Photovoltaic: Technology of Next Millennium

Introduction 

Photovoltaic, or PV for short, is a solar power technology that uses solar cells or solar photovoltaic arrays to convert light directly into electricity with no emission of dangerous gases and with least amount of industrials waste. The first few years of 21^st century have witnessed a large development in the Photovoltaic energy generation and utilization. Due to variety of reasons including the concerns of deteriorating earth atmosphere and global warming, the PV technology has seen large increase in solar panel manufacturing and deployment world over, particularly in Japan and Germany. About 30-40% growth in the sector in last few years is a great incentive for investment. Entrepreneurs, venture capitalists and big industrial houses in the country are coming forward to establish industries in this area. The technology is expected to make a big splash in the Indian industrial world and solve the power crisis being faced in many states.

The main issue with the technology is its affordability. Intense R&D efforts are being made world over to find new materials, processes, and device structures to increase power conversion efficiency of basic unit so as to bring cost of solar generated power equivalent to that of power obtained from conventional sources.

Technology

The photovoltaic effect was first observed in 1839 by Alexandre-Edmond Becquerel. The first modern solar cell was patented by Russel Ohl in 1946. In 1954 Bell Laboratories found that silicon doped with certain impurities was very sensitive to light. This signaled the start of the modern age of solar power technology. The first practical application of photovoltaic was to power orbiting satellites and other spacecraft and pocket calculators, but today, the majority of photovoltaic modules are used for terrestrial grid connected power generations. There is a smaller market for off grid power for remote dwellings, roadside lamp posts, emergency telephones, remote sensing, and cathodic protection of pipelines. Based on different technologies and materials, the solar cells can be grouped into 4 different generations:

First generation photovoltaic cell: The cell consists of a large-area, single-crystal, single layer p-n junction diode, capable of generating usable electrical energy from light sources with the wavelengths of sunlight. The cells are typically made using a diffusion process with silicon wafers. These silicon wafer-based solar cells are the dominant technology in the commercial production of solar cells, accounting for more than 86% of the terrestrial solar cell market.

Second generation photovoltaic cell: These cells are based on the use of thin epitaxial deposits of semiconductors on lattice-matched wafers. There are two classes of epitaxial photovoltaic – space and terrestrial. Space cells typically have higher AM0 efficiencies (28-30%) in production, but have a higher cost per watt. Their “thin-film” cousins have been developed using lower-cost processes, but have lower AM0 efficiencies (7-9%) in production. There are currently a number of technologies/semiconductor materials under investigation or in mass production. Examples include amorphous silicon, polycrystalline silicon, micro-crystalline silicon, cadmium telluride, copper indium selenide/sulfide. An advantage of thin-film technology theoretically results in reduced mass so it allows fitting panels on light or flexible materials, even on textiles. Second generation solar cells now comprise a small segment of the terrestrial photovoltaic market, and approximately 90% of the space market.

Third-generation photovoltaic cell: They are proposed to be very different from the previous semiconductor devices as they do not rely on a traditional p-n junction to separate photo-generated charge carriers. For space applications quantum well devices (quantum dots, quantum ropes, etc.) and devices incorporating carbon nanotubes are being studied – with a potential up to 45% AM0 production efficiency. For terrestrial applications, these new devices include photoelectrochemical cells, polymer solar cells, nano-crystal solar cells, dye-sensitized solar cells and are still in the research phase.

Fourth Generation Photovoltaic cell: This hypothetical generation of solar cells may consist of composite photovoltaic technology, in which polymers with nano-particles can be mixed together to make a single multi-spectrum layer. The multi-spectrum layers can be stacked to make multi-spectrum solar cells more efficient and cheaper.

Out of the four generations listed above, first two have been commercialized. Bulk of the photovoltaic modules deployed so far consist of crystalline silicon. The efficiency of crystalline silicon modules varies from 17-22%, though theoretical limit is around 29%. Using these modules, large solar farms connected to grid, stand alone power stations to electrify villages and small localities have been established. The silicon modules have also been integrated to house hold and commercial buildings to provide the main or alternate electrical energy.

Efficiency of a solar cell depends on its ability to absorb solar radiation. Larger the fraction of solar radiation it absorbs, larger will be its efficiency and larger power it will generate. Taking this into account, multi-junction solar cells have been fabricated. The efficiency of triple junction, state of the art solar cell of 40.7%  has been recorded by Spectrolab, USA. The R&D work to improve efficiency further is going on by using 4, 5 or 6 junction solar cells. Using these high efficiency solar cells and focusing solar light to 500X, high efficiency solar concentrator has been devised to give electrical power of few KWp, enough to light up a household of small family. The use of light reflector have reduced the actual device size of the device, thus reducing the usage and price of much costlier semiconductor materials.

The efficiency of basic solar cell unit has further been increased to 42.8% using an altogether novel concept. The credit goes to a group of scientists under the leadership of University of Delaware, USA. The cells achieve this by splitting the incoming light into high, low and medium energy chunks. The light is then directed to different state of the art devices optimized to respective chunks of radiation leading to higher efficiency. The concept is yet to be commercialized.

The last two generations of solar cells are still at research and development stage. It will take some more years to understand the underlying science and technology to bring them to commercial level.

Polysilicon prices expected to stabilise in the near term

Demand for silicon used to make solar cells is likely to rise from 41,000 tonnes in 2006 to 120,000 tonnes in 2010E. Although polysilicon prices have risen the world over, causing concerns on cell/ module makers’  profitability, we believe these prices are due for a correction by Q1/Q2CY09 as more capacities come on stream.

What drives cost reduction for solar energy?

While polysilicon prices are a key cost driver, cell efficiency as well as wafer thickness play important roles in driving costs down. Currently, wafer thickness ranges between 200 nm and 300 nm; it is expected to come down with a theoretical limit of 130 nm, after which wafer becomes transparent and difficult to handle.

A 70 micron decrease in thickness, leads to a 10-15% increase in wafer output or 10-15% cost reduction. A 1% increase in cell efficiency leads to a ~7% cost reduction at all levels in the value chain. Economies of scale also play an important role in cost reduction. Other levers for cost reduction include increase in throughput, reduced breakages, increased proportion of A cells, and increase in uptimes. With a robust 40% demand CAGR expected over CY06-10E, cell capacities are also growing at a rapid pace.

Solar PV: The sun is rising on the global power scenario

From a low base (<1% of global energy consumption, 2.5 GW of sales in CY06), solar photovoltaic over green-house emissions, decreasing cost of solar energy, and increasing government support  are the major growth drivers.

 

Demand side perspective: Opportunity for solar power

Solar constitutes a very small fraction (<1%) of the global electricity production, as of now. However,it has been growing rapidly on a low base, driven by cost reductions, favorable government policies and subsidies and also due to its being an environment-friendly source of energy.

The global solar industry is poised to expand from ~USD 11.7 bn in CY06 to >USD 52 bn in CY11E (in terms of solar systems sold). 

The cost of solar electricity (at > 25 cents/ Kwh) continues to be significantly higher than those of conventional electricity (<10 cents/ Kwh) and this has limited market growth historically.

 

Although production from solar systems varies according to the time of the day, peak output coincides with peak electricity demand. As peak electricity tariffs tend to be substantially higher than normal and comparable to solar power costs, PV systems are becoming economically more viable and hence, can be used as sources of peak power.

Supply side perspective

Technology is the key in this industry because it is a necessary component to create cost reductions. Within the supply chain in photovoltaic, the cell step affords a degree of differentiation, which drives cost reduction. The efficiency and production costs of solar cells are crucial for the efficiency of the entire solar module.

As the industry evolves, multiple players are expected to put up capacities in different stages of the solar value chain. As cell and module making stages require fewer investments and are quicker to ramp up, it is not surprising that capacity has built up more rapidly in these stages, which has resulted in raw material (silicon)  shortage. 

Silicon capacities take a long time to scale up as the manufacturing process is technologically cumbersome and requires huge scale for economic viability.

Photovoltaics -Information

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Photovoltaic, or PV for short, is a solar power technology that uses solar cells or solar photovoltaic arrays to convert light directly into electricity. Solar cells produce direct current (DC) electricity from light, which can either be used directly or through batteries to power equipments. Though the photovoltaic effect was first observed in 1839 by Alexandre-Edmond Becquerel, the first modern solar cell was patented by Russel Ohl in 1946.

 

 In 1954 Bell Laboratories found that silicon doped with certain impurities was very sensitive to light. This signaled the start of the modern age of solar power technology. The first practical application of photovoltaics was to power orbiting satellites and other spacecraft and pocket calculators, but today the majority of photovoltaic modules are used for grid connected power generation. In this case an inverter is required to convert the DC to AC. There is a smaller market for off grid power for remote dwellings, roadside lamp posts, emergency telephones, remote sensing, and cathodic protection of pipelines. The most important issue with solar panels is capital cost (installation and materials). Because of much increased demand, the price of silicon has risen and shortages occurred in 2005 and 2006.

 

Newer alternatives to standard crystalline silicon modules includes casting wafers instead of sawing , thin film (CdTe, CIGS, amorphous Si, microcrystalline Si), concentrator modules, ‘Sliver’ cells, and continuous printing processes. Due to economies of scale solar panels get less costly as people use and buy more as manufacturers increase production to meet demand, the cost and price is expected to drop in the years to come.

There are many materials and structures used in manufacturing of solar cells so as to increase their efficiency. Following solar cell configurations have been used to increase the conversion efficiency:

1. Back Surface Field: In this the front surface is made in the normal way, but the back of the cell, instead of containing just a metallic ohmic contact, has a very heavily doped region adjacent to the contact.

2. Violet Cell: The violet cell is fabricated with reduced surface doping concentration and smaller junction depth. The combination of higher lifetime near the surface and narrower junction greatly improves the response at high photon energies.

3. Textured Cell: The textured cell has pyramidal surfaces produced by anisotropical etching of <100>-oriented Si surface. The incident light on the side of a pyramid gets reflected onto another pyramid instead of getting reflected. Thus the reflection of incident light is reduced and efficiency increases.

4. Vertical Junction Solar Cell: It has both the junction and metallization perpendicular to the cell surface. The diffusion and metal contacts are embedded in deeply etched grooves normal to the surface, formed by anisotropic etching of <100> Si.

5. Heterojunction Solar Cell: the solar cells are made by using semiconductors of different bandgaps. The higher bandgap material is placed on the front and the lower bandgap material at the back of the cell.

6. Thin Film Solar Cell: The active semiconductor layers poly crystalline or disordered films that have been deposited or formed on electrically active or passive substrates.

7. Optical Concentrator: Sunlight is focused, using mirrors and lenses, on a solar cell so as to achieve higher efficiency. As the incident power increases from 1 SUN to 1000 SUNS the short circuit current increases linearly along with open circuit voltage and thus the efficiency.

Based on different technologies and materials, the solar cells can also be grouped into 4 different generations:

First generation photovoltaic cell: The cell consists of a large-area, single-crystal, single layer p-n junction diode, capable of generating usable electrical energy from light sources with the wavelengths of sunlight. The cells are typically made using a diffusion process with silicon wafers. These silicon wafer-based solar cells are the dominant technology in the commercial production of solar cells, accounting for more than 86% of the terrestrial solar cell market.

Second generation photovoltaic cell: These cells are based on the use of thin epitaxial deposits of semiconductors on lattice-matched wafers. There are two classes of epitaxial photovoltaics – space and terrestrial. Space cells typically have higher AM0 efficiencies (28-30%) in production, but have a higher cost per watt. Their “thin-film” cousins have been developed using lower-cost processes, but have lower AM0 efficiencies (7-9%) in production. There are currently a number of technologies/semiconductor materials under investigation or in mass production. Examples include amorphous silicon, polycrystalline silicon, micro-crystalline silicon, cadmium telluride, copper indium selenide/sulfide. An advantage of thin-film technology theoretically results in reduced mass so it allows fitting panels on light or flexible materials, even textiles. Second generation solar cells now comprise a small segment of the terrestrial photovoltaic market, and approximately 90% of the space. market.

Third-generation photovoltaics cell: They are proposed to be very different from the previous semiconductor devices as they do not rely on a traditional p-n junction to separate photogenerated charge carriers. For space applications quantum well devices (quantum dots, quantum ropes, etc.) and devices incorporating carbon nanotubes are being studied – with a potential for up to 45% AM0 production efficiency. For terrestrial applications, these new devices include photoelectrochemical cells, polymer solar cells, nanocrystal solar cells, Dye-sensitized solar cells and are still in the research phase.

Fourth Generation Photovoltaic cell: This hypothetical generation of solar cells may consist of composite photovoltaic technology, in which polymers with nano particles can be mixed together to make a single multispectrum layer. Then the thin multispectrum layers can be stacked to make multispectrum solar cells more efficient and cheaper based on polymer solar cell and multijunction technology used by NASA on Mars missions. The layer that converts different types of light is first, then another layer for the light that passes and last is an infra-red spectrum layer for the cell�thus converting some of the heat for an overall solar cell composite.

Solar cells just got a boost in efficiency thanks to a couple of scientists at the University of Delaware. The new cells can convert 42.8% of the light that strikes them into electricity, which is a step up from the previous record of 40.7%. The cells do this by splitting light into high energy, low energy, and medium energy chunks. The light is then directed to different materials depending on what type it is, eeking electrons out of it. Government is pushing for a goal of 50% efficiency, which would go a long way towards making solar power a more practical alternative energy source.

 

 

CO2 and Climate Change

We know that carbon dioxide (CO2) is increasing in the atmosphere from human activities such as burning of fossil fuels and deforestation. This increase is one of the major factors in global warming. There is no longer any scientific debate about this. The most recent report by the Intergovernmental Panel on Climate Change has confirmed this.

In 2007 the concentration of CO2 in the atmosphere is approximately 380 parts per million (ppm). Every year human activities add to that. Some scientists and economists in the climate science world such as David Stern and James Hansen have identified a concentration of 450 ppm as a maximum goal for CO2 that may avoid the most significant damage to the Earth’s ecosystems and economies. There is a great deal of uncertainty about the severity of the effects associated with this or any other target level for CO2.  We have chosen to use it for this simulation, but we could have set it higher or lower.  As you play with the simulation consider how the three scenarios would play out if the bathtub overflowed at a level other than 450 ppm.

Already there is much more CO2 in the atmosphere than at any time in the past 425,000 years. Follow the green line to the right edge of this graph. This is where we are today.

The Carbon Cycle

Carbon is constantly entering the atmosphere in the form of carbon dioxide, methane, and other gases. At the same time, it is being removed by green plants, the oceans, and in other ways. This is the carbon cycle. The balance in the cycle is critical in determining the Earth’s climate.

Carbon atoms are continuously being exchanged between living and dead organisms, the atmosphere, oceans, rocks, and soil. With every outward breath, we release CO2 from our lungs into the atmosphere, containing atoms of carbon from plants and animals that we have eaten. Atoms of carbon in our bodies today might previously have been in many different plants and animals – perhaps including dinosaurs and other extinct creatures.

The distribution of carbon among atmosphere, organisms, land, and oceans has changed over time. About 550 million years ago the concentration of CO2 in the atmosphere was 7,000 parts per million – more than 18 times what it is today. Where did all that atmospheric carbon go? For the most part it ended up as sedimentary rocks such as limestone. How that happened is part of the larger story of the carbon cycle.

The carbon cycle is a combination of many biological, chemical, and physical processes that move carbon around.

Carbon Dioxide Sinks and Sources

A carbon dioxide sink is something that removes CO2 from the atmosphere. For example, green plants consume CO2 during the process of photosynthesis. Burning wood and fossil fuels are sources of CO2. The oceans are both a source of CO2 and a CO2 sink. This is because CO2 in air that is in contact with the surface of the ocean dissolves in water and is therefore removed from the atmosphere. At the same time, dissolved CO2 is released into the atmosphere. The balance between these two processes depends on many factors and is changeable over time. Presently there is more CO2 dissolving into the oceans than is being released. This means that right now the oceans are a CO2 sink.

Most of the carbon on Earth is in compounds found in sediments and sedimentary rocks. Comparatively little is in the atmosphere.
		Billion Metric Tons
	Underwater sediments and sedimentary rocks	80,000,000
	Ocean water, shells, and organisms	40,000
	Fossil fuels (oil, gas, and coal)	4,000
	Organic material in soil	1,500
	Atmosphere	825
	Land plants	580

Questions about CO2 and Climate Change

What are CO2  emissions?

 Carbon dioxide (CO2) is a gas that makes up a tiny fraction of the Earth’s atmosphere. It occurs naturally, mostly as a result of breathing, of decay, from the burning of wood and the release of CO2 from the oceans. CO2 emissions also result from the burning of fossil fuels and other human activities.  It is this human-generated CO2 that we are showing in our simulation.

What are CO2 removals?

Carbon sinks remove carbon from the atmosphere. The main carbon sinks responsible for removals are photosynthesis and absorption by the oceans.

The oceans are both a carbon sink and a source of CO2. There is an ongoing exchange of CO2 between the atmosphere and the oceans. The balance depends upon factors including water temperature and the concentrations of CO2 in both the oceans and the atmosphere.

For hundreds of thousands of years emissions and removals remained roughly in balance with the concentration of CO2 in the atmosphere varying between 180 and 300 parts per million (ppm). This was true until humans began to burn fossil fuels during the Industrial Revolution. These additional CO2 emissions are the problem. Currently much more CO2 is being released than can be taken up by plants or absorbed by the ocean. The concentration of CO2 in the atmosphere is now 380 ppm and rising.

Why do removals seem to follow emissions?

Carbon dioxide flows between the atmosphere, biosphere, and oceans in order to maintain a balanced distribution. When the concentration of CO2 in the atmosphere increases, two things happen:

• “CO2 fertilization” occurs. Plants use more CO2 for photosynthesis, growing more leaves and woody material.
• The surface ocean—mixed by wind-driven waves— quickly absorbs CO2, which then diffuses more gradually into the deep ocean.

Both processes have limits. The oceans can only absorb so much CO2 before releasing as much CO2 back to the atmosphere as was taken up. For plants, the limitations on growth from water and other nutrients become important. This is called “sink saturation.”

In the “Allow Increased Emissions” future, removals increase because the rapidly-growing concentration of CO2 in the atmosphere continues to drive uptake. Part of the excess CO2 is absorbed by plants and the oceans.

In the “Reduce CO2 Emissions” future, removals fall because the excess of CO2 in the atmosphere above that in the biosphere and oceans is not so great.

What’s the connection between CO2  and climate change?  

We know that CO2 absorbs heat from the Sun and releases it into the atmosphere. Going back millions of years, when the concentration of CO2 was higher, the Earth was warmer. Eventually CO2 concentration dropped and the world became cooler. Since the 1740s CO2 concentration has increased significantly, and the average temperature on Earth has also increased.

 Why does the CO2 level in the atmosphere continue to rise even when emissions are leveled off?
This scenario corresponds to clicking the middle button in our simulation: “LEVEL OFF CO2 EMISSIONS.” After about 2045 emissions are no longer increasing. At that point removals are also level from year to year. But since emissions are greater than removals, each year more CO2 goes into the atmosphere than is removed. So the amount of CO2 in the atmosphere continues to rise. 

It’s like a bus traveling through the city with people getting on and off. Let’s say that at one stop 5 people get on the bus and 3 get off. At the next stop the same thing happens: 5 people get on and 3 get off. If this pattern continues the bus will get very crowded. The number of people getting on the bus is level: 5 at each stop. But since only three people get off there is an increase of 2 people each time the bus stops. In order to keep the crowding from getting worse, the same number of people have to get off the bus as get on.  And to reduce the crowding, more people have to get off than get on.

In order to keep the concentration of CO2 in the atmosphere at a given level, say 450 ppm, emissions and removals have to be equal.  In order to reduce the concentration of CO2 in the atmosphere, removals have to be greater than emissions.

CO2 capture-and -storage project

The Sleipner field is in the North Sea, about 250 km (155 mi) west of Stavanger, Norway. It is operated by Statoil, Norway’s largest oil company. The Sleipner field produces natural gas and condensate (light oil) from the Heimdal sandstones, which are about 2,500 m (8,200 ft) below sea level. Natural gas is a mixture of gases. It is typically at least 90% methane, plus other hydrocarbons such as ethane and propane. Natural gas often also contains gases such as nitrogen, oxygen, and carbon dioxide; sulfur compounds; and water. Gas containing small volumes of these impurities can still be used as fuel, but gas with high volumes of them cannot be burned efficiently and safely.

The natural gas produced at Sleipner contains unusually high levels (about 9%) of carbon dioxide (CO2), but the customers buying the gas from Statoil need less than 2.5%.

A special platform, Sleipner-T, has been built to support a 20-m-high (65-ft), 8,000-ton treatment plant that separates CO2 from the natural gas. The Sleipner-T plant produces about 1 million tons of CO2 per year.

To encourage companies to reduce their carbon emissions, the Norwegian government imposes a carbon tax equivalent to about $50 per ton of CO2 released into the atmosphere. To avoid paying this tax, and as a test of alternative technology, all of the CO2 extracted since 1996, when gas production started at Sleipner, has been pumped back deep underground.

 It is not put back where it came from, because that would further contaminate the natural gas. Instead, it is put into a 200-m-thick (650-ft) sandstone layer called the Utsira formation, about 800 m (2,600 ft) beneath the bottom of the North Sea. The Utsira formation contains no commercial oil or gas; like most rocks deep underground, it is filled with salt water. The Utsira formation has high porosity and permeability, so the CO2 moves rapidly sideways and upward through the rock layer, replacing the water between the sand grains.

It is estimated that it would take about 600 billion tons of CO2 to fill all the pore space of the Utsira sandstone. That is equivalent to all the human-made CO2 production for over 20 years, at current rates. It is likely that CO2 sequestration will continue at Sleipner long after the abandonment of the field as a hydrocarbon producer. The Utsira formation is just one of many similar deep saline aquifers around the world that could be used to help slow down or reverse the rate at which CO2 and other greenhouse gases are released into the atmosphere.

The Sleipner project is the first commercial example of CO2 storage in a deep saline aquifer, so there is a lot of interest from around the world in its success. In particular, scientists want to know how the CO2 moves inside the aquifer and if there is a risk that it could escape back to the surface.

Carbon dioxide capture, transport and storage technologies

Carbon dioxide capture and storage (CCS) in geological formations is a potentially important climate change mitigation measure in the coming decades, as geological formations can store large amounts of CO2 (as well as other gases or liquids) for thousands of years. However, CCS has not been widely used to date. The largest CO2 storage project to date (at the Sleipner field in the North Sea) has been injecting approximately 1 million tonnes of CO2 per year since 1996 into a saline formation.

         

CO2 emissions are produced from a wide variety of combustion-related and industrial processes sources.CO2 emissions from large point sources, such as power plants, refineries or cement plants, could be captured, transported and stored in several different ways.The technologies used within each step of the carbon dioxide capture and storage chain are at different stages of development. Some are mature/widely applied; some are economically feasible in some conditions, while other technologies are at the demonstration phase. Geological CCS projects (developed as CDM projects, or not) could involve   different combinations of capture, transportation and storage technologies. In turn, this could lead to a wide range of potential CDM project types.

 

CO2 capture already occurs in some energy and industrial activities. For example, CO2 separation routinely occurs in industries where CO2 is required as an input to a manufacturing process (e.g. production of urea). CO2 must be stripped from natural gas during exploitation of fields with significant CO2 content. CO2 is also extracted in refineries, ammonia plants and hydrogen plants.

 

“Pre-combustion” separation of CO2 can occur during the partial combustion of fossil fuels, used for example in the production of hydrogen or hydrogen-rich fuels. Electricity generation from Integrated Gasification Combined Cycle (IGCC) plants also requires pre-combustion separation of CO2.

 

“Postcombustion” CO2 capture from flue gases is also possible, e.g. via absorption or flue gas treatment (IPCC 2005). This can be used to capture CO2 from electricity generation plants (and indeed is the only option for CO2 capture from existing power plants), although it is energy-intensive and so entails an energy penalty. Oxyfuel combustion refers to a technology under development whereby fuel is combusted in oxygen and re-circulated flue gas, rather than air (which is mainly made up of nitrogen). The exhaust gases from oxyfuel combustion contains thus mainly CO2 and H2O (water vapour), rather than nitrogen. As the vapour can be easily condensed, the waste gas is largely CO2.

 

Transport of CO2 can be done by pipeline or ship. Commercial-scale transport of CO2 via pipeline and ship/tankers already occurs (IPCC 2005). Pipeline transport is normally of compressed (gaseous) CO2, whereas transport on ships is often of liquefied CO2, as this takes less volume. Liquefaction of gases is routinely used, e.g. for the transport of liquefied petroleum gas (LPG) or liquefied natural gas (LNG).

 

There are also different ways in which CO2 can be stored. These include various underground geological formations such as oil and gas fields (in use or abandoned), saline formations or coal seams (mineable or unmineable). Experience with storing CO2 in these types of formations varies. For example, the largest CO2 storage project to date (at the Sleipner field in the North Sea) has been injecting approximately 1 million tons of CO2 per year since 1996 into a saline formation. Other demonstrations, pilot or commercial projects, exist to inject CO2 into depleted gas fields and coal mines.

 

CO2 is being re-injected into various oil fields to increase the rate and amount of oil produced. Such enhanced oil recovery (EOR) can also use other fluids for the same purpose, notably water and steam.

 

Globally, CO2-based EOR projects inject around 40 million tonnes of CO2 per year – of which 30 million come from natural underground sources of CO2 and about 10 million tonnes is captured from industrial  plants. The use of CO2 for EOR can provide a valuable near-term opportunity for gaining storage  experience, but this needs to be done at the right time in the life of a particular field. Enhanced gas recovery and enhanced coal bed methane recovery are in development phase.