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