“Mircea cel Batran” Naval Academy Scientific Bulletin, Volume XIX – 2016 – Issue 2
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ULTRACAPACITORS FOR TORPEDO PROPULSION
1
Deniz ÜNSALAN
2
Kunsel IZET-ÜNSALAN
Prof. Dr. Institute of Marine Sciences and Technology, Dokuz Eylül University, İzmir, Turkey
Assoc. Prof. Dr.Ing. Faculty of Mechanical, Industial and Maritime Engineering, Ovidius University,
Constanta, Romania
1
2
Abstract- Ultracapacitors are electrochemical devices that are able to supply high time rates of energy and
have the advantages of having virtually infinite charge-discharge cycles and are fast charging. Those
properties have made them attractive for the storage of intermittent energy. Their main drawback so far has
been considered their lower energy storage capacities. However, in the last few years, breakthroughs in
efforts to improve their energy capacities have been achieved mainly by the use of carbon nanotube
technology and carbon aerogel materials.
Propulsion systems of torpedoes launched from both surface and submarine ships have to provide high
powers for higher speeds, have to have a sufficient supply of energy to achieve higher ranges, have to be reuseable and be serviceable onboard, as well as being silent and free from exhaust gas signature, if possible.
Those requirements has lead the authors to consider ultracapacitors as sources for torpedo propulsion- able
to supply higher powers, re-useable for exercises, rechargeable onboard the ship, silent and free from
exhaust gas. Higher ranges are achievable by the allocation of the space saved from the air supply/fuel
storage requirements and the replacement of heat engine by the more compact electric motor for
ultracapacitor banks. It is deemed that the ultracapacitors of the near future shall also be feasible
alternatives for Aluminum-Silveroxide torpedo batteries since they are rechargeable.
A parametric analysis involving the comparison of the proposed system by an existing hypothetical 21 inch
(533 mm) torpedo propelled by a heat engine running on the Otto fuel.
Keywords: Ultracapacitor, Nanotechnology, Torpedo propulsion, Otto fuel, Aluminum-Silveroxide battery
Introduction
Torpedoes have been formidable naval weapons,
as witnessed by their wide usage and high kill
rates during the wars of the last 150 years,
following the first self propelled torpedo of
Whitehead in 1866. They can be launched from
surface ships, submarines or aircraft, using
torpedo tubes or missiles, and against both
surface and underwater targets.
Recent
developments in torpedoes are concentrated on
increasing their range and speed by using
advanced propulsion systems, that can use both
chemical fuels via heat engines or by advanced
batteries.
A torpedo propulsion system can be considered to
be composed of three units: the energy source,
the engine and the propulsor. Energy source can
be a fuel and an oxidizer, a single-component
fuel, or an electric battery or another source of
stored energy. The engine can be a heat engine
or an electric motor. A torpedo propulsor can be a
classical propeller, a dual-rotation propeller or a
pumpjet.
The demands from the torpedo propulsion system
are high range, high speed and stealth - silent
operation and no visible exhaust gases. Electric
propulsion provides a silent operation with no
exhaust gases, but however, until the recent
developments in battery technology, electric
propulsion has been regarded inferior to heat
engine propulsion due to the lower energy and
power concentrations. Another property should
be re-useability, because a torpedo is used mainly
as an exercise weapon at peace and non-combat
periods and has a single-use in combat periods where it is used for the delivery of an explosive
charge against the target. Also, the torpedo
should be maintainable on board, due to logistic
considerations. Electric propulsion has also the
advantages of constant weight and constant
power output: As the stored energy is depleted,
the buoyancy characteristics of the torpedo do not
change and the need for depth control is reduced.
Heat engines used for torpedo propulsion have to
operate against an exhaust backpressure, which
reduces the engine outpult with increasing depths
of operation. Electric propulsion eliminates that
problem.
Modern torpedo batteries, which use either lithium
or silver, are mainly of the primary type, which
means they should be replaced after each usage,
which is a process that requires dedicated shore
or tender ship based facilities.
A modern development in the storage of energy is
the recent and continuing development of
ultracapacitors- which provide the advantages of
323
DOI: 10.21279/1454-864X-16-I2-047
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“Mircea cel Batran” Naval Academy Scientific Bulletin, Volume XIX – 2016 – Issue 2
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high power capacities, short charging periods,
virtually an infinite number of charge-discharge
cycles, while being inferior to other energy storage
modes from the point of energy capacity.
However, recent developments have resulted in
increasing the energy capacity of ultracapacitors,
which has lead the authors to consider them as
energy sources for torpedoes. This presentation
is about this proposed usage, highlighting the
possible advantages of its adaptation as an
energy source for torpedoes.
Ultracapacitors- Principles of operation and
characteristics
An ultracapacitor stores energy by physically
separating positive and negative charges. The
charges are stored on two sets of electrodes
divided by an insulator. Energy is stored by the
polarization of ions in the electrolyte filling the
space within the electrodes.
There are no
chemical changes on the electrodes, therefore,
ultracapacitors have a long cycle life but their
energy density is limited by the surface area
exposed by the reversely charged electrodes. So
far, the reason that their energy storage capacity
is limited has been due to that reason. However,
their capacity to be rapidly charged and
discharged has made them attractive for short
time energy storage and supply, with high power
outputs for short periods.
One of such
applications is the energy storage for regenerative
breaking and acceleration for hybrid and electric
cars. A study of the history, fundamentals and
current technology of ultracapacitors has been
given by Yu et al [1].
The amount of energy stored in an ultracapacitor
can be written in a way similar to the conventional
capacitors:
E = 12 CV 2
As can be seen, the energy storage capacity of an
ultracapacitor depends both on the capacitance
and the potential difference of the two collectors.
Figure 1. Principle of an ultracapacitor (From
Khaligh and Lee, 2010)
Unfortunately,
the
electrolytes
used
in
ultracapacitors limit the voltage to about 2.7 Volts,
albeit it has been reported that the use of nonaqueous (“ionic”) electrolytes make the use of
about 4 Volts possible [1]. Therefore, the second
way to increase the energy storage capacity lies
in the increase of the capacitance, C. Since the
capacitance of an ultracapacitor is proportional to
the surface area, research has been made to
increase the exposed surface area by using
nanotechnology: Use of carbon-nanotubes and
graphene sheets is one of the solutions, where
specific capacitances of 150.9 F/g corresponding
to energy densities of about 85 W.h/kg (306
kW/kg) while delivering 4kW/kg power (at room
temperature) has been achieved. [2], [3]. It is
worth to note that the capacity increases with
o
temperature: 136 W.h/kg (490 kW/kg) at 80 C [3].
Another approach has been the use of aerogels,
where specific capacitances up to 366 F/g that do
not deteriorate with increasing number of chargedischarge cycles have been achieved [4]. It is of
no doubt that the capacities of available
ultracapacitors shall reach to that of the rival
Lithium-ion and silver-metal based primary
324
DOI: 10.21279/1454-864X-16-I2-047
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batteries in the near future, reaching the present
200-300 W.h/kg levels, while offering the obvious
advantages of stealth, re-useability, and quick
onboard and even in-tube charging.
Speed and range considerations for torpedoes
Torpedoes should be both fast enough to engage
their submarine or surface targets in the shortest
time possible, and also should have a range
sufficient enough for the distant targets. Those
two requirements do conflict, and it becomes
necessary to seek an optimization. To optimize
the torpedo speed (V t ) for a given torpedo with
the given propulsive energy content (E), and
realizing that the torpedo can be treated as a ship
obeying the standard procedures used in naval
architecture, the time endurance (t e ) for a torpedo
is given as:
te =
E
PD
where P D is the delivered power to the propulsor.
In terms of torpedo speed (V t ), wetted surface
area of the torpedo (S), density of seawater (ρ),
coefficient of resistance (C t ) and propulsive
efficiency (η P ), time endurace becomes [5]:
te =
E
ρVt 3SCt
2ηP
Assuming all the terms in the denominator with
the exception of torpedo speed be a constant, k 1 ;
te =
E
k1Vt3
On the other hand, time to engage a target ship
(τ eng ), with speed V s escaping the torpedo
(assuming perfect chase, there both the torpedo
and target ship follow the same course),
τ eng =
from which the optimum torpedo speed is
obtained for a given target ship:
3
Vt = Vs
2
and the corresponding range:
R=
4 E
27 kVs2
Effective firing range, L, also in the “chase-mode”,
can be found by equating the distance travelled by
the torpedo (V t × t e ) and the distance travelled by
the target at the same period (L+V s × t e )):
Vt te = L + Vs te
From which,
1
L = Vt te
3
Since time endurance is R/V t , it follows that L =
R/3, or the maximum firing range in the chase
mode is one-third of the torpedo range.
Taking the fact that a fast surface or a subsurface
target seldom has a speed exceeding 35 knots
(probably the upper limit for modern displacement
type hull surface warships and submarines, the
most feasible target for a high-speed torpedo), the
torpedo speed of 52.5 knots (27 m/s) is selected
for the evaluation of the concept.
Case study of a typical torpedo
A hypothetical torpedo, with the external
characteristics of U.S. Navy’s Mk.48 torpedo is
taken as the demonstration of performance of
ultracapacitor power unit (Figure 2).
R
Vt − Vs
Equating the time to engage the target ship (τ eng )
and time endurance, the range of the torpedo at
the rated speed V t is obtained:
R=
dR
=0
dVt
E Vt − Vs
⋅
k
Vt 3
For the optimum torpedo speed with a maximum
range, the last equation is derived with respect to
the torpedo speed and set to zero:
Figure 2. Mk.48 torpedo
(http://www.navysite.de/weapons/mk-48.htm)
The characteristic dimensions of the torpedo are
given as:
Length
5.79 m
325
DOI: 10.21279/1454-864X-16-I2-047
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External diameter
0.533 m
Bow sonar dome diameter
0.30 m
Length of tail run
1.20 m
Parallel body length
3.80 m
Weight
1663 kg
Warhead
292.5 kg
Coefficient of resistance of the torpedo was
calculated from the correlations by Brooks and
Lang [6] at 52.5 knots to be C T = 0.0030,
corresponding to a propulsive power of 370 kW,
assuming a propulsive efficiency η P = 0.65 by a
pumpjet system, driven by an electric motor. For
a range of 40 000 meters at that speed, fired to an
escaping target from a distance of about 13000
meters, the torpedo shall consume about 550 MJ
of energy, which shall be supplied by the
ultracapacitors. If approximately 3 meters of
parallel body is allocated for the ultracapacitors,
the energy storage capacity of the ultracapacitors
should be about 1 MJ/liter, and for a neutral
buoyancy contribution, about 1 MJ/kg. However,
the latest reported capacity is 306 kJ/kg [2], about
one-third of the targetted capacity. Nevertheless,
it does not mean that ultacapacitor technology is
not feasible for torpedoes. On the contrary, taking
the developments in nanotechnology into account,
there is no doubt that within a few years
ultracapacitors shall be feasible alternatives for
torpedo propulsion and also for both flying and
undersea autonomous unmanned vehicles, due to
its acoustic and bubble stealth advantages, being
re-chargeable and re-useableness and elimination
of battery life considerations.
CONCLUSIONS
Present day state of the art for ultra capacitors is still below the existing primary batteries and heat engine
fuels, as can be seen from the table below. Expected efficiency (from the fuel to the propeller) of heatengine based systems (using petrol, ethyl alcohol, hydrogen peroxide and Otto fuel) was taken to be 0.25,
due to the maximum power conditions that they have to operate.
Energy source
Petrol
Ethyl alcohol
Hydrogen
peroxide
Otto Fuel
Li-Ion Battery
AgO-Al Battery
Ultracapacitor
(Best achieved)
Table 1. Comparison of torpedo energy sources
Lower
Stochiometric LHV, fuel/air Expected
heating value Air/Fuel ratio
mixture
efficiency
(kJ/kg)
(kJ/kg)
Delivered
energy
(kJ/kg)
Visible
track
43500
14.7
2771
0.25
693
Yes
28865
9
2887
0.25
722
Yes
2888
0
-
0.25
722
Yes
2556
0
-
0.25
639
Yes
460
-
-
0.90
414
No
468
-
-
0.90
421
No
306
-
-
0.90
275
No
Despite the present inferiority of ultra capacitors as torpedo energy sources, the advantages achievable by
their usage as the propulsive power sources are:
•
•
•
•
•
•
Silent operation, with no trailing gas bubbles
Weight and buoyancy characteristics do not change during the period of operation
Engine performance is not affected with operation depth
Are rechargeable and re-useable
They can be charged onboard and in the torpedo tube.
Battery life is long, with infinite charge /discharge cycles.
It should be stated that the results from the continuing research are encouraging and there is no doubt that
the torpedoes of the near future shall take the advantage of ultra capacitors as energy sources, as well as all
other applications so far using lithium-ion batteries. The main obstacle, limited energy storage capability can
326
DOI: 10.21279/1454-864X-16-I2-047
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be overcome with the development of nano-technological materials to increase the capacitor effective
surface area and with electrolytes that can endure higher voltages without chemical decomposition.
BIBLIOGRAPHY
[1] Yu, A., Chabot, V. Zhang, J., “Electrochemical Supercapacitors for Energy Storage and Delivery,
Fundamentals and Applications”, CRC Press, 2013
[2] Kannappan S., Kaliyappan K., Manian R.K., Pandian A.S., Yang H., Lee Y.S.,, Jang J.H., Lu W.,
“Graphene based Supercapacitors with Improved Specific Capacitance and Fast Charging Time at High
Current Density”, arXiv preprint arXiv:1311.1548, 2013.
[3] Liu,C., Yu,Z., Neff, D., Zhamu, A., and Jang B.Z., “Graphene-Based Supercapacitor with an Ultrahigh
Energy Density”, American Chemical Society Nano Lett. 2010, 10, 4863–4868
[4] Ye, S., Feng, J., Wu, P., “Deposition of Three-Dimensional Graphene Aerogel on Nickel Foam as a
Binder-Free Supercapacitor Electrode”, ACS Applied Materials & Interfaces, 2013, 5, 7122−7129
[5] Larsson, L., Raven, H.C., Pauling, J.R., “Ship Resistance and Flow”, Society of Naval Architercts and
Marine Engineers, 2010
[6] Brooks, T.D., Lang, T.G., “Hydrodynamic Drag of Torpedoes”, U.S. Naval Ordnance Test Station,
NAVORD Report 5842, 18 February 1958
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DOI: 10.21279/1454-864X-16-I2-047
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