Are Steam-Powered Batteries Feasible?
Thermal rechargeable energy storage technology was introduced to the transportation world during the same time period as the early electrical storage batteries. The technology was used on early submarines as well as in railway shunting service. Combining advances in pressure vessel technology with new developments in thermo-electric conversion technology can create new uses for thermal energy storage technology. One of the applications could involve the short-haul or short-line railway industry for the development of a modern fireless steam locomotive. The essence of the classical fireless steam locomotive was the storage of highly pressurized saturated water well in excess of 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 567° F or 297° C. 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 400° F or 204° C even though the maximum accumulator temperature would be 567° F or 297° C. 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 6.5 Feet inner diameter by 55 Feet inside length would have a volume of over 1600 ft3 of which some 1300 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 58,000-lb of saturated water contracts. The accumulator would store 58,000-lb x 215-BTU/lb = 12,521,600-BTU or 4900-Hp-hr of useable thermal energy that could be converted at 25% to 31% efficiency to traction. This would translate to 1200-Hp to almost 1500-Hp of which 900-Hp to 1100-Hp could be available at the drawbar for up to 1-hour duration.
The accumulator could be built to hold pressure of 2000-psia at 636° F or 335° C with density at 38.98-lb/ft3 and a weight of 50,700-lb for 1300 ft3. The drop in temperature from 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 15,970,500-BTU or 6275-Hp-hr. The locomotive electrical output could approach 1500-Hp for 1-hour of which up to 1,200-Hp may be available for traction 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 than 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. Saturated water at high pressure and temperature behaves like a refrigerant that may be used in a high-temperature heat pump.
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. The mixtures could include compounds such as aluminum oxide (Al2O3 or O=Al-O-Al=O), lithium aluminum oxide (Li-O-Al=O), one of the hydroxides (O=Al-O-H), lithium carbonate (Li2CO3) and/or lithium hydroxide (Li-O-H). These mixtures could provide high heat storage capacity at useable temperatures. They 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° C 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.
Thermal energy can be stored chemically by thermally decomposing certain metallic carbonates that occur naturally as ores. Calcium carbonate (CaCO3) can be carefully heated and decomposed into calcium oxide (Ca-O) and carbon dioxide (CO2). The carbon dioxide can be stored in a pressurized tank or it could be stored as carbonate in a metallic compound that decomposes at very low temperature. The carbon dioxide can be reacted under pressure with calcium oxide to generate a heat of formation that would be hot enough to energize a closed-cycle air turbine engine driving electrical generation equipment. The exhaust heat from the turbine could energize a solid-state thermo-electric engine that may provide added electric power for the traction motors.
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%.
Combined cycle technology such as a hot air turbine engine compounded into solid-state thermo-electric technology can provide higher overall thermodynamic efficiency between 55% and 60%. A thermo-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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