Compressed air energy storage

Compressed Air Energy Storage (CAES) refers to the compression of air to be used later as energy source. 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 vehicles, or even tools.



Types
Compressed air energy storage can be done in two ways:
 * Compression is done with an electrically powered turbo-compressor.
 * Expansion is done with a natural-gas powered 'expander' (heater) which drives a combustion turbine or air engine (generator) to produce renewable electricity.

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.


 * Compressed air energy storage can also be used to describe technology on a smaller scale such as exploited by air cars or wind farms in carbon-fiber tanks.

Physics of isothermal compressed air storage
One type of reversible air compression and expansion is described by the isothermal process, where the heat of compression and expansion is removed or added to the system at the same rate as it is produced. 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:
 * $$PV=nRT=constant $$

By the definition of work, where A and B are the initial and final states of the system:


 * $$W_{A\to B}=\int_{V_A}^{V_B} P\,dV =\int_{V_A}^{V_B} \frac{nRT}{V}\,dV = nRT\int_{V_A}^{V_B} \frac{1}{V}\,dV = nRT(\ln{V_B}-\ln{V_A}) = nRT\ln{\frac{V_B}{V_A}} = nRT\ln{\frac{P_A}{P_B}}$$

where, $${P_A}{V_A} = {P_B}{V_B} $$, and so, $$\frac{V_B}{V_A} = \frac{P_A}{P_B} $$


 * $$\ P $$ is the absolute pressure,
 * $$\ V $$ is the volume of the vessel,
 * $$\ n $$ is the amount of substance of gas,
 * $$\ R $$ is the ideal gas constant,
 * $$\ T $$ is the absolute temperature,
 * $$\ W $$ is the energy stored or released.

This amounts to about 101 $$\ln{\frac{P_A}{P_B}}$$ kJ/m³ at 0 degrees Celsius (273.15 kelvin) or 110 $$\ln{\frac{P_A}{P_B}}$$ kJ/m³ at 25 °C (298 K), per mole. '''YOU MUST REMOVE THE "PER MOLE" COMMENT! THAT IS INCORRECT AND WILL MAKE PEOPLE MAKE A 44.6 TIMES ERROR. It IS a simple energy measured in KJ/M^3. '''

'''SINCE ONE IS CALCULATING ENERGY PER UNIT VOLUME $V_A$ ($1m^3$) AND $ 1ATMOSPHERE$ $nRT_A$ REMAINS CONSTANT. $101\frac{kJ}{m^3}\ln{\frac{P_A}{P_B}$ IS CORRECT FOR ANY $T_A$ AND CONSTANT $P_A$. DON'T APPLY THIS TO THE VOLUME OF THE CYLINDER, THIS REPRESENTS THE ENERGY TO COMPRESS A UNIT VOLUME OF UNCOMPRESSED AIR.'''

A mole of gas molecules at standard temperature and pressure (0 °C, 0.1 MPa), occupies 22.4 liters. Also there are 1000 liters in 1 m³. So there are about $$n = 1000 / 22.4 = 44.6 $$ moles of gas molecules in 1 m³.

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, then the energy storage efficiency will be reduced. Energy storage systems are 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.

An air engine or air motor is a device for converting potential energy from compressed air into kinetic energy to drive other machines. As in a steam engine, expansion of externally supplied pressurized gas performs work against one or more pistons or rotors to move wheels or other tools.

By Boyle's law it is known that

"For a fixed mass of ideal gas at fixed temperature, the product of pressure and volume is a constant. The formula is P1V1=P2V2 making pressure and volume indirectly related"

Therefore under identical temperature:
 * the pressure multiplied by the volume of a gas contained in a tank corresponds to a constant;
 * the variation of pressure of the gas is inversely proportional to its volume.

If either the pressure or the volume are altered, the factor T can be modified accordingly. It is what brings to the concepts of thermodynamic, of adiabatic expansion of compressed air.

Energy density and efficiency
Ideal air compression and expansion is described by the isothermal process. Compressing air however 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 110 $$ln{\frac{P_A}{P_B}}$$ kJ/Nm3 at 24°Celsius. A Nm3 is a cubic meter of gas volume at normal, i.e. atmospheric pressure, conditions. Thus if 1.0 m3 of ambient air is very slowly compressed into a 5-liter bottle at 200 bar, the potential energy stored is 583 kJ (or 0.16 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). This is practically impossible and if the bottle is emptied down to 10 bar, the energy extractable is about 330 kJ. 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 various 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 can be below 2 kg in this size, always regarding legal safety codes. Thus we get energy densities from roughly 75 up to 300 kJ/kg. Ordinary steel bottles thus have about the same energy density as lead-acid batteries and advanced fiber-reinforced bottles that of superior electrochemical storage batteries. However, modern batteries provide almost their full energy at a nearly constant voltage, whereas the pressure of compressed air storage varies greatly. It is technologically difficult for air engines to maintain high efficiency and sufficient power values over such pressure swings.

The advantage of compressed air over electric storage is the longer lifetime of pressure vessels compared to batteries and the lower toxicity of the materials used. However for this to count, air engines must become as light, efficient and cheap as available electric motors. Compressed air tanks can also be charged more safely than those with inflammable fuels.

As with electric technology, it must be stressed that compressed air is only an energy vector therefore can only be as clean as its source.

Safety
As with most technologies, compressed air has safety concerns, mainly the catastrophic rupture of the tank. Rigid safety codes make this a rare occurrence at the cost of weight: codes may require the working pressure to less than 40% of the rupture pressure for steel bottles and less than 20% for fiber-wound bottles. High pressure bottles are fairly strong so that they stay unruptured in crashes and follows the ISO 11439 standard.

Technical boundaries
For practical application to transportation, several technical problems must be first addressed:
 * As the pressurised air expands, it is cooled, which limits the efficiency (see combined gas law). This cooling reduces the amount of energy that can be recovered by expansion, so practical engines apply ambient heat to increase the expansion available.
 * Conversely, the compression of the air by pumps (to pressurize the tanks) will heat the air. If this heat is not recovered it represents a further loss of energy and so reduces efficiency.
 * Storage of air at high pressure requires strong containers, which if not made of exotic materials will be heavy, reducing vehicle efficiency, while exotic materials (such as carbon fiber composites) tend to be expensive.
 * Energy recovery in a vehicle during braking by compressing air also generates heat, which must be conserved for efficiency.
 * It should be noted that the air engine is not necessarily emission-free, since the power to compress the air initially may produce emissions at the point of generation. However such emissions from the power to compress the air initially would be far less than the emissions from gasoline powered cars and trucks already on the streets based on petroleum.

History
The air engine and its idea of using air as an energy carrier is not new. Air has been used since the 19th century to power mine locomotives, and was at one time the basis of naval torpedo propulsion.

Compressed air is still currently used in racecars to provide the initial energy needed to start the car's main power plant, the internal combustion engine (ICE).

Many people have been working on the idea of compressed air vehicles, with renewed interest since the 1990s.

Many shop tools use a small turbine expander for power, and many larger tools for use in high risk environments use pneumatic power.

Engine design
A compressed air engine uses the expansion of compressed air to drive the pistons of an engine, or to drive a turbine, to propel the vehicle or generate electricity.

Sometimes efficiency is increased by the following methods:


 * several stages of expansion are used
 * use of environmental heat at normal temperature to warm the otherwise cold expanded air from the storage tank
 * use of environmental heat at normal temperature to warm the otherwise cold expanded air between expansion stages.

This non-adiabatic expansion has the potential to greatly increase the thermodynamic efficiency of the machine, which also improves the energy that can be extracted from a storage tank with a given pressure and volume.

The most efficient arrangement to date uses high, medium and low pressure pistons, followed by an airblast venturi that draws ambient air over an air to air heat exchanger that warms the exhaust between each expansion stage.

The only exhaust gas from each stage is cold air which can be as cool as (−15 °C), this may also be used for air conditioning in a car.

At some cost in thermodynamic efficiency additional heat can be supplied by burning fuel, either to warm the air in the tanks, or even inside the tank itself. This was invented in 1904 for Whitehead's torpedoes. This improves the range and speed available for a given tank volume and pressure, but thermodynamically it may not be as efficient.

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.

Other uses of air engine
Besides the use of compressed air engines for power generation, air engines are also found in vehicles for their propulsion.

Cars
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 must be designed to safety standards appropriate for a pressure vessel, such as ISO 11439.

The storage tank may be made of:
 * carbon-fiber in order to reduce its weight while achieving the necessary strength.
 * kevlar.

Compressed air is a heavy way of storing fuel, 300l air at 300 MPa only amounts to about 12kWh (the equivalent of 1.4 liter (0.37 gallons) of gasoline). During rupture testing, the tank cracks, but does not break up, producing no splinters or fragments.

All four major manufacturers who are developing air cars have designed safety features into their containers as opposed to hydrogen's issues of damage and danger involved in high-impact crashes. Air, on its own, is also non-flammable. Though no company has yet demonstrated the effectiveness of an imploding engine (ZAP) vs a quick release (MDI) standard, and other safety designs; it is expected that large-scale production may lead specific governments to set their own standards. It was reported on Discovery's Beyond Tomorrow that on its own carbon-fiber is brittle and splits; but creates no shrapnel.

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.

Boats and aircraft
As of present, no boats or aircraft other than toys currently use an air engine for their propulsion (although it is possible, if unlikely).

Plain compressed air engine
With this system, a sole air engine is hooked up to a compressed air tank.

Hybrid 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 a electric battery. The compressed air engine, drawing its energy from compressed air tanks, recharge the electric battery. This system (called a PHEV or Pneumatic Hybrid Electric Vehicle-system) and is being marketed by companies as Energine. At the moment however, this PHEV-system is used solely on vehicles for their propulsion, and their use in energy generation is not yet present.

Existing hybrid systems
A hybrid 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 Mckintosh 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.