The International Steam Pages

The Ultimate Fireless Steam Locomotive and the Modern Power Industry Part 5

Harry Valentine develops previous ideas:

During its halcyon days, the fireless steam locomotive set the standard in terms of reliability, ease of operation, ease of repair and maintenance along with low overall operating costs. The fireless steam locomotive was compatible with industries that could supply the steam required for the frequent thermal recharges. Fireless steam locomotives were used to shunt railcars of coal to and from sidings at several coal fired power stations. The readily available source of steam from the earlier generation of fire-tube and low-pressure water-tube boilers made for easy thermal recharging.

There have been new developments and advances in thermal technology since the demise of the classical fireless steam locomotive. This technology includes spherical pressure vessels capable of storing supercritical steam. The solar thermal power sector has advanced heat-of-fusion, phase change thermal storage technology to keep the steam turbines operating after sunset. Modern gas-cooled nuclear reactors now circulate gas to energize boilers to generate superheated steam using light water. These advances can be applied to a modern fireless steam locomotive.

Steam-based Recharging:

During the period of the demise of traditional fireless steam locomotive, there have been numerous improvements in the technology that could form the basis of a modern thermally rechargeable locomotive. The development of spherical pressure vessels made of alloy steel allows for the storage of steam at the supercritical or ultra-critical phase at pressure levels in excess of 3220-psia or 22.2Mpa and at temperatures in excess of 400C or 750F. Several modern coal-fired power stations generate steam at over 3500-psia (24Mpa) at over 550C or 1020F.

There is scope to tap into the ultra-critical steam from a modern thermal power station to recharge modern, ultra-high pressure fireless steam locomotives during the overnight off-peak periods. Alternatively, groups such as Enginion (Amovis) of Germany have developed modern mono-tube coiled boilers capable of raising steam to some 4000-psia (27.6Mpa) at some 600C or 1110F. The recharging of fireless steam locomotives using ultra-critical steam will require special safety precautions, perhaps even some automation when it comes to connecting and disconnecting high-pressure steam lines to and from the locomotives.

Developments in grid-scale compressed air energy storage (CAES) technology provide a possible means by which to undertake the thermal recharge. There are salt caverns of enormous proportions that occur naturally in the earths bedrock. In many nations, the natural gas industry flushes the salt from these caverns that may measure up to 1-mile (1600m) in diameter by up to 9000m in vertical height. These caverns occur at sufficient depth to hold natural gas or compressed air at pressure levels of 1600-psia (11Mpa) to 2300-psia (16Mpa).

The sheer size of the caverns allows for seasonal storage of energy with pressure levels dropping to some 1000-psia (7Mpa) by end of the generating cycle during the season of peak demand for electric power. The caverns are recharged during the seasons of low demand for electric power. The compressed air is superheated prior to being expanded in turbine engines that drive electrical generators, with the exhaust air being used to energize a bottom-cycle engine. There is an alternate use for small amounts of compressed air being tapped from a giant cavern of compressed air.

Pneumatic Thermal Recharging:

There is pneumatic technology that can extract heat from a flow of heated compressed air and transfer that heat into a thermal storage system. The highly compressed air has to flow through a restriction called a nozzle followed by a diverging diffuser, that is, a conduit of steadily increasing cross-sectional area and that exhausts to the atmosphere (14.696-psia or 100kPa). Some heat recovery technology may be in order to make more efficient use of the exhaust heat.

The nozzle causes a sudden drop in both air pressure and temperature and absorbs the heat. A cooling system may remove heat from the nozzle and transfer it elsewhere, including into a thermal storage medium. The compressed air will reach sonic speed in the nozzle and accelerate to supersonic speed in the diverging diffuser. The ratio of cross-sectional area between the diffuser exit and the nozzle will determine the drop in temperature and depending on the ratio of the air pressure upstream of the nozzle and downstream of the diffuser exit.

If the upstream pressure is 900-psia (6Mpa) and the exit pressure is 15-psia (100kPa for a 60 to 1 pressure ratio) and the diffuser exit area is 6-times the nozzle area, the air would accelerate to sonic speed in the nozzle and to Mach 3.3 at the exit. The air may be superheated upstream of the nozzle and its temperature could drop from 1200F (649C) to 56F (14C) and transfer an enormous amount of heat into the nozzle. The nozzle may be surrounded by thermal storage material that would absorb the heat.

Heat Retrieval from Thermal Storage:

There is an evolving precedent in the nuclear power industry involving gas-cooled reactors where a circulating flow of helium, carbon dioxide or air transfers heat from a nuclear reactor to a boiler. The same method may be used to transfer heat from a thermal storage system to the saturated water inside the accumulator of a fireless steam locomotive, to maintain constant temperature and pressure for the purpose of extending operating range. Steam pipes that carry steam from the accumulator will connect to the thermal storage tank so as to superheat the steam prior to expansion in the engine.

The thermal storage tank and the accumulator/boiler would include air tubes through which air may flow. A micro-turbine engine comprising a compressor and a turbine would operate using heat from the thermal storage tank to circulate air through the air tube system to transfer heat into the accumulator/boiler. Air pressure in the tube system would vary, as would the air density to vary the amount of heat being carried from the thermal storage tank into the accumulator/boiler. The micro-turbine engine may operate at a thermal efficiency of 5% to 10%, its primary purpose being to circulate air through a sealed and closed circuit.

Thermal Storage Material:

The thermal storage material would be a mixture of variations of a metallic oxide/hydroxide/carbonate that would absorb a tremendous amount of thermal energy as it melts at constant temperature. Much research has already gone into the thermal storage capability of molten lithium aluminates that include the naturally occurring LiAlO2 ore and Li3AlO3 that is made from reacting aluminum with lithium hydroxide. The addition of lithium hydroxide (LiOH) that melts at 460C or lithium carbonate (Li2CO3) that melts at 720C offers to produce a melting temperature of 400C to 600C in containers that are made from or lined with silicon carbide.

There is scope to add aluminum oxide (Al2O3) to the mixture to increase thermal storage capacity. A mixture of aluminum-oxide-hydroxide (AlO2H) and lithium aluminate (LiAlO2) plus lithium hydroxide (LiOH) as a catalyst also promises to provide high thermal storage capability. Other mixtures include sodium aluminates such as the ore cryolite (Na3AlF6) added to Na3AlO3 that is made from reacting aluminum with sodium hydroxide (NaOH). A mixture of Na2CO3 and Na3AlO3 may also offer useful thermal storage properties, with potential to add NaOH to adjust the melting temperature and thermal storage capacity to a useable range.

Combined Cycle Operation:

A combined-cycle system would involve higher storage temperatures that would allow an externally heated air turbine engine to simultaneously pump air and drive an electrical generator. The temperature of the exhaust heat from the turbine engine would raise saturated steam, with additional thermal energy being transferred from thermal storage to the boiler to produce superheated steam. An overall combined efficiency of 45% to over 50% may be possible.

Mixtures of thorium oxides combine high density and high thermal storage capacity at a usable temperature range. A container made of silicon carbide may contain a mixture of thorium oxide (ThO2) plus thorium hydroxide [Th(OH)4] that could melt near 1000C, with scope to add thorium carbonate to adjust the melting temperature. The greatly extended life expectancy of the thermal storage compound would offset its high cost over many years of operation. It would provide much heat at the appropriate temperature to sustain the operation of an externally heated air turbine engine, the exhaust heat of which could sustain part of the operation of a steam engine.

There are several metallic carbonates that decompose thermally to yield a metallic oxide and carbon dioxide. When the carbon dioxide is reacted under pressure with the metallic oxide, a metallic carbonate forms and releases a large quantity of thermal energy during the heat of formation process. Much research has already gone into using the heat of decomposition of calcium carbonate as a form of thermal energy storage. During the heat of decomposition process, the carbon dioxide gas is pumped into a pressure vessel.

Future Prospects:

The nuclear power industry has successfully used gas to transfer heat from a heat source (the reactor) to a boiler while the solar thermal power industry has made advances in heat-of-fusion thermal storage technology. There is scope to apply some of this technology into the development of a thermal rechargeable railway locomotive that be recharged using either superheated steam or a superheated compressed gas. While some thermal rechargeable locomotives may still be used in shunting operations, there would be scope to develop such locomotives to pull commuter trains during rush hour periods, operate tourist excursion trains or provide the traction for freight trains that operate along short lines and/or branch lines.

The classical fireless steam locomotive incurred very low operating costs over the long term. They outperformed competing locomotive designs in terms of reliability, durability, ease of maintenance and longevity in service. There is potential for a modern thermal rechargeable locomotive to do likewise and offer greatly increased service duration between recharges. The extended useful service life of thermal storage material that may be used in rechargeable railway locomotives could make them into a viable motive power option along non-electrified railway lines.

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Rob Dickinson