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