Reviving the Classic Heat of Fusion Fireless Steam Locomotives

Using the latent heat of fusion of molten metal as a means to store energy in a steam railway locomotive, dates back to the 1920's when the concept was first originated. The two original heat of fusion fireless locomotives were intended to offer extended operating range with constant power, over conventional accumulator fireless locomotives. While the locomotive was in operation, energy from the heat of fusion would release at constant temperature over a long period of time, as opposed to the constantly dropping temperature that occurred in an accumulator containing pressurize saturated steam.

While the scientific theory behind using the heat of fusion of molten metal as a means to raise steam is a sound theory, the heat of fusion locomotives' in-service performance record proved to be disappointing on units which operated in both the England as well as in Europe. By re-examining the causes of the performance defficiencies in the original heat of fusion fireless steam locomotives, clues may emerge to reveal the source of these shortcomings, thereby providing guidelines as to how to resolve such problems, as well as to guide future research into advancing the performance capabilities of the fireless steam locomotive concept. Resolving the shortcomings of the original heat of fusion fireless locomotives may reveal that they may have been a concept ahead of their time.

The original fireless locomotives stored thermal energy in tanks of molten sodium hydroxide (NaOH), a corrosive and hygroscopic compound that melts at 318-degrees C/ 515-degrees F, with a latent heat of fusion of 77.5-Btu/lb. Being hygroscopic means that any leakage of steam into the NaOH could cause a chemical reaction, rapidly raising temperature. Being corrosive, NaOH would corrode most metals. Both locomotives were built at a time when stainless steel was only being introduced into industry, in very limited quantities. The discovery corrosion-resistant, high-temperature ceramic materials was still several decades away in the future. One of the fireless locomotives entered service even before chemist Dr.Rudolf Loebl published his ground-breaking chemistry handbook that included the heats of fusion of a wide range of materials, compounds and elements.

Corrosion:

The European sodium hydroxide fireless locomotive was used in commuter service and for a short while, it ran successfully. Its deterioration in performance was attributed to "caking" inside the thermal storage tank. Part of this phenomena can be attributed to some chemical interaction between the molten material and the material of its container or heat transfer equipment (water tubes). Even a small amount of such chemical interaction/reaction would have had the effect of altering the chemical properties of the molten material, namely, lowering both its melting temperature as well as its latent heat of fusion. As an example, if a small amount salt (NaCl) is added to iced water (H₂O), it begins to melt (brine can remain in the molten, or liquid state, at temperatures below -17.77-degrees C (0-degrees F). In a fireless steam locomotive operating on heat of fusion, a reduction in both melting temperature and latent heat of fusion in the thermal storage material, would reduce locomotive power output and operating range, perhaps even rendering the unit unusable for further service.

Heat Exchanger:

A heat of fusion fireless locomotive would have needed to transfer heat from the molten material to the water/steam via water tubes. The use of water tubes immediately limits the power reserve of the locomotive (locomotives using accumulators have a very high power reserve). The power reserve is essential when positive displacement steam engines (pistons) are being used. At temperatures over 300-degrees C/500-degrees F, the thermal conductivity of iron/steel is well below 60-W/M-degree C. Insufficient heat may have transferred from molten metal to water in the water tubes, resulting in low-grade saturated steam entering the locomotive cylinders during operation. Given the state of technology during the era when the heat of fusion fireless steam locomotives were introduced, the heat of fusion technology may have been better suited to supplement the heat loss from an accumulator.

Chemical Stability:

Several compounds involving the bonding of sodium to oxygen, have a propensity for dissociation at elevated temperatures. The vibrations that would transmit from the track into the locomotive thermal storage tank, could affect the propensity of sodium to dissociate from oxygen in the molten sodium hydroxide (the bond is Na-O-H, sodium atom bonded to an oxygen atom, in turn bonded to a hydrogen atom). A small amount of corrosion of tank or heat exchanger material, could provide material that could bond to an vacant oxygen atom, should a sodium aton be displaced. In the original heat of fusion steam locomotive thermal storage tanks, there was a high propensity for a range of chemical reactions to occur, with the end result of lowered melting temperature and reduced latent heat of fusion.

Hygroscopy:

Hygroscopy refers to solubility in water. Sugar and salt are hygroscopic while a glass tumbler is not. Sodium hydroxide is water soluble, meaning that even a small leak of steam into the thermal storage tank would alter chemical properties. Initially, if the steam leak begins while the locomotive is in service, it would deliver superb performance due to the increased availability of thermal energy ... for one trip only. After one spectacular trip, the thermal energy storage capabilities of the hygroscopic molten metal would decrease drastically, to the point where the deterioration in the locomotive's performance would render it unfit for service.

Future Fireless Prospects

The present known world oil reserves are projected to be sufficient for the next 20 to 30-years, a time period over which oil prices are projected to rise steadily, making alternate forms of energy economically viable. In mainline railway operation in the more developed nations, electric traction would likely predominate along heavily travelled, high-frequency and high tonnage routes. This would likely be the case in nations that have large coal reserves (clean coal technologies in power stations), large amounts of hydro-electricity, large amounts of nuclear energy or even access to hydrogen-fusion fueled electric power stations.

Fireless Competitors:

Alternate propulsion technologies may prove more viable along the less dense rail routes. There are two main energy sources for such propulsion: thermodynamic (solar, geothermal, biomass, nuclear, atomic fusion) and kinetic energy (wind, tidal power, ocean wave energy conversion, hydro-electricity). Where low cost kinetic energy is available, locomotive propulsion alternatives may include flywheel-electric propulsion, battery-electric (lithium ion or other advanced technology), hydrogen fuel cell with electric motor, or even compressed liquid air (including liquid nitrogen which can hold 3.5-times the enrgy density of lead-acid batteries). The latter concept is a distant relative of the fireless steam locomotive: both use the same types of pressure vessels or accumulators.

Compressed-air Locomotive:

Modern off-the-shelf tank technology allows air compressed to be contained 4,000-psi in spherical tanks. Modern off-the-shelf hydraulic technology allows for air-over-oil pumping systems to pump air to over 5,000-psi (350-bar) and be stored in stationary tanks, allowing for rapid re-charging of compressed-air locomotives during brief stop-overs. The original compressed air locomotives were small units used in coal mines, yet were capable of pulling ore trains for up to 10-miles. Porter (USA) built several such units, holding some 800-psi in its main tank and 280-psi in an intermediate tank, which fed into compound expansion cylinders.

Large modern units could be built to the maximum right-of-way clearance dimensions allowed by railway compaies. They may be able to pull short trains for up to 25-miles. The life expectancy of compressed air technology would far exceeding the life expectancy of battery or fuel cell technology, while also incurring much lower parts replacement costs. The compressed air locomotive would be competitive in terms of overall energy efficiency as well as longterm costs. It would be a competitive option in regions where electricity to drive air compression equipment is sourced from kinetic energy. Its low operating cost, reliability, longevity and long term durability would be competitive with fireless steam power. However, in terms of operating range, power output and overall energy efficiency in regions where low-cost thermal energy sources predominate, the fireless steam locomotive would be the more competitive alternative.

Fireless Steam Options:

In areas where low-cost thermal energy (biomass, clean-coal technology) is available, a new/future generation of onboard-combustion steam powered locomotives is possible Where large amounts of low-cost thermal energy (solar thermal energy, low-cost stationary incineration technology, geothermal energy, mini-nuclear energy) is available, the fireless locomotive would be a propulsion option to consider. Much scope exists to upgrade its performance capabilities of the original fireless locomotives, to raise power output, extend operating range and improve logistics. It terms of overall energy efficiency and overall costs, a modernized fireless steam locomotive could source its energy directly from the same thermal energy sources as battery-electric, hydrogen fuel cell electric or even compressed air locomotives, at much lower costs. In terms of overall energy efficiency, from the thermal fuel source (solar thermal, geothermal, clean coal, mini-nuclear, biomass) to the drive axle or drawbar, the fireless steam locomotive would exceed the efficiency levels of competitors operating on battery, hydrogen fuel cell and compressed air technology (these latter three require a thermal power station to generate electric power ..... typically 37% - 40% power station efficiency).

Material Requirements (Heat of Fusion):

The experiences of the classical heat of fusion fireless steam locomotives can provide a valuable guide by which to direct future research into fireless steam railway traction. The thermal storage material need to be non-hygroscopic, while the inner surface of the container (thermal storage tank) and outer surface of the water tubes and superheaters need to be both chemically inert as well as corrosion resistant. The thermal storage material also would need to have a (1) high level of thermal conductivity, a (2) high level of latent heat of fusion, a (3) high thermal capacity (specific heat) and (4) be chemically stable and not be subject to dissociation or breakdown at high temperature. Many materials (including aluminium) that can endure several thousand repeated deep-cycle heating/cooling cycles already meet this criterior, while being within usage temperature ranges for fireless locomotive operation.

Heat Storage and Utilization:

There are several ways of using heat of fusion material as a means of thermal energy storage in a steam locomotive. One method is described in another paper and involves using a traditional (modernised) accumulator along with a tank of molten metal to both superheat the steam prior to expansion in the engine, as well as supplement the heat depletion rate from the accumulator. A second method would involve using two thermal storage tanks together with the accumulator and a water tank: one thermal tank to pre-heat water into high pressure saturated steam prior to entry into the accumulator (through a perforated pipe); the second tank for superheating the steam prior to expansion. The third concept would involve the use of a thermal tank and a water tank, with water being pumped under pressure through water tubes (located inside the thermal tank, which would duplicate the role of a water-tube boiler). This concept would have a very low power reserve and be better suited for steam-turbine-electric operation.

Heat Transfer using Heat Exchangers:

Due to concern over installing high-pressure water/steam lines inside a container of molten metal, an intermediate heat transfer method may be used. This method would involve circulating low-pressure steam or gas through the tubes/heat exchanger located in the thermal storage tank, then transferring the heat to an external heat exchanger which may contain high-pressure water tubes/steam lines. The low pressure fluid could exchange heat with high pressure (1000-psi/70-bar) tubes carrying water/steam, which if in the superheated state, may be used for expansion in a turbine engine. If the high-pressure steam is saturated, it may instead be injected into an accumulator for use in a system having a large power reserve and operating on pistons (or a comparable positive-displacement expander). Prior to entering the cylinders for expansion, the steam may be superheated via a secondary high-pressure/low-pressure heat exchanger technology, located outside the superheating thermal storage tank.

A variation of this low-pressure/high-pressure heat exchanger method would see the low-pressure hot gas being propogated either through a heater tube located inside a 1,000-psi accumulator (to maintain thermal energy levels), or through "firetubes" located in a 300-psi firetube-type boiler to raise steam for propulsion. In the latter case, a superheating heat exchanger would need to be used, where low-pressure hot fluid from the thermal tank would superheat higher-pressure saturated steam leaving the boiler, prior to being expanded in a positive displacement steam engine. Alternatively, a separate superheating thermal tank may also be used. The use of an intermediate low-pressure fluid allows for low-pressure lines to be used in the thermal tank(s).

All interior surfaces inside the thermal storage tanks may then be coated and lined with chemically-neutral, corrosion resistant and inert ceramic material. Low pressure heat exchanger equipment located inside the thermal tanks may be made from ceramic materials such as silicon-carbide (SC-1000 from Kyocera, 130-W/M-deg C thermal conductivity) or even silicon-nitride (SN-281 from Kyocera, 59-watts/metre - degree C thermal conductivity). The high thermal conductivity of silicon-carbide allows for relatively thick-walled heater tubes. By comparison, irons and steels have lower levels of thermal conductivity at temperatures over 300-deg C/500-deg F.

Heat Exchanger Material:

The nature and properties of the heat exchanger material will ultimately be one of the major factors influencing theuse of heat of fusion technology in fireless steam railway traction. Most metals in the solid state would be apt to corrode when immersed in another molten metal. Corrosion-resistant metals such as chrome-nickel alloy stainless steel have very relative low levels of thermal conductivity (19 - 26 W/M-deg C) at elevated temperatures (300 - 800 -deg C/ 500 - 1400-deg C), with thermal expansion coefficients of 0.0000065/degree F. Corossion-resistant ceramics like silicon-carbide and silicon-nitride maintain their mechanical and thermal properties to temperatures well excess of 1,000-deg C/1800-deg F. In terms of thermal expansion, silicon-nitride has a coefficient of 0.0000019/degree F while silicon-carbide has a coefficient of 0.0000025/degree F. There is, however, concern about the UTS (ultimate tensile strength) of ceramic materials carrying high-pressure water/steam.

Due to differences in rates of thermal expansion, stainless steel and ceramic cannot be used together inside the same tank. Even a joint between two pipes of different materials may be problematic as far as carrying high-pressure heated fluid is concerned: there is a high propensity for fluid leakage or stress cracking of the material, perhaps even both. The nature of the materials used in the construction of the thermodynamic system of a heat of fusion fireless steam locomotive, will influence its design layout. To minimize problems of leakage or stress cracking, the heat storage tank, heat exchangers, water pump, accumulator and steam engine may have to all be made from the same material.

Stainless-Steel System:

In an all (chrome-nickel alloy) stainless steel system, the low level of material thermal conductivity would require that the heat of fusion tank be used along with an accumulator. It is as yet uncertain as to whether molten aluminium (at 700-deg C/1300-deg F) would corrode chrome-nickel alloy stainless steel. If not, then aluminium may be used as a thermal storage medium, for the purposes of maintaining heat levels inside the accumulator(s). Heat would re-circulate between the thermal storage tank and the accumulator(s) via heat exchangers in both tanks, with pump-driven low-pressure fluid flowing continuously between the two while the locomotive is in service. The low thermal conductivity of the stainless steel would allow for a very small amount of water to be raised into high-pressure saturated steam that could be injected into the accumulator(s) via perforated pipe(s). This restraint allows for Ranotor condensor/coolers to be used, allowing for some re-use of exhaust steam and extending locomotive operating range.

Stainless steel boilers are made in Peru, allowing an accumulator to be beuilt from the same material and operate at 1,000-psi. The fairly rapid task of heating the fluid in the accumulator would be done by flowing steam through one or more perforated pipes. The time consuming task of heating of the aluminium would need to be done either by using heat pipes, or transmitting infrared solar thermal energy through fibre-optic cables made from artificial sapphire (an aluminium-oxide). The stainless steel system may even require the use of stainless steel cylinders.

Ceramic System:

The Spilling company as well as the Enginion Company, both of Germany, have manufactured oil-free ceramic steam piston engines. The Enginion ceramic steam piston engine was successfully operated in an automobile. Turbine blades are also being made from ceramics such as silicon-nitride and silicon-carbide. This indicates that some ceramic materials have high enough levels of tensile strength to be able handle some pressure levels (350-psi) of superheated steam, perhaps even as high as 800-psi may also be possible. In a 350-psi system, heat exchangers made from ceramic may be installed in a ceramic thermal storage tank containing molten material.

Due to the expansion/contraction rates of materials like aluminium (coefficient of 0.0000131/deg F) and lithium carbonate, the heat exchangers need to be positioned so as to compensate for the occasional solidification of the molten material. Heat pipes to heat the material would have their exteriors made from ceramic (storage material heating could be done either by heat pipe, or by infrared solar energy transmitted via optical cable). Several ceramics have higher levels of thermal shock (up to 750-deg C/1380-deg F) and higher levels of thermal conductivity than alloy stainless steel, a ceramic system may be used to heat water into the high-pressure saturated state, for injection into the accumulator.

Heat from the thermal storage tank may also be used to superheat steam leaving the accumulator and prior to entry into the engine, as well as to maintain heat levels in the accumulator while the locomotive is inoperation. A Ranotor condensor/cooler may be used to re-use some exhaust steam, re-circulating it as water for propulsion and extending locomotive operating range. The use of oil-free ceramic piston engines allows for both higher engine inlet temperatures, as well as allowing for easier condensing of the exhaust steam. The UTS of the ceramic material would determine the size of and pressure level used in the accumulator. Carbon fibre embedded in the ceramic material may be used to add tensile strength/hoop strength to the accumulator.

A high-pressure (1,000-psi or higher) ceramic system would either operate with a small accumulator, or without one. In the latter case, as steam-turbine-electric propulsion may be used, using a turbine made from ceramic material. The Turboflux company of Arizona (USA) is presently working on an efficient small turbine that may be applicable for use in railway traction. A great deal of research potential still exists in the field of fireless steam railway traction. It was not within the scope of this paper to explore the potential of combined-cycle fireless (steam) railway traction.

Harry Valentine, Transportation Researcher, harrycv@hotmail.com

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