Harry Valentine considers the advantages of a supercapacitor-powered 'slug' unit. By Harry Valentine
Open Access Article Originally Published: March 03, 2011
There is much ongoing research being aimed at developing super capacitor energy storage devices for automotive transportation applications. Most of this research is aimed at developing super capacitors for use in transportation applications where power-to-weight ratios are quite substantial. As transportation vehicles increase in magnitude, the power-to-weight ratio decreases significantly.
There are a few ready markets for cost competitive, large-scale super capacitor energy storage devices that may be applied to such applications as railway traction operations. While hybrid and battery powered industrial and shunting locomotive depend on lead-acid battery technologies, these batteries do not deliver a surge of power and can only be recharged over extended time durations. The addition of super capacitors could greatly enhance the performance of battery-powered locomotives, except that technology of such magnitude is currently unavailable.
Research into super capacitors has indicated that bi-metallic oxides can store large amounts of energy. For applications such as railway shunting and railway commuter train services, banks of mega-sized super capacitors would need to store energy in low-cost bi-metallic oxides. One such oxide ore actually occurs quite naturally in the bedrock of Madagascar. Its technical name is ILMENITE (iron titanate FeTiO3) and will store considerably fewer watt-hours per kilogram as barium titanate.
Another naturally occurring iron-based mineral is the bi-metallic oxide known as chromite (FeCr2O4). Like ilmenite, it may also have possible application in large-scale super capacitors intended for severe service applications. The molecule barium chromate (BaCrO4) is produced in large quantities at competitive prices across China and India. It is also a bi-metallic oxide capable of holding an electrostatic charge in a large-scale, commercial transportation application.
The engine of a family car that weighs 2500-lb may produce an output of some 75-Hp to 100-Hp output. Engines of 150-Hp to 200-Hp have powered buses that weigh some 25,000-lb. A locomotive of 2500-Hp may pull a passenger train that weighs some 750,000-lb. An engine of some 24,000-Hp may propel a container ship of some 25,000-metric tons deadweight (55,000,000-lbs). As the size of the transportation technology increases, the power-to-weight ratio decreases.
In large-scale transportation applications, the power-to-weight ratio is a fraction of that of a private automobile. There may actually be a market for a super capacitor that can store enough energy to move a train over a short distance. In railway shunting service, the energy stored in a super capacitor way be sufficient to move a train from a standing stop to maximum shunting speed. As the train reaches shunting speed, the batteries would blend in to keep the train traveling at constant speed.
In railway freight operations, there exists a traction technology called a “slug” unit. It is essentially the chassis and traction technology of a diesel-electric locomotive that receives electrical power from a companion diesel-electric locomotive. Several American railways use ballasted slug units to provide additional traction to pull heavy freight trains. The slug unit may be the ideal candidate for large-scale super capacitor technology that stores energy in low-cost, naturally occurring bi-metallic oxides.
The weight of the energy storage units can replace the ballast in the slug units and assist to provide addition traction. A tough, rugged energy storage technology that can operate in extreme cold and extreme heat would assist several types of railway motive requirements. It would also need to quickly dump massive amounts of power into traction motors to start a heavy train and be capable of rapid recharge as the train uses electro-dynamic braking to reduce speed.
A rechargeable railway slug unit could be assigned to service assisting diesel-electric locomotive to pull heavy commuter trains. A single diesel-electric unit and a companion rechargeable slug unit may replace a compliment of 2 x diesel-electric units on a multi-stop 12-coach commuter train. In service, the rechargeable slug unit would absorb energy as the train slows to a stop. That energy may provide 60% to 65% of the energy needed to accelerate the train. During the service stop, the slug unit would also receive additional energy from the companion diesel locomotive.
A rechargeable slug unit may also operate as a companion to an electric locomotive, absorbing deceleration energy and recharging during service stops. Electric locomotives cause severe power swings on the distribution grid. During acceleration, the electric locomotive could draw minimal energy from the power grid as the stored energy in the rechargeable slug unit provides energy to accelerate the train. Power from the grid would gently blend in as the train reaches its cruising speed.
A large rechargeable slug locomotive equipped with 6-axles and a driving cab may be assigned to a commuter train of 7 x bi-level coaches. The locomotive may weigh some 350,000-lb (158,000-kg) and store energy in some 25,000-kg of ilmenite from the mines of Madagascar. It may store some 80 to 100-Watt-hours per kilogram (w-h/kg) of energy, or 2000 to 2500 kW-hr of power, enough to propel the train for a distance of up to 30-miles at a speed of 40-miles power hour. Should the train make a service stop every few miles, it may partially recharge at the stations during the stops.
Conclusions:
While much research is focused on developing a super capacitor technology capable of propelling an automobile for some 100-miles or more, there is possible opportunity to develop mega-scale super capacitor technology for railway traction applications. Such technology could store electrostatic energy using low-cost, naturally occurring ores and minerals. While such storage technology may not provide the energy storage densities of a barium titanate super capacitor, they may do the required tasks in a variety of railway traction applications.
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