It is well known that supercapacitors can send out their entire charge almost instantaneously and are capable of millions of charge/discharge cycles without degradation. What's more they have extremely low internal resistance or ESR and offer cycle stability in a wide temperature range – from +65 °C to
-40 °C (batteries can't perform well below 0 °C).
Supercaps are perfect when a large power demand is required in a short period of time.
Let's focus on the phrase I just used: “..when a large power demand is required in a short period of time….”
How about using a capacitor bank to generate enough charge to provide the 64 Megajoules of power needed to shoot the U.S. Navy's future electromagnetic railgun, now under development by BAE Systems? Here, the stored electrical energy in capacitor bank will be used to accelerate a projectile to hypersonic speeds. First, electricity generated by the ship is stored over several seconds in a pulsed power system. Next, an electric pulse is sent to the railgun, creating an electromagnetic force accelerating 14.5kg (23lb), high velocity tungsten slugs from a 155mm cannon to around 5,800 m/sec (19,000 ft/sec or about 13,000 miles per hour). At such high velocities railgun projectiles can be non-explosive, relying on their enormous speed to destroy the target. Using stored electrical energy in capacitor banks to accelerate a projectile to high speeds thus avoids transporting hazardous chemical explosives traditionally used in firing shells.
But the energy needed to do this is equal to about 18 kilowatt hours, which is equivalent to the amount of power an average American household uses in an entire day. And the power supply must be able to deliver large currents, sustained and controlled over a useful amount of time.
A railgun operates via a homopolar motor armature. The name homopolar indicates that the electrical polarity of the conductor and the magnetic field poles do not change (i.e., that it does not require commutation). The railgun is comprised a pair of parallel conducting rails, along which a sliding armature is accelerated by the electromagnetic effects of a current flow. The principle is to pump about a million amps down two parallel conductors joined by the movable conductive projectile. Lorenz magnetic forces push the projectile forward.
You can think of an electromagnetic railgun as a flat electric motor. Like an ordinary motor it uses an alternating electromagnetic field to move an armature, only in this case instead of traveling in a circle the armature is a sliding metal conductor holding a projectile between the two conductive rails. The armature builds up speed traveling along the length of the rails and when it reaches the end of the rails it releases the projectile at very high speed. Navy planners are targeting a 50- to 100-nautical mile initial range capability for the projectile with eventual expansion of up to 220 nautical miles.
The Navy has chosen high-performance batteries from K2 Energy to power its electromagnetic railgun capacitors. K2 Energy specializes in lithium iron phosphate battery technology and will provide the self-contained battery that acts as an intermediate energy store system to power the capacitor bank. The Navy's plan is to deploy these 64 Megajoule cannons on warships sometime around 2020-2025. In July of this year the Navy showed off two working prototypes aboard the USS Millinocket at Naval Base San Diego. It also announced that a prototype electromagnetic railgun will be installed onto this joint high speed vessel (JHSV) for at-sea testing in FY 2016. The Millinocket is a high-speed, shallow draft vessel intended for rapid transport of conventional or special forces as well as equipment and supplies. The JHSV will reach speeds of 35–45 knots (65–83 km/h; 40–52 mph). The ship was chosen for its available cargo and topside space (the cannon and massive bank of capacitors required take up a lot of room on board).
In its initial land-based trials, conducted in 2012, a railgun projectile achieved speeds of Mach 5, and traveling in a straight line with no elevation penetrated a steel plate an eighth of an inch thick 100 yards away, then travelled seven kilometers down range before stopping.
The following two photographs show the railgun prototype launcher tested at the Naval Surface Warfare Center (NSWC) facility in Dahlgren, VA.
In the top photo the honing machine is used to precisely size the bore after each change of gun components. The muzzle chamber is a one-inch plate steel structure to protect the building from blast overpressure. The bottom photo shows the pulsed power system, which uses capacitors to store and release the energy needed to fire the gun. The inductor shapes the current pulse. A Rotary Arc Gap (RAG) switch receives high voltage signal from the trigger generator, which creates a 60,000 volt pulse to trigger the switch.
The RAG switch was designed as a pulse closing switch to discharge the stored energy in the capacitor bank. The switch often is one of the key issues in pulsed power generation and applications. Spark-gap switches are a popular choice but these switches have very limited lifetime due to electrode erosion resulting from localized arc heating in high current. Rotary arc gap switches have a coaxial cylindrical structure. The arc is driven by an axial magnetic field which is generated by the coils that are placed both at the top and bottom of the switch. The RAG in Dahlgren has two ring electrodes. According to the Navy the soft dump resistor shown in the photograph is a water and detergent filled and is used to bleed off stored energy in the event a shot has to be aborted. Charging diodes also shown protect the power supplies.
The magnetic fields generated during launch can cause damage to electronic equipment, so the railgun ship must be shielded. But while the engineering behind the weapon’s electrical and electromagnetic systems appears to be well in hand, additional design work needs to be done.
Here’s the reason why: The rails are subjected to enormous repulsive forces during shooting, forces that will push them apart and away from the projectile. As the rail/projectile clearances increase, arcing can develop, causing extensive damage to the rail surfaces. This limited some early research rail guns to one shot with an extended—and unacceptable for naval use—delay for repair/replacement.
Apart from the rails’ need to survive the mechanical violence of an accelerating projectile, both rails and projectiles need to withstand an enormous amount of heating due to the large currents and frictions involved. The heat generated potentially can melt the equipment, as well as cause a personnel safety threat and make the railgun easy to detect by enemy forces via thermal (infrared) means. Continuing design work is needed to create cooling systems that will allow these hot, high-energy rail guns to fire multiple rounds in rapid succession.
While its performance specs are admittedly impressive, the Navy is also interested in the weapon as a cost saving measure. According to Rear Adm. Matthew Klunder, Chief of Naval Research, a rail gun round costs about $25,000, which is a lot less than the $1.5M price tag of a Tomahawk cruise missile which has become the military’s primary long range weapon. What’s more, ships can carry dozens of missiles, but they could be loaded with hundreds of smaller railgun projectiles.