Like Hydrogen fuel, compressed air is not a direct renewable fuel but a secondary fuel option, derived as an energy storage technology. Compressed air has been used for well over a century to drive equipment – think compressed air guns in the building industry. Compressed air energy storage (CAES) uses electricity to compress air that be used to drive a turbine generator to produce electricity on demand (when needed), or even to drive a pneumatic engine as a transport fuel.
Electricity and Storage
CAES has been used for many decades with the compressed air either produced via a pump on-site or stored in high pressure cylinders. More recently, underground caverns (e.g. solution-mined caverns in a salt deposit) are being considered because of their exceptionally large storage capacity. The cavern can be insulated and compressed air stored with little temperature change and heat loss. The low cost of construction for these compressed gas storage systems is an advantage, using the cave walls to help contain the pressure.
As with all energy sources, CAES is only as cleanly renewable as the fuel used to initially create the energy. Providing that the energy used to compress the air is from a renewable source such as wind or solar, then it is a clean source. In the past, fossil fuels sources were often used to compress the air, which both maintains the pollution problems of fossil fuels and greatly diminishes the efficiency of even using compressed air as an alternate energy storage. The lifetime of these storage systems is expected to be well over a century, which makes their investment a good option to consider.
The big difference between storing compressed into high pressure storage tanks on the surface or compressing it into caverns is a crucial one. Air compressed on the surface is simply stored directly in to the storage container. Compressing air for storage underground requires a multi stage pumping and retrieval system – it isn’t just a large-scale version of a compressed air tank. This means energy is used to manage the cavern storage systems thereby greatly reducing the efficiency of the system. Its advantage comes when large amounts of renewable electricity are being produced that can be used to compress the air for larger scale on-demand turbine electrical generation, otherwise the electricity would simply be discharged in to the atmosphere.
A compressed-air vehicle (CAV) is simply a vehicle with a pressurized tank of air as the fuel supply. The pressure of the air expands to drive a Pneumatic motor. These kinds of motors have been in use for many decades and have applications in torpedoes, vehicles used in digging tunnels, and early prototype submarines. More recently, research has been on passenger cars. The main problem has been the need to show CAES competiveness with other options such as hydrogen fuel. Air compressed using renewable energy sources is completely non-polluting in both its production and in use as a fuel – nothing is burned, it is just air compressing and expanding. The only danger from CAES is being too close to a direct rupture of a storage tank, which is quite a minimal risk when compared to other forms of energy storage. Gasoline and Diesel, the transport fuels we currently accept and use every day, are extremely risky when it comes to potential hazards. Economists have analyzed the cost benefits of using compressed air in various CAES transport systems. Some vehicles could have on-board compressor units plugged into the electrical mains (assumes renewable electricity sourced) while others could refill at large compressed air service stations. The cost ranges from less than $1 to $1.25 per each 50 mile driven depending on the storage source. This makes it comparable to modern transport costs.
There are several prototype vehicles out there. The range of the cars or larger transports (trucks, buses, etc…) is only limited by the size of the storage tanks. The top speed of most current prototypes run only on compressed air is about 50 mph. This makes them currently ideal for urban use (especially with the zero emissions) but more marginal for long distance travel where time is of the essence. Tata Motors India was a pioneer with the use of the CAES transport in 2007 and now has a small lightweight model produced with (Motor Development International France) called the ‘Airpod’ that gets about 50 mph top speed with a range of 160 miles.
There are currently several CAES-hydrid vehicles operating efficiently while research continues on using CAES as a main transport fuel. According to a recent interview of Loveland, Colorado’s, truck fleet, the fleet manager Steve Kibler, says, that hybrid hydraulic drive system called ‘RunWise’ uses a technique that stores braking energy and hydraulic fluid and then releases it to accelerate the truck up to 35 miles per hour, with less reliance on the engine, Kibler said, “With very little operator training, we were able to achieve 48 percent fuel savings, and a Return on Investment of 6 years on hydrid trash trucks compared to regular diesel trash trucks.