by Keith D. Foote
Compressed air can be a way to store energy. It can be stored in simple, low cost air storage tanks. For very large installations, compressed air can be stored in caverns. For economic purposes, energy produced and stored during times of low demand can be released and used during times of high demand. Small storage systems have been used for some time as the propulsion for mining trains. Large scale applications focus on conserving the heat energy involved in compressing air (heat loss reduces the amount energy being stored).
Compressed air energy has been used to power cities since 1870. Cities such as Paris, France and Buenos Aires, Argentina have used compressed air systems. The first use of compressed air as an energy source was constructed by Victor Popp. He used it to power clocks. He sent a pulse of air every moment to change the clock's pointer arms. Compressed air quickly evolved to powering people's homes and to industrial work. Compressed air is still used as a source of energy for a number of industrial and construction jobs. The systems provided energy to the machines of seamstresses, dentists, printers, and bakeries. It is quite popular with roofers using compressed air nailing guns
Large amounts of compressed air are often stored in underground caverns. This type of system has a large storage capacity. The cavern can be insulated and compressed with little temperature change and heat loss. The low cost of construction for these compressed gas storage system is an advantage, using the cave walls to help contain the pressure.
If ambient air is very slowly compressed 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 the air from its initial 20 MPa pressure down to 100 kPa (the bottle is completely "empty" at ambient pressure). Achieving high efficiency is a technical challenge both due to heat loss and unrecoverable internal gas heat. If the bottle is emptied to 1 MPa, the extractable energy is about 300 kJ at the motor shaft.
Fiber-reinforced bottles can be compared to rechargeable lead-acid batteries. Batteries produce a nearly constant voltage during their entire discharge. The pressure in a container continuously lowers from full to empty. It is a technical challenge to design compression tanks able to maintain both high efficiency and consistent power from a broad range of internal pressures. Compressed air can also be used to transfer power at high flux rates, which is useful for the acceleration and deceleration of transportation vehicles. Hybrid vehicles using compressed air are now being investigated.
Compressed air tanks have some advantages over traditional batteries. This includes longer lifespans and a lack of poisonous acids. New battery designs, like those using Lithium Iron Phosphate don't have these problems, but can overheat. Compressed air is low cost, but "advanced" pressure containers are expensive to develop and test.
Compressed air, as an energy source, is only as "clean" as the fuel used to initially create the energy. Using coal to generate electricity, which is transformed into compressed gas and stored in compression tanks, is not any cleaner then using the coal-sourced energy directly.
Compressed air has safety concerns, as do most technologies. A catastrophic tank rupture is the biggest concern. Intelligent safety codes have made this a rare, occasional occurrence. However, it comes at the cost of increased container weight and safety devices, like pressure relief valves. A broken valve can send a tank shooting off like a bullet, or a misguided missile. High pressure containers are designed to be quite strong, and generally don't rupture during auto accidents. Compressed air in autos or aircraft has great potential, but the containers must be lightweight and compact.
Engines running on compressed air have been in use since the 1800s to power mining trains, drills, and pumps, by using a centralized distribution system. An engine running on compressed air uses the expansion process to push the pistons in the engine, which will turn an axle or a turbine.
A number of companies, since the 1990s, have made claims they were developing cars that run on compressed air, but curiously, there none are available. In spite of solid evidence it is inexpensive to recharge, leaves no roadside pollution, and uses a lubricant made of cooking oil.
The amount of time needed to refill an empty tank is important. Compressed air cars can fill their tanks almost instantaneously, if a supply of compressed air is available. There is also the option of having a stationary compressor in your garage or a built-in, on-board compressor, but both of these situations may take several hours to recharge your vehicle. There is at least one air pressure car that claims you can use a gas station tire pump to recharge the tank in a few minutes. (Recently aired on Shark Tank.)
Air storage systems offer a low power density and don't travel very far. It is the vehicle's high efficiency that makes it a good match for hybrid vehicles using a conventional internal combustion engine. The stored gas can be used for braking purposes and to optimize an engine's piston cycle, which does not run completely balanced during all RPM/power levels.
PSA Peugeot Citroën and Bosch have developed hybrid systems using hydraulics to transfer energy to and from a tank containing compressed nitrogen. They claim to have achieved a 45 percent reduction in fuel use. They claim the system is more affordable than competing electric cars and flywheel KERS systems. They plan for vehicle to be on the road cars by 2016.