In classical Euclidean geometry, a point is a primitive notion,
defined as "that which has no part". Points and other primitive notions
are not defined in terms of other concepts, but only by certain formal
properties, called axioms, that they must satisfy; for example, "there is exactly one straight line that passes through two distinct points". As physical diagrams, geometric figures are made with tools such as a compass, scriber,
or pen, whose pointed tip can mark a small dot or prick a small hole
representing a point, or can be drawn across a surface to represent a
curve.
A point can also be determined by the intersection of two curves or three surfaces, called a vertex or corner.
Since the advent of analytic geometry, points are often defined or represented in terms of numerical coordinates. In modern mathematics, a space of points is typically treated as a set, a point set.
An isolated point is an element of some subset of points which has some neighborhood containing no other points of the subset.
Points in Euclidean geometry
Points, considered within the framework of Euclidean geometry, are one of the most fundamental objects. Euclid originally defined the point as "that which has no part". In the two-dimensional Euclidean plane, a point is represented by an ordered pair (x, y) of numbers, where the first number conventionally represents the horizontal and is often denoted by x, and the second number conventionally represents the vertical and is often denoted by y. This idea is easily generalized to three-dimensional Euclidean space, where a point is represented by an ordered triplet (x, y, z) with the additional third number representing depth and often denoted by z. Further generalizations are represented by an ordered tuplet of n terms, (a1, a2, … , an) where n is the dimension of the space in which the point is located.
Many constructs within Euclidean geometry consist of an infinite collection of points that conform to certain axioms. This is usually represented by a set of points; As an example, a line is an infinite set of points of the form
where c1 through cn and d are constants and n is the dimension of the space. Similar constructions exist that define the plane, line segment, and other related concepts. A line segment consisting of only a single point is called a degenerate line segment.
In addition to defining points and constructs related to points,
Euclid also postulated a key idea about points, that any two points can
be connected by a straight line. This is easily confirmed under modern extensions of Euclidean geometry,
and had lasting consequences at its introduction, allowing the
construction of almost all the geometric concepts known at the time.
However, Euclid's postulation of points was neither complete nor
definitive, and he occasionally assumed facts about points that did not
follow directly from his axioms, such as the ordering of points on the
line or the existence of specific points. In spite of this, modern
expansions of the system serve to remove these assumptions.
Dimension of a point
There are several inequivalent definitions of dimension in mathematics. In all of the common definitions, a point is 0-dimensional.
The dimension of a vector space is the maximum size of a linearly independent subset. In a vector space consisting of a single point (which must be the zero vector 0),
there is no linearly independent subset. The zero vector is not itself
linearly independent, because there is a non-trivial linear combination
making it zero: .
The topological dimension of a topological space is defined to be the minimum value of n, such that every finite open cover of admits a finite open cover of which refines in which no point is included in more than n+1 elements. If no such minimal n exists, the space is said to be of infinite covering dimension.
A point is zero-dimensional with respect to the covering dimension because every open cover of the space has a refinement consisting of a single open set.
Hausdorff dimension
Let X be a metric space. If S ⊂ X and d ∈ [0, ∞), the d-dimensional Hausdorff content of S is the infimum of the set of numbers δ ≥ 0 such that there is some (indexed) collection of balls covering S with ri > 0 for each i ∈ I that satisfies
The Hausdorff dimension of X is defined by
A point has Hausdorff dimension 0 because it can be covered by a single ball of arbitrarily small radius.
Geometry without points
Although
the notion of a point is generally considered fundamental in mainstream
geometry and topology, there are some systems that forgo it, e.g. noncommutative geometry and pointless topology. A "pointless" or "pointfree" space is defined not as a set, but via some structure (algebraic or logical respectively) which looks like a well-known function space on the set: an algebra of continuous functions or an algebra of sets respectively. More precisely, such structures generalize well-known spaces of functions in a way that the operation "take a value at this point" may not be defined. A further tradition starts from some books of A. N. Whitehead in which the notion of region is assumed as a primitive together with the one of inclusion or connection.
Often in physics and mathematics, it is useful to think of a point as
having non-zero mass or charge (this is especially common in classical electromagnetism, where electrons are idealized as points with non-zero charge). The Dirac delta function, or δ function, is (informally) a generalized function on the real number line that is zero everywhere except at zero, with an integral of one over the entire real line. The delta function is sometimes thought of as an infinitely high,
infinitely thin spike at the origin, with total area one under the
spike, and physically represents an idealized point mass or point charge. It was introduced by theoretical physicist Paul Dirac. In the context of signal processing it is often referred to as the unit impulse symbol (or function). Its discrete analog is the Kronecker delta function which is usually defined on a finite domain and takes values 0 and 1.
Planting trees is a nature-based way to remove carbon dioxide from the atmosphere; however, the effect may only be temporary in some cases.
Carbon dioxide removal (CDR) is a process in which carbon dioxide (CO2)
is removed from the atmosphere by deliberate human activities and
durably stored in geological, terrestrial, or ocean reservoirs, or in
products. This process is also known as carbon removal, greenhouse gas removal or negative emissions. CDR is more and more often integrated into climate policy, as an element of climate change mitigation strategies. Achieving net zero emissions
will require first and foremost deep and sustained cuts in emissions,
and then—in addition—the use of CDR ("CDR is what puts the net into net zero emissions").
In the future, CDR may be able to counterbalance emissions that are
technically difficult to eliminate, such as some agricultural and
industrial emissions.
As of 2023, CDR is estimated to remove around 2 gigatons of CO2 per year. This is equivalent to about 4% of the greenhouse gases emitted per year by human activities. There is potential to remove and sequester up to 10 gigatons of carbon
dioxide per year by using those CDR methods which can be safely and
economically deployed now. However, quantifying the exact amount of carbon dioxide removed from the atmosphere by CDR is difficult.
Definition
Carbon dioxide removal (CDR) is defined by the IPCC as: "Anthropogenic activities removing CO2
from the atmosphere and durably storing it in geological, terrestrial,
or ocean reservoirs, or in products. It includes existing and potential
anthropogenic enhancement of biological or geochemical sinks and direct
air capture and storage, but excludes natural CO2 uptake not directly caused by human activities."
Synonyms for CDR include greenhouse gas removal (GGR),negative emissions technology, and carbon removal. Technologies have been proposed for removing non-CO2 greenhouse gases such as methane from the atmosphere, but only carbon dioxide is currently feasible to remove at scale. Therefore, in most contexts, greenhouse gas removal means carbon dioxide removal.
The term geoengineering (or climate engineering) is sometimes used in the scientific literature for both CDR or SRM (solar radiation management), if the techniques are used at a global scale. The terms geoengineering or climate engineering are no longer used in IPCC reports.
Categories
CDR methods can be placed in different categories that are based on different criteria:
Role in the carbon cycle (land-based biological; ocean-based biological; geochemical; chemical); or
Timescale of storage (decades to centuries; centuries to millennia; thousand years or longer)
Concepts using similar terminology
CDR can be confused with carbon capture and storage (CCS), a process in which carbon dioxide is collected from point-sources such as gas-fired power plants, whose smokestacks emit CO2 in a concentrated stream. The CO2 is then compressed and sequestered or utilized. When used to sequester the carbon from a gas-fired power plant, CCS
reduces emissions from continued use of the point source, but does not
reduce the amount of carbon dioxide already in the atmosphere.
Role in climate change mitigation
Use of CDR reduces the overall rate at which humans are adding carbon dioxide to the atmosphere. The Earth's surface temperature will stabilize only after global emissions have been reduced to net zero, which will require both aggressive efforts to reduce emissions and deployment of CDR.
It is not feasible to bring net emissions to zero without CDR as
certain types of emissions are technically difficult to eliminate. Emissions that are difficult to eliminate include nitrous oxide emissions from agriculture, aviation emissions, and some industrial emissions. In climate change mitigation strategies, the use of CDR counterbalances those emissions.
After net zero emissions have been achieved, CDR could be used to reduce atmospheric CO2 concentrations, which could partially reverse the warming that has already occurred by that date. All emission pathways that limit global warming to 1.5 °C or 2 °C by
the year 2100 assume the use of CDR in combination with emission
reductions.
Critique and risks
Critics
point out that CDR must not be regarded as a substitute for the
required cuts in greenhouse gas emissions. Oceanographer David Ho
formulated it like this in 2023 "We must stop talking about deploying
CDR as a solution today, when emissions remain high—as if it somehow
replaces radical, immediate emission cuts.
Reliance on large-scale deployment of CDR was regarded in 2018 as
a "major risk" to achieving the goal of less than 1.5 °C of warming,
given the uncertainties in how quickly CDR can be deployed at scale. Strategies for mitigating climate change that rely less on CDR and more on sustainable use of energy carry less of this risk.
The possibility of large-scale future CDR deployment has been described as a moral hazard, as it could lead to a reduction in near-term efforts to mitigate climate change. However, the 2019 NASEM report concludes: "Any argument to delay
mitigation efforts because NETs will provide a backstop drastically
misrepresents their current capacities and the likely pace of research
progress."
CDR is meant to complement efforts in hard-to-abate sectors
rather than replace mitigation. Limiting climate change to 1.5 °C and
achieving net-zero emissions would entail substantial carbon dioxide
removal (CDR) from the atmosphere by the mid-century, but how much CDR
is needed at country level over time is unclear. Equitable allocations
of CDR, in many cases, exceed implied land and carbon storage
capacities. Many countries have either insufficient land to contribute
an equitable share of global CDR or insufficient geological storage
capacity.
Experts also highlight social and ecological limits for carbon
dioxide removal, such as the land area required. For example, the
combined land requirements of removal plans as per the global Nationally
Determined Contributions in 2023 amounted to 1.2 billion hectares,
which is equal to the combined size of global croplands.
Permanence
Forests, kelp beds,
and other forms of plant life absorb carbon dioxide from the air as
they grow, and bind it into biomass. However, these biological stores
are considered volatile carbon sinks as the long-term sequestration cannot be guaranteed. For example, natural events, such as wildfires
or disease, economic pressures and changing political priorities can
result in the sequestered carbon being released back into the
atmosphere.
Biomass, such as trees, can be directly stored into the Earth's subsurface. Furthermore, carbon dioxide that has been removed from the atmosphere can be stored in the Earth's crust by injecting it into the subsurface, or in the form of insoluble carbonate
salts. This is because they are removing carbon from the atmosphere and
sequestering it indefinitely and presumably for a considerable duration
(thousands to millions of years).
Current and potential scale
As of 2023, CDR is estimated to remove about 2 gigatons of CO2 per year, almost entirely by low-tech methods like reforestation and the creation of new forests. This is equivalent to 4% of the greenhouse gases emitted per year by human activities. A 2019 consensus study report by NASEM assessed the potential of all forms of CDR other than ocean fertilization
that could be deployed safely and economically using current
technologies, and estimated that they could remove up to 10 gigatons of
CO2 per year if fully deployed worldwide. In 2018, all analyzed mitigation pathways that would prevent more than 1.5 °C of warming included CDR measures.
Some mitigation pathways propose achieving higher rates of CDR
through massive deployment of one technology; however, these pathways
assume that hundreds of millions of hectares of cropland are converted
to growing biofuel crops. Further research in the areas of direct air capture, geologic sequestration of carbon dioxide, and carbon mineralization could potentially yield technological advancements that make higher rates of CDR economically feasible. Investing in nature-based solutions is considered a way to buy time for
the advancement of engineered carbon removal methods, enabling their
full deployment in the second half of the 21st century.
Methods
Overview listing based on technology readiness level
The following is a list of known CDR methods in the order of their technology readiness level
(TRL). The ones at the top have a high TRL of 8 to 9 (9 being the
maximum possible value, meaning the technology is proven), the ones at
the bottom have a low TRL of 1 to 2, meaning the technology is not
proven or only validated at laboratory scale.
Blue carbon
management in coastal wetlands (restoration of vegetated coastal
ecosystems; an ocean-based biological CDR method which encompasses mangroves, salt marshes and seagrass beds)
The CDR methods with the greatest potential to contribute to climate
change mitigation efforts as per illustrative mitigation pathways are
the land-based biological CDR methods (primarily
afforestation/reforestation (A/R)) and/or bioenergy with carbon capture
and storage (BECCS). Some of the pathways also include direct air
capture and storage (DACCS).
Afforestation, reforestation, and forestry management
Trees use photosynthesis to absorb carbon dioxide and store the carbon in wood and soils. Afforestation is the establishment of a forest in an area where there was previously no forest.Reforestation is the re-establishment of a forest that has been previously cleared. Forests are vital for human society, animals and plant species. This is
because trees keep air clean, regulate the local climate and provide a
habitat for numerous species.
As trees grow they absorb CO2 from the atmosphere and store it in living biomass, dead organic matter and soils.
Afforestation and reforestation – sometimes referred to collectively as
'forestation' – facilitate this process of carbon removal by
establishing or re-establishing forest areas. It takes forests
approximately 10 years to ramp- up to the maximum sequestration rate.
Depending on the species, the trees will reach maturity after
around 20 to 100 years, after which they store carbon but do not
actively remove it from the atmosphere. Carbon can be stored in forests indefinitely, but the storage can also
be much more short-lived as trees are vulnerable to being cut, burned,
or killed by disease or drought. Once mature, forest products can be harvested and the biomass stored in long-lived wood products, or used for bioenergy or biochar. Consequent forest regrowth then allows continuing CO2 removal.
Risks to deployment of new forest include the availability of
land, competition with other land uses, and the comparatively long time
from planting to maturity.
Carbon farming is a set of agricultural methods that aim to store carbon in the soil, crop roots, wood and leaves. The overall goal of carbon farming is to create a net loss of carbon from the atmosphere. This is done by increasing the rate at which carbon is sequestered into
soil and plant material. One option is to increase the soil organic
carbon content using practices of soil regeneration. This can also aid plant growth, improve soil water retention capacity and reduce fertilizer use. Sustainable forest management is another tool that is used in carbon farming.
Agricultural methods for carbon farming include adjusting how tillage and livestock grazing is done, using organic mulch or compost, working with biochar and terra preta, and changing the crop types. Methods used in forestry include for example reforestation and bamboo farming. Carbon farming is not without its challenges or disadvantages. This is because some of its methods can affect ecosystem services. For example, carbon farming could cause an increase of land clearing, monocultures and biodiversity loss.
Biomass carbon removal and storage
Biomass carbon removal and storage (frequently abbreviated as BiCRS) is a family of technologies for Carbon dioxide removal, which collect biomass (such as agricultural waste or biproducts of biomass energy systems) and sequesters that carbon through a permanent or semi-permanent method of storage. The family of technologies is often compared with direct air capture. Unlike direct air capture that use human engineered technologies to
remove carbon dioxide from the atmosphere (which is expensive and energy
intensive), BiCRS technologies rely on photosynthesis of plants and then engineering solutions for taking the carbon-rich residue of that plant life and sequestering it.
BiCRS technologies introduce a number of challenges for carbon
dioxide removal, including uncertainty about measuring sequestration of
buried biomass, andcomplexity in sourcing biomass (it introduces
additional demand for agricultural land and organic bioproducts). Researchers and policy think tanks like World Resources Institute recommend policy that put limits on which kind of biomass can be used for these process.
Biochar is created by the pyrolysis of biomass, and is under investigation as a method of carbon sequestration.
Biochar is a charcoal that is used for agricultural purposes which also aids in carbon sequestration,
the capture or hold of carbon. It is created using a process called
pyrolysis, which is basically the act of high temperature heating
biomass in an environment with low oxygen levels. What remains is a
material known as char, similar to charcoal but is made through a
sustainable process, thus the use of biomass. Biomass is organic matter produced by living organisms or recently
living organisms, most commonly plants or plant based material. A study done by the UK Biochar Research Center has stated that, on a
conservative level, biochar can store 1 gigaton of carbon per year.
With greater effort in marketing and acceptance of biochar, the benefit
of Biochar Carbon Removal could be the storage of 5–9 gigatons per year in soils. However, at the moment, biochar is restricted by the terrestrial carbon
storage capacity, when the system reaches the state of equilibrium, and
requires regulation because of threats of leakage.
Direct air capture with carbon sequestration (DACCS)
Direct air capture (DAC) is the use of chemical or physical processes to extract carbon dioxide (CO2) directly from the ambient air. If the extracted CO2 is then sequestered
in safe long-term storage, the overall process is called direct air
carbon capture and sequestration (DACCS), achieving carbon dioxide
removal. Systems that engage in such a process are referred to as
negative emissions technologies (NET).
There are several methods of sequestering carbon from the ocean, where dissolved carbonate in the form of carbonic acid is in equilibrium with atmospheric carbon dioxide. These include ocean fertilization, the purposeful introduction of plant nutrients to the upper ocean. While one of the more well-researched carbon dioxide removal
approaches, ocean fertilization would only sequester carbon on a
timescale of 10–100 years. While surface ocean acidity may decrease as a
result of nutrient fertilization, sinking organic matter will
remineralize, increasing deep ocean acidity. A 2021 report on CDR
indicates that there is medium-high confidence that the technique could
be efficient and scalable at low cost, with medium environmental risks. Ocean fertilization is estimated to be able to sequester 0.1 to 1
gigatonnes of carbon dioxide per year at a cost of US$8 to $80 per
tonne.
Ocean alkalinity enhancement involves grinding, dispersing, and dissolving minerals such as olivine, limestone, silicates, or calcium hydroxide to precipitate carbonate sequestered as deposits on the ocean floor. The removal potential of alkalinity enhancement is uncertain, and
estimated at between 0.1 and 1 gigatonnes of carbon dioxide per year at a
cost of US$100 to $150 per tonne.
Electrochemical techniques such as electrodialysis
can remove carbonate from seawater using electricity. While such
techniques used in isolation are estimated to be able to remove 0.1 to 1
gigatonnes of carbon dioxide per year at a cost of US$150 to $2,500 per
tonne, these methods are much less expensive when performed in conjunction with seawater processing such as desalination, where salt and carbonate are simultaneously removed. Preliminary estimates suggest that the cost of such carbon removal can
be paid for in large part if not entirely from the sale of the
desalinated water produced as a byproduct.
The cost of CDR differs substantially depending on the maturity of
the technology employed as well as the economics of both voluntary
carbon removal markets and the physical output; for example, the
pyrolysis of biomass produces biochar that has various commercial
applications, including soil regeneration and wastewater treatment. DAC cost from $94 to $600 per tonne, biochar from $200 to $584 per tonne and nature-based solutions (such as reforestation and afforestation) to be less than $50 per tonne. The fact that biochar commands a higher price in the carbon removal
market than nature-based solutions reflects the fact that it is a more
durable sink with carbon being sequestered for hundreds or even
thousands of years while nature-based solutions represent a more
volatile form of storage, which risks related to forest fires, pests,
economic pressures and changing political priorities. It is important to note that different CDR removal technologies could
have their design and operational advantages, for example, while
nature-based solutions are cheap, DAC plant that captures 1 MtCO2 per year using a land area of 0.4–1.5 km2 (99–371 acres) is equivalent to the CO2 capture rates of roughly 46 million trees, requiring approximately 3,098–4,647 km2 (765,494–1,148,241 acres) of land. The Oxford Principles for Net Zero Aligned Carbon Offsetting states
that to be compatible with the Paris Agreement: "...organizations must
commit to gradually increase the percentage of carbon removal offsets
they procure with the view of exclusively sourcing carbon removals by
mid-century." These initiatives along with the development of new industry standards
for engineered carbon removal, such as the Puro Standard, will help to
support the growth of the carbon removal market.
Although CDR is not covered by the EU Allowance as of 2021, the European Commission is preparing for carbon removal certification and considering carbon contracts for difference. CDR might also in future be added to the UK Emissions Trading Scheme. As of end 2021 carbon prices for both these cap-and-trade schemes
currently based on carbon reductions, as opposed to carbon removals,
remained below $100.After the diffusion of net-zero targets, CDR plays a more important
role in key emerging economies (e.g. Brazil, China, and India).
As of early 2023, financing has fell short of the sums required
for high-tech CDR methods to contribute significantly to climate change
mitigation. Though available funds have recently increased
substantially. Most of this increase has been from voluntary private
sector initiatives. Such as a private sector alliance led by Stripe with prominent members including Meta, Google and Shopify,
which in April 2022 revealed a nearly $1 billion fund to reward
companies able to permanently capture & store carbon. According to
senior Stripe employee Nan Ransohoff, the fund was "roughly 30 times the
carbon-removal market that existed in 2021. But it's still 1,000 times
short of the market we need by 2050." The predominance of private sector funding has raised concerns as
historically, voluntary markets have proved "orders of magnitude" smaller than those brought about by government policy. As of 2023
however, various governments have increased their support for CDR; these
include Sweden, Switzerland, and the US. Recent activity from the US
government includes the June 2022 Notice of Intent to fund the Bipartisan Infrastructure Law's $3.5 billion CDR program, and the signing into law of the Inflation Reduction Act of 2022, which contains the 45Q tax to enhance the CDR market.
Removal of other greenhouse gases
Although some researchers have suggested methods for removing methane, others say that nitrous oxide would be a better subject for research due to its longer lifetime in the atmosphere.
Carbon dioxide usually behaves as a gas in air at standard temperature and pressure (STP), or as a solid called dry ice when cooled and/or pressurised sufficiently. If the temperature and pressure are both increased from STP to be at or above the critical point for carbon dioxide, it can adopt properties midway between a gas and a liquid. More specifically, it behaves as a supercritical fluid above its critical temperature (304.128 K, 30.9780 °C, 87.7604 °F) and critical pressure (7.3773 MPa, 72.808 atm, 1,070.0 psi, 73.773 bar), expanding to fill its container like a gas but with a density like that of a liquid.
Supercritical CO 2 is becoming an important commercial and industrial solvent due to its role in chemical extraction, in addition to its relatively low toxicity and environmental impact. The relatively low temperature of the process and the stability of CO 2 also allows compounds to be extracted with little damage or denaturing. In addition, the solubility of many extracted compounds in CO 2 varies with pressure, permitting selective extractions.
Carbon dioxide is gaining popularity among coffee manufacturers looking to move away from classic decaffeinating solvents. sCO 2
is forced through green coffee beans which are then sprayed with water
at high pressure to remove the caffeine. The caffeine can then be
isolated for resale (e.g., to pharmaceutical or beverage manufacturers)
by passing the water through activated charcoal filters or by distillation, crystallization or reverse osmosis. Supercritical carbon dioxide is used to remove organochloridepesticides and metals from agricultural crops without adulterating the desired constituents from plant matter in the herbal supplement industry.
Supercritical carbon dioxide can be used as a solvent in dry cleaning.
Supercritical carbon dioxide is used as the extraction solvent for creation of essential oils and other herbal distillates. Its main advantages over solvents such as hexane and acetone
in this process are that it is non-flammable and does not leave toxic
residue. Furthermore, separation of the reaction components from the
starting material is much simpler than with traditional organic solvents. The CO 2 can evaporate into the air or be recycled by condensation into a recovery vessel. Its advantage over steam distillation is that it operates at a lower temperature, which can separate the plant waxes from the oils.
In laboratories, sCO 2
is used as an extraction solvent, for example for determining total
recoverable hydrocarbons from soils, sediments, fly-ash, and other
media, and determination of polycyclic aromatic hydrocarbons in soil and solid wastes. Supercritical fluid extraction has been used in determining hydrocarbon components in water.
Processes that use sCO 2 to produce micro and nano scale particles, often for pharmaceutical uses, are under development. The gas antisolvent process, rapid expansion of supercritical solutions, and supercritical antisolvent precipitation (as well as several related methods) process a variety of substances into particles.
Due to its ability to selectively dissolve organic compounds and assist enzyme functioning, sCO 2 has been suggested as a potential solvent to support biological activity on Venus- or super-Earth-type planets.
Manufactured products
Environmentally beneficial, low-cost substitutes for rigid thermoplastic and fired ceramic are made using sCO 2 as a chemical reagent. The sCO 2 in these processes is reacted with the alkaline components of fully hardened hydraulic cement or gypsumplaster to form various carbonates. The primary byproduct is water.
sCO 2 is used in the foaming of polymers.
Supercritical carbon dioxide can saturate the polymer with solvent.
Upon depressurization and heating, the carbon dioxide rapidly expands,
causing voids within the polymer matrix, i.e., creating a foam. Research is ongoing on microcellular foams.
sCO 2 is chemically stable, reliable, low-cost, non-flammable and readily available, making it a desirable candidate working fluid for transcritical cycles.
Supercritical CO2 is used as the working fluid in domestic water heat pumps. Manufactured and widely used, heat pumps are available for domestic and business heating and cooling. While some of the more common domestic water heat pumps remove heat
from the space in which they are located, such as a basement or garage,
CO2 heat pump water heaters are typically located outside, where they remove heat from the outside air.
Power generation
The unique properties of sCO 2
present advantages for closed-loop power generation and can be applied
to power generation applications. Power generation systems that use
traditional air Brayton and steam Rankine cycles can use sCO 2 to increase efficiency and power output.
The relatively new Allam power cycle uses sCO2 as the working fluid in combination with fuel and pure oxygen. The CO2 produced by combustion mixes with the sCO2 working fluid. A corresponding amount of pure CO2 must be removed from the process (for industrial use or sequestration). This process reduces atmospheric emissions to zero.
sCO2 promises substantial efficiency improvements. Due to its high fluid density, sCO2 enables compact and efficient turbomachinery. It can use simpler, single casing body designs while steam turbines
require multiple turbine stages and associated casings, as well as
additional inlet and outlet piping. The high density allows more
compact, microchannel-based heat exchanger technology.
For concentrated solar power, carbon dioxide critical temperature
is not high enough to obtain the maximum energy conversion efficiency.
Solar thermal plants are usually located in arid areas, so it is
impossible to cool down the heat sink to sub-critical temperatures.
Therefore, supercritical carbon dioxide blends, with higher critical temperatures, are in development to improve concentrated solar power electricity production.
Further, due to its superior thermal stability and
non-flammability, direct heat exchange from high temperature sources is
possible, permitting higher working fluid temperatures and therefore
higher cycle efficiency. Unlike two-phase flow, the single-phase nature of sCO 2
eliminates the necessity of a heat input for phase change that is
required for the water to steam conversion, thereby also eliminating
associated thermal fatigue and corrosion.
The use of sCO 2 presents corrosion engineering, material selection and design issues. Materials in power generation components must display resistance to damage caused by high-temperature, oxidation and creep.
Candidate materials that meet these property and performance goals
include incumbent alloys in power generation, such as nickel-based superalloys for turbomachinery components and austenitic stainless steels for piping. Components within sCO 2
Brayton loops suffer from corrosion and erosion, specifically erosion
in turbomachinery and recuperative heat exchanger components and intergranular corrosion and pitting in the piping.
Testing has been conducted on candidate Ni-based alloys,
austenitic steels, ferritic steels and ceramics for corrosion resistance
in sCO 2
cycles. The interest in these materials derive from their formation of
protective surface oxide layers in the presence of carbon dioxide,
however in most cases further evaluation of the reaction mechanics and
corrosion/erosion kinetics and mechanisms is required, as none of the
materials meet the necessary goals.
In 2016, General Electric announced a sCO2-based turbine that enabled a 50% efficiency of converting heat energy to electrical energy. In it the CO2
is heated to 700 °C. It requires less compression and allows heat
transfer. It reaches full power in 2 minutes, whereas steam turbines
need at least 30 minutes. The prototype generated 10 MW and is
approximately 10% the size of a comparable steam turbine. The 10 MW US$155-million Supercritical Transformational Electric Power
(STEP) pilot plant was completed in 2023 in San Antonio. It is the size
of a desk and can power around 10,000 homes.
Other
Work is underway to develop a sCO 2closed-cycle gas turbine
to operate at temperatures near 550 °C. This would have implications
for bulk thermal and nuclear generation of electricity, because the
supercritical properties of carbon dioxide at above 500 °C and 20 MPa
enable thermal efficiencies approaching 45 percent. This could increase
the electrical power produced per unit of fuel required by 40 percent or
more. Given the volume of carbon fuels used in producing electricity,
the environmental impact of cycle efficiency increases would be
significant.
Supercritical CO 2 is an emerging natural refrigerant, used in new, low carbon solutions for domestic heat pumps. Supercritical CO 2
heat pumps are commercially marketed in Asia. EcoCute systems from
Japan, developed by Mayekawa, develop high temperature domestic water
with small inputs of electric power by moving heat into the system from
the surroundings.
Supercritical CO 2 has been used since the 1980s to enhance recovery in mature oil fields.
"Clean coal" technologies are emerging that could combine such enhanced recovery methods with carbon sequestration. Using gasifiers
instead of conventional furnaces, coal and water is reduced to hydrogen
gas, carbon dioxide and ash. This hydrogen gas can be used to produce
electrical power In combined cycle gas turbines, CO 2
is captured, compressed to the supercritical state and injected into
geological storage, possibly into existing oil fields to improve yields.
Supercritical CO 2 can be used as a working fluid for geothermal electricity generation in both enhanced geothermal systemsand sedimentary geothermal systems (so-called CO 2 Plume Geothermal).EGS systems utilize an artificially fractured reservoir in basement
rock while CPG systems utilize shallower naturally-permeable sedimentary
reservoirs. Possible advantages of using CO 2
in a geologic reservoir, compared to water, include higher energy yield
resulting from its lower viscosity, better chemical interaction, and
permanent CO 2 storage as the reservoir must be filled with large masses of CO 2. As of 2011, the concept had not been tested in the field.
Aerogel production
Supercritical carbon dioxide is used in the production of silica, carbon and metal based aerogels. For example, silicon dioxide gel is formed and then exposed to sCO 2. When the CO 2 goes supercritical, all surface tension is removed, allowing the liquid to leave the aerogel and produce nanometer sized pores.
Sterilization of biomedical materials
Supercritical CO 2 is an alternative for thermal sterilization of biological materials and medical devices with combination of the additive peracetic acid (PAA). Supercritical CO 2
does not sterilize the media, because it does not kill the spores of
microorganisms. Moreover, this process is gentle, as the morphology,
ultrastructure and protein profiles of inactivated microbes are
preserved.
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