A Thermo-Pneumatic Tesla Turbine Locomotive
Pneumatic locomotives operating on compressed air were successfully used in coalmines over a period of several decades. The Porter Company (USA) built a model that used an 800-psi accumulator tank, an operating tank set at 280-psi and compound expansion piston engines. These original pneumatic locomotives operated successfully and safely over short distances (up to 60,000-ft) in tunnels, hauling ore cars out of mines. None of the locomotives that were used in both American and in Europe used an air heating system to increase power or raise operating efficiency.
Modern pressure vessel technology allows air to be stored at pressures of up to 45,000-psi (310-Mpa) in reinforced spherical tanks. For pneumatic locomotive operation, spherical tanks of up to 2.75m (external diameter) can hold compressed air at 10,000-psi (68.95-Mpa). Several such tanks may be installed on an articulated locomotive frame, holding up to 40,000-lb (18,000-Kg) of compressed air at 80-degrees F (26-degrees C), air which may be fed into these tanks from larger stationary accumulator tanks holding compressed air up to 20,000-psi (137.9-Mpa). This arrangement would allow for rapid replenishing of locomotive air supply. The stationary tanks may be replenished during off-peak hours, reducing demand for high-priced electric power. Extreme high pressure pumping of air into the stationary tanks would have to be undertaken in stages, with air-over-oil pumping technology being used to achieve pressures of 10,000-psi. Cooling of air under compression would be essential to maximize storage density.
The air-compression technology may operate in a combined cycle with either a building complex or district heating system, allowing for the reject heat obtained for air compression to be put to productive use during winter months. During summer months, the reject heat from air compression may also be used to drive new generation absorption building complex cooling systems. The reject heat from air compression may also contribute to the thermal energy supply being stored in a stationary thermal storage tank, for later transfer into the locomotive thermal storage tank. Both storage tanks could be made from corrosion resistant materials such as silicon-nitride, which could hold a compound such as molten aluminum (melts at 645-degrees C, 170-Btu/lb heat of fusion) or molten lithium carbonate (melts at 723-degrees C, 260-Btu/lb heat of fusion). Heating of the molten metal could be accomplished by using concentrated solar thermal energy, garbage incineration, and heat pumping of geothermal energy to a high temperature, biomass combustion, fusion energy or even fission energy (micro-nuclear, using low-radiation pebble-bed technology).
Simultaneous pressurization of the accumulators and heating of the thermal storage material is possible, using cascade heat pumping technology. A low temperature heat pumping circuit using sulphur dioxide could remove heat from the air as it is being compressed. It could reject heat at 260-deg F (125-deg C) with a coefficient of performance (COP) of 6:1 to 9:1, to a higher temperature heat pump circuit using a different working fluid, such as saturated water. The saturated water would be pumped at 80% compressor efficiency from a low pressure of 25-psia (240-deg F) to a high pressure of 250-psia (401 deg F), with a COP of 3.84:1 to 4:1. High temperature heat pumping circuits (COP's of 3:1) on stationary tanks could involve such working fluids as mercury or a mixture of 56% sodium and 44% potassium (circuit material and compressor made from silicon-nitride) to raise the temperature to melt aluminum or lithium carbonate. The high temperature heat pumping could be supplemented with concentrated solar thermal energy during summer months (year round if the locomotives are in arid tropical nations).
To improve thermal efficiency while the locomotive is in operation, the compressed air would need to be heated to a high temperature, prior to expansion in an engine. The locomotive could be a 3-section articulated unit, with a thermal storage tank located at the centre, between the sections carrying the spherical pressure tanks. Cylindrical operating tanks set at 1,000-psi (6,895-Mpa) could be located below the spherical tanks. The operating tanks would feed air to the expander, via the thermal tanks. The on-board thermal tanks could also be made from silicon-nitride and contain molten aluminum or molten lithium carbonate. Heat transfer between stationary and mobile thermal tanks could be accomplished using superheated air as the heat transfer fluid, for reasons of safety.
One possible engine option for this application would be a relatively compact 3 or 4-stage Tesla turbine system. Exhaust air from a higher-pressure turbine would be reheated prior to expansion in a larger capacity lower-pressure turbine. The 3 to 4-stage compound reheat-expansion could raise adiabatic efficiency levels to over 90%. The Tesla turbine delivers optimal efficiency over a narrow range of operating speed, requiring the use of an electrical transmission. The power output of the Tesla turbine may be varied by varying inlet air pressure levels, at a constant maximum temperature. Inlet air velocity would need to vary between Mach 1 to Mach 1.25.
Heat transfer between stationary and mobile thermal tanks may use air being pumped through the heat transfer circuit using a compressor (a radial-flow bladed turbine) made from silicon-nitride, a material capable of handling extremes of temperature. The multi-pass air lines required to heat the mobile tank would each contain a series of venturies, each causing a successive pressure drop of 0.528 and a successive temperature drop of 0.833 (absolute temperature). After leaving the mobile tank, the heat exchanger air would pass through an air turbine, which would further reduce air temperature while driving a low-pressure air turbo-compressor placed upstream of the main compressor. The airline inside each stationary tank would contain a series of multi-pass tubes, to enhance the transfer of heat. With an adiabatic efficiency of 80%, the compressor would raise the temperature being fed into the mobile thermal tank.
If the compressor has a pressure ratio of 4:1, it would raise absolute air temperature by 48.6%, while a 6:1 pressure ratio would see a 66.85% rise in absolute temperature. If the air leaving the stationary tank is at 1350-degrees R, it would rise to 2170-deg R (1710-deg F) using a 4:1 pressure ratio at 80% adiabatic compressor efficiency. Using a 6:1 pressure ratio, this would increase temperature to 2478-deg R (2018-deg F/1103-deg C) at 80% compressor adiabatic efficiency. Absolute temperatures entering the mobile tanks could be raised an extra 6% - 10%, by using a diffuser downstream of the compressor. This could raise heating temperature to 2166-degrees F (1185-deg C), sufficient to rapidly remelt/reheat the aluminum or lithium carbonate during a thermal recharge.
While the locomotive is in operation, the maximum air mass flow rate passing through the engine could be set between 10,000-lb/hr to 15,000-lb/hr. With a 40,000-lb maximum compressed air capacity (density of 49.98-lb/cu.ft), the usable air supply could last for 2 to 3-hours. With 1,000-psi operating tank pressure and 14.7-psi atmospheric pressure (assume exhaust to atmosphere), the outlet pressure ration would be 68:1, which would translate to an absolute temperature ratio of 3.339 at 100% adiabatic expansion efficiency. With molten aluminum held at 645-degrees C (1193-degrees F), the air could be heated to 1000-deg F prior to expansion. In a single stage expansion, exhaust air would drop to -22.75-deg F, yielding a temperature drop of 1022.75-deg F. Multiplying this by an air specific heat of 0.24-Btu/lb-deg R, an adiabatic efficiency of 90% and a mass flow rate of 10,000-lb/hr, dividing by 2545-Btu/Hp-hr, yields 868-Hp. Increasing air mass flow rate to 15,000-lb/hr raises power to 1302-Hp. This power level allows the locomotive to pull a short commuter train for up to 2-hours at speeds of 60-miles/hour.
Using lithium carbonate as thermal storage material could raise air temperature to 1140-deg F, dropping to 19-deg F after expansion (100% adiabatic efficiency). At 90% adiabatic engine efficiency (single pass expansion), the engine would deliver 951-HP using 10,000-lb-air/hr (1427-Hp using 15,000-lb-air/hr). Using the Tesla turbine engine in a multi-stage reheat expansion system, would economize on air consumption and increase locomotive operating duration/distance, enabling short-distance intercity routes up to 150-miles to be served at moderate rates of speed. Using heat pumping between the thermal tanks to further heat the air may be possible, using a corrosion resistant heat pump circuit and compressor made from silicon-nitride. Possible working fluids would include mercury or a sodium/potassium mixture, enabling COP's of at least 3:1 (this is the minimum COP that will yield a net gain), enabling a net of 1250-Hp at 10,000-lb-air/hr (1875-Hp at 15,000-lb-air/hr). High COP heat pumping could raise compressed air temperatures to 1600 to 1800-deg F prior to expansion, raising engine efficiency levels sufficiently to allow for power to be used to drive the heat pump compressor (240 to 340-Hp for a COP over 3:1; a COP of 1:1 requires 950-Hp at the compressor and yields zero net gain) .
Further improvements in locomotive performance are possible, using "renewable combustion" technology. Certain chemical compounds release heat during bonding and dissociate when heated to a high temperature. Magnesium hydride is one such compound, while potassium oxide is another. During heating, the hydrogen can be disassociated from the magnesium and stored in a separate chamber, or oxygen from potassium. In operation, the heat of formation of the magnesium hydride or potassium oxide could be used to superheat the air prior to expansion in the engine. Some "renewable combustion" combinations (heat of formation) could heat the air to 2000-deg F prior to expansion and raise exhaust temperature to 276-deg F, allowing heat from exhaust air to be re-introduced into the operating tank and into the spherical accumulators, which would be cooling as internal pressure dropped. Re-circulating reject heat would further increase the operating range of the thermo-pneumatic locomotive.
Heat from the atmosphere could be heat-pumped into the accumulators to reduce pressure loss during operation. With 2000-deg F air temperature, 1465-Hp would be available at 90% single-pass adiabatic efficiency, using 10,000-lb-air/hr (3-hours in service operation at speeds up to 75-mi/hr or 120-Km/hr) while 2193-Hp would become available over 2-hours using 15,000-lb-air/hr (train speed 90-mi/hr or 1145-Km/hr). Heat exchangers made from silicon-carbide may be used in the high-temperature heating of the compressed air, whether from "renewable combustion" or from combustion of a fuel that would otherwise be unsuitable for use in an internal combustion piston engine (e.g.: low-rank coal-water fuel, corrosive liquid fuels or similar gaseous fuels). These fuels may either destroy engine lubrication or build sludge and engine deposits that would impair efficient internal combustion engine operation. External combustion engines can yield lower exhaust pollutant emission levels, due to greater scope to manage and refine the combustion process.
In a resource constrained future where oil prices rise to 3 to 4-time present day levels, a thermo-pneumatic Tesla turbine locomotive may be able to operate some types of commuter train services and short-distance intercity passenger train services, both along relatively low-density routes where the cost of railway electrification could not be justified. Alternatively, a thermo-pneumatic Tesla turbine locomotive could operate along rail lines in small nations. Such a locomotive and its energy storage systems would have longer longevity that present competing technologies, resulting in less need to replace worn or expended parts.
An efficient TESLA gas turbine concept, burning problematic fuel
The bladeless Tesla turbine is able to operate under conditions that would either harm conventional bladed turbines, or use fuels that could not be used in them. One such fuel is coal-water fuel, an emulsion that has successfully been used in external combustions such as boiler fuel. When coal-water fuel was burned in conventional internal combustions engines, deposits formed on engine parts such as piston rings, valves and on turbine blades. Similar results occurred when powdered coal was used in internal combustion engines.
The very nature of the design of the Tesla turbine suggests that it may be able to operate as the power turbine in internal-combustion gas turbine engines burning either powdered coal or liquid coal-water fuel.
If the coal-fired gas turbine were intended to generate under 2,000-Hp, it could use a radial-flow bladed compressor of smaller diameter than the Telsa power turbine. A higher powered unit would use an axial-flow compressor with its air intake located between the compressor and Tesla turbine. Air leaving the radial-flow compressor would flow through180-degrees before entering the combustion chambers, while the air from the axial-flow compressor would flow through 270-degrees before doing likewise. The route undertaken by the compressed air to the combustion chambers forms the basis of an approach that could raise the overall part-load efficiency of a single-shaft gas turbine using a Tesla power turbine.
Most gas turbine engines operate at reduced engine thermal efficiency when running at part-load power output. During part-load operation, the air mass flowrate entering the combustion chambers of conventional bladed turbine engines would be reduced, resulting in a drop in turbine inlet temperature. As a result, peak engine efficiency of single-shaft, internal combustion bladed gas turbine engines would occur when both the turbine inlet temperature as well as the turbine rotational speed are at their maximum. To overcome this shortcoming, a 3-shaft, reheat free-turbine concept was developed during the 1950's and 1960's to raise the part-load engine thermal efficiency of conventional internal-combustion bladed gas turbine engines. This engine proved to be temperamental when used in marine service during that time period.
The Tesla power turbine proposed for use in the concept gas turbine engine would have air from the compressor divided between 3-combustion chambers sized in a 1:2:4 mass flow rate ratio. The Tesla power turbine would have 7-nozzle inlets that would correspond to the 3-combustion chambers. At full power, all 3-combustion chambers and all 7-nozzles would be in operation. As power demand is reduced, the air mass flow rate entering the compressor would be reduced. To maintain maximum combustion temperature and maximum turbine inlet temperature, the reduced air mass flow rate would be directed into fewer combustion chambers. Special inlet valves in the air circuit would direct compressed air to the operating combustion chambers.
The inlet nozzles that would remain in operation would continue to deliver hot gas into the Tesla discs at maximum inlet temperature and optimal inlet velocity. Using this approach would enable the Tesla power turbine to deliver 7-equally spaced levels of power output at peak engine thermal efficiency, even when the Tesla turbine operates at part-load. This "digital" approach to turbine power operation could be expanded to use 4-combustion chambers built in a 1:2:4:8: mass flowrate ratio. Such a concept could supply hot gas to a large-diameter Tesla power turbine through 15-inlet nozzles. The multiple nozzle concept (or "digital" system) for Tesla turbines is being applied to a test unit being developed for the natural gas industry by the Centripetal Dynamics group. It is also being proposed for use in steam-driven Tesla turbine concepts to regulate power while the steam remains at constant temperature and pressure.
In the "digital" power system, combustion chambers would either be on or off, as would the inlet nozzles supplied by each combustion chamber. Each operating nozzle will deliver combustion gas to the Tesla discs at a constant maximum turbine inlet temperature and at optimal gas flow velocity. Such an arrangement would enable an internal-combustion Tesla turbine to operate at peak engine thermal efficiency over a wide range of power output. It would also be able to achieve this while burning fuels that would otherwise foul conventional internal-combustion bladed turbine engines as well as comparable piston engines. The coal-water fuels and powdered coal would sell at a lower cost per BTU (or KJ) than diesel fuel, making a clean-coal burning Tesla engine a competitive powerplant for railway locomotives as well as for large boats. In both applications, the turbine would drive high-speed electrical generation gear.
One possible alternative combustion system for the proposed concept Tesla turbine, would be to use 8-identical combustion chambers feeding into 8-identical turbine inlet nozzles using identical fuel burner/combustor/injectors. This approach could be more cost effective and make for a less complex spare parts inventory. As engine power output is reduced, fewer combustion chambers and fewer inlet nozzles would remain in operation. The remaining combustion chambers and inlet nozzles will deliver hot gas at the constant maximum inlet temperature and at optimal inlet velocity into the Tesla turbine, ensuring optimal engine thermal efficiency.
The combustion chambers, the inlet nozzles and the inside surface of the Telsa turbine casing would be lined with silicon-nitride, a modern ceramic that can operate at sustained high temperatures of 2500-degrees F. Coal-water fuel can be combusted at temperatures of over 2000-degrees C. Silicon-nitride is being used to make modern turbine blades. It may be possible for the discs of the Tesla turbine to be made from the same compound. If the Tesla turbine exhaust has sufficient pressure and temperature, the gas could drive a second, lower-pressure Tesla unit that would also drive electrical generation gear. Space considerations inside railway locomotive carbodies as well as in boats may require that the Tesla turbine units be installed using vertical shafts. The development of new materials and new fuels (such as coal-water fuel) create new opportunities for the Tesla turbine.
Reference:
Harry Valentine,
Transportation Researcher,
harrycv@hotmail.com
Another fine article by Mr. Harry Valentine; harrycv@hotmail.com