical reaction (experimental as of 2013).[1][2][3]

CSP is being widely commercialized and the CSP market has seen about 740 MW of generating capacity added between 2007 and the end of 2010. More than half of this (about 478 MW) was installed during 2010, bringing the global total to 1095 MW. Spain added 400 MW in 2010, taking the global lead with a total of 632 MW, while the US ended the year with 509 MW after adding 78 MW, including two fossil–CSP hybrid plants.[4] The Middle East is also ramping up their plans to install CSP based projects and as a part of that Plan, Shams-I the largest CSP Project in the world has been installed in Abu Dhabi, by Masdar.[5]

CSP growth is expected to continue at a fast pace. As of January 2014, Spain had a total capacity of 2,204 MW making this country the world leader in CSP. 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 account for 90% of CSP plants.[4]

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

## History

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.[6]

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 Maadi, Egypt for irrigation.[7][8][9][10] 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.[11]

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.[12] 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, and will remain so until the 390 MW Ivanpah power tower project comes online.

## 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 common forms, namely parabolic trough, enclosed trough, dish Stirlings, concentrating linear Fresnel reflector, and solar power tower.[13] Although simple, these solar concentrators are quite far from the theoretical maximum concentration.[14][15] For example, the parabolic-trough concentration gives about 1/3 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.

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.[16]

### Parabolic trough

Main article: Parabolic trough

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[17]) is heated to 150–350 °C (423–623 K (302–662 °F)) as it flows through the receiver and is then used as a heat source for a power generation system.[18] 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, alongside with Plataforma Solar de Almería's SSPS-DCS test facilities in Spain.[19]

#### Enclosed trough

Enclosed trough systems are used to produce process heat. 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.[20] 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.[21] Water is carried throughout the length of the pipe, which is boiled to generate steam when intense sun 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.[20]

### 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 as 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.[22][23]

### Dish Stirling

Main article: 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 (523–973 K (482–1,292 °F)) and then used by a Stirling engine to generate power.[18] 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. As of 2008, The world record for solar to electric efficiency was set at 31.25% by SES dishes at the National Solar Thermal Test Facility (NSTTF)[24] . 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 power tower

Main article: 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 sea water. The working fluid in the receiver is heated to 500–1000 °C (773–1,273 K (932–1,832 °F)) and then used as a heat source for a power generation or energy storage system.[18] 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. eSolar's 5 MW Sierra SunTower, located in Lancaster, California, is the only CSP tower facility operating in North America. The National Solar Thermal Test Facility, NSTTF located in Albuquerque, NM, is an experimental solar thermal test facility with a heliostat field capable of producing 6 MW.

## Deployment around the world

The commercial deployment of CSP plants started by 1984 in the US with the SEGS plants until 1990 when the last SEGS plant was completed. From 1991 to 2005 no CSP plants were built anywhere in the world.

Concentrated Solar Power (MWp)
Year 1984 1985 1989 1990 ... 2006 2007 2008 2009 2010 2011 2012
Installed 14 60 200 80 0 1 74 55 178.50 306.50 628.5 802.5
Cumulative 14 74 274 354 354 355 429 484 662.5 969 1597.5 2553

Source:[25][26]

## Efficiency

total spectrum solar concentrator

To optimize the efficiency of solar power, a total spectrum solar concentrator is used. The suns rays are first concentrated using parabolic mirrors or a fresnel lens, and then spread out into a light spectrum via a prism. Each different part of the spectrum is aimed at a solar cell manufactured to operate most efficiently in that wavelength range. This technology has multiple problems, most notably in that it results in only marginal gains in efficiency[citation needed].

For thermodynamic solar systems, the maximum solar-to-work (ex: electricity) efficiency $\eta$ can be deduced by considering both thermal radiation properties and Carnot's principle.,[27][28] Indeed, solar irradiation must first be converted into heat via a solar receiver with an efficiency $\eta_{Receiver}$; then this heat is converted into work with Carnot efficiency $\eta_{Carnot}$. Hence, for a solar receiver providing a heat source at temperature TH and a heat sink at temperature T° (e.g.: atmosphere at T° = 300 K) :

$\eta = \eta_\mathrm{Receiver} \cdot \eta_\mathrm{Carnot}$
with $\eta_\mathrm{Carnot} = 1 - \frac{T^0}{T_H}$
and $\eta_\mathrm{Receiver} = \frac{Q_\mathrm{absorbed}-Q_\mathrm{lost}}{Q_\mathrm{solar}}$
where $Q_\mathrm{solar}$, $Q_\mathrm{absorbed}$, $Q_\mathrm{lost}$ are respectively the incoming solar flux and the fluxes absorbed and lost by the system solar receiver.

For a solar flux I (e.g. I = 1000 W/m2) concentrated C times with an efficiency $\eta_{Optics}$ on the system solar receiver with a collecting area A and an absorptivity $\alpha$:

$Q_\mathrm{solar} = \eta_\mathrm{Optics} I C A$,
$Q_\mathrm{absorbed} = \alpha Q_\mathrm{solar}$,

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 $\epsilon$ applying the Stefan-Boltzmann law yields:

$Q_\mathrm{lost} = A \epsilon \sigma T_H^4$

Simplifying these equations by considering perfect optics ($\eta_\mathrm{Optics}$ = 1), collecting and reradiating areas equal and maximum absorptivity and emissivity ($\alpha$ = 1, $\epsilon$ = 1) then substituting in the first equation gives

$\eta = \left(1 - \frac {\sigma T_H^4 }{IC} \right) \cdot \left( 1 - \frac{T^0}{T_H} \right)$

One sees that efficiency does not simply increase monotonically with the receiver temperature. Indeed, the higher the temperature, the higher the Carnot efficiency, but also the lower the receiver efficiency. Hence, the maximum reachable temperature (i.e.: when the receiver efficiency is null, blue curve on the figure below) is: $T_\mathrm{max} = \left({\frac {IC}{\sigma}} \right)^{0.25}$

There is a temperature Topt for which the efficiency is maximum, i.e. when the efficiency derivative relative to the receiver temperature is null:

$\frac{d\eta}{dT_H}(T_\mathrm{opt}) = 0$

Consequently, this leads us to the following equation:

$T_{opt}^5-(0.75T^0)T_\mathrm{opt}^4-\frac{T^0IC}{4\sigma} = 0$

Solving this equation numerically allows us to obtain the optimum process temperature according to the solar concentration ratio 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

## Costs

As of 9 September 2009, the cost of building a CSP station was typically about US$2.50 to$4 per watt,[29] while the fuel (the sun's radiation) is free. Thus a 250 MW CSP station would have cost $600–1000 million to build. That works out to$0.12 to 0.18 USD/kWh.[29] New CSP stations may be economically competitive with fossil fuels. Nathaniel Bullard, a solar analyst at Bloomberg New Energy Finance, has calculated that the cost of electricity at the Ivanpah Solar Power Facility, a project under construction in Southern California, will be lower than that from photovoltaic power and about the same as that from natural gas.[30] However, in November 2011, Google announced that they would not invest further in CSP projects due to the rapid price decline of photovoltaics. Google invested US$168 million on BrightSource.[31][32] 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. As of March 2012, there were 1.9 GW of CSP installed, with 1.8 GW of that being parabolic trough.[33] ## Incentives ### Spain Solar-thermal electricity generation is eligible for feed-in tariff payments (art. 2 RD 661/2007), if the system capacity does not exceed the following limits: Systems registered in the register of systems prior to 29 September 2008: 500 MW for solar-thermal systems. Systems registered after 29 September 2008 (PV only). The capacity limits for the different system types are re-defined during the review of the application conditions every quarter (art. 5 RD 1578/2008, Annex III RD 1578/2008). Prior to the end of an application period, the market caps specified for each system type are published on the website of the Ministry of Industry, Tourism and Trade (art. 5 RD 1578/2008).[34] Since 27 January 2012, Spain has halted acceptance of new projects for the feed-in-tariff.[35][36] Projects currently accepted are not affected, except that a 6% tax on feed-in-tariffs has been adopted, effectively reducing the feed-in-tariff.[37] ### Australia At the federal level, under the Large-scale Renewable Energy Target (LRET), in operation under the Renewable Energy Electricity Act 2000 (Cth), large scale solar thermal electricity generation from accredited RET power stations may be entitled to create large-scale generation certificates (LGCs). These certificates can then be sold and transferred to liable entities (usually electricity retailers) to meet their obligations under this tradeable certificates scheme. However as this legislation is technology neutral in its operation, it tends to favour more established RE technologies with a lower levelised cost of generation, such as large scale onshore wind, rather than solar thermal and CSP.[38] At State level, renewable energy feed-in laws typically are capped by maximum generation capacity in kWp, and are open only to micro or medium scale generation and in a number of instances are only open to solar PV (photovoltaic) generation. This means that larger scale CSP projects would not be eligible for payment for feed-in incentives in many of the State and Territory jurisdictions. ## Future 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. The increase in investment would be from 2 billion euros worldwide to 92.5 billion euros in that time period.[39] Spain is the leader in concentrated solar power technology, with more than 50 government-approved projects in the works. Also, it exports its technology, further increasing the technology's stake in energy worldwide. Because the technology works best with areas of high insolation (solar radiation), experts predict the biggest growth in places like Africa, Mexico, and the southwest United States. It indicates that the thermal storage systems based in nitrates (calcium, potassium, sodium,...) will make the CSP plants more and more profitable. 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 Annual Investment Cumulative 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 the current range of €0.23–0.15/kwh to €0.14–0.10/kwh.[39] Recently the EU has begun to look into developing a €400 billion ($774 billion) network of solar power plants based in the Sahara region using CSP technology known as Desertec, 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.[40]

Algeria has the biggest area of desert, and private Algerian firm Cevital has signed up for Desertec.[40] With its wide desert (the highest CSP potential in the Mediterranean and Middle East regions ~ about 170 TWh/year) and its strategic geographical location near Europe Algeria is one of the key countries to ensure the success of Desertec project. Moreover, with the abundant natural-gas reserve in the Algerian desert, this will strengthen the technical potential of Algeria in acquiring Solar-Gas Hybrid Power Plants for 24-hour electricity generation.

Other organizations expect CSP to cost $0.06(US)/kWh by 2015 due to efficiency improvements and mass production of equipment.[41] 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. On 9 September 2009; 4 years ago, Bill Weihl, Google.org's green-energy spokesperson 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.[29]

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.[42]

## Very large scale solar power plants

There are several proposals for gigawatt size, very large scale solar power plants. They include the Euro-Mediterranean Desertec proposal, Project Helios in Greece (10 gigawatt), and Ordos (2 gigawatt) in China. 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[43] (in 2003). The gigawatt size projects are arrays of single plants. The largest single plant in operation is 80 MW (SEGS VIII and SEGS IX) and the largest single plant in construction is 370 MW (Ivanpah Solar). In 2012, the BLM made available 97,921,069 acres of land in the southwestern United States for solar projects, enough for between 10,000 and 20,000 gigawatts (GW).[44]

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