Trains in Transition
Electric trains that draw current from overhead cables are usually confined to operating along railway lines where the traffic density is high enough to justify the high cost of installing the overhead cables and related power distribution technology. There are low-density railway lines that may have been built at locations where locomotives have to operate free from sparks and exhaust emissions. Such operating environments may include mines, tunnels and specialized industrial locations where explosive vapours may be present the surrounding atmosphere.
The Classical Era
During the late 19th century mines that had access to electric power used that power to drive compressors to recharge pneumatic mining locomotives. These units shared much of their componentry with steam locomotives from which they had been developed. Builders of steam locomotives such as Porter and Heisler offered a range of fireless steam locomotives as well as pneumatic locomotives for mining operations.
Most of the pneumatic mining locomotives from the classical era stored the pressurised air (800-psia to 1200-psia) in multiple cylindrical tanks that fed into a small running tank that operated at 200-psia. Unheated air was expanded in many of these locomotives in a series of up to 3-stages using compound piston engines. Some models of compressed air locomotives carried a tank of hot water to preheat the air prior to expansion in an engine. One of the last pneumatic locomotives was built in Germany in 1955 and stored air in three off-the-shelf tanks the held pressurise up to 2900-psia.
Battery powered locomotives originally appeared during the early years of the 20th century. The largest of these units had a battery capacity of up to 150-Kilowatt-hours. By the mid-20th century they had gained acceptance in mining operations and in industrial shunting applications. They eventually supplanted the pneumatic locomotives in many such operations and especially after electric traction motors could be modified to operate without emitting sparks (commutator flashover). The German State Railways even operated a small fleet of battery electric self-propelled (2-axle) railbuses that carried passengers on a few lightly travelled rural routes.
Railway operations are extremely rough and locomotive components are continually subjected to longitudinal jolts. These shocks and jolts are espe in railway humping yards. The jolts and shocks from such operations caused the breakage of turbine blades in an experimental steam turbine electric locomotive owned by the Chesapeake and Ohio Railroad during the 1950's. Pneumatic locomotives could easily endure these shocks and jolts while special care had to be undertaken when coupling battery powered locomotives to short trains.
The Modern Era
A new type of electrically rechargeable locomotive has recently appeared in the USA and is expected to debut elsewhere around the world. The experimental prototype locomotive is fueled by hydrogen and uses fuel cells to generate electricity to power the driving wheels. Fuel cells are an old technology that date from the middle of the 19th century and that can generate electricity through reverse electrolysis. The technology is presently prohibitively expensive and may be susceptible to damage by the severe jolts and shocks and vibration that are typical to modern railway operations.
The fuel cell locomotive makes its debut at a time when new developments are beginning to appear in competing technologies. These technological developments could greatly improve the performance of electrically rechargeable locomotives. One of the manufacturers of lead-acid batteries developed the spiral cell several years ago where a layer of fibreglass fabric was rolled between thin plates of lead and lead oxide. Spiral cell battery technology is more tolerant of the severe jolts and shocks and vibration and can hold more energy (per unit volume and per unit weight) than conventional lead-acid batteries. They offer a longer useful life expectancy when used in deep-cycle operation.
Flow batteries may offer some advantages in useful life expectancy and energy storage. There have been recent advances in that technology however they may not be quite suitable for the severity of railway operations. Research is underway to develope ceramic ultracapacitor devices that operate like batteries that can offer the combination of high-energy storage density, rapid recharge time and extended life expectancy (over 15,000-deep cycle recharges). Such energy storage devices may appear commercially within the next decade and are aimed at automotive operations. It is as yet unknown as to whether such technology may be suitable for certain railway applications.
Reviving an Earlier Technology
The hydrogen fuel cell is an old technology that never took off in the free market. It was a brilliant concept never proved itself commercially and needed massive government assistance to advance. Its cost is still prohibitive. Pneumatic locomotives were developed in the free market and proved themselves in some highly specialised railway traction applications. At the present day there are any number of researchers and builders who are investigating pneumatically powered automobiles.
If pneumatic energy storage and propulsion technology were to be revived, the place to begin would be an application where the technology had previously operated successfully. That application is railway traction. Modern pressure vessels can hold extreme pressure (up to 45,000-psia) when built as a hollow sphere. Modern cooling technology could keep the stored air (or nitrogen in some applications) in a saturated or liquid state. Multiple pressure vessels can be carried aboard a railway chassis that rides on multiple axles. Some steam locomotives such as UP's "Big Boy" rode on 12-axles.
Compressed air stored in a battery of spherical tanks (up to 4,500-psia) could sequentially be fed into a lower pressure running tank (at some 1500-psia). Computer activated valves would progressively release air into the running tank. The high-pressure (HP) and low-pressure (LP) engines would both operate using a pressure ratio of 10 to 1. Exhaust air from the HP-engine would be reheated prior to being expanded in the LP-engine.
Heat from engine exhaust and from the electrical transmission could initially be transferred into the running tank and then into the accumulators to maintain pressure at a usable level. The isentropic efficiency of the axial flow turbines could exceed 90%, as could the efficiency of the electrical transmission. A positive displacement engine that runs of pressurised air could drive through a multispeed hydraulic or mechanical transmission to transmit power to the rails.
When air is compressed it produces heat (Boyle's Law). Air can be pumped to higher pressure in a series of stages. Heat can be removed at each stage and pumped into an onboard thermal storage system that would preheat the air prior to it being expanded in an engine. A variety of thermal storage systems are possible. The heat can be used to decompose a metallic carbonate like calcium carbonate (CaCO3) into calcium oxide (CaO) and carbon dioxide (CO2) that may be stored in separate onboard tanks. When the locomotive is in operation the metallic oxide and carbon dioxide would recombine (CaO + CO2 = CaCO3) to generate the thermal energy needed to preheat the compressed air prior to expansion.
The thermal energy that is extracted while the air is being compressed may also be stored in a molten mixture of metallic oxides. Lithium aluminate (LiAlO2) may be mixed with either lithium carbonate (Li2CO3) or lithium hydroxide (LiOH). The melting temperature of the mixture would depend on the relative concentration of these oxides to each other. The molten mixture would be contained in sealed cone-shaped containers made from either silicon carbonate or silicon nitride. The conical containers would be cast (embedded) into blocks of steel into which pipes that carry pressurised air and heating fluid are also embedded. The heater pipes would contain choke valves to enhance the transfer of heat into the thermal storage system.
The conical shape of the containers serves 2-purposes. It compensates for differences in thermal expansion between the steel, the container and its contents. The conical shape also offers good surface contact with the steel to allow for the optimal transfer of heat and to more easily endure the jolts and shocks and vibration that are typical in railway locomotives. The molten metallic oxide mixture stores thermal energy in the heat-of-fusion. It is sufficiently durable to withstand many thousands of repeated deep-cycle discharges and recharges of thermal energy with little or no deterioration.
A modern pneumatic locomotive that uses off-the-shelf spherical accumulators and onboard thermal storage technology is possible. It is an electrically rechargeable locomotive that could offer comparable energy storage density to locomotives that use competing electrical storage technology. It would offer comparable efficiency to its competitors in terms of "energy-in" and "usable energy-out". The pneumatic locomotive would operate like a compressorless gas turbine locomotive. Compressors can consume 55% to 70% of total engine output (hence the engine efficiencies between 30% and 45%). The absence of a compressor would allow the heated pneumatic locomotive to operate at a relatively high efficiency over a wide range of power output.
Electrically rechargeable locomotives may be used to pull trains through non-electrified tunnels. These tunnels may previously have been electrified and may no longer be so due to reduced traffic. The locomotives may be used in short-haul freight, short-haul intercity passenger service and in rush hour commuter service. They could be built to operate for distances of up to 200-miles. Electrically rechargeble locomotives may best be suited for operation along low-density railway lines in regions that receive electric power from kinetic sources such as hydroelectric dams, windmills, ocean wave power converters and tidal power installations.
At no time should any electrically rechargeable locomotive be recharged using power from a thermal power station. Most thermal power stations produce power at below 40% thermal efficiency. The worst case scenario would be the energy efficiency of the hydrogen fuel cell locomotive being recharged from such a power station. The typical efficiency of electrolysis is under 70%. Fuel cells are touted as being able to operate at an efficiency of 50%. Given that an electric motor will return a part-load efficiency of 90%, the overall energy efficiency would be 40% x 70% x 50% x 90% = 12.6%. The finest steam locomotives designed by Andre Chapelon in France and by Livio Dante Porta in Argentina operated at that level of efficiency.
Modern thermally rechargeable locomotives are possible and their energy supply can be replenished directly from thermal power installations. The thermal energy may be stored using heat-of-decomposition technology, heat-of-fusion technology, solid state ceramic thermal storage technology or even by using saturated water inside a pressure vessel. A steamless variant could convert heat to energy in excess of 30% thermal efficiency. An externally heated Brayton-cycle engine (an air turbine) would be suitable. The alternative would be a battery of thermoacoustic engines.
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