The International Steam Pages
Researching the Ultimate Fireless Steam Locomotive – Part 4
Harry Valentine firstname.lastname@example.org develops previous ideas:
Ongoing research and developments in contemporary energy conversion technology can be applied to stored thermal technology for the purpose of developing a modern fireless steam locomotive. The essence of the classical fireless steam locomotive was the storage of highly pressurised saturated water well in excess of its boiling point at atmospheric pressure. That storage system has many advantages. The storage medium is readily available at a very low cost, is non-toxic, renewable and infinitely recyclable.
The highest pressures that were used in accumulators on German-built fireless steam locomotives held pressure at 1200 psia at 297°C (567°F). The thermal energy stored inside such accumulators can be used to energize a new generation of heat engine that can generate electricity for electric traction motors. At least 3 groups are undertaking research into thermo-acoustic engines that convert heat to low-frequency sound waves that drive linear alternators to produce electricity. There are 3 other parties undertaking research and developments into solid-state thermo-electric converters that will operate at higher efficiency than traditional heat engines.
One possibility in modern fireless steam traction would be a locomotive that stores thermal energy using saturated water inside an accumulator. Instead of driving cylinders or an innovative design of rotary mechanical engine, the energy in the accumulator would be used to energize either a bank of thermo-acoustic engines or a modern solid-state thermo-electric converter. One such technology is the Johnson Thermoelectric Energy Conversion System or JTEC that is being developed by former NASA research engineer Lonnie Johnson in Texas.
Johnson’s technology operates on a version of the Ericsson cycle and purports to be able achieve higher conversion efficiencies than the best large thermal engines that are currently in production. His ambient heat engine technology is intended to operate at high efficiency on thermal energy that is at lower temperature than his JTEC technology. There appears to be scope to combine Johnson’s cutting edge technology with an old and proven thermal energy storage technology.
The mass of water inside the accumulator would remain unchanged and would be heated by flowing either superheated or saturated steam under high pressure (over 1000 psia) through a heat exchange line that passes through the accumulator. The heat exchange line would contain a series of choke valves that would reduce steam pressure and temperature. Heat taken from that steam by the choke valves would be transferred by conduction and some convection into the saturated water inside the accumulator. The steam could be heated by any of a variety of technologies that would include concentrated solar power, conventional nuclear power or radiation-free nuclear power, geothermal energy, burning garbage or biomass.
Thermal energy would be transferred from the accumulator to the solid-state or thermo-acoustic technology using a closed-loop steam circuit that would include a thermostat line. An electrically driven pump would circulate the steam through the closed-loop steam line. The thermostat could be set so that the heat engines receive thermal energy at a constant temperature of 204°C (400°F) even though the maximum accumulator temperature would be 297°C (567°F). Heat transfer from the steam line could be enhanced by including choke valves in the steam lines directly underneath the solid-state thermal engines. The enthalpy of the saturated water would drop from 572 BTU/lb to 357 BTU/lb for a difference of 215 BTU/lb. Accumulator pressure would drop from 1200 psia to 400 psia over the duration of the operating cycle.
A tank of 6feet inner diameter by 50 feet inside length would have a volume of over 1400 ft3 of which 1100 ft3 may be occupied by saturated water at an initial pressure of 1200 psia. The density of the saturated water would increase from 44.8 lb/ft3 to 51.7 lb/ft3 as the volume of 49,280 lb of saturated water contracts. The accumulator would store 49,280 lb x 215 BTU/lb = 10,595,200 BTU or 4163 Hp hr of useable thermal energy that could be converted at 25% to 30% efficiency to traction.
The accumulator could be built to hold pressure of 2000 psia at 335°C (636°F ) with a density at 38.98 lb/ft3 and a weight of 42,884 lb for 1100 ft3. The drop in temperature from 335°C to 204°C (636°F to 400°F) would reduce enthalpy from 672 BTU/lb to 357 BTU/lb for a change of 315 BTU/lb that would translate to 13,508,771 BTU or 5307 Hp hr. The locomotive output at the wheel could approach 1200 Hp for 1 hour. This would mean up to 1,000 Hp available for traction over a period of 1 hour which would be sufficient to pull an excursion train, a commuter train or a freight train along a branch line or short line.
Steam as the Heat Transfer Agent:
There are thermal storage materials that could store thermal energy at lower pressure that saturated water inside an accumulator. Steam pumped under pressure can be used as the heat transfer fluid to add heat to such thermal storage technology or to remove heat and transfer it to solid-state thermal-electric conversion technology. Choke valves may be used to transfer heat into the storage technology as well as to transfer heat into the solid-state technology when the locomotive is in operation. Steam lines that each contain multiple choke valves could increase the rate at which thermal recharging could be achieved.
There are possibilities to use the latent heat of fusion of various molten mixtures of very similar metallic-oxides that occur naturally as ores to store thermal energy. These mixtures are corrosive and would have to be contained in specialized sealed cone-shaped containers made from materials such as silicon-nitride or silicon carbide. The containers along with steam lines may be cast into blocks of alloy steel or aluminum oxide to assure safety.
Fusion technology could store energy at up to 500°C (or 932°F) and offer higher thermal efficiency that what may be possible using highly pressurized saturated water. A volume of 500 ft3 of a metallic oxide mixture having a specific gravity of 2.5 could weigh 78,000 lb and store up to 300 BTU/lb of thermal energy. The 23,400,000 BTU of energy would translate to over 9000 Hp hr of which some 40% could be converted to traction energy by the solid-state thermal conversion technology. A stored thermal energy locomotive could theoretically operate at an output of up to 1200 Hp for up to 3 hours in various types of service.
In terms of low operating cost the accumulator fireless steam locomotive could still be considered as a traction option for short-haul purposes in regions where there is easy access to thermal recharging at low cost. Rechargeable electrical batteries used in railway traction would have a limited life-expectancy due to the limited number of deep-cycle recharges and discharges to which such technology would be subject.
Thermal rechargeable technology can withstand hundreds of times more deep-cycle recharges and discharges that electrical storage technology. Electrical rechargeable technology is at a competitive disadvantage at locations where electricity is produced at thermal power stations that operate at below 50% thermodynamic efficiency. This would especially be the case if hydrogen is produced at 65% efficiency and converted in fuel cells that operate at 40% to 50% efficiency for an overall efficiency of less than 15%.
A modern fireless steam-electric locomotive that recharges from a thermal power station could operate at over 20% overall thermodynamic efficiency. Its operation could have little negative impact on the natural environment and it could operate in any of several niche services. The solid-state electrical equipment could include ultra-capacitor technology to reclaim energy during braking and also to help reduce energy consumption during acceleration.
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