Concentrated solar power

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https://en.wikipedia.org/wiki/Concentrated_solar_power 

A solar power tower at Crescent Dunes Solar Energy Project concentrating light via 10,000 mirrored heliostats spanning thirteen million sq ft (1.21 km²).

 

The three towers of the Ivanpah Solar Power Facility

 

Part of the 354 MW SEGS solar complex in northern San Bernardino County, California

 

Bird's eye view of Khi Solar One, South Africa

Concentrated solar power (CSP, also known as concentrating solar power, concentrated solar thermal) systems generate solar power by using mirrors or lenses to concentrate a large area of sunlight onto a receiver. Electricity is generated when the concentrated light is converted to heat (solar thermal energy), which drives a heat engine (usually a steam turbine) connected to an electrical power generator or powers a thermochemical reaction.

CSP had a global total installed capacity of 5,500 MW in 2018, up from 354 MW in 2005. Spain accounted for almost half of the world's capacity, at 2,300 MW, despite no new capacity entering commercial operation in the country since 2013. The United States follows with 1,740 MW. Interest is also notable in North Africa and the Middle East, as well as India and China. The global market was initially dominated by parabolic-trough plants, which accounted for 90% of CSP plants at one point. Since about 2010, central power tower CSP has been favored in new plants due to its higher temperature operation – up to 565 °C (1,049 °F) vs. trough's maximum of 400 °C (752 °F) – which promises greater efficiency.

Among the larger CSP projects are the Ivanpah Solar Power Facility (392 MW) in the United States, which uses solar power tower technology without thermal energy storage, and the Ouarzazate Solar Power Station in Morocco, which combines trough and tower technologies for a total of 510 MW with several hours of energy storage.

As a thermal energy generating power station, CSP has more in common with thermal power stations such as coal, gas, or geothermal. A CSP plant can incorporate thermal energy storage, which stores energy either in the form of sensible heat or as latent heat (for example, using molten salt), which enables these plants to continue to generate electricity whenever it is needed, day or night. This makes CSP a dispatchable form of solar. Dispatchable renewable energy is particularly valuable in places where there is already a high penetration of photovoltaics (PV), such as California because demand for electric power peaks near sunset just as PV capacity ramps down (a phenomenon referred to as duck curve).

CSP is often compared to photovoltaic solar (PV) since they both use solar energy. While solar PV experienced huge growth in recent years due to falling prices, Solar CSP growth has been slow due to technical difficulties and high prices. In 2017, CSP represented less than 2% of worldwide installed capacity of solar electricity plants. However, CSP can more easily store energy during the night, making it more competitive with dispatchable generators and baseload plants.

The DEWA project in Dubai, under construction in 2019, held the world record for lowest CSP price in 2017 at US$73 per MWh for its 700 MW combined trough and tower project: 600 MW of trough, 100 MW of tower with 15 hours of thermal energy storage daily. Base-load CSP tariff in the extremely dry Atacama region of Chile reached below $50/MWh in 2017 auctions.

History

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

A legend has it that Archimedes used a "burning glass" to concentrate sunlight on the invading Roman fleet and repel them from Syracuse. In 1973 a Greek scientist, Dr. Ioannis Sakkas, curious about whether Archimedes could really have destroyed the Roman fleet in 212 BC, lined up nearly 60 Greek sailors, each holding an oblong mirror tipped to catch the sun's rays and direct them at a tar-covered plywood silhouette 49 m (160 ft) away. The ship caught fire after a few minutes; however, historians continue to doubt the Archimedes story.

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, invеntors such as John Ericsson and Frank Shuman developed concentrating solar-powered dеvices for irrigation, refrigеration, and locomоtion. In 1913 Shuman finished a 55 horsepower (41 kW) parabolic solar thermal energy station in Maadi, Egypt for irrigation. 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.

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 power tower 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. The 10 MW Solar One power tower was developed in Southern California in 1981. Solar One was converted into Solar Two in 1995, implementing a new design with a molten salt mixture (60% sodium nitrate, 40% potassium nitrate) as the receiver working fluid and as a storage medium. The molten salt approach proved effective, and Solar Two operated successfully until it was decommissioned in 1999. The parabolic-trough technology of the nearby Solar Energy Generating Systems (SEGS), begun in 1984, was more workable. The 354 MW SEGS was the largest solar power plant in the world, until 2014.

No commercial concentrated solar was constructed from 1990 when SEGS was completed until 2006 when the Compact linear Fresnel reflector system at Liddell Power Station in Australia was built. Few other plants were built with this design although the 5 MW Kimberlina Solar Thermal Energy Plant opened in 2009.

In 2007, 75 MW Nevada Solar One was built, a trough design and the first large plant since SEGS. Between 2009 and 2013, Spain built over 40 parabolic trough systems, standardized in 50 MW blocks.

Due to the success of Solar Two, a commercial power plant, called Solar Tres Power Tower, was built in Spain in 2011, later renamed Gemasolar Thermosolar Plant. Gemasolar's results paved the way for further plants of its type. Ivanpah Solar Power Facility was constructed at the same time but without thermal storage, using natural gas to preheat water each morning.

Most concentrated solar power plants use the parabolic trough design, instead of the power tower or Fresnel systems. There have also been variations of parabolic trough systems like the integrated solar combined cycle (ISCC) which combines troughs and conventional fossil fuel heat systems.

CSP was originally treated as a competitor to photovoltaics, and Ivanpah was built without energy storage, although Solar Two had included several hours of thermal storage. By 2015, prices for photovoltaic plants had fallen and PV commercial power was selling for 1⁄3 of recent CSP contracts. However, increasingly, CSP was being bid with 3 to 12 hours of thermal energy storage, making CSP a dispatchable form of solar energy. As such, it is increasingly seen as competing with natural gas and PV with batteries for flexible, dispatchable 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. Parabolic trough and concentrating linear Fresnel reflectors are classified as linear focus collector types, while dish and solar tower are point focus types. Linear focus collectors achieve medium concentration factors (50 suns and over), and point focus collectors achieve high concentration factors (over 500 suns). Although simple, these solar concentrators are quite far from the theoretical maximum concentration. 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.

Parabolic trough

Main article: 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 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) 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. 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.

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. 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. 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.

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.

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. 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.

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. 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. 

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.

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.

Dish Stirling

See also: Solar thermal energy § Dish designs

 

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. 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. 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. 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.

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. 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. 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. 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. 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. 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.

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. These are also small in capacity (<0.4 MW) with flexibility to install in few acres area. Waste heat from the power plant can also be used for process steam generation and HVAC needs. 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.

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

Carbon neutral fuels production

Carbon neutral synthetic fuels production using concentrated solar thermal energy at nearly 1500 °C temperature is feasible technically and viable commercially in near future with declining costs of CSP plants. Also carbon neutral hydrogen can be produced with solar thermal energy (CSP) using Sulfur–iodine cycle, Hybrid sulfur cycle, Iron oxide cycle, Copper–chlorine cycle, Zinc–zinc oxide cycle, Cerium(IV) oxide–cerium(III) oxide cycle, etc.

Deployment around the world

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

 

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

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. 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.

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%. 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.

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. 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

As early as 2011, the rapid decline of the price of photovoltaic systems lead to projections that CSP will no longer be economically viable. As of 2020, the least expensive utility-scale concentrated solar power stations in the United States and worldwide are 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 for utility-scale PV. This five-fold price difference has been maintained since 2018.

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 the cost were approaching the upper end of the fossil fuel cost range at the beginning of the 2020s driven by support schemes in several countries, including Spain, the US, Morocco, South Africa, China, and the UAE: 

 

CSP deployment has slowed down considerably as most of the above-mentioned markets have cancelled their support, as the technology turned out to be more expensive on a per kWH basis than solar PV and wind power. CSP in combination with Thermal Energy Storage (TES) is expected by some to become cheaper than PV with lithium batteries for storage durations above 4 hours per day, while NREL expects that by 2030 PV with 10-hour storage lithium batteries will cost the same as PV with 4-hour storage used to cost in 2020.

Incentives and Markets

Spain

Andasol Solar Power Station in 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. 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). Because of cost concerns Spain has halted acceptance of new projects for the feed-in-tariff on 27 January 2012  Already accepted projects were affected by a 6% "solar-tax" on feed-in-tariffs, effectively reducing the feed-in-tariff.

In this context, the Spanish Government enacted the Royal Decree-Law 9/2013 in 2013, aimed at the adoption of urgent measures to guarantee the economic and financial stability of the electric system, laying the foundations of the new Law 24/2013 of the Spanish electricity sector. This new retroactive legal-economic framework applied to all the renewable energy systems was developed in 2014 by the RD 413/2014, which abolished the former regulatory frameworks set by the RD 661/2007 and the RD 1578/2008 and defined a new remuneration scheme for these assets.

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. Towards this end bi-annual auctions of 200 MW of CSP capacity starting in 2021 are expected, but details are not yet known.

Australia

Main article: Solar power in Australia

Several CSP dishes have been set up in remote Aboriginal settlements in the Northern Territory: Hermannsburg, Yuendumu and Lajamanu.

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 AUD$0.08/kWh or close to USD$0.06/kWh. 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 is looking to start construction of a 50MW combines CSP and PV facility in Mt. Isa of North-West Queensland in 2021.

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. 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. 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. At the end of 2020, China operated a total of 545 MW in 12 CSP plants, seven plants (320 MW) are molten-salt towers; another two plants (150MW) use the proven Eurotrough 150 parabolic trough design, 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.

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.

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. 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.

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. Algeria has the biggest area of desert, and private Algerian firm Cevital signed up for Desertec. 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. In other countries especially Spain and South Africa CSP plants have met their designed parameters 

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. 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.

Very large scale solar power plants

There have been several proposals for gigawatt size, very-large-scale solar power plants. They include the Euro-Mediterranean Desertec proposal and Project Helios in Greece (10 GW), both now canceled. A 2003 study concluded that the world could generate 2,357,840 TWh each year from very large scale solar power plants using 1% of each of the world's deserts. Total consumption worldwide was 15,223 TWh/year (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. 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.

Environmental effects

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

Water use

Concentrating solar power plants with wet-cooling systems have the highest water-consumption intensities of any conventional type of electric power plant; only fossil-fuel plants with carbon-capture and storage may have higher water intensities. A 2013 study comparing various sources of electricity found that the median water consumption during operations of concentrating solar power plants with wet cooling was 3.1 cubic metres per megawatt-hour (810 US gal/MWh) for power tower plants and 3.4 m³/MWh (890 US gal/MWh) for trough plants. This was higher than the operational water consumption (with cooling towers) for nuclear at 2.7 m³/MWh (720 US gal/MWh), coal at 2.0 m³/MWh (530 US gal/MWh), or natural gas at 0.79 m³/MWh (210 US gal/MWh). A 2011 study by the National Renewable Energy Laboratory came to similar conclusions: for power plants with cooling towers, water consumption during operations was 3.27 m³/MWh (865 US gal/MWh) for CSP trough, 2.98 m³/MWh (786 US gal/MWh) for CSP tower, 2.60 m³/MWh (687 US gal/MWh) for coal, 2.54 m³/MWh (672 US gal/MWh) for nuclear, and 0.75 m³/MWh (198 US gal/MWh) for natural gas. The Solar Energy Industries Association noted that the Nevada Solar One trough CSP plant consumes 3.2 m³/MWh (850 US gal/MWh). The issue of water consumption is heightened because CSP plants are often located in arid environments where water is scarce.

In 2007, the US Congress directed the Department of Energy to report on ways to reduce water consumption by CSP. The subsequent report noted that dry cooling technology was available that, although more expensive to build and operate, could reduce water consumption by CSP by 91 to 95 percent. A hybrid wet/dry cooling system could reduce water consumption by 32 to 58 percent. A 2015 report by NREL noted that of the 24 operating CSP power plants in the US, 4 used dry cooling systems. The four dry-cooled systems were the three power plants at the Ivanpah Solar Power Facility near Barstow, California, and the Genesis Solar Energy Project in Riverside County, California. Of 15 CSP projects under construction or development in the US as of March 2015, 6 were wet systems, 7 were dry systems, 1 hybrid, and 1 unspecified.

Although many older thermoelectric power plants with once-through cooling or cooling ponds use more water than CSP, meaning that more water passes through their systems, most of the cooling water returns to the water body available for other uses, and they consume less water by evaporation. For instance, the median coal power plant in the US with once-through cooling uses 138 m³/MWh (36,350 US gal/MWh), but only 0.95 m³/MWh (250 US gal/MWh) (less than one percent) is lost through evaporation. Since the 1970s, the majority of US power plants have used recirculating systems such as cooling towers rather than once-through systems.

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. Federal wildlife officials were quoted by opponents as calling the Ivanpah power towers "mega traps" for wildlife.

Some media sources have reported that concentrated solar power plants have injured or killed large numbers of birds due to intense heat from the concentrated sunrays. Some of the claims may have been overstated or exaggerated.

According to rigorous reporting, in over six months, 133 singed birds were counted. 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.

Biochar

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Biochar 

Biochar produced from residual wood in Namibia

 

Smaller pellets of biochar

 

Biochar after production, in a large pile

Biochar is charcoal that is produced by pyrolysis of biomass in the absence of oxygen; it is used as a soil conditioner usually directly but occasionally via being fed to animals. Biochar is defined by the International Biochar Initiative as "The solid material obtained from the thermochemical conversion of biomass in an oxygen-limited environment". Biochar is a stable solid that is rich in carbon and can endure in soil for thousands of years.

Biochar is being investigated as a means of carbon sequestration, and it may be a means to mitigate climate change. It results from processes related to pyrogenic carbon capture and storage (PyCCS). Biochar may increase the soil fertility of acidic soils and increase agricultural productivity.

History

The word "biochar" is a late 20th century English neologism derived from the Greek word βίος, bios, "life" and "char" (charcoal produced by carbonisation of biomass). It is recognised as charcoal that participates in biological processes found in soil, aquatic habitats and in animal digestive systems.

Pre-Columbian Amazonians produced biochar by smoldering agricultural waste (i.e., covering burning biomass with soil) in pits or trenches. It is not known if they intentionally used biochar to enhance soil productivity. European settlers called it terra preta de Indio. Following observations and experiments, a research team working in French Guiana hypothesized that the Amazonian earthworm Pontoscolex corethrurus was the main agent of fine powdering and incorporation of charcoal debris in the mineral soil.

Production

Biochar is a high-carbon, fine-grained residue that is produced via pyrolysis; it is the direct thermal decomposition of biomass in the absence of oxygen (preventing combustion), which produces a mixture of solids (the biochar proper), liquid (bio-oil), and gas (syngas) products. The specific yield from pyrolysis is dependent on process conditions such as temperature, residence time, and heating rate. These parameters can be tuned to produce either energy or biochar.^[16] Temperatures of 400–500 °C (673–773 K) produce more char, whereas temperatures above 700 °C (973 K) favor the yield of liquid and gas fuel components. Pyrolysis occurs more quickly at higher temperatures, typically requiring seconds rather than hours. The increasing heating rate leads to a decrease of biochar yield, while the temperature is in the range of 350–600 °C (623–873 K). Typical yields are 60% bio-oil, 20% biochar, and 20% syngas. By comparison, slow pyrolysis can produce substantially more char (≈35%); this contributes to soil fertility. Once initialized, both processes produce net energy. For typical inputs, the energy required to run a "fast" pyrolyzer is approximately 15% of the energy that it outputs. Pyrolysis plants can use the syngas output and yield 3–9 times the amount of energy required to run.

Besides pyrolysis, torrefaction and hydrothermal carbonization processes can also thermally decompose biomass to the solid material. However, these products cannot be strictly defined as biochar. The carbon product from the torrefaction process contains some volatile organic components, thus its properties are between that of biomass feedstock and biochar. Furthermore, even the hydrothermal carbonization could produce a carbon-rich solid product, the hydrothermal carbonization is evidently different from the conventional thermal conversion process. Therefore, the solid product from hydrothermal carbonization is defined as "hydrochar" rather than "biochar".

The Amazonian pit/ trench method harvests neither bio-oil nor syngas, and releases CO₂, black carbon, and other greenhouse gases (GHGs) (and potentially, toxins) into the air, though less greenhouse gasses than captured during the growth of the biomass. Commercial-scale systems process agricultural waste, paper byproducts, and even municipal waste and typically eliminate these side effects by capturing and using the liquid and gas products. The production of biochar as an output is not a priority in most cases.

Centralized, decentralized, and mobile systems

In a centralized system, unused biomass is brought to a central plant for processing into biochar. Alternatively, each farmer or group of farmers can operate a kiln. Finally, a truck equipped with a pyrolyzer can move from place to place to pyrolyze biomass. Vehicle power comes from the syngas stream, while the biochar remains on the farm. The biofuel is sent to a refinery or storage site. Factors that influence the choice of system type include the cost of transportation of the liquid and solid byproducts, the amount of material to be processed, and the ability to supply the power grid.

Common crops used for making biochar include various tree species, as well as various energy crops. Some of these energy crops (i.e. Napier grass) can store much more carbon on a shorter timespan than trees do.

For crops that are not exclusively for biochar production, the Residue-to-Product Ratio (RPR) and the collection factor (CF), the percent of the residue not used for other things, measure the approximate amount of feedstock that can be obtained. For instance, Brazil harvests approximately 460 million tons (MT) of sugarcane annually, with an RPR of 0.30, and a CF of 0.70 for the sugarcane tops, which normally are burned in the field. This translates into approximately 100 MT of residue annually, which could be pyrolyzed to create energy and soil additives. Adding in the bagasse (sugarcane waste) (RPR=0.29 CF=1.0), which is otherwise burned (inefficiently) in boilers, raises the total to 230 MT of pyrolysis feedstock. Some plant residue, however, must remain on the soil to avoid increased costs and emissions from nitrogen fertilizers.

Various companies in North America, Australia, and England sell biochar or biochar production units. In Sweden the 'Stockholm Solution' is an urban tree planting system that uses 30% biochar to support urban forest growth.

At the 2009 International Biochar Conference, a mobile pyrolysis unit with a specified intake of 1,000 pounds (450 kg) was introduced for agricultural applications.

Thermo-catalytic depolymerization

Alternatively, "thermo-catalytic depolymerization", which utilizes microwaves, has been used to efficiently convert organic matter to biochar on an industrial scale, producing ≈50% char.

Properties

The physical and chemical properties of biochars as determined by feedstocks and technologies are crucial. Characterization data explain their performance in a specific use. For example, guidelines published by the International Biochar Initiative provide standardized evaluation methods. Properties can be categorized in several respects, including the proximate and elemental composition, pH value, and porosity. The atomic ratios of biochar, including H/C and O/C, correlate with the properties that are relevant to organic content, such as polarity and aromaticity. A van-Krevelen diagram can show the evolution of biochar atomic ratios in the production process. In the carbonization process, both the H/C and O/C ratios decrease due to the release of functional groups that contain hydrogen and oxygen.

Applications

Carbon sink

Biomass burning and natural decomposition releases large amounts of carbon dioxide and methane to the Earth's atmosphere. The biochar production process also releases CO₂ (up to 50% of the biomass), however, the remaining carbon content becomes indefinitely stable. Biochar carbon remains in the ground for centuries, slowing the growth in atmospheric greenhouse gas levels. Simultaneously, its presence in the earth can improve water quality, increase soil fertility, raise agricultural productivity, and reduce pressure on old-growth forests.

Biochar can sequester carbon in the soil for hundreds to thousands of years, like coal. Early works proposing the use of biochar for cabon dioxide removal to create a long-term stable carbon sink were published in the 2010s. This technique is advocated by scientists including James Hansen and James Lovelock.

A 2010 report estimated that sustainable use of biochar could reduce the global net emissions of carbon dioxide (CO
₂), methane, and nitrous oxide by up to 1.8  billion tonnes carbon dioxide equivalent (CO
_2e) per year (compared to the about 50 billion tonnes emitted in 2021), without endangering food security, habitats, or soil conservation. However a 2018 study doubted enough biomass would be available to achieve significant carbon sequestration. A 2021 review estimated potential CO₂ removal from 1.6 to 3.2 billion tonnes per year.

In 2021 the cost of biochar ranged around European carbon prices, but was not yet included in the EU or UK Emissions Trading Scheme.

In developing countries, biochar derived from improved cookstoves for home-use can contribute to negative carbon emissions, while achieving other benefits for sustainable development.

Soil amendment

Biochar in preparation as a soil amendment

Biochar offers multiple soil health benefits in degraded tropical soils, but is less beneficial in temperate regions. Its porous nature is effective at retaining both water and water-soluble nutrients. Soil biologist Elaine Ingham highlighted its suitability as a habitat for beneficial soil micro organisms. She pointed out that when pre-charged with these beneficial organisms, biochar becomes promotes good soil, and plant health.

Biochar reduces leaching of E-coli through sandy soils depending on application rate, feedstock, pyrolysis temperature, soil moisture content, soil texture, and surface properties of the bacteria.

For plants that require high potash and elevated pH, biochar can improve yield.

Biochar can improve water quality, reduce soil emissions of greenhouse gases, reduce nutrient leaching, reduce soil acidity, and reduce irrigation and fertilizer requirements. Under certain circumstances biochar induces plant systemic responses to foliar fungal diseases and to improve plant responses to diseases caused by soilborne pathogens.

Biochar's impacts are dependent on its properties, as well as the amount applied, although knowledge about the important mechanisms and properties is limited. Biochar impact may depend on regional conditions including soil type, soil condition (depleted or healthy), temperature, and humidity. Modest additions of biochar reduce nitrous oxide (N
₂O) emissions by up to 80% and eliminate methane emissions, which are both more potent greenhouse gases than CO₂.

Studies reported positive effects from biochar on crop production in degraded and nutrient–poor soils. The application of compost and biochar under FP7 project FERTIPLUS had positive effects on soil humidity, crop productivity and quality in multiple countries. Biochar can be adapted with specific qualities to target distinct soil properties. In Colombian savanna soil, biochar reduced leaching of critical nutrients, created a higher nutrient uptake, and provided greater nutrient availability. At 10% levels biochar reduced contaminant levels in plants by up to 80%, while reducing chlordane and DDX content in the plants by 68 and 79%, respectively. However, because of its high adsorption capacity, biochar may reduce pesticide efficacy. High-surface-area biochars may be particularly problematic.

Biochar may be ploughed into soils in crop fields to enhance their fertility and stability, and for medium- to long-term carbon sequestration in these soils. It has meant a remarkable improvement in tropical soils showing positive effects in increasing soil fertility and in improving disease resistance in West European soils. The use of biochar as a feed additive can be a way to apply biochar to pastures and to reduce methane emissions.

One study reported that biochar helps build soil carbon by an average 3.8%.

Application rates of 2.5–20 tonnes per hectare (1.0–8.1 t/acre) appear to be required to produce significant improvements in plant yields. Biochar costs in developed countries vary from $300–7000/tonne, generally impractical for the farmer/horticulturalist and prohibitive for low-input field crops. In developing countries, constraints on agricultural biochar relate more to biomass availability and production time. A compromise is to use small amounts of biochar in lower cost biochar-fertilizer complexes.

Slash-and-char

Switching from slash-and-burn to slash-and-char farming techniques in Brazil can decrease both deforestation of the Amazon basin and carbon dioxide emission, as well as increase crop yields. Slash-and-burn leaves only 3% of the carbon from the organic material in the soil. Slash-and-char can retain up to 50%. Biochar reduces the need for nitrogen fertilizers, thereby reducing cost and emissions from fertilizer production and transport. Additionally, by improving soil's till-ability, its fertility and its productivity, biochar-enhanced soils can indefinitely sustain agricultural production, whereas slash/ burn soils quickly become depleted of nutrients, forcing farmers to abandon the fields, producing a continuous slash and burn cycle. Using pyrolysis to produce bio-energy does not require infrastructure changes the way, for example, processing biomass for cellulosic ethanol does. Additionally, biochar can be applied by the widely used machinery.

Water retention

Biochar is hygroscopic due to its porous structure and high specific surface area. As a result, fertilizer and other nutrients are retained for plants' benefit.

Stock fodder

A Western Australian farmer explored the use of biochar mixed with molasses as stock fodder. He asserted that in ruminants, biochar can assist digestion and reduce methane production. The farmer also used dung beetles to work the resulting biochar-infused dung into the soil without using machinery. The nitrogen and carbon in the dung are both incorporated into the soil rather than staying on the soil surface, reducing the production of nitrous oxide and carbon dioxide. The nitrogen and carbon add to soil fertility. On-farm evidence indicates that the fodder led to improvements of liveweight gain in Angus-cross cattle.

Doug Pow won the Australian Government Innovation in Agriculture Land Management Award at the 2019 Western Australian Landcare Awards for this innovation. Pow's work led to two further trials on dairy cattle, yielding reduced odour and increased milk production.

Research

Biochar applied to the soil in research trials in Namibia

Research into aspects involving pyrolysis/biochar is underway around the world, but as of 2018 was still in its infancy. From 2005 to 2012, 1,038 articles included the word "biochar" or "bio-char" in the topic indexed in the ISI Web of Science. Research is in progress by Cornell University, University of Edinburgh (which has a dedicated research unit), University of Georgia, the Agricultural Research Organization (ARO) of Israel, Volcani Center, and University of Delaware.

Long-term effects of biochar on C sequestration has been examined using soil from arable fields in Belgium with charcoal-enriched black spots dating from before 1870 from charcoal production mound kilns. Topsoils from these 'black spots' had a higher organic C concentration [3.6 ± 0.9% organic carbon (OC)] than adjacent soils outside these black spots (2.1 ± 0.2% OC). The soils had been cropped with maize for at least 12 years which provided a continuous input of C with a C isotope signature (δ13C) −13.1, distinct from the δ13C of soil organic carbon (−27.4 ‰) and charcoal (−25.7 ‰) collected in the surrounding area. The isotope signatures in the soil revealed that maize-derived C concentration was significantly higher in charcoal-amended samples ('black spots') than in adjacent unamended ones (0.44% vs. 0.31%; p = 0.02). Topsoils were subsequently collected as a gradient across two 'black spots' along with corresponding adjacent soils outside these black spots and soil respiration, and physical soil fractionation was conducted. Total soil respiration (130 days) was unaffected by charcoal, but the maize-derived C respiration per unit maize-derived OC in soil significantly decreased about half (p < 0.02) with increasing charcoal-derived C in soil. Maize-derived C was proportionally present more in protected soil aggregates in the presence of charcoal. The lower specific mineralization and increased C sequestration of recent C with charcoal are attributed to a combination of physical protection, C saturation of microbial communities and, potentially, slightly higher annual primary production. Overall, this study evidences the capacity of biochar to enhance C sequestration through reduced C turnover.

Biochar sequesters carbon (C) in soils because of its prolonged residence time, ranging from years to millennia. In addition, biochar can promote indirect C-sequestration by increasing crop yield while, potentially, reducing C-mineralization. Laboratory studies have evidenced effects of biochar on C-mineralization using ¹³
C signatures.

Fluorescence analysis of biochar-amended soil dissolved organic matter revealed that biochar application increased a humic-like fluorescent component, likely associated with biochar-carbon in solution. The combined spectroscopy-microscopy approach revealed the accumulation of aromatic-carbon in discrete spots in the solid-phase of microaggregates and its co-localization with clay minerals for soil amended with raw residue or biochar. The co-localization of aromatic-C:polysaccharides-C was consistently reduced upon biochar application. These finding suggested that reduced C metabolism is an important mechanism for C stabilization in biochar-amended soils.

Research and practical investigations into the potential of biochar for coarse soils in semi-arid and degraded ecosystems are ongoing. In Namibia biochar is under exploration as climate change adaptation effort, strengthening local communities' drought resilience and food security through the local production and application of biochar from abundant encroacher biomass.

In recent years, biochar has attracted interest as a wastewater filtration medium as well as for its adsorbing capacity for the wastewater pollutants.

Situated cognition

From Wikipedia, the free encyclopedia
https://en.wikipedia.org/wiki/Situated_cognition

Situated cognition is a theory that posits that knowing is inseparable from doing by arguing that all knowledge is situated in activity bound to social, cultural and physical contexts.

Under this assumption, which requires an epistemological shift from empiricism, situativity theorists suggest a model of knowledge and learning that requires thinking on the fly rather than the storage and retrieval of conceptual knowledge. In essence, cognition cannot be separated from the context. Instead knowing exists, in situ, inseparable from context, activity, people, culture, and language. Therefore, learning is seen in terms of an individual's increasingly effective performance across situations rather than in terms of an accumulation of knowledge, since what is known is co-determined by the agent and the context.

History

While situated cognition gained recognition in the field of educational psychology in the late twentieth century, it shares many principles with older fields such as critical theory, (Frankfurt School, 1930; Freire, 1968) anthropology (Jean Lave & Etienne Wenger, 1991), philosophy (Martin Heidegger, 1968), critical discourse analysis (Fairclough, 1989), and sociolinguistics theories (Bakhtin, 1981) that rejected the notion of truly objective knowledge and the principles of Kantian empiricism.

Lucy Suchman's work on situated action at Xerox Labs was instrumental in popularizing the idea that an actor's understanding of how to perform work results from reflecting on interactions with the social and material (e.g. technology-mediated) situation in which she or he acts. More recent perspectives of situated cognition have focused on and draw from the concept of identity formation as people negotiate meaning through interactions within communities of practice. Situated cognition perspectives have been adopted in education, instructional design, online communities and artificial intelligence (see Brooks, Clancey). Grounded Cognition, concerned with the role of simulations and embodiment in cognition, encompasses Cognitive Linguistics, Situated Action, Simulation and Social Simulation theories. Research has contributed to the understanding of embodied language, memory, and the representation of knowledge.

Situated cognition draws a variety of perspectives, from an anthropological study of human behavior in the context of technology-mediated work, or within communities of practice to the ecological psychology of the perception-action cycle and intentional dynamics, and even research on robotics with work on autonomous agents at NASA and elsewhere (e.g., work by W. J. Clancey). Early attempts to define situated cognition focused on contrasting the emerging theory with information processing theories dominant in cognitive psychology.

Recently theorists have recognized a natural affinity between situated cognition, New Literacy Studies and new literacies research (Gee, 2010). This connection is made by understanding that situated cognition maintains that individuals learn through experiences. It could be stated that these experiences, and more importantly the mediators that affect attention during these experiences is affected by the tools, technologies and languages used by a socio-cultural group and the meanings given to these by the collective group. New literacies research examines the context and contingencies that language and tool use by individuals and how this changes as the Internet and other communication technologies affect literacy.

Glossary

Term	Definition
affordance	properties of the environment, specified in the information array (flow field) of the individual, that present possibilities for action and are available for an agent to perceive directly and act upon
attention and intention	Once an intention (goal) is adopted, the agent's perception (attention) is attuned to the affordances of the environment.
attunement	attunement is a persisting state of awareness of the affordances in the environment and how they may be acted upon
community of practice	The concept of a community of practice (often abbreviated as CoP) refers to the process of social learning that occurs and shared sociocultural practices that emerge and evolve when people who have common goals interact as they strive towards those goals.
detection of invariants	perception of what doesn't change across different situations
direct perception (pick up)	describes the way an agent in an environment senses affordances without the need for computation or symbolic representation
effectivities	The agents ability to recognize and use affordances of the environment.
embodiment	as an explanation of cognition emphasizes first that the body exists as part of the world. In a dynamic process, perception and action occurring through and because of the body being in the world, interact to allow for the processes of simulation and representation.
legitimate peripheral participation	the initial stage(s) of a person's active membership in a community of practice to which he or she has access and the opportunity to become a full participant.
perceiving and acting cycle	Gibson (1986) described a continuous perception-action cycle, which is dynamic and ongoing. Agents perceive and act with intentionality in the environment at all times.

Key principles

Affordances/effectivities

James J. Gibson introduced the idea of affordances as part of a relational account of perception. Perception should not be considered solely as the encoding of environmental features into the perceiver's mind, but as an element of an individual's interaction with her environment (Gibson, 1977). Central to his proposal of an ecological psychology was the notion of affordances. Gibson proposed that in any interaction between an agent and the environment, inherent conditions or qualities of the environment allow the agent to perform certain actions with the environment. He defined the term as properties in the environment that presented possibilities for action and were available for an agent to perceive directly and act upon. Gibson focused on the affordances of physical objects, such as doorknobs and chairs, and suggested that these affordances were directly perceived by an individual instead of mediated by mental representations such as mental models. It is important to note that Gibson's notion of direct perception as an unmediated process of noticing, perceiving, and encoding specific attributes from the environment, has long been challenged by proponents of a more category-based model of perception.

This focus on agent-situation interactions in ecological psychology was consistent with the situated cognition program of researchers such as James G. Greeno (1994, 1998), who appreciated Gibson's apparent rejection of the factoring assumptions underlying experimental psychology. The situated cognition perspective focused on "perception-action instead of memory and retrieval…A perceiving/acting agent is coupled with a developing/adapting environment and what matters is how the two interact". Greeno (1994) also suggested that affordances are "preconditions for activity," and that while they do not determine behavior, they increase the likelihood that a certain action or behavior will occur.

Shaw, Turvey, & Mace (as cited by Greeno, 1994) later introduced the term effectivities, the abilities of the agent that determined what the agent could do, and consequently, the interaction that could take place. Perception and action were co-determined by the effectivities and affordances, which acted 'in the moment' together. Therefore, the agent directly perceived and interacted with the environment, determining what affordances could be picked up, based on his effectivities. This view is consistent with Norman's (1988) theory of "perceived affordances," which emphasizes the agent's perception of an object's utility as opposed to focusing on the object itself.

An interesting question is the relationship between affordances and mental representations as set forth in a more cognitivist perspective. While Greeno (1998) argues that attunements to affordances are superior to constructs such as schemata and mental models, Glenberg & Robertson (1999) suggested that affordances are the building blocks of mental models.

Perception (variance/invariance)

The work of Gibson (1986) in the field of visual perception greatly influences situated cognition. Gibson argued that visual perception is not a matter of the eye translating inputs into symbolic representation in the brain. Instead the viewer perceives and picks up on the infinite amount of information available in the environment. Specifically, an agent perceives affordances by discovering the variants, what changes, and more importantly the invariants, what does not change across different situations. Given a specific intention (or intentional set), perceptions of invariants are co-determined by the agent and the affordances of the environment, and are then built upon over time.

Memory

Situated cognition and ecological psychology perspectives emphasize perception and propose that memory plays a significantly diminished role in the learning process. Rather, focus is on the continuous tuning of perceptions and actions across situations based on the affordances of the environment and the interaction of the agent within that environment (Greeno, 1994). Representations are not stored and checked against past knowledge, but are created and interpreted in activity (Clancey, 1990).

Situated cognition understands memory as an interaction with the world, bounded by meaningful situations, that brings an agent toward a specified goal (intention). Thus, perception and action are co-determined by the effectivities and affordances, which act 'in the moment' together. Therefore, the agent directly perceives and interacts with the environment, determining what affordances can be picked up, based on his effectivities, and does not simply recall stored symbolic representations.

Knowing

Situativity theorists recast knowledge not as an entity, thing, or noun, but as knowing as an action or verb. It is not an entity which can be collected as in knowledge acquisition models. Instead knowing is reciprocally co-determined between the agent and environment. This reciprocal interaction can not be separated from the context and its cultural and historical constructions. Therefore, knowing isn't a matter of arriving at any single truth but instead it is a particular stance that emerges from the agent-environment interaction.

Knowing emerges as individuals develop intentions through goal-directed activities within cultural contexts which may in turn have larger goals and claims of truth. The adoption of intentions relates to the direction of the agent's attention to the detection of affordances in the environment that will lead to accomplishment of desired goals. Knowing is expressed in the agent's ability to act as an increasingly competent participant in a community of practice. As agents participate more fully within specific communities of practice, what constitutes knowing continuously evolves. For example, a novice environmentalist may not look at water quality by examining oxygen levels but may consider the color and smell. Through participation and enculturation within different communities, agents express knowing through action.

Learning

Since knowing is rooted in action and can not be decontextualized from individual, social, and historical goals teaching approaches that focus on conveying facts and rules separately from the contexts within which they are meaningful in real-life do not allow for learning that is based on the detection of invariants. They are therefore considered to be impoverished methods that are unlikely to lead to transfer. Learning must involve more than the transmission of knowledge but must instead encourage the expression of effectivities and the development of attention and intention through rich contexts that reflect real life learning processes.

Learning, more specifically literacy learning is affected by the Internet and other communication technologies as also evidenced in other segments of society. As a result of this youth are recently using affordances provided by these tools to become experts in a variety of domains. These practices by youth are viewed as them becoming "pro-ams" and becoming experts in whatever they have developed a passion for.

Language

Individuals don't just read or write texts, they interact with them, and often these interactions involve others in various socio-cultural contexts. Since language is often the basis for monitoring and tracking learning gains in comprehension, content knowledge and tool use in and out of school the role of situated cognition in language learning activities is important. Membership and interaction in social and cultural groups is often determined by tools, technologies and discourse use for full participation. Language learning or literacy in various social and cultural groups must include how the groups work with and interact with these texts. Language instruction in the context of situated cognition also involves the skilled or novice use of language by members of the group, and instruction of not only the elements of language, but what is needed to bring a student to the level of expert. Originating from emergent literacy, specialist-language lessons examines the formal and informal styles and discourses of language use in socio-cultural contexts. A function of specialist-language lessons includes "lucidly functional language", or complex specialist language is usually accompanied by clear and lucid language used to explain the rules, relationships or meanings existing between language and meaning.

Legitimate peripheral participation

According to Jean Lave and Wenger (1991) legitimate peripheral participation (LPP) provides a framework to describe how individuals ('newcomers') become part of a community of learners. Legitimate peripheral participation was central to Lave and Wenger's take on situated cognition (referred to as "situated activity") because it introduced socio-cultural and historical realizations of power and access to the way thinking and knowing are legitimated. They stated, "Hegemony over resources for learning and alienation from full participation are inherent in the shaping of the legitimacy and peripherality of participation in its historical realizations" (p. 42). Lave and Wenger's (1991) research on the phenomenon of apprenticeship in communities of practice not only provided a unit of analysis for locating an individual's multiple, changing levels and ways of participation, but also implied that all participants, through increased involvement, have access to, acquire, and use resources available to their particular community.

To illustrate the role of LPP in situated activity, Lave and Wenger (1991) examined five apprenticeship scenarios (Yucatec midwives, Vai and Gola tailors, naval quartermasters, meat cutters, and non-drinking alcoholics involved in AA). Their analysis of apprenticeship across five different communities of learners lead them to several conclusions about the situatedness of LPP and its relationship to successful learning. Key to newcomers' success included:

• access to all that community membership entails,
• involvement in productive activity,
• learning the discourse(s) of the community including "talking about and talking within a practice," (p. 109), and
• willingness of the community to capitalize on the inexperience of newcomers, "Insofar as this continual interaction of new perspectives is sanctioned, everyone's participation is legitimately peripheral in some respect. In other words, everyone can to some degree be considered a 'newcomer' to the future of a changing community" 

Planning vs. action

Suchman's book, Plans and Situated Actions: The Problem of Human-machine Communication (1987), provided a novel approach to the study of human-computer interaction (HCI). By adopting an anthropological approach to sensemaking and interpretation, Suchman was able to demonstrate how both action and planning were situated in the context of a flow of socially- and materially-mediated activities - an idea that stimulated many of the later conceptualizations of situated cognition. Her studies contrasted the deterministic approach to planning assumed by technology designers with the situated nature of planning as people make sense of the status of their workflow and adjust their course of action accordingly. For example, a photocopier will instruct its user to reload all pages in the original order after a jam, whereas the user understands that they only need to copy the last page again. By arguing that plans were the result of ongoing processes of prospective/retrospective sense making, Suchman identified the limits of technology system access to relevant social and material resources as a major cause of limitations in how technology supports human work.

This position led to a major debate with Vera and Simon (1993), who argued that cognition is based on symbolic representations and that planning must therefore be deterministic, based on a pre-determined repertoire of learned response. Most organizational theorists would now see this debate as reflecting individual/cognitive vs. socially-situated levels of analysis (requiring a similar need for paradigmatic co-existence as Wave–particle duality). Suchman (1993) argues that planning in the context of work-activity is similar to navigating a canoe through rapids: you know what point on the river you are aiming for, but you constantly adjust your course as you interact with rocks, swells, and currents on the way. As a result, many organizational theorists argue that plans can only be viewed as post-hoc justifications of action, while Suchman herself appears to view plans and actions as interrelated in the moment of action.

Representation, symbols, and schemata

In situated theories, the term "representation" refers to external forms in the environment that are created through social interactions to express meaning (language, art, gestures, etc.) and are perceived and acted upon in the first person sense. "Representing" in the first person sense is conceived as an act of re-experiencing in the imagination that involves the dialectic of ongoing perceiving and acting in coordination with the activation of neural structures and processes. This form of reflective representation is considered to be a secondary type of learning, while the primary form of learning is found in the "adaptive recoordination that occurs with every behavior". Conceptualizing is considered to be a "prelinguistic" act, while "knowing" involves creative interaction with symbols in both their interpretation and use for expression. "Schema" develop as neural connections become biased through repeated activations to reactivate in situations that are perceived and conceived as temporally and compositionally similar to previous generalized situations.

Goals, intention, and attention

Young-Barab Model (1997)

The Young-Barab Model (1997) pictured to the left, illustrates the dynamics of intentions and intentional dynamics involved in the agent's interaction with his environment when problem solving.

Dynamics of Intentions: goal (intention) adoption from among all possible goals (ontological descent). This describes how the learner decides whether or not to adopt a particular goal when presented with a problem. Once a goal is adopted, the learner proceeds by interacting with their environment through intentional dynamics. There are many levels of intentions, but at the moment of a particular occasion, the agent has just one intention, and that intention constrains his behavior until it is fulfilled or annihilated.

Intentional Dynamics: dynamics that unfold when the agent has only one intention (goal) and begins to act towards it, perceiving and acting. It is a trajectory towards the achievement of a solution or goal, the process of tuning one's perception (attention). Each intention is meaningfully bounded, where the dynamics of that intention inform the agent of whether or not he is getting closer to achieving his goal. If the agent is not getting closer to his goal, he will take corrective action, and then continue forward. This is the agent's intentional dynamics, and continues on until he achieves his goal.

Transfer

There are various definition of transfer found within the situated cognition umbrella. Researchers interested in social practice often define transfer as increased participation. Ecological psychology perspectives define transfer as the detection of invariance across different situations. Furthermore, transfer can only "occur when there is a confluence of an individual's goals and objectives, their acquired abilities to act, and a set of affordances for action".

Embodied cognition

Main article: Embodied cognition

The traditional cognition approach assumes that perception and motor systems are merely peripheral input and output devices. However, embodied cognition posits that the mind and body interact 'on the fly' as a single entity. An example of embodied cognition is seen in the area of robotics, where movements are not based on internal representations, rather, they are based on the robot's direct and immediate interaction with its environment. Additionally, research has shown that embodied facial expressions influence judgments, and arm movements are related to a person's evaluation of a word or concept. In the latter example, the individual would pull or push a lever towards his name at a faster rate for positive words, than for negative words. These results appeal to the embodied nature of situated cognition, where knowledge is the achievement of the whole body in its interaction with the world.

Externalism

As to the mind, by and large, situated cognition paves the way to various form of externalism. The issue is whether the situated aspect of cognition has only a practical value or it is somehow constitutive of cognition and perhaps of consciousness itself. As to the latter possibility, there are different positions. David Chalmers and Andy Clark, who developed the hugely debated model of the extended mind, explicitly rejected the externalization of consciousness. For them, only cognition is extended. On the other hand, others, like Riccardo Manzotti or Teed Rockwell, explicitly considered the possibility to situate conscious experience in the environment.

Pedagogical implications

Since situated cognition views knowing as an action within specific contexts and views direct instruction models of knowledge transmission as impoverished, there are significant implications for pedagogical practices. First, curriculum requires instructional design that draws on apprenticeship models common in real life. Second, curricular design should rely on contextual narratives that situate concepts in practice. Classroom practices such as project-based learning and problem-based learning would qualify as consistent with the situated learning perspective, as would techniques such as Case Base Learning, Anchored Instruction, and cognitive apprenticeship.

Cognitive apprenticeship

Cognitive apprenticeships were one of the earliest pedagogical designs to incorporate the theories of situated cognition (Brown, Collins, & Duguid, 1989). Cognitive apprenticeship uses four dimensions (e.g., content, methods, sequence, sociology) to embed learning in activity and make deliberate the use of the social and physical contexts present in the classroom (Brown, Collins, & Duguid, 1989; Collins, Brown, & Newman, 1989). Cognitive apprenticeship includes the enculturation of students into authentic practices through activity and social interaction (Brown, Collins, & Duguid, 1989). The technique draws on the principles of Legitimate Peripheral Participation (Lave & Wenger, 1991) and reciprocal teaching (Palincsar & Brown, 1984; 1989) in that a more knowledgeable other, i.e. a teacher, engages in a task with a more novice other, i.e. a learner, by describing their own thoughts as they work on the task, providing "just in time" scaffolding, modeling expert behaviors, and encouraging reflection. The reflection process includes having students alternate between novice and expert strategies in a problem-solving context, sensitizing them to specifics of an expert performance, and adjustments that may be made to their own performance to get them to the expert level (Collins & Brown, 1988; Collins, Brown, & Newman, 1989). Thus, the function of reflection indicates "co-investigation" and/or abstracted replay by students.

Collins, Brown, and Newman (1989) emphasized six critical features of a cognitive apprenticeship that included observation, coaching, scaffolding, modeling, fading, and reflection. Using these critical features, expert(s) guided students on their journey to acquire the cognitive and metacognitive processes and skills necessary to handle a variety of tasks, in a range of situations Reciprocal teaching, a form of cognitive apprenticeship, involves the modeling and coaching of various comprehension skills as teacher and students take turns in assuming the role of instructor.

Anchored instruction

Anchored instruction is grounded in a story or narrative that presents a realistic (but fictional) situation and raises an overarching question or problem (compare with an essential question posed by a teacher). This approach is designed to 1) engage the learner with a problem or series of related problems, 2) require the learner to develop goals and discover subgoals related to solving the problem(s), and 3) provide the learner with extensive and diverse opportunities to explore the problem(s) in a shared context with classmates. For example, a Spanish teacher uses a video drama series focused on the murder of a main character. Students work in small groups to summarize parts of the story, to create hypotheses about the murderer and motive, and to create a presentation of their solution to the class. Stories are often paired so that across the set students can detect the invariant structure of the underlying knowledge (so 2 episodes about distance-rate-time, one about boats and one about planes, so students can perceive how the distance-rate-time relationship holds across differences in vehicles). The ideal smallest set of instances needed provide students the opportunity to detect invariant structure has been referred to as a "generator set" of situations.

The goal of anchored instruction is the engagement of intention and attention. Through authentic tasks across multiple domains, educators present situations that require students to create or adopt meaningful goals (intentions). One of the educator's objectives can be to set a goal through the use of an anchor problem. A classic example of anchored instruction is the Jasper series. The Jasper series includes a variety of videodisc adventures focused on problem formulation and problem solving. Each videodisc used a visual narrative to present an authentic, realistic everyday problem. The objective was for students to adopt specific goals (intentions) after viewing the story and defining a problem. These newly adopted goals guided students through the collaborative process of problem formulation and problem solving.

Perceiving and acting in avatar-based virtual worlds

Virtual worlds provide unique affordances for embodied learning, i.e. hands on, interactive, spatially oriented, that ground learning in experience. Here "embodied" means acting in a virtual world enabled by an avatar.

Contextual affordances of online games and virtual environments allow learners to engage in goal-driven activity, authentic interactions, and collaborative problem-solving – all considered in situated theories of learning to be features of optimal learning. In terms of situated assessment, virtual worlds have the advantage of facilitating dynamic feedback that directs the perceiving/acting agent, through an avatar, to continually improve performance.

Research methodologies

The situative perspective is focused on interactive systems in which individuals interact with one another and physical and representational systems. Research takes place in situ and in real-world settings, reflecting assumptions that knowledge is constructed within specific contexts which have specific situational affordances. Mixed methods and qualitative methodologies are the most prominently used by researchers.

In qualitative studies, methods used are varied but the focus is often on the increased participation in specific communities of practice, the affordances of the environment that are acted upon by the agent, and the distributed nature of knowing in specific communities. A major feature of quantitative methods used in situated cognition is the absence of outcome measures. Quantitative variables used in mixed methods often focus on process over product. For example, trace nodes, dribble files, and hyperlink pathways are often used to track how students interact in the environment.

Critiques of situativity

In "Situated Action: A Symbolic Interpretation" Vera and Simon wrote: " ... the systems usually regarded as exemplifying Situated Action are thoroughly symbolic (and representational), and, to the extent that they are limited in these respects, have doubtful prospects for extension to complex tasks" Vera and Simon (1993) also claimed that the information processing view is supported by many years of research in which symbol systems simulated "broad areas of human cognition" and that there is no evidence of cognition without representation.

Anderson, Reder and Simon (1996) summarized what they considered to be the four claims of situated learning and argued against each claim from a cognitivist perspective. The claims and their arguments were:

1. Claim: Activity and learning are bound to the specific situations in which they occur. Argument: Whether learning is bound to context or not depends on both the kind of learning and the way that it is learned.
2. Claim: Knowledge does not transfer between tasks. Argument: There is ample evidence of successful transfer between tasks in the literature. Transfer depends on initial practice and the degree to which a successive task has similar cognitive elements to a prior task.
3. Claim: Teaching abstractions is ineffective. Argument: Abstract instruction can be made effective by combining of abstract concepts and concrete examples.
4. Claim: Instruction must happen in complex social contexts. Argument: Research shows value in individual learning and on focusing individually on specific skills in a skill set.

Anderson, Reder and Simons summarize their concerns when they say: "What is needed to improve learning and teaching is to continue to deepen our research into the circumstances that determine when narrower or broader contexts are required and when attention to narrower or broader skills are optimal for effective and efficient learning" (p. 10).

Considerations

However, it is important to remember that a theory is neither wrong nor right but provides affordances for certain aspects of a problem. Lave and Wenger recognized this in their ironic comment, "How can we purport to be working out a theoretical conception of learning without engaging in the project of abstraction [decontextualized knowledge] rejected above?" (Lave & Wenger, 1991, p. 38).