The International Steam Pages
Heat Pumping the Fireless Steam Locomotive
It may be possible to increase the thermal efficiency and extend the operating range of the fireless steam locomotive, by using heat pump technology. Such technology could be used on either an accumulator locomotive, or a variant using heat of fusion thermal storage technology. As well, it may also be possible to use cascade heat pump technology to heat/reheat either the accumulator of a fireless steam locomotive, or the firetube boiler of a live-flame steam locomotive. The heat to achieve this may come from mainly large stationary thermal storage tanks. The working fluid used by the heat pumps, may be saturated compressed steam.
An examination of the refrigeration/heat pump cycle reveals that water may actually be used as a refrigerant/working fluid when the low temperature (evaporator) is near or above the atmospheric pressure level boiling temperature of water. In a heat pump (like an air conditioner) a compressor pumps fluid under pressure from the low temperature heat exchanger (the evaporator) to the high temperature heat exchanger (the condenser). An expansion valve at the exit of the condenser maintains a high fluid pressure between the condenser and the compressor, while the evaporator operates at both lower pressure and lower temperature.
One attractive feature of a heat pump is the COP, of coefficient of performance, a parameter analogous to "efficiency". Household air conditioners and heat pumps operate at a COP of 4:1, that is, 4-units of thermal energy are transferred for every 1-unit of energy being consumed by the compressor. The COP value for most saturated fluids can be directly calculated using the enthalpy values at various temperature and pressure levels. A COP value greater than 4:1 would provide a fireless steam locomotive with a small net overall gain in power output, engine efficiency and operating duration, all as a result of using heat pump technology. The table below gives some COP values for saturated steam, using 80% compressor efficiency.
The lower COP values are based on a worse case scenario, where the higher values would be more accurate. In a normal high-pressure (1000-psia) Gilli fireless steam locomotive built by Henschel, the steam would enter the cylinders at 544.5 deg F, until the accumulator temperature and pressure began to drop. A heat pump expansion valve at the exit of the condenser can be adjusted to compensate for dropping accumulator pressures and temperatures. As well, the heat pump can be used to maintain a constant level of "superheat" on the steam entering the cylinders (preferably a well designed uniflow, using exhaust ports). The heat pump evaporator would be located right inside the accumulator, where it would source its "low-temperature" heat supply.
The propulsive steam leaving the accumulator would be superheated by the condenser, prior to expansion in the steam engine. To assure a constant power level over time in service, the high-pressure accumulator could supply propulsive steam to a lower pressure accumulator, which would also contain the heat pump condenser. Pressure levels in this lower pressure accumulator would remain constant over a longer time duration, allowing superheated steam to enter the cylinders at constant temperature and pressure.
This arrangement would enable the locomotive to operate at a constant level of power output over prolonged time periods at constant throttle and constant inlet valve cut-off settings. The higher steam inlet temperatures would raise overall engine efficiency, reducing overall steam consumption and allowing inlet valves to operate at minimum cut-off ratios for extended durations. The heat pumped superheated accumulator fireless steam locomotive would offer a net power/efficiency gain over a non-heat pumped version. To maximize the potential of this concept, it would need to be built to the maximum right-of-way clearance dimensions of the rail system upon which it operates.
A heat pumped fireless steam locomotive featuring compound accumulators, would be able to pull trains at low to moderate speeds (20 to 40-miles per hour), over distances of 30 to 50-miles in under-developed and developing nations. Example, a train covers 100-miles in 10-hours on one route in Thailand. Trains in such nations stop frequently, usually every 20 to 50-miles, where thermal recharging may be undertaken during a layover. The locomotive may actually used multiple high-pressure accumulators (each with its own perforated recharge pipe), all connected to a single low-pressure accumulator. The use of multiple recharge pipes allows for more rapid thermal recharging during en route stop-overs.
A variety of fuel sources, including biomass incineration and heat pumping energy from stationary thermal sources, may be used for locomotive recharging. Stationary tanks may be heated directly by concentrated solar heat in arid tropical nations. In some nations, heat from geothermal sources (steam at 400-degrees F) may be heat pumped to a higher temperature, using pressurised saturated steam as the "refrigerant". Geothermal heat may be used to heat, then superheat steam to be injected into the locomotive accumulators. In an environment where railway fuel oil prices continue to rise and threaten the viability of rail operations, the heat pumped fireless steam option could evolve into a usable and viable form of motive traction.
A heat-pumped accumulator fireless steam locomotive built to the maximum dimensions on a railway system, may need to carry its weight on up to 12-axles or more, on 2 or more trucks/bogies using 42" wheels. Six axles on one truck/bogie may be powered, with wheels on the inner two axles and outer two axles being flangeless, with wheels on the remaining two-axles being flanged, so as to reduce rail wear and enable easier operation on tight curves. The drive train may utilize two closely spaced counter-balanced side rods having their oil-lubricated roller bearing cranks being spaced at 90-degrees to each other. This drive system would be enclosed inside a casing which would be carried on one side of the truck/bogie (as unsprung weight).
On the opposite side, a fully counterbalanced 90-degree double-acting Vee-2 (uniflow) steam engine would drive into the side rod drive system, enabling power to be transmitted to 6-driving axles. Each drive-case crank would be connected to a power axles via a spline in a quill drive. Quill drives are well proven in railway operations. Such a system could maintain low unsprung (axle) weight and be gentle on railway tracks, while maximizing locomotive tractive effort. An enclosed, lubricated roller bearing side-rod power delivery system would operate more efficiently and reliably for a longer time, than a comparable (and vastly more expensive and more troublesome) gear-drive system.
A heat pumped accumulator fireless steam engine would not only be able to offer useful services along rail routes in under-developed and developing countries, the concept can also be used in tourist/excursion train operations (though it would need to be built to a more traditional steam locomotive appearance. In terms of operating cost, energy cost, maintenance costs (and related downtime), a heat-pumped fireless steam locomotive could prove to be a very cost competitive concept. Heat pumping technology can be used on fireless steam locomotives using heat of fusion thermal storage systems, a combination that could offer even higher levels of power output, increased thermal efficiency and greatly extended operating distances that an accumulator variant, while still being very competitive in terms of capital cost and operating cost compared to other non-electric locomotive types.
Harry Valentine, Transportation Researcher, firstname.lastname@example.org
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