Info

2nd LCAUV

600h

3,000 km

10 kW (Max)

5000 kWh

2016 or later

3rd LCAUV

?

10,000 km

?

?

Table 4. Power system specifications

Table 4. Power system specifications

Urashima, the first prototype of a fuel cell-driven underwater vehicle built by Mitsubishi Heavy Industry Ltd., has the following specifications: length; 10 m, weight; 10 tons, maximum depth rating; 3500 m, maximum cruising speed; 3.2 knots, and endurance; 60 hours. Fuel cells for underwater vehicles should run on pure-hydrogen and pure-oxygen since no air exists underwater. The water byproduct produced in the fuel cell should be stored in the vehicle body to keep its buoyancy constant. If the reactant water is pumped into the external environment, the vehicle consumes much more energy and the vehicle loses weight and will start to float. We have thus developed a completely closed fuel cell system, which confines energy resources and reactant water to the system, namely the closed-cycle PEFC system as shown in Figure 10. This FC system consists of two stacks, recirculation blowers, humidifiers, a heat exchanger and a reactant water storage tank, generating power of 4 kW. All devices are installed into a titanium pressure vessel. The coolant water from the FC stack is reused to humidify the hydrogen gas. A metal hydride (MH) vessel and a high-pressure oxygen tank are included. The heat generated in the FC stack is applied to heating the MH to extract hydrogen from the MH and the excess heat is radiated into seawater. Figure 11 shows a typical I-V plot of the PEFC system obtained in the 317 km sea trial. The maximum FC system efficiency was about 54 % at typical cruising speed.

Fig. 10. System configuration of the closed-cycle PEFC for Urashima.
Fig. 11. I-V plotting of the closed-cycle PEFC during a 317 km.

The target cruising range of the 2nd LCAUV has increased tenfold from that of the Urashima. JAMSTEC has set target specifications for the fuel cell system, with system efficiency of over 60 % and downsizing of the pressure vessel and tanks as shown in Table 2. They made and tested a single cell which consists of a solid polymer electrolyte membrane, carbon black, platinum-alloy, carbon paper, and metallic separators. The test was done under the following conditions: cell temperature of 60 degrees Celsius, process pressure of 300 kPaA, and gas utility factor of 50 %. Figure 12 shows an I-V plot of the new cell compared with the Urashima system. In the figure the circle and the square show that of Urashima and that of the new cell, respectively. They have a single cell efficiency of 60 % at a higher heating value in the target current point. Now JAMSTEC has designed a blower- and humidifier-less system using the new cells.

Platform

Efficiency

Volume ratio

Urashima

54 %

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