Concentrated solar power

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.

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 some 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 market saw about 740 megawatt (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.[7] The Middle East is also ramping up their plans to install CSP based projects. Shams-I has been installed in Abu Dhabi, by Masdar.[8] The largest CSP projects in the world is Ivanpah Solar Power Facility in the United States (which uses solar power tower technology) and Mojave Solar Project (which uses parabolic troughs).

As of January 2014, 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. In Italy, a handful of companies are trying to get authorization for 14 plants, totalling 392 MW, despite a strong local and political opposition.[9] The global market has been dominated by parabolic-trough plants, which account for 90% of CSP plants.[7]

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 of the panels and much smaller operating costs.[10] However, the Chile Copiapó reached ¢6.3/kWh.[11] In 2015, CSP represented less than 2% of worldwide installed capacity of solar electricity plants.

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

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.[12]

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.[13][14][15][16] 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.[17]

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.[18] 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 reaches full power.

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.[19] Although simple, these solar concentrators are quite far from the theoretical maximum concentration.[20][21] 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.[20][21][22]

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.[23]

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[24]) 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.[25] 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.[26]

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.[27] 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.[28] 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.[27]

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.[29]

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 sea water. 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.[25] 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.[30] 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. [31][32][33]

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.[34]

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.[35][36]

Dish Stirling

A 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.[25] 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.[37] 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.[38] 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.

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[39]

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 has increased nearly tenfold since 2004 and grew at an average of 50 percent per year during the last five years.[40]: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. There is a notable trend towards developing countries and regions with high solar radiation.

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.[41][42] In addition, a novel solar CPV/CSP hybrid system has been proposed recently.[43]

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[39][44]:146 [40]:51 · CSP-world.com[45] · IRENA[46]

Efficiency

The conversion efficiency 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 and subsequently the heat is converted into work by the heat engine with the efficiency , using Carnot's principle.[47][48] For a solar receiver providing a heat source at temperature and a heat sink at room temperature , the overall conversion efficiency can be calculated as follows:

with
and
where , , are respectively the incoming solar flux and the fluxes absorbed and lost by the system solar receiver.

For a solar flux (e.g. ) concentrated times with an efficiency on the system solar receiver with a collecting area and an absorptivity :

,
,

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 applying the Stefan-Boltzmann law yields:

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

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:

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

Consequently, this leads us to the following equation:

Solving this equation numerically allows us to obtain the optimum process temperature according to the solar concentration ratio (red curve on the figure below)

C 500 1000 5000 10000 45000 (max. for Earth)
Tmax 1720 2050 3060 3640 5300
Topt 970 1100 1500 1720 2310

Theoretical efficiencies aside, real-world experience of CSP reveals a 25%–60% shortfall in projected production. 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 once thought to be economically superior to photovoltaics, up to 2015 it has proven not to be the case. Recent PV commercial power is selling for  1⁄3 recent CSP contracts.[49][50]

Costs

As of 9 September 2009, the cost of building a CSP station was typically about US$2.50 to US$4 per watt,[51] 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.[51] 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.[52] 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.[53][54] 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.[55]

At the 2017 Chile auction, SolarReserve bid $63/MWh (¢6.3/kWh) for 24-hour CSP power with no subsidies, competing with other types such as LNG gas turbines.[11]

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:

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).[56]

Since 27 January 2012, Spain has halted acceptance of new projects for the feed-in-tariff.[57][58] Projects currently accepted are not affected, except that a 6% tax on feed-in-tariffs has been adopted, effectively reducing the feed-in-tariff.[59]

Australia

At the federal level, under the Large-scale Renewable Energy Target (LRET), in operation under the Renewable Energy Electricity Act 2000, 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.[60] 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.[61] 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:

Year Annual
Investment 		Cumulative
Capacity
2015 €21 billion 4,755 MW
2050 €174 billion 1,500,000 MW

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.[61]

The EU looked into developing a €400 billion (US$774 billion) network of solar power plants based in the Sahara region using CSP technology to be known as Desertec, to create "a new carbon-free network linking Europe, the Middle East and North Africa". The plan was backed mainly by German industrialists and predicted production of 15% of Europe's power by 2050. Morocco was a major partner in Desertec and as it has barely 1% of the electricity consumption of the EU, it could produce more than enough energy for the entire country with a large energy surplus to deliver to Europe.[62] Algeria has the biggest area of desert, and private Algerian firm Cevital signed up for Desertec.[62] 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. Most of the participants pulled out of the effort at the end of 2014.

Other organizations had predicted CSP to cost $0.06(US)/kWh by 2015 due to efficiency improvements and mass production of equipment.[63] 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.[64]

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.[65]

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 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[66] (in 2003). The gigawatt size projects are arrays of 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.[67]

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.[68][69][70][71] Federal wildlife officials have begun calling these power towers "mega traps" for wildlife.[72][73][74]

According to rigorous reporting, in over six months, actually only 133 singed birds were counted.[75] 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.[76]

See also

References

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  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". Solar Energy. 108: 287–307. doi:10.1016/j.solener.2014.07.008.
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  55. ↑ Renewable Energy Cost Analysis – Concentrating Solar Power. irena.org
  56. ↑ Feed-in tariff (Régimen Especial). res-legal.de (12 December 2011).
  57. ↑ Spanish government halts PV, CSP feed-in tariffs Archived 5 August 2012 at the Wayback Machine.. Solarserver.com (30 January 2012). Retrieved on 22 April 2013.
  58. ↑ Spain Halts Feed-in-Tariffs for Renewable Energy. Instituteforenergyresearch.org (9 April 2012). Retrieved on 22 April 2013.
  59. ↑ Spain introduces 6% energy tax. Evwind.es (14 September 2012). Retrieved on 22 April 2013.
  60. ↑ A Dangerous Obsession with Least Cost? Climate Change, Renewable Energy Law and Emissions Trading Prest, J. (2009) in Climate Change Law: Comparative, Contractual and Regulatory Considerations, W. Gumley & T. Daya-Winterbottom (eds.) Lawbook Company, ISBN 0455226342
  61. 1 2 Concentrated solar power could generate 'quarter of world's energy' Guardian
  62. 1 2 Tom Pfeiffer (23 August 2009) Europe's Saharan power plan: miracle or mirage? Reuters
  63. ↑ CSP and photovoltaic solar power, Reuters (23 August 2009).
  64. ↑ "Video: Concentrating photovoltaics inspired by telescope design". SPIE Newsroom. 2011. doi:10.1117/2.3201107.02.
  65. ↑ Cassandra Sweet (2015-06-13). "High-Tech Solar Projects Fail to Deliver". WSJ.
  66. ↑ A Study of Very Large Solar Desert Systems with the Requirements and Benefits to those Nations Having High Solar Irradiation Potential. geni.org.
  67. ↑ Solar Resource Data and Maps. Solareis.anl.gov. Retrieved on 22 April 2013.
  68. ↑ John Roach. "Burned Birds Become New Environmental Victims of the Energy Quest". NBC News.
  69. ↑ Michael Howard (2014-08-20). "Solar Thermal Plants Have a PR Problem, And That PR Problem Is Dead Birds Catching on Fire". Esquire.
  70. ↑ "Emerging solar plants scorch birds in mid-air". Fox News.
  71. ↑ http://bigstory.ap.org/article/emerging-solar-plants-scorch-birds-mid-air
  72. ↑ "How a Solar Farm Set Hundreds of Birds Ablaze". Nature World News.
  73. ↑ "Ivanpah Solar Power Tower Is Burning Birds".
  74. ↑ http://www.kcet.org/news/redefine/rewire/Avian-mortality%20Report%20FINALclean.pdf
  75. ↑ "For the Birds: How Speculation Trumped Fact at Ivanpah". RenewableEnergyWorld.com. Retrieved 2015-05-04.
  76. ↑ "One Weird Trick Prevents Bird Deaths At Solar Towers". CleanTechnica.com. Retrieved 4 May 2015.

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