Compressed air energy storage (CAES) is a way to store the energy generated at one time for use at a later time by using compressed air. On a utility scale, the energy generated during the low-energy demand period (off-peak) can be released to meet the higher demand period (peak load). Small-scale systems have long been used in applications such as mine locomotive propulsion. Large-scale applications should conserve heat energy associated with compressed air; Wasting heat lowers the energy efficiency of storage systems.
Video Compressed air energy storage
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Air compression creates heat; warmer air after compression. Expansion removes heat. If no additional heat is added, the air will cool down after expansion. If heat generated during compression can be stored and used during expansion, storage efficiency is greatly increased. There are three ways in which CAES systems can handle heat. Air storage can be adiabatic, diabatic, or isothermal.
Adiabatic
Adiabatic storage keeps the heat generated by compression and returns it to the air when it is expanded to produce power. This is the subject of ongoing studies, without any utility-scale plant by 2015, but the German project ADELE plans to bring a pilot plant (360 MWH storage capacity) to service by 2016. The theoretical efficiency of adiabatic storage is close to 100% perfect, but in practice the round trip efficiency is estimated to reach 70%. Heat may be stored in solid form such as concrete or stone, or more likely in liquids such as hot oil (up to 300 ° C) or aqueous salt solution (600 ° C).
Diabatic
Dynamic storage removes much heat of compression with the intercooler (thus approaching isothermal compression) into the atmosphere as waste; essentially discarding, thus, the renewable energy used to perform compression work. Once removed from storage, this compressed air temperature is one indicator of the amount of stored energy left in this air. Consequently, if the air temperature is low for the energy recovery process, air must be heated substantially before the expansion in the turbine to power the generator. This reheating can be done with a fired natural gas burner for utility level storage or with a heated metal mass. Since recovery is often most needed when renewable sources are stationary, fuel must be burned to make up for the wasted heat . This lowers the efficiency of the storage-recovery cycle; and while this approach is relatively simple, fuel combustion adds to the cost of recovered electrical energy and jeopardizes the ecological benefits associated with most renewable energy sources. Nevertheless, it is by far the only system that has been commercially deployed.
The McIntosh, Alabama CAES plant requires 2.5 MJ of electricity and 1.2 MJ lower heating value (LHV) of gas for each MJ of energy output, corresponding to an energy recovery efficiency of about 27%. The 2X1 General Electric 7FA combined power plant, one of the most efficient natural gas plants in operation, uses 1.85 MJ (LHV) of gas per MJ produced, 54% thermal efficiency.
Isothermal
The isothermal compression and expansion approach seeks to maintain operating temperatures with constant heat exchange to the environment. They are only practical for low power levels, without very effective heat exchangers. Theoretical efficiency of isothermal energy storage is close to 100% for perfect heat transfer to the environment. In practice none of these perfect thermodynamic cycles can be obtained, as some heat loss is inevitable.
Near Isothermal
Near isothermal compression (and expansion) is a process in which air is compressed in very close proximity to a large uncompressed thermal mass such as heat absorbing and releasing structures (HARS) or water sprays. A HARS usually consists of a series of parallel fins. When the air is compressed, the heat of compression is quickly transferred to the thermal mass, so that the gas temperature becomes stable. The external cooling circuit is then used to maintain thermal mass temperature. Isothermal efficiency (Z) is the measure at which the process lies between adiabatic and isothermal processes. If the efficiency is 0%, then it is fully adiabatic; with 100% efficiency, it is completely isothermal. Usually with an isothermal process near the 90-95% efficiency can be expected.
More
One implementation of isothermal CAES uses a high, medium and low pressure piston in series, with each stage followed by venturi airblast pumps that draw the surrounding air through an air-to-air (or air-to-sea) air exchanger between each expansion stage. The initial compressed air torpedo design uses a similar approach, replacing seawater. Venturi warms the exhaust from the previous stage and recognizes this heated air to the next stage. This approach is widely adopted in various compressed air vehicles such as mining and tram locomotives belonging to H. K. Porter, Inc. Here the heat of compression is effectively stored in the atmosphere (or ocean) and returned later.
Maps Compressed air energy storage
Compressor and expander
Compression can be done with an electric-powered turbo-compressor and expansion with a turbo 'expanders' or an air machine that drives an electric generator to generate electricity.
Storage
The CAES (Compressed Air Energy Storage) storage system is one of the most interesting characteristics of this technology, and it is strongly linked to economic feasibility, energy density and flexibility. There are several categories of air-storage vessels, based on storage thermodynamic conditions, and on selected technologies:
- Constant Volume Storage (Mined cave solution, ship above ground, aquifer, automotive applications, etc.)
- Constant Pressure Storage (Underwater Pressure Vessels, Hybrid Pumped Hydro - Compressed Air Storage)
Constant Volume Storage.
This storage system uses space with rigid boundaries to store large amounts of air. This means from a thermodynamic point of view, that this system is a Constant Volume and a Variable Pressure system. This causes some operational problems on the compressors and turbines operating on them, so pressure variations must be kept below a certain limit, as does the pressure induced on the storage vessel.
Ship storage is often an underground cave created by solution mining (salt dissolved in water for extraction) or by utilizing abandoned mines; the use of porous rock formations (rocks that have a hole through which fluid or air may pass) such as where natural gas reservoirs have been found have also been studied.
In some cases also the above ground pipe is tested as a storage system, providing some good results. Obviously the cost of the system is higher, but it can be placed wherever the designer chooses, while the underground system requires some particular geological formation (salt domes, aquifers, gas mines run out.. etc.).
Constant Pressure Storage
In this case the storage vessel is kept at a constant pressure, while the gas is contained in a variable volume vessel. Many types of storage vessels have been proposed, but the operating conditions follow the same principle: The storage vessel is positioned hundreds of meters below water and the hydrostatic pressure of the water column above the storage vessel allows maintaining the pressure at the desired level.
This configuration allows:
- Increase the energy density of the storage system, since all the contained air can be used (constant pressure in all load conditions, full or empty, the pressure is the same, so the turbine has no problem exploiting it, while with a constant volume system after some when the pressure is below the security limit and the system must stop)
- Increase the efficiency of the turbomachinery, which will work under constant entry conditions.
- Open for use of different geographic locations to position CAES plants (shoreline, floating platforms, etc.)
On the other hand, the cost of this storage system is higher, because of the need to position the storage vessel at the bottom of the selected water reservoir (often sea or sea) and because of the cost of the vessel itself.
Plants operate on a daily cycle, charging at night and use during the day. The heating of compressed air using natural gas or geothermal to increase the amount of extracted energy has been studied by the Northwest Pacific National Laboratory.
The compressed air energy storage can also be used on a smaller scale as exploited by air-driven cars and locomotives, and can use high-carbon carbon-fiber storage tanks. To maintain energy stored in compressed air, this tank must be thermally insulated from the environment; otherwise, stored energy will pass under a hot form because it compresses the air to raise its temperature.
History
Transmission
The city's compressed air energy system has been built since 1870. Cities like Paris, France; Birmingham, England; Dresden, Rixdorf and Offenbach, Germany and Buenos Aires, Argentina installed such a system. Victor Popp built the first system to turn on the clock by sending air pulses every minute to change their pointing arms. They are rapidly evolving to produce power to home and industry. In 1896, the Paris system had a 2.2 MW generation that was distributed at 550 kPa at 50 km of air pipe for motors in light and heavy industries. Usage is measured by cubic meter. This system was the main source of energy delivered home at the time and also supported by dentist machines, tailors, printing facilities and bakeries.
Storage
- 1978 - The first utility-scale compressed air energy storage project is 290 megawatts of Huntorf in Germany using a salt dome.
- 1991 - A 110-megawatt plant with a capacity of 26 hours built in McIntosh, Alabama (1991). The facility cost in Alabama for $ 65 million succeeds up to $ 590 per kW of generating capacity and about $ 23 per kW-hr of storage capacity, using a 19 million cubic feet caked salt cave solution to store air up to 1100 psi. Although the compression phase is approximately 82% efficient, the expansion phase requires the combustion of natural gas at one third the rate of gas turbines generating the same amount of electricity.
- December, 2012 - General Compression completes construction of a 2-MW CAES near-isothermal project in Gaines, TX; the third CAES project in the world. This project does not use fuel.
Project
- The Huntorf plant in Germany (290 MW) is diabetic. Energy 580 MWh, efficiency 42%.
- McIntosh Factory in Alabama, USA (110 MW) diabatis. 2,860 MWh of energy, 54% efficiency.
- November 2009 - US Department of Energy delivers $ 24.9 million matching fund for phase one of 300 million MW, $ 356 million Pacific Gas and Electric CAES using a salty porous rock formation developed near Bakersfield in Kern County, California. The purpose of this project is to build and validate advanced designs.
- December, 2010 - The US Department of Energy provides $ 29.4 million in funding to do initial work on a 150 MW salt-based CAES project developed by Iberdrola USA in Watkins Glen, New York. The goal is to combine smart grid technology to balance renewable intermittent energy sources.
- 2013 - The first adiabatic CAES project, a 200 megawatt facility named ADELE, is planned for development in Germany. This project has been suspended for undisclosed reasons until at least 2016.
- 2017 (projected) - Storelectric Ltd plans to build a 40Ã, 40 MW renewable energy plant in Cheshire, UK, with storage capacity of 800Ã, MWh. "It will be 20 times larger than the CAES 100% renewable energy built so far, representing a step change in the storage industry." according to their website.
- 2020 (projected) - Apex has planned a CAES plant for Anderson County, Texas to go online by 2016. This project has been suspended and will not run until Summer 2020.
- Larne, Northern Ireland - the 330 MW CAES project for a two-cave mining solution in a salt deposit, backed by the EU with EUR90 million.
- The EU-funded RICAS 2020 (adiabatic) project in Austria uses crushed stones to store heat from the compression process to improve efficiency. The system is expected to achieve 70-80% efficiency.
Storage thermodynamics
To achieve a reversible thermodynamic process approaching so much of the energy is stored in the system and can be taken, and the stored losses are ignored, isothermal process near reversible or desirable isentropic processes.
Isothermal Storage
In the isothermal compression process, the gas in the system is stored at a constant temperature. It certainly requires a heat exchange with gas, otherwise the temperature will rise during charging and down during discharge. This heat exchange can be achieved by intercooling between the next stage in the compressor, the regulator and the tank. To avoid wasted energy, intercooler must be optimized for high heat transfer and low pressure drop. Smaller compressors can estimate isothermal compression even without intercooling, due to the relatively high ratio of surface area to the volume of compression chamber and the resulting increase in heat dissipation from the compressor body itself.
When a person gets perfect isothermal storage (and discharge), this process is said to be "reversible". This requires that the heat transfer between the environment and the gas occurs above a very small temperature difference. In this case, there is no loss of excess in the heat transfer process, so the compression work can be recovered completely as an expansion job: 100% storage efficiency. However, in practice, there is always a temperature difference in every heat transfer process, so that all practical energy storage obtains an efficiency lower than 100%.
Untuk memperkirakan pekerjaan kompresi/ekspansi dalam proses isotermal, dapat diasumsikan bahwa udara terkompresi mematuhi hukum gas ideal,
- .
di mana , dan begitu, . Di sini, adalah tekanan absolut, adalah volume kapal, adalah jumlah substansi gas (mol) dan adalah konstanta gas ideal.
Jika ada tekanan konstan di luar kapal yang sama dengan tekanan awal , pekerjaan positif dari tekanan luar mengurangi energi yang dapat dieksploitasi (nilai negatif). Ini menambahkan istilah untuk persamaan di atas:
Contoh
Berapa banyak energi yang dapat disimpan dalam bejana penyimpanan 1 m 3 pada tekanan 70 bar (7,0 MPa), jika tekanan ambient adalah 1 bar (0,10 MPa). Dalam hal ini, proses kerjanya adalah
- =
- = 7.0Â MPa ÃÆ' 1 m 3 ÃÆ' ln (0,1 MPa/7,0 MPa) (7,0 MPa - 0,1 MPa) x 1 m 3 = -22.8 MJ (setara 6.33 KWh).
A negative sign means that work is done on the surrounding gas. The irreversibilities process (as in heat transfer) will result in less energy being recovered from the expansion process than is required for the compression process. If the environment is at a constant temperature, for example, the thermal resistance in the intercooler will mean that the compression occurs at a temperature somewhat higher than the ambient temperature, and the expansion will occur at a temperature somewhat lower than the ambient temperature. Thus a perfect isothermal storage system is impossible to achieve.
Storage Adiabatic (isentropic)
The adiabatic process is a process in which there is no heat transfer between the liquid and its surroundings: the system is insulated against heat transfer. If the next process is internally reversible (smooth, slow and frictionless, to an ideal limit) then it will also be isentropic.
An adiabatic storage system does away with intercooling during the compression process, and only allows the gas to heat up during compression, and also to cool during expansion. This is interesting, since the loss of energy associated with heat transfer is avoided, but the downside is that the storage vessel must be isolated against heat loss. It should also be mentioned that the real compressor and turbine are not isentropic, but have an approximately 85% isentropic efficiency, with the result that the efficiency of round-trip storage for adiabatic systems is also less than perfect.
Large thermodynamic storage system
Energy storage systems often use large underground caverns. This is a preferred system design, because of its very large volume, and thus a large amount of energy that can be stored with only a slight change of pressure. The cave spaces can be easily isolated, adiabatically compressed with slight temperature changes (approaching reversible isothermal systems) and heat loss (approaching the isentropic system). This advantage is in addition to the low cost of building a gas storage system, using an underground wall to assist in withstanding pressure.
It has recently developed an insulated underwater pocket, with similar thermodynamic properties for large underground cave storage.
Practical constraints in transport
To use air storage in a vehicle or aircraft for practical ground or air transport, the energy storage system should be compact and lightweight. The specific energy and energy density is the technical term that determines this desired quality.
Specific energy, energy density and efficiency
As explained in the thermodynamics of the gas storage section above, compressing the air heats it up and extends it to cool it. Therefore, practical air engines require heat exchangers to avoid too high or low temperatures and yet do not achieve ideal ideal temperature conditions, or ideal thermal insulation.
However, as stated above, it is useful to describe the maximum energy that can be stored using isothermal cases, which works up to about 100 kJ/m 3 [Ã, ln sub> A / P B )].
So if 1.0 m 3 air from the atmosphere is compressed very slowly into a 5 liter bottle at 20 MPa (200 bar), the potential energy stored is 530 kJ. A highly efficient air motor can transfer this into kinetic energy if it runs very slowly and manages to expand air from the initial 20 MPa pressure to 100 kPa (the bottle is completely "empty" at atmospheric pressure). Achieving high efficiency is a good technical challenge due to heat loss to the ambient and the heat of internal gas that can not be repaired. If the above bottle is emptied to 1 MPa, the energy that can be extracted is about 300 kJ on the motor shaft.
Standard 20 MPa, 5 liter steel bottle has a mass of 7.5 kg, which is 5 kg higher. High tensile strength fibers such as carbon fiber or Kevlar can weigh below 2 kg in this measure, consistent with the legal security code. One cubic meter of air at 20 ° C has a mass of 1.204 kg at standard temperature and pressure. Thus, the specific theoretical of the specific energy comes from about 70 kJ/kg on the motor shaft for a plain steel bottle up to 180 kJ/kg for advanced fibers, whereas practically > reachable the special energy for the same container is from 40 to 100 kJ/kg.
Comparison with battery
Advanced fiber-reinforced bottles are comparable to rechargeable lead-acid batteries in terms of energy density. Batteries provide almost constant voltage across their fill levels, while pressure varies greatly when using pressure vessels from full to empty. It is technically challenging to design an air machine to maintain high efficiency and sufficient power over various pressures. Compressed air can transfer power at very high flux rates, which meet the ultimate acceleration and deceleration goals of the transport system, especially for hybrid vehicles.
The compressed air system has advantages over conventional batteries including durability of pressure vessels and lower material toxicity. Newer battery designs such as those based on Lithium Iron Phosphate chemistry do not have this problem. The cost of compressed air is potentially lower; However, advanced pressure vessels are expensive to develop and safety testing and are currently more expensive than mass-produced batteries.
As with any electric storage technology, compressed air is just as "clean" as the source of energy it stores. The lifecycle assessment addresses the question of the overall emission of a given energy storage technology combined with a mixture of generations provided on the power grid.
Security
Like most technologies, compressed air has security issues, especially the outbreak of a disaster tank. Safety regulations make this a rare occurrence with higher bodyweight costs and additional safety features such as pressure relief valves. The regulation may limit the legal work pressure to less than 40% of the breakage pressure for steel bottles (safety factor 2.5), and less than 20% for fiber-wound bottles (safety factor 5). The commercial design adopts the ISO 11439 standard. The high pressure bottle is strong enough so it generally does not break in the vehicle collision.
Vehicle app
History
Air machines have been in use since the 19th century to power mine, pump, drill and tram locomotives, through centralized city-level distribution. Racing cars use compressed air to start their internal combustion engines (ICE), and large Diesel engines may have started pneumatic motors.
Engine
A pressurized air machine uses a compressed air expansion to drive the engine piston, rotate the shaft, or move the turbine.
The following methods can improve efficiency:
- High-efficiency continuous expansion turbines
- Multiple expansion steps
- Waste heat usage, especially in hot hybrid engine design
- Environmental heat usage
Highly efficient settings use high, medium and low pressure pistons in series, with each stage followed by airblast venturi drawing the air around through an air-to-air heat exchanger. It warms the exhaust from the previous stage and recognizes this heated air to the next stage. The only exhaust from each stage is cold air which can be as cold as -15 ° C (5 ° F); cold air can be used for air conditioning in car.
Additional heat can be supplied by fuel combustion as in 1904 for Whitehead torpedoes. This increases the range and speed available for a given tank volume with additional fuel costs.
Car
Since about 1990 some companies claim to have developed pressurized air cars, but none are available. Usually the main advantages claimed are: no roadside pollution, low cost, use of cooking oil for lubrication, and integrated air conditioning.
The time required to recharge the exhausted tank is essential for vehicle applications. "Transfer volume" moves compressed air from stationary tanks to the vehicle tank almost instantly. Alternatively, stationary or on-board compressors can compress air on demand, may take several hours.
Ship
Large marine diesel engines start using compressed air, usually between 20 and 30 bar and stored in two or more large bottles, working directly on the piston through a special starter valve to rotate the crankshaft before starting fuel injection. This arrangement is more compact and less expensive than electric starter motors on the scale, and is capable of supplying the very high bursts of power required without putting too much weight on power generators and ship distribution systems. Compressed air is generally also used, at lower pressure, to control the engine and act as a spring force acting on the cylinder exhaust valve, and to operate other auxiliary systems and electrical equipment on the board, sometimes including Pneumatic PID controllers. One advantage of this approach is that in the event of a power outage, the vessel system supported by stored compressed air can continue to function uninterruptedly, and the generator can be restarted without power supply. Another is that pneumatic tools can be used in an environment that is usually wet without the risk of electric shock.
Hybrid vehicles
While the air storage system offers relatively low power density and vehicle coverage, high efficiency appeals to hybrid vehicles that use conventional internal combustion engines as a major power source. Air storage can be used for regenerative braking and to optimize piston engine cycles that are not equally efficient at all power levels/RPMs.
Bosch and PSA Peugeot CitroÃÆ'¡n have developed hybrid systems that use hydraulics as a way to transfer energy to and from compressed nitrogen tanks. A reduction of up to 45% in fuel consumption is claimed, corresponding to 2.9l/100 km (81 mpg, 69 g CO2/km) on the NEDC cycle for compact frameworks such as the Peugeot 208. The system is claimed to be much more affordable than competing with the KERS electrical system and flywheel and expected on a road car in 2016.
System types
Hybrid system
Brayton cycle engines condense and heat the air with suitable fuel for internal combustion engines. For example, natural gas or compressed biogas hot air, and then a conventional gas turbine engine or the rear of a jet engine extends it to produce a job.
The wind compressor engine can recharge the electric battery. The apparently dead energine promotes the Pne-PHEV or Pneumatic Plug-in Hybrid Electric Vehicle-system.
Existing hybrid system
Huntorf, Germany in 1978, and McIntosh, Alabama, USA in 1991 commissioned a hybrid power plant. Both systems use off-peak energy for compressed air and burn natural gas in compressed air during the power generation phase.
Future hybrid system
Iowa Stored Energy Park (ISEP) will use aquifer storage rather than cave storage. The displacement of water in the aquifer produces the regulation of air pressure by constant hydrostatic pressure of water. A spokesperson for ISEP claims, "You can optimize your equipment for better efficiency if you have constant pressure." The output power of the McIntosh and Iowa systems is in the range of 2-300 MW.
Additional facilities are under development in Norton, Ohio. FirstEnergy, Akron, Ohio electric utility acquired development rights for the Norton 2,700 MW project in November 2009.
The RICAS2020 project tries to use a derelict mine for adiabatic CAES with heat recovery. The heat of compression is stored in the tunnel section filled with loose stones, so that the compressed air is almost cold when entering the main pressure storage chamber. The cold compressed air regains heat stored in the rock when it is released back through the turbine surface, leading to higher overall efficiency.
Storage lake or ocean
Deep water in lakes and oceans can provide pressure without the need for high pressure vessels or drill into salt or aquifer caves. Air goes into cheap and flexible containers like plastic bags down on deep or offshore lakes with steep drops. Obstacles include a limited number of locations and the need for high pressure pipes between surfaces and containers. Because the container will be so cheap, the need for big (and very deep) pressure may not be that important. The main benefit of the system built on this concept is that the charging and discharging pressures are a constant function of the depth. Carnot inefficiency can thus be reduced in power generation. Carnot efficiency can be improved by using several stages of filling and discharging and using cheap and sinking heat sources such as cold water from rivers or hot water from solar ponds. Ideally, the system should be very smart - for example, by cooling the air before pumping on summer days. This must be engineered to avoid inefficiencies, such as changes in wasteful pressure caused by inadequate piping diameter.
A nearly isobaric solution is possible if the compressed gas is used to drive the hydroelectric system. However, this solution requires a large pressure tank located on land (as well as undersea airbags). Also, hydrogen gas is the preferred fluid, since other gases suffer substantial hydrostatic pressures at relatively simple depths (such as 500 meters).
E.ON, one of Europe's leading electricity and gas companies, has provided EUR1.4 million (£ 1.1 million) to develop underwater storage pockets. Hydrostor in Canada is developing a commercial underwater storage system "accumulator" for compressed air energy storage, starting on a scale of 1 to 4 MW.
There are plans for some types of compressed air energy storage in underwater caves by Northern Ireland.
Near Isothermal
A number of close isothermal compression methods are being developed. Fluid Mechanics has a system with a heat absorbing and discharge structure (HARS) attached to a reciprocating piston. Light Sailing injects water spray into reciprocating cylinders. SustainX uses a mixture of air-water foam inside the compressor. All of these systems ensure that air is compressed with high thermal diffusivity compared to compression speed. Usually this compressor can run at speeds up to 1000 rpm. To ensure high thermal diffusivity, the average distance of the gas molecule from the heat absorbing surface is about 0.5 mm. This close isothermal compressor can also be used as near isothermal builders and is being developed to improve the efficiency of CASE travel rounds.
See also
- Compressed air vehicle
- Compressed aircar
- Alternate movers
- Locomotive is not lit
- Energy storage grid
- Hydraulic accumulator
- List of energy storage projects
- Pneumatic
- The zero-emission vehicle
- Cryogenic energy storage
References
External links
- The Paris Compressed Air System - technical notes Part 1 Part 2 Part 3 Part 4 Part 5 Part 6 (Special supplement, Scientific American, 1921)
- Solutions for some of the country's energy problems may be little more than hot air (Sandia National Labs, DoE).
- MSNBC Articles, Cities for Saving Wind Power for Further Use, January 4, 2006
- Power storage: The trapped wind
- Capturing the Wind In Bottles A group of Midwest utilities are building a factory that will store excess wind power underground
- New York Times article: Technology; Using Compressed Air To Store Power
- Air Energy Storage, Entropy, and Compression Efficiency
Source of the article : Wikipedia
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