Compressed Air Energy Storage (CAES) refers to the compression of air to be used later as energy source. At utility scale, it can be stored during periods of low energy demand (off-peak), for use in meeting periods of higher demand (peak load).Alternatively it can be used to power tools, or even vehicles. (see also: Pressure vessel)
In practice neither of these perfect thermodynamic cycles are obtainable, as some heat losses are unavoidable.
A highly efficient arrangement, which fits neatly into none of the above categories, uses high, medium and low pressure pistons in series, with each stage followed by an airblast venturi that draws ambient air (or seawater as in early compressed air torpedo designs) over an air-to-air (or air-to-seawater) heat exchanger between each expansion stage. This warms the exhaust of the preceding stage and admits this preheated air to the following stage. This was widely practiced in various compressed air vehicles such as H. K. Porter, Inc's mining locomotivesand trams.. Here the heat of compression is effectively stored in the atmosphere (or sea) and returned later on.
Air is stored in mass quantity in underground in a cavern created by solution mining (salt is dissolved away) or an abandoned mine. Plants are designed to operate on a daily cycle, charging at night and discharging during the day.
City-wide compressed air energy systems have been around since the early 19th century (1870). In the early days, cities as Paris, Birmingham, Rixdorf, Offenbach, Dresden and Buenos Aires were equipped with these systems. The first systems were constructed by Victor Popp for powering clocks (by sending a pulse of air every minute to change the pointer), yet quickly evolved as a means to deliver power to industry and to homes.As of 1896, the Paris system had 3000 hp of generation distributed at 80psi in 30 miles of air pipes whose use for motors in light as well as heavy industry. Usage was measured with meters. The systems were the main source of (house-delivered) energy in these days and were also used to power the machines of dentists, seamstresses, printing facilities and bakeries.
Physics of isothermal compressed air storage
One type of reversible air compression and expansion is described by the isothermal process, where the temperature remains constant. Compressing air heats it up and the heat must therefore be able to flow to the environment during compression for the temperature to remain constant. In practice this is often not the case, because to properly intercool a compressor requires a compact internal heat exchanger that is optimized for high heat transfer and low pressure drop. Without an internal heat exchanger, isothermal compression can be approached at low flow rates, particularly for small systems. Small compressors have higher inherent heat exchange, due to a higher ratio of surface area to volume. Nevertheless it is useful to describe the limiting case of ideal isothermal compression of an ideal gas:
The ideal gas law, for an isothermal process is:
By the definition of work, where A and B are the initial and final states of the system:
where, PAVA = PBVB , and so,
This amounts to about 2.271 kJ at 0 degrees Celsius (273.15 kelvin) or 2.478 kJ at 25 °C (298 K), per mole, or simply 100 kJ per m³ of gas (at 0.1 MPa = aprox. atmospheric pressure).
An isothermal process is thermodynamically reversible, so to the extent the processes are isothermal, the efficiency of compressed air storage will approach 100%. The equation above represents the maximum energy that can be stored. In practice, the process will not be perfectly isothermal and the compressors and motors will have heat-related energy losses.
When gas is compressed adiabatically, some of the compression work goes into heating the gas. If this heat is then lost to the surroundings, and assuming the same quantity of heat is not added back to the gas upon expansion, the energy storage efficiency will be reduced. Energy storage systems often use large natural underground caverns. This is the preferred system design, due to the very large gas volume, and thus the large quantity of energy that can be stored with only a small change in pressure. The cavern space can be compressed adiabatically and the resulting temperature change and heat losses are small.
Practical constraints in transportation
Energy density and efficiency
Compressing air heats it up and expanding it cools it down. Therefore practical air engines require heat exchangers in order to avoid excessively high or low temperatures and even so don't reach ideal constant temperature conditions. Nevertheless it is useful to describe the maximum energy storable using the isothermal case, which works out to about 100 kJ/m. Thus if 1.0 m of ambient air is very slowly compressed into a 5-liter bottle at 200 bar, the potential energy stored is 530 kJ (or 0.15 kWh). A highly efficient air motor could transfer this into kinetic energy if it runs very slowly and manages to expand the air from its initial 200 bar pressure completely down to 1 bar (bottle completely "empty" at ambient pressure). Achieving high efficiency is a technical challenge both due to nonlinear energy storage and the thermodynamic considerations. If the bottle above is emptied down to 10 bar, the energy extractable is about 300 kJ at the motor shaft. The efficiency of isothermal compressed gas storage is theoretically 100% but in practice the process is not isothermal and the two engines (compressor and motor) have additional types of losses.
A standard 200 bar 5 liter steel bottle has a mass of 7.5 kg, a superior one, 5 kg. Bottles reinforced with, or built from, high-tensile fibers such as carbon-fiber or kevlar can be below 2 kg in this size, consistent with the legal safety codes. 1 m3 of air contained inside such a full bottle has a mass of 1.225 kg (at 20°C). Thus, theoretical energy densities are from roughly 70 kJ/kg at the motor shaft for a plain steel bottle to 180 kJ/kg at the motor shaft for an advanced fiber-wound one, whereas practical achievable energy densities for the same containers would be from 40 kJ/kg to 100 kJ/kg. Comparing to the data given for rechargeable batteries, this makes the advanced fiber-reinforced bottle example comparable to the lead-acid battery in terms of energy density and advanced battery systems are several times better. Batteries also provide nearly constant voltage over their entire charge level, whereas the pressure of compressed air storage varies greatly with charge level. It is technically challenging to design air engines to maintain high efficiency and sufficient power over such a wide range of pressures. Compressed air can transfer power at very high rates, which is a principal objective of transportation prime-movers, for acceleration and deceleration; particularly for hybrid vehicles.
Advantages of compressed air over electric storage are the longer lifetime of pressure vessels compared to batteries and the lower toxicity of the materials used. Costs are thus potentially lower, however advanced pressure vessels are costly to develop and safety-test and at present are more expensive than mass-produced batteries.
As with electric technology, it must be stressed that compressed air energy storage depends on external energy sources and overall consumption can only be as "clean" as these.
As with most technologies, compressed air has safety concerns, mainly the catastrophic rupture of the tank. Highly conservative safety codes make this a rare occurrence at the tradeoff of higher weight. Codes may limit the legal working pressure to less than 40% of the rupture pressure for steel bottles (safety factor of 2.5), and less than 20% for fiber-wound bottles (safety factor of 5). Design rules are according to the ISO 11439 standard.High pressure bottles are fairly strong so that they generally do not rupture in crashes.
Compressed air vehicles
The idea of using air as an energy carrier is not new. Air engines have been used since the 19th century to power mine locomotives, pumps, drills and trams in cities such as Paris, Birmingham, Rixdorf, ... in the 19th century, via a central, city-level, compressed air energy distribution. Also, at one time compressed air was the basis of naval torpedo propulsion.
Many people have been working on the idea of compressed air vehicles with renewed interest since the 1990s energy crisis.
A compressed air engine uses the expansion of compressed air to drive the pistons of an engine, an axle, or to drive a turbine.
Sometimes efficiency is increased by the following methods:
A highly efficient arrangement uses high, medium and low pressure pistons in series, with each stage followed by an airblast venturi that draws ambient air over an air-to-air heat exchanger between each expansion stage. This warms the exhaust of the preceding stage and admits this preheated air to the following stage..
The only exhaust gas from each stage is cold air which can be as cold as (−15 °C), this may also be used for air conditioning in a car.
Additional heat can be supplied by burning fuel as in 1904 for Whitehead's torpedoes. This improves the range and speed available for a given tank volume at the cost of the additional fuel.
As an alternative to pistons or turbines, the Quasiturbine is also capable of running on compressed air, and is thus also a compressed air engine.
Several companies claim to have been developing compressed air cars for public use, since about 1990, but none are available yet. Typically the main advantages are claimed to be: no roadside emissions, low cost technology, engine uses food oil for lubrication, and integrated air conditioning.
The tanks may be refilled at a service station (using volume transfer), or in a few hours at home or in parking lots plugging the car into the electric grid via an on-board compressor. The cost of driving such car is typically projected to be around €0.75 per 100 km, with a complete refill at the "tank-station" at about US$3.
Besides the use of compressed air engines for propulsion, compressed air is used for power generation and in paintball. Many dentists and shop tools use small turbine expanders for power, and many larger tools used in high electrical shock risk environments are pneumatic driven instead of electrically powered.
Certain segments of the Amish population also use compressed air to power household appliances. In many cases they will take a standard electric household appliance (such as a KitchenAid Mixer), remove the electric motor and install a compressed air turbine instead.
Types of systems
The system can be a hybrid power generation system, with the stored compressed air mixed with a fuel suitable for an internal combustion engine. For example, natural gas or biogas can be added, then combusted to heat the compressed air, and then expanded in a conventional gas turbine engine (or the rear portion of a jet engine), using the Brayton cycle.
In addition, Compressed air engines can be used in conjunction with an electric battery. The compressed air engine, drawing its energy from compressed air tanks, recharge the electric battery. This system (called a Pne-PHEV or Pneumatic Plug-in Hybrid Electric Vehicle-system) and was being promoted by the apparently defunct Energine.
Existing hybrid systems
A hybrid plant was commissioned in Huntorf (Germany) in 1978, and again in McIntosh, Alabama in 1991 (USA).Both systems use off-peak energy for the air compression.
The operating duration of the McIntosh plant is 24 hours, with the extended operation being achieved through the combined burning of a natural gas/compressed air mix.
Future hybrid systems
A proposed hybrid power plant is under consideration in Iowa. The design calls for a 75 - 150 MW wind farm, where the wind power will be used for air compression. Power output of the McIntosh and Iowa gas/compressed air generation systems is in the range of 2-300 MW.
Additional facilities are under development in Norton, Ohio and Iowa Stored Energy Park (ISEP). This 2700 MW Norton project has been started in 2001, but in early 2007 construction had not actually begun.
Increased efficiency is expected at ISEP, due to the use of aquifer storage rather than cavern storage. The displacement of water in the aquifer results in regulation of the air pressure by the constant hydrostatic pressure of the water. A spokesperson for ISEP claims "you can optimize your equipment for better efficiency if you have a constant pressure."It is planned to have 75 - 150 MW of capacity.
Lake or ocean storage
The need for pressurized vessels or for mining (into salt caverns or aquifers) can be obviated by placing the pressurized air underwater in flexible containers (e.g. plastic bags) - at the bottom of deep lakes or off sea coasts with steep drop-offs. Challenges include the limited number of suitable locations and the need for very-high-pressure pipelines between shore and depth. However, since the containers would be very inexpensive, the need for great pressure (at great depth) may not be as important. A key benefit of systems built on this concept is that charge and discharge pressures are always constant (as determined by depth): Thus all Carnot inefficiencies can be reduced in the controlled environment of the power plant. Carnot efficiency can be increased by using multiple stages for charge and discharge and by taking advantage of inexpensive heat sinks and heat sources, such as cold water from rivers or hot water from solar ponds. Ideally, the system must be very adaptive in this regard - for example, by cooling air before pumping on summer days; also, it must be engineered to avoid inefficiency, such as wasteful pressure changes caused by inadequate piping diameter.
Sea bed anchored energy bags are being researched as solutions to the problem of storing energy generated by renewable sources at The University of Nottingham.
Academics have received €1.4m (£1.1m) in funding from E.ON, one of Europe's leading power and gas companies, to develop a new generation of and undersea storage bags that will collect energy in the form of compressed air.
Energy storage in submerged, open bottomed, anchored caissons
This is energy storage by the displacement of water by compressed air. To recover the energy compressed air is an intermediate energy carrier. The efficiency and cost of compressing the air and to recover the energy stored in the compressed air must be considered. One possibility is to sink the type of caisson used in the Normandy Mulberry Harbours of which - about 115 of these huge structures were built in about 9 months during WW 2 http://www.everything2.org/index.pl?node_id=1306625
In theory similar structures could be constructed and floated into much deeper water, sunk and anchored to the bottom using rock anchors. A flexible air pipe could be brought to the surface, to say a wind turbine base, housing an air compressor and an air turbine generator set. By pumping compressed air down to the caisson, water would be ejected lowering the internal water surface to the bottom of the caissons. To recover the stored energy the air valve would be opened and the energy recovered via the air turbine as the water re enters and air is forced out. Because the container is open bottomed, there is no internal or external pressure force - both inside and outside the caisson will be at near identical pressures at all times. However there will be massive buoyancy forces to be restrained. The flexible pipe would simply be a solid steel pipe which over 100 m can cope with significant bending - the PLUTO pipelines, to bring fuel to allied troops after D Day were basically a 3 inch (75 mm) diameter steel pipe unreeled from a 30 foot (10 m) diameter reel.
Since the compressed air will be at high pressure (10 bar at 100m depth) a lower cost air turbine can be used (compared to a low pressure one). The air pressure will be fairly constant as well, meaning an efficient air motor configuration can be chosen.
Note 90% of the energy will be in the displaced water only 10% in the compressed air itself.
Published - July 2009
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