Many will argue that the U.S. needs 130 or more attack submarines to meet the present Soviet naval threat — and Secretary Lehman stated this need recently before a Congressional committee. Today, 96 u.s. SSNs are confronted by a Soviet force of nearly 300 attack submarines, so additional numbers of u.s. attack boats is certainly logical. The low-cost way to acquire efficient subs, in addition to the 100 SSNs programmed by the u.s. Navy, seems better directed towards fuel cell powered submarines than modern diesels. At one-fourth the cost of a new SSN, the fuel cell submarine offers a far more practical, expendable, quiet, and long submerged endurance -yet limited capability — approach to meeting the Soviet threat, particularly: under the ice, in shallow waters, in defense of homeland waters and offensively in sea areas where a concentration of several submarines tend to be more effective than a single high-quality nuclear submarine.
In effect, fuel cell power either drives the submarine directly through a d.c. motor or it stores electrical energy in a battery system which can augment the fuel cell’s electrical output -for high speed submerged operations. It’s like a diesel-electric submarine, but it is far better adapted for today’s naval threats.
Why this power system is practical today, how it works, and what its potential is for future operations are the ingredients of this article. That a fuel cell submarine can’t compete with a nuclear-powered SSN for most of the submarine jobs, is understood. But as a solution to greatly increased numbers of useful attack submarines in an environment of belt-tightened budgets, it appears attractive.
Background
Fuel cells have been, and are being, used extensively in the NASA manned space flight programs. The United States Army employs fuel cells as portable field power units. American power companies such as Consolidated Edison are operating fuel cell plants which can generate 4.8 megawatts of power — or enough electricity for 2,000 customers. Telephone companies use smaller, 40-kilowat fuel cell plants which generate power for their telephone electric switching equipment. And, many East coast utility industries have invested over $200 million since 1980 on 50 fuel cell units to power apartment buildings, offices, and factories in order to lessen the dependence on centrally situated power plants.
How the Fuel Cell Power System Works
A fuel cell power system generates hydrogen and oxygen in a “reformer,” from stored hydrogen peroxide and JP-5 aviation fuel. The hydrogen and oxygen produced then passes through fuel cells which power a d-e propulsion motor. There are three types of hydrogen and oxygen generating systems which the Western nations see as feasible for use in submarines. One operates on a chemical reaction that utilizes boron hydride. A second uses the principle or hydrolysis. While a third operates on the principle of reforming hydrocarbons into hydrogen and oxygen. The latter has been used in a 1981 Massachusetts Institute of Technology design study and is described here as a feasible and safe way or producing H 2 and o 2 for fuel cells. This uses a reformer system that utilizes hydrogen peroxide (H2O2) and marine JP-5 distillate fuel. The important feature of this fuel cell system is the emphasis on the safe handling feature of the H2O2 solution.
The important reason for an H 2 and 0 2 “reformer” generator is that it does not require noisy internal combustion to produce power. Pollutants are not emitted nor is there a requirement for moving parts. Collectively, these characteristics produce a very low acoustic signature highly desired in a submarine propulsion system which is estimated to provide power conversion at efficiencies of 40 to 70 percent.
The fuel cell resembles a large battery that can be constantly recharged. A simplified diagram of one of these “batteries” is shown. During operation, the anode side of the fuel cell is bathed with hydrogen-rich gas or pure hydrogen, while oxygen bathes the cathode side. The electro-chemical reactions as the two gases pass by a solid polymer electrolyte are shown. The useful product of the chemical reaction is a very high direct current flow of electrons between the electrodes and through the de propulsion ~otor circuits of the submarine or to the sub’s batteries. The reaction product in a fuel cell is pure water.
FUEL CELL
Several fuel cells can be physically arranged into modular stacks. The stacked fuel cells are connected to the propulsion motor circuits in series to provide the desired voltage output levels. They can also be parallel-connected to obtain the required current or power levels. Computer controlled switchers and rheostat circuits are then used to make the series-parallel electrical connections and to control the speed of the propulsion motor. These circuits and fuel cell stacks, in essence, form an efficient powergeneration matrix.
The “reformer” part of the fuel cell propulsion system generates the hydrogen and oxygen gases for the fuel cells. Figure 1 is a simplified diagram of the reformer used in the MIT study. The hydrogen peroxide solution is fed to a “decomposer.” The “decomposer” is a catalytic device made of a silver palladium screen pack. This device decomposes the H 2 o2 solution to make steam and oxygen. These produc~s are “cooled” and then the oxygen is extracted from the water by a “separator” unit. The water is sent to holding ballast tanks or pumped overboard, while the oxygen is directed to the fuel cells or to the internal atmosphere control system of the submarine for life support. The heat from the steam being cooled by the oxygen “cooler” unit is used to “heat” the JP-5 distillate prior to its injection into a “converter.” The “converter” is another catalytic device which causes the JP-5 to decompose in the presence of oxygen and sufficient beat — increased by the reaction of JP-5, o2, and steam in a “combustion chamber” — to form co, co 2 , and hydrogen. The heat, oxygen, and steam are supplied to the “converter” from the H 2 o2 “decomposer” via the combustion chamber. A concurrent reaction occurs when the steam is introduced into the “converter:” H 2 0 + C 0 + Heat = C O, C o2 + H 2 .
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The converter system removes the heavier co 2 gas and discharges it overboard with the cooling water. The remaining lighter gases are sent to a “diffusion” unit, where hydrogen is separated from the other gases with a silver palladium membrane device. The H2 gases are then sent via a “saturator,” to the fuel cells for consumption. Because many light gases, including hydrogen, are sent through the “diffusion” unit, a constant recirculation of these gases from the “diffusion” unit, to the combustion chamber must be maintained to prevent the diffusion unit from becoming saturated with unwanted gases.
The problem of corrosion from using the hydrogen peroxide solution can be substantially reduced with plastics and teflon. These materials line feeder-lines and fuel tanks. The modularization of fuel cell stacks also promotes the control of corrosion by providing better maintainability through modularity.
The Fuel Cell Submarine
The propulsion system configuration for a fuel cell submarine is similar to that used in a 1981 design-study presented at tbe Massachusetts Institute of Technology. It is comprised of two reformer systems and ninety ~2-kilowatt fuel cells that are packaged into stacks to form a power generation matrix. The fuel cell stacks and the two reformer systems are readily sound-quieted by using equipment containment vessels, sound suppression deck mounts for the containment vessels, and sound reduction of auxilary systems. The only moving parts for the fuel cell sub’s propulsion system are the machine-controlled gas distribution valves, the electrical rheostats, the solid-state switching circuits in the fuel cell matrix, and the bearings in the electrical drive motor. The end product is extremely quiet. It will be less complex with less moving parts and more efficient than nuclear reactor-steam propulsign systems or diesel-electric systems. The 400 F nominal operating temperature or the “reformer” and “combustion chambers” produce another desirable feature that is paramount in future attack submarines — i.e. a very low infrared (IR) signature.
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Maintenance requirements are significantly reduced and a modest crew of about half the number of an SSN is required for wartime scenarios.
With only short piping runs between the adjacently located fuel and oxidizer tanks and the reformer systems, — as shown in the fuel cell submarine picture — safety problems are minimized. The pipes will be of double-wall design, the inner pipes carrying the JP-5 and ~02 to the “reformer” systems. The outer pipes wlll contain any leaks that might occur. By implanting monitoring devices in the outer pipes, leakage from the inner pipes can be detected and corrective action taken to prevent leakage to the internal atmosphere or the submarine.
A typical getting-underway scenario involves bringing five to six fuel cells on line fifteen to twenty minutes after the reformer system has started generating oxygen and hydrogen gases. During the fifteen minute interval, half of the submarine battery system would be used to drive the electrical propulsion motor until the fuel cell electrical output was great enough to be brought on-line. After the fuel cell electrical current is sufficient to propel the submarine at about six knots, the fuel cell output can be connected to the de propulsion motor. At the same time, the fuel cells would recharge the battery system to replace the electricity lost by the initial steaming surge and carry the necessary hotel load. The same technique would be used for sudden emergency flank-speed requirements — but using most of the fuel cell units. A lesser number of fuel cells would be used for speeds of less than its top speed of 32 knots.
Conclusions
The small dimensions (about 2000T) of a fuel cell submarine helps to make it approach an SSN’s 60-day under-ice capability — with fuel for about 6 knots submerged endurance over the 60 days. A larger submarine fleet composed of a high-low mix of SSNs and fuel cell submarines would allow the nuclear attack submarines to be more readily available for operations where they are needed the most; particularly for remote ocean operations which require highspeed long range translt capabilities.
The fuel cell subs could be forward based in Allied countries to eliminate long ocean transits to their patrol areas. They would be well suited for Mediterranean operations, and be very good for mine laying operations. And, remember that with the towed linear array, the wire-guided HK 48 torpedo and the TO~AHAWK missile, they are far more effective than the diesel boats of the past.
The fuel cell sub can be the key to a rapid and effective expansion of the United States submarine fleet when war is imminent, or after the start of a general war.
Michael D. Fulgham
