Concentrated solar power

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Concentrated solar power
The PS10 Solar Power Plant concentrates sunlight from a field of heliostats onto a central solar power tower.

Concentrated solar power (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.

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.[1]

CSP growth is expected to continue at a fast pace. As of April 2011, another 946 MW of capacity was under construction in Spain with total new capacity of 1,789 MW expected to be in operation by the end of 2013. A further 1.5 GW of parabolic-trough and power-tower plants were under construction in the US, and contracts signed for at least another 6.2 GW. 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 percent of CSP plants.[1]

History

Concentrated sunlight has been used[citation needed] 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 (Sicily). 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.[2]

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.[3][4][5][6] 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.[7]

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 degrees Celsius.[8] 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.

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, dish Stirlings, concentrating linear Fresnel reflector, and solar power tower.[9] Although simple, these solar concentrators are quite far from the theoretical maximum concentration.[10][11] 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.[12]

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[13]) 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.[14] 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.[15]

Fresnel reflectors

Liddell Power Station's Compact Linear Fresnel reflectors are not as efficient as parabolic mirrors but are much cheaper.

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.[16][17]

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 (482–1292 °F)) and then used by a Stirling engine to generate power.[14] 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.

Solar power tower

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 (773–1273 K (932–1832 °F)) and then used as a heat source for a power generation or energy storage system.[14] 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. eSolar's 5 MW Sierra SunTower located in Lancaster, California and is the only CSP tower facility operating in North America.

Efficiency

For thermodynamic solar systems, the maximum solar-to-work (ex: electricity) efficiency η can be deduced by considering both thermal radiation properties and Carnot's principle.[18] Indeed, solar irradiation must first be converted into heat via a solar receiver with an efficiency ηReceiver; then this heat is converted into work with Carnot efficiency ηCarnot. 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) :

with $\eta_{Carnot} = 1 - \frac{T^0}{T_H}$
and $\eta_{Receiver} = \frac{Q_{absorbed}-Q_{lost}}{Q_{solar}}$
where Qsolar, Qabsorbed, Qlost 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 ηOptics on the system solar receiver with a collecting area A and an absorptivity α:

Qsolar = ηOpticsICA,
Qabsorbed = αQsolar,

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:

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

Simplifying these equations by considering perfect optics (ηOptics = 1), collecting and reradiating areas equal and maximum absorptivity and emissivity (α = 1, $\epsilon$ = 1) then substituting in the first equation gives

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

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: $T_{max} = ({\frac {IC}{\sigma}})^{0.25}$

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

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

Consequently, this lead us to the following equation:

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

Solving numerically this equation allows 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,[19] 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/kwh.[19] To put this in perspective, Arizona Public Service (APS), Arizona‘s largest utility company, purchases power from the Palo Verde Nuclear Generating Station at a cost of$0.0165/kwh[citation needed]. Nonetheless, 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.[20]

Future of CSP

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.[21] 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. 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.[21] 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.[22] Algeria has the biggest area of desert, and private Algerian firm Cevital has signed up for Desertec.[22] 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.[23] 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.[24]

On September 9, 2009; 2 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.[19]

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. [25]

References

1. ^ a b Janet L. Sawin and Eric Martinot (29 September 2011). "Renewables Bounced Back in 2010, Finds REN21 Global Report". Renewable Energy World.
2. ^ Archimedes through the Looking Glass, Thomas W. Africa, February 1975, Classical Association of the Atlantic States, p. 305
3. ^ Butti and Perlin (1981), p.60–100
4. ^ From troughs to triumph: SEGS and gas
5. ^ Encyclopedia of Earth, Shuman, Frank
6. ^ Cabinet Magazine, The Beautiful Possibility
7. ^ "A New Invention To Harness The Sun" Popular Science, November 1929
8. ^ A Golden Thread: 2500 Years of Solar Architecture and Technology, Butti and Perlin (1981), p.68
9. ^ Types of solar thermal CSP plants
10. ^ Julio Chaves, Introduction to Nonimaging Optics, CRC Press, 2008 [ISBN 978-1420054293]
11. ^ Roland Winston et al.,, Nonimaging Optics, Academic Press, 2004 [ISBN 978-0127597515]
12. ^ New innovations in solar thermal
13. ^ Molten salt as CSP plant working fluid
14. ^ a b c Martin and Goswami (2005), p.45
15. ^ "Linear-focusing Concentrator Facilities: DCS, DISS, EUROTROUGH and LS3". Plataforma Solar de Almería. Archived from the original on 28 September 2007. Retrieved 2007-09-29.
16. ^ Compact CLFR
17. ^ Ausra compact CLFR introducing cost-saving solar rotation features
18. ^ Fletcher, E.A., 2001. Solarthermal processing: A review. ASME Journal of Solar Energy Engineering 123(2):63-74.
19. ^ a b c Poornima Gupta; Laura Isensee (Fri Sep 11, 2009 3:51pm EDT). "Google Plans New Mirror For Cheaper Solar Power". In Carol Bishopric. Global Climate and Alternative Energy Summit. San Francisco: Reuters & businessworld.in. Archived from the original on 14 September 2009. Retrieved 2010-03-21.
20. ^ Robert Glennon and Andrew M. Reeves, Solar Energy's Cloudy Future, 1 Ariz. J. Evtl. L. & Pol'y, 91, 106 (2010) available at http://ajelp.com/documents/GlennonFinal.pdf
21. ^ a b Concentrated solar power could generate 'quarter of world's energy' Guardian
22. ^ a b Europe's Saharan power plan: miracle or mirage? Reuters
23. ^ CSP and photovoltaic solar power Reuters
24. ^ Concentrating Solar Power desertec-australia
25. ^ Video: Concentrating photovoltaics inspired by telescope design SPIE Newsroom

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