Energía solar concentrada

No confundir con concentradores fotovoltaicos .

Una torre de energía solar que concentra la luz a través de 10,000 heliostatos espejados que abarcan 13 millones de pies cuadrados (1.21 km ² ).

Las tres torres de la planta de energía solar de Ivanpah

Parte del complejo solar SEGS de 354 MW en el norte del condado de San Bernardino, California

Vista de pájaro de Khi Solar One , Sudáfrica

Los sistemas de energía solar concentrada ( CSP , también conocida como energía solar de concentración , energía solar térmica concentrada ) generan energía solar mediante el uso de espejos o lentes para concentrar una gran área de luz solar en un receptor. ^{[1] La} electricidad se genera cuando la luz concentrada se convierte en calor ( energía solar térmica ), que impulsa un motor térmico (generalmente una turbina de vapor ) conectado a un generador de energía eléctrica ^{[2] [3] [4]} o alimenta un termoquímico reacción. ^{[5] [6] [7]}

La CSP tenía una capacidad instalada total global de 5.500  MW en 2018, frente a los 354 MW de 2005. España representó casi la mitad de la capacidad mundial, con 2.300 MW, a pesar de que desde 2013 no ha entrado en operación comercial nueva capacidad en el país ^[8]. Le sigue Estados Unidos con 1.740 MW. El interés también es notable en el norte de África y Oriente Medio, así como en India y China. El mercado mundial estuvo inicialmente dominado por plantas de cilindro parabólico, que representaron el 90% de las plantas de CSP en un momento. ^[9] Desde aproximadamente 2010, la CSP de torre central de energía se ha favorecido en las nuevas plantas debido a su operación a temperaturas más altas, hasta 565 ° C (1049 ° F) frente al máximo de 400 ° C (752 ° F) de la depresión, lo que promete mayor eficiencia.

Entre los proyectos de CSP más grandes se encuentran la planta de energía solar de Ivanpah (392 MW) en los Estados Unidos, que utiliza tecnología de torre de energía solar sin almacenamiento de energía térmica, y la estación de energía solar de Ouarzazate en Marruecos, ^[10] que combina tecnologías de canal y torre para un total de 510 MW con varias horas de almacenamiento de energía.

Como central generadora de energía térmica, la CSP tiene más en común con las centrales térmicas como las de carbón, gas o geotermia. Una planta termosolar puede incorporar almacenamiento de energía térmica , que almacena energía ya sea en forma de calor sensible o como calor latente (por ejemplo, utilizando sal fundida ), lo que permite que estas plantas sigan generando electricidad siempre que sea necesario, de día o de noche. Esto hace que la CSP sea una forma de energía solar distribuible . La energía renovable distribuible es particularmente valiosa en lugares donde ya existe una alta penetración de energía fotovoltaica (PV), como California ^[11] porque la demanda de energía eléctrica alcanza su punto máximo cerca de la puesta del sol justo cuando la capacidad fotovoltaica disminuye (un fenómeno conocido comocurva de pato ). ^[12]

La CSP a menudo se compara con la energía solar fotovoltaica (PV), ya que ambos utilizan energía solar. Si bien la energía solar fotovoltaica experimentó un gran crecimiento en los últimos años debido a la caída de los precios, ^{[13] [14]} El crecimiento de la CSP solar ha sido lento debido a dificultades técnicas y altos precios. En 2017, la CSP representó menos del 2% de la capacidad instalada mundial de plantas de energía solar. ^[15] Sin embargo, la CSP puede almacenar energía más fácilmente durante la noche, lo que la hace más competitiva con los generadores despachables y las plantas de carga base. ^{[16] [17] [18] [19]}

El proyecto DEWA en Dubai, en construcción en 2019, mantuvo el récord mundial de precio más bajo de CSP en 2017 a $ 73 por MWh ^[20] para su proyecto combinado de canal y torre de 700 MW: 600 MW de canal, 100 MW de torre con 15 horas de almacenamiento de energía térmica diaria. La tarifa base de CSP en la región extremadamente seca de Atacama en Chile alcanzó menos de ¢ 5,0 / kWh en las subastas de 2017. ^{[21] [22]}

Historia

Motor de vapor solar para bombeo de agua, cerca de Los Ángeles, circa 1901

Una leyenda dice que Arquímedes usó un "vidrio ardiente" para concentrar la luz del sol en la flota romana invasora y repelerlos de Siracusa . En 1973, un científico griego, el Dr. Ioannis Sakkas, que tenía curiosidad por saber si Arquímedes realmente podría haber destruido la flota romana en el 212 a. C., alineó a casi 60 marineros griegos, cada uno sosteniendo un espejo alargado con la punta para captar los rayos del sol y dirigirlos hacia un alquitrán. silueta de madera contrachapada cubierta a 49 m (160 pies) de distancia. El barco se incendió después de unos minutos; sin embargo, los historiadores continúan dudando de la historia de Arquímedes. ^[23]

En 1866, Auguste Mouchout utilizó un cilindro parabólico para producir vapor para la primera máquina de vapor solar. La primera patente para un colector solar fue obtenida por el italiano Alessandro Battaglia en Génova, Italia, en 1886. Durante los años siguientes, inventores como John Ericsson y Frank Shuman desarrollaron dispositivos de energía solar de concentración para riego, refrigeración y locomoción. En 1913, Shuman terminó una estación de energía solar térmica parabólica de 55 caballos de fuerza (41 kW) en Maadi, Egipto para riego. ^{[24] [25] [26] [27]} El primer sistema de energía solar que utiliza un plato de espejo fue construido por el Dr. RH Goddard, quien ya era bien conocido por su investigación sobre cohetes de combustible líquido y escribió un artículo en 1929 en el que afirmaba que se habían abordado todos los obstáculos anteriores. ^[28]

El profesor Giovanni Francia (1911-1980) diseñó y construyó la primera planta de energía solar concentrada, que entró en funcionamiento en Sant'Ilario, cerca de Génova, Italia en 1968. Esta planta tenía la arquitectura de las plantas de torre de energía actuales con un receptor solar en el centro de un campo de colectores solares. La planta pudo producir 1 MW con vapor sobrecalentado a 100 bar y 500 ° C. ^[29] La torre de energía Solar One de 10 MW se desarrolló en el sur de California en 1981. Solar One se convirtió en Solar Twoen 1995, se implementó un nuevo diseño con una mezcla de sales fundidas (60% de nitrato de sodio, 40% de nitrato de potasio) como fluido de trabajo receptor y como medio de almacenamiento. El método de sales fundidas demostró ser eficaz, y Solar Two operó con éxito hasta que fue dado de baja en 1999. ^[30] La tecnología de colectores cilindro-parabólicos de los cercanos Sistemas de Generación de Energía Solar (SEGS), iniciada en 1984, era más viable. La SEGS de 354 MW fue la planta de energía solar más grande del mundo, hasta 2014.

No se construyó ningún solar concentrado comercial desde 1990, cuando se completó SEGS, hasta 2006, cuando se construyó el sistema de reflector Fresnel lineal compacto en la central eléctrica de Liddell en Australia. Se construyeron pocas otras plantas con este diseño, aunque la Planta de Energía Solar Térmica Kimberlina de 5 MW se inauguró en 2009.

En 2007, se construyó Nevada Solar One de 75 MW, un diseño de canal y la primera gran planta desde SEGS. Entre 2009 y 2013, España construyó más de 40 sistemas cilindroparabólicos, estandarizados en bloques de 50 MW.

Debido al éxito de Solar Two, en 2011 se construyó en España una central eléctrica comercial, denominada Solar Tres Power Tower , que más tarde pasó a denominarse Planta Termosolar Gemasolar. Los resultados de Gemasolar allanaron el camino para otras plantas de este tipo. La instalación de energía solar de Ivanpah se construyó al mismo tiempo, pero sin almacenamiento térmico, utilizando gas natural para precalentar el agua cada mañana.

La mayoría de las plantas de energía solar concentrada utilizan el diseño de canal parabólico, en lugar de la torre de energía o los sistemas Fresnel. También ha habido variaciones de sistemas de colectores cilindro-parabólicos como el ciclo combinado solar integrado (ISCC) que combina colectores y sistemas convencionales de calor de combustibles fósiles.

La CSP se trató originalmente como un competidor de la energía fotovoltaica, e Ivanpah se construyó sin almacenamiento de energía, aunque Solar Two había incluido varias horas de almacenamiento térmico. En 2015, los precios de las plantas fotovoltaicas habían caído y potencia comercial PV se vendía a 1 / 3 de los contratos de CSP recientes. ^{[31] [32]} Sin embargo, cada vez más, la CSP se licitaba con entre 3 y 12 horas de almacenamiento de energía térmica, lo que hacía que la CSP fuera una forma de energía solar despachable. ^[33] Como tal, se considera cada vez más que compite con el gas natural y la energía fotovoltaica con baterías por energía flexible y distribuible.

Tecnología actual

La CSP se utiliza para producir electricidad (a veces llamada termoelectricidad solar, generalmente generada a través de vapor ). Los sistemas de tecnología solar concentrada utilizan espejos o lentes con sistemas de seguimiento para enfocar un área grande de luz solar en un área pequeña. La luz concentrada se utiliza luego como calor o como fuente de calor para una central eléctrica convencional (termoelectricidad solar). Los concentradores solares utilizados en los sistemas CSP a menudo también se pueden utilizar para proporcionar calefacción o refrigeración de procesos industriales, como en el aire acondicionado solar .

Existen tecnologías de concentración en cuatro tipos ópticos, a saber, colector cilindro parabólico , plato , reflector de Fresnel lineal de concentración y torre de energía solar . ^[34] Los reflectores de Fresnel lineales de concentración y de colector cilindro parabólico se clasifican como tipos de colectores de enfoque lineal, mientras que los de plato y torre solar son tipos de enfoque puntual. Los colectores de enfoque lineal logran factores de concentración medios (50 soles y más) y los colectores de enfoque puntual logran factores de concentración altos (más de 500 soles). Aunque simples, estos concentradores solares están bastante lejos de la concentración máxima teórica. ^{[35] [36]} Por ejemplo, la concentración de colectores cilindro-parabólicos da aproximadamente 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.^[35][36][37]

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.^[38][39]

Parabolic trough

Parabolic trough at a plant near Harper Lake, California

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 at the longitudinal focal line 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^[40]) 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.^[41] 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.^[42]

Enclosed trough

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.^[43] 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.^[44] 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.^[43]

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 290 kWh (1,000,000 BTU) in sunny regions, compared to between $10 and $12 for other conventional solar thermal technologies.^[45]

Solar power tower

Main article: Solar power tower

Ashalim Power Station, Israel, on its completion the tallest solar tower in the world. It concentrates light from over 50,000 heliostats.

The PS10 solar power plant in Andalusia, 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 heat-transfer fluid, which can consist of water-steam or 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.^[41] 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. Beam down tower application is also feasible with heliostats to heat the working fluid.^[46]

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.^[47] 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.^[48][49][50]

Fresnel reflectors

Main article: Compact linear Fresnel reflector

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.^[51][52]

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.^[53]

Dish Stirling

A dish Stirling

See also: Solar_thermal_energy § Dish_designs

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.^[41] 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 31 January 2008, a cold, bright day.^[54] 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.^[55] 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

Main article: 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.^[56]

CSP with thermal energy storage

See also: Thermal energy storage and Solar thermal energy

In a CSP plant that includes storage, the solar energy is first used to heat the molten salt or synthetic oil which is stored providing thermal/heat energy at high temperature in insulated tanks.^[57][58] Later the hot molten salt (or oil) is used in a steam generator to produce steam to generate electricity by steam turbo generator as per requirement.^[59] 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 or solar peaker plant.^[60][61] 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, but without the pollution.^[62] CSP with thermal energy storage plants can also be used as cogeneration plants to supply both electricity and process steam round the clock. As of December 2018, CSP with thermal energy storage plants generation cost have ranged between 5 c € / kWh and 7 c € / kWh depending on good to medium solar radiation received at a location.^[63] Unlike solar PV plants, CSP with thermal energy storage plants can also be used economically round the clock to produce only process steam replacing pollution emitting fossil fuels. CSP plant can also be integrated with solar PV for better synergy.^[64][65][66]

CSP with thermal storage systems are also available using Brayton cycle with air instead of steam for generating electricity and/or steam round the clock. These CSP plants are equipped with gas turbine to generate electricity.^[67] These are also small in capacity (<0.4 MW) with flexibility to install in few acres area.^[67] Waste heat from the power plant can also be used for process steam generation and HVAC needs.^[68] In case land availability is not a limitation, any number of these modules can be installed up to 1000 MW with RAMS and cost advantage since the per MW cost of these units are cheaper than bigger size solar thermal stations.^[69]

Centralized district heating round the clock is also feasible with concentrated solar thermal storage plants.^[70]

Deployment around the world

Main articles: List of solar thermal power stations and Solar power by country

1,000

2,000

3,000

4,000

5,000

6,000

7,000

1984

1990

1995

2000

2005

2010

2015

Worldwide CSP capacity since 1984 in MW_p

National CSP capacities in 2018 (MW_p)

Country	Total	Added
Spain	2,300	0
United States	1,738	0
South Africa	400	100
Morocco	380	200
India	225	0
China	210	200
United Arab Emirates	100	0
Saudi Arabia	50	50
Algeria	25	0
Egypt	20	0
Australia	12	0
Thailand	5	0
Source: REN21 Global Status Report, 2017 and 2018[71][72][8]

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.^[73]: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.

Worldwide Concentrated Solar Power (MW_p)

Year	1984	1985	1989	1990	1991-2005	2006	2007	2008	2009	2010	2011	2012	2013	2014	2015	2016	2017	2018	2019
Installed	14	60	200	80	0	1	74	55	179	307	629	803	872	925	420	110	100	550	381
Cumulative	14	74	274	354	354	355	429	484	663	969	1,598	2,553	3,425	4,335	4,705	4,815	4,915	5,465	6,451[74]
Sources: REN21[71][75]:146[73] :51[72]  · CSP-world.com[76] · IRENA[77] · HeliosCSP[8]

Efficiency

The efficiency of a concentrating solar power system will depend on the technology used to convert the solar power to electrical energy, the operating temperature of the receiver and the heat rejection, thermal losses in the system, and the presence or absence of other system losses; in addition to the conversion efficiency, the optical system which concentrates the sunlight will also add additional losses.

Real-world systems claim a maximum conversion efficiency of 23-35% for "power tower" type systems, operating at temperatures from 250 to 565 °C, with the higher efficiency number assuming a combined cycle turbine. Dish Stirling systems, operating at temperatures of 550-750 °C, claim an efficiency of about 30%.^[78] Due to variation in sun incidence during the day, the average conversion efficiency achieved is not equal to these maximum efficiencies, and the net annual solar-to- electricity efficiencies are 7-20% for pilot power tower systems, and 12-25% for demonstration-scale Stirling dish systems.^[78]

Theory

The maximum conversion efficiency of any thermal to electrical energy system is given by the Carnot efficiency, which represents a theoretical limit to the efficiency that can be achieved by any system, set by the laws of thermodynamics. Real-world systems do not achieve the Carnot efficiency.

The conversion efficiency ${\displaystyle \eta }$ of the incident solar radiation into mechanical work 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 mechanical energy by the heat engine with the efficiency ${\displaystyle \eta _{mechanical}}$, using Carnot's principle.^[79][80] The mechanical energy is then converted into electrical energy by a generator. For a solar receiver with a mechanical converter (e.g., a turbine), the overall conversion efficiency can be defined as follows:

${\displaystyle \eta =\eta _{\mathrm {optics} }\cdot \eta _{\mathrm {receiver} }\cdot \eta _{\mathrm {mechanical} }\cdot \eta _{\mathrm {generator} }}$

where ${\displaystyle \eta _{\mathrm {optics} }}$ represents the fraction of incident light concentrated onto the receiver, ${\displaystyle \eta _{\mathrm {receiver} }}$ the fraction of light incident on the receiver that is converted into heat energy, ${\displaystyle \eta _{\mathrm {mechanical} }}$ the efficiency of conversion of heat energy into mechanical energy, and ${\displaystyle \eta _{\mathrm {generator} }}$ the efficiency of converting the mechanical energy into electrical power.

${\displaystyle \eta _{\mathrm {receiver} }}$ is:

${\displaystyle \eta _{\mathrm {receiver} }={\frac {Q_{\mathrm {absorbed} }-Q_{\mathrm {lost} }}{Q_{\mathrm {incident} }}}}$

with ${\displaystyle Q_{\mathrm {incident} }}$, ${\displaystyle Q_{\mathrm {absorbed} }}$, ${\displaystyle Q_{\mathrm {lost} }}$ respectively the incoming solar flux and the fluxes absorbed and lost by the system solar receiver.

The conversion efficiency ${\displaystyle \eta _{\mathrm {mechanical} }}$ is at most the Carnot efficiency, which is determined by the temperature of the receiver ${\displaystyle T_{H}}$ and the temperature of the heat rejection ("heat sink temperature") ${\displaystyle T^{0}}$,

${\displaystyle \eta _{\mathrm {Carnot} }=1-{\frac {T^{0}}{T_{H}}}}$

The real-world efficiencies of typical engines achieve 50% to at most 70% of the Carnot efficiency due to losses such as heat loss and windage in the moving parts.

Ideal case

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 ${\displaystyle \alpha }$:

${\displaystyle Q_{\mathrm {solar} }=ICA}$,

${\displaystyle Q_{\mathrm {absorbed} }=\eta _{\mathrm {optics} }\alpha Q_{\mathrm {solar} }}$,

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) and without considering the ultimate conversion step into electricity by a generator, 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 (Q_lost) 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), T_max is:${\displaystyle T_{\mathrm {max} }=\left({\frac {IC}{\sigma }}\right)^{0.25}}$

There is a temperature T_opt 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	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 good part of which is due to the practical Carnot cycle losses not included in the above analysis.

Cost and value of CSP

As of 2020, the least expensive utility-scale concentrated solar power stations in the United States and worldwide are still five times more expensive than utility-scale photovoltaic power stations, with a projected minimum price of 7 cents per kilowatt-hour for the most advanced CSP stations against record lows of 1.32 cents per kWh^[81] for utility-scale PV.^[82] This five-fold price difference has been maintained since 2018.^[83]

Even though overall deployment of CSP remains limited the levelized cost of power from commercial scale plants has decreased significantly in recent years. With a learning rate estimated at around 20% cost reduction of every doubling in capacity ^[84] the cost were approaching the upper end of the fossile fuel cost range at the beginning of the 2020s driven be support schemes in several countries, including Spain, the US, Morocco, South Africa, China, and the UAE:

In markets around the world CSP is facing a difficult situation and deployment has slowed down considerably as most of the above-mentioned markets have cancelled their support,^[85] as the technology turned out to be more expensive on a per kWH basis than solar PV and wind power. However, the value of CSP is today the combination with Thermal Energy Storage(TES) that makes the plants dispatchable and a good addition for power systems rich in fluctuating generation from PV and wind. Power from CSP with TES is expected to remain cheaper than PV with lithium batteries for storage durations above 4 hours per day ^[86] allowing, for example, cheap solar base-load that could be interesting for energy intensive processes such as smelting or hydrolysis.

Incentives and Markets

Spain

In 2008 Spain launched the first commercial scale CSP market in Europe. Until 2012, solar-thermal electricity generation was initially eligible for feed-in tariff payments (art. 2 RD 661/2007) - leading to the creation of the largest CSP fleet in the world which at 2.3 GW of installed capacity contributes about 5TWh of power to the Spanish grid every year.^[87]The initial requirements for plants in the FiT were:

• Systems registered in the register of systems prior to 29 September 2008: 50 MW for solar-thermal systems.
• Systems registered after 29 September 2008 (PV only).

The capacity limits for the different system types were 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).^[88] Because of cost concerns Spain has halted acceptance of new projects for the feed-in-tariff on 27 January 2012 ^[89][90] Already accepted projects were affected by a 6% "solar-tax" on feed-in-tariffs, effectively reducing the feed-in-tariff.^[91]

After a lost decade for CSP in Europe, Spain announced it its National Energy and Climate Plan the intention of adding 5GW of CSP capacity between 2021 and 2030.^[92] Towards this end bi-annual auctions of 200 MW of CSP capacity starting in 2021 are expected, but details are not yet know.^[93]

Australia

See also: Energy policy of Australia

So far no commercial scale CSP project has been commissioned in Australia, but several projects were suggested. In 2017 now bankrupt American CSP developer SolarReserve got awarded a PPA to realize the 150MW Aurora Solar Thermal Power Project in South Australia at a record low rate of just AUD78/MWh or close to US$0.06/kWh.^[94] Unfortunately the company failed to secure financing and the project got cancelled. Another promising application for CSP in Australia are mines that need 24/7 electricity but often have no grid connection. Vast Solar a startup company aiming to commercialize a novel modular third generation CSP design ^{[95] [96]} is looking to start construction of a 50MW combines CSP and PV facility in Mt. Isa of North-West Queensland in 2021.^[97]

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.^[98]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.

China

In 2016 China announced its intention to build a batch of 20 technologically diverse CSP demonstration projects in the context of the 13th Five-Year Plan, with the intention of building up an internationally competitive CSP industry.^[99] Since the first plants were completed in 2018, the generated electricity from the plants with thermal storage is supported with an administratively set FiT of RMB 1.5 per kWh.^[100] At the end of 2020, China operated a total of 545 MW in 12 CSP plants,^[101] seven plants (320 MW) are molten-salt towers; another two plants (150MW) use the proven Eurotrough 150 parabolic trough design,^[102] three plants (75 MW) use liner fresnel collectors. Plans to build a second batch of demonstration projects were never enacted and further technology specific support for CSP in the upcoming 14th Five-Year Plan is unknown. Current support is set for remaining projects from the demonstration batch and will run out at the end of 2021.^[103]

India

In March 2020, SECI called for 5000 MW tenders which can be combination of Solar PV, Solar thermal with storage and Coal based power (minimum 51% from renewable sources) to supply round the clock power at minimum 80% yearly availability.^[104][105]

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 worldwide to €92.5 billion in that time period.^[106]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.^[106]

The European Union 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.^[107] Algeria has the biggest area of desert, and private Algerian firm Cevital signed up for Desertec.^[107] 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.

Experience with first-of-a-kind CSP plants in the USA was mixed. Solana in Arizona, and Ivanpah in California indicate large production shortfalls in electricity generation between 25% and 40% in the first years of operation. 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.^[108] In other countries especially Spain and South Africa CSP plants have met their designed parameters ^[109]

CSP has other uses than electricity. Researchers are investigating solar thermal reactors for the production of solar fuels, making solar a fully transportable form of energy in the future. These researchers use the solar heat of CSP as a catalyst for thermochemistry to break apart molecules of H₂O, to create hydrogen (H₂) from solar energy with no carbon emissions.^[110] By splitting both H₂O and CO₂, other much-used hydrocarbons – for example, the jet fuel used to fly commercial airplanes – could also be created with solar energy rather than from fossil fuels.^[111]

Very large scale solar power plants

There have been several proposals for gigawatt size, very-large-scale solar power plants.^[112] 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^[113] (in 2003). The gigawatt size projects would have been arrays of standard-sized single plants. 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.^[114] The largest single plant in operation is the 510 MW Noor Solar Power Station. In 2022 the 700 MW CSP 4th phase of the 5GW Mohammed bin Rashid Al Maktoum Solar Park in Dubai will become the largest solar complex featuring CSP.

Suitable sites

The locations with highest direct irradiance are dry, at high altitude, and located in the tropics. These locations have a higher potential for CSP than areas with less sun.

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.^[115]

Environmental effects

CSP has a number of environmental effects, particularly on water use, land use and the use of hazardous materials.^[116]Water is generally used for cooling and to clean mirrors. Some projects are looking into various approaches to reduce the water and cleaning agents use, including the use of barriers, non-stick coatings on mirrors, water misting systems, and others.^[117]

Effects 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 by being burned if they fly near the point where light is being focused. This can also affect raptors who hunt the birds.^{[118][119][120][121]} Federal wildlife officials were quoted by opponents as calling the Ivanpah power towers "mega traps" for wildlife.^{[122][123][124]}

According to rigorous reporting, in over six months, 133 singed birds were counted.^[125] 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.^[126] Other than in the US, no bird deaths have been reported at CSP plants internationally.

See also

• Concentrated photovoltaics (CPV)
• Daylighting
• List of solar thermal power stations
• Luminescent solar concentrator
• Photovoltaic thermal hybrid solar collector#PV/T concentrator (CPVT) (CPVT)
• Salt evaporation pond
• Solar air conditioning
• Solar thermal energy
• Solar thermal collector
• Solar water heating
• Thermal energy storage
• Thermochemical cycle

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External links

• Concentrating Solar Power Utility
• NREL Concentrating Solar Power Program
• Plataforma Solar de Almeria, CSP research center
• ISFOC (Institute of Concentrating Photovoltaic Systems)
• Baldizon, Roberto (5 March 2019). "Innovations on Concentrated Solar Thermal Power". Medium. Retrieved 18 January 2020.