Multifunctional Cryogenic Power System for Unmanned Underwater Vehicles Bradley N. Stoops, Chinh T. Nguyen, Mark S. Haberbusch Sierra Lobo, Inc., Milan, Ohio Phone: (419) 499-9653 Fax: (419) 499-7700 bstoops@sierralobo.com www.sierralobo.com ABSTRACT This project involves the development of a fuel cell power system for unmanned underwater vehicles (UUV), which uses cryogenic liquid hydrogen and liquid oxygen to provide increased energy and operating efficiency for longer missions and the ability to refuel vessels for rapid deployment. Underwater vehicles serve in as a key element in integrated operations of surface ships and submarines, providing a range of support functions including autonomous surveillance and mine counter measures. Fuel cells offer a viable power option for meeting mission energy requirements, and, at the same time, they can reduce the host vessel logistics burden. Cryogenic underwater power sources are efficient, quiet, compact, and easy to maintain. The total energy delivered by a fuel cell system is limited only by the amount of fuel and oxygen available to the fuel cell energy conversion stack. Since cryogenic liquid oxygen and hydrogen are more dense than compressed oxygen and hydrogen, both the weight and volume to accommodate the volume constraints of the vehicle design and the longevity of the missions can be obtained. This paper will discuss the design of a cryogenic power section for a 21 diameter UUV. Sierra Lobo, Inc. Page 1 of 12
I INTRODUCTION Undersea vehicles serve as key elements in integrated operations of future surface ships and submarines, providing a range of mission support functions including autonomous surveillance, mine counter measures, submarine track and trail, and Special Forces transport. These vehicles include the heavy-weight vehicles (21 diameter) and large-diameter vehicles (>36 diameter). The energy densities required to power these vehicles to meet the stated mission requirements are significantly higher than what can be provided by traditional technologies. Current power sources for these vehicles, such as rechargeable silver-zinc batteries (100 Whr/kg) or high-energy primary Lithium batteries (400 Whr/kg), do not meet the energy requirements for future missions. Furthermore, the non-rechargeable primary Lithium batteries only last 60 hours, cost more than $100 K each mission to replace, and put a tremendous logistics burden on the host vessel by having to change-out and store the hazardous energy section of the UUV after each mission. Fuel cell power system technologies can meet increased mission energy requirements. Fuel cell power systems can currently achieve 400-1,100 Whr/kg and, at the same time, reduce the host vessel logistics burden by using fuel and oxidizer sources that can be readily generated onboard the host vehicle at no cost through host vehicle electrolysis systems. Fuel cells typically have lifetimes that exceed 3,000 hours without significant performance degradation enabling more than 50 sorties before changing-out the fuel cell stack. The cost of a replacement fuel cell stack is at least 1/5 the cost of the primary batteries. In addition, fuel cells operating on hydrogen or more complex fuels (such as high-energy density hydrocarbons) and oxygen are an efficient, quiet, compact, and easy-to-maintain undersea power source. Sierra Lobo, Inc. Page 2 of 12
The total energy delivered by the fuel cell system is limited only by the amount of fuel and oxygen available to the fuel cell stack, since undersea vehicles must carry both the fuel and the oxygen sources. Consequently, storing the fuel and oxygen onboard the UUV is a critical requirement. A full-scale liquid oxygen storage system has been designed, fabricated, and tested with a 1 kw PEM fuel cell under a Navy SBIR Phase II contract through the Office of Naval Research. Sierra Lobo has developed a cryogenic liquid oxygen storage system that can store oxygen safely at the maximum possible density and provide gaseous oxygen to a fuel cell powered underwater vehicle at the required flow rates [Ref. 1]. Currently, under a Navy SBIR Phase III contract, Sierra Lobo is developing a cryogenic liquid oxygen and liquid hydrogen fuel cell system, which will be integrated into a 21 UUV. II UUV POWER SYSTEM ALTERNATIVES Batteries are the primary power system used in the Navy s UUV programs. Silverzinc and rechargeable and expendable primary Lithium batteries were baselined for use on the LMRS vehicle. These battery systems are compared in Table 1 to a Proton Exchange Membrane (PEM) fuel cell power system coupled with the Sierra Lobo liquid oxygen and liquid hydrogen storage system. Comparisons are shown between the fuel cell and the battery systems in all four of the most important parameters to the Navy: mission endurance, specific energy, reusability (sorties), and operating cost. The strength of silver-zinc batteries is their relatively low cost and ability to be recharged, which allows up to 15 sorties or missions to be completed before the batteries are replaced. The major weakness of silver-zinc batteries is the low-energy density or energy storage per unit mass of the system. Sierra Lobo, Inc. Page 3 of 12
The strength of the primary Lithium battery is the large-energy density, which enables increased mission duration. The major weakness of the primary Lithium battery is that it is not rechargeable. It is an expendable battery that must be replaced after each mission at a prohibitive cost of $100,000 a replacement. A primary Lithium battery has safety and logistical issues regarding maintenance and storage. The strength of the hydrogen and oxygen fuel cell is that it can deliver greater energy densities, mission endurance, and more than three times the number of sorties compared to silver-zinc batteries and at a reasonable replacement cost. The fuel cell specific energy range displayed in Table 1 is based on the mass of the engineering prototype as calculated in a final production prototype system mass. Larger diameter UUV systems will have larger specific energy for a cryogenic fuel cell system due to the ability to store more reactants with little additional structural mass required. Energy Source Silver-Zinc Batteries Secondary Lithium Batteries Table 1 - Energy Source Comparisons for 21 UUV (six knots, NUWC profile and 100V output) Endurance (hrs) Energy (kwhr) Specific Energy (Whr/kg) No. of Sorties 16 21 110 15 28 40 120 300 Expendables Cost ($) Recharge time is 2X endurance; recharge up to 20 times Recharge cost is minimal; cell maintenance cost Primary Lithium Batteries Fuel Cell (LOX/LH 2 ) 60 100 420 1 85 120 350-760 50 Not rechargeable, $100,000 per use <$200 Reactant cost per use Sierra Lobo, Inc. Page 4 of 12
III CRYOGENIC UUV POWER SYSTEM DESIGN TECHNOLOGY Sierra Lobo developed a unique Liquid Oxygen Storage and Delivery System for fuel cell powered Unmanned Underwater Vehicles (UUV) for the Navy. The Phase II engineering prototype system was a full-scale design to store 50 kg of Liquid Oxygen (LOX) storage capacity in a compact modular design for a 21 diameter UUV. The system is scaleable and modular to accommodate large vehicles, such as other heavy-weight vehicles and large-diameter vehicles, as defined in the Navy UUV Master Plan [Ref. 2]. The system is designed to deliver gaseous oxygen at a flow rate of between 0.1 to 100 g/min. The 1:1000 flow-rate ratio supports the operation of a fuel cell with a power range of between 10 W and 10 kw. The Phase II engineering prototype is shown in Figure 1. H 2, O 2, and N 2 Gas Control 1 kw PEM Fuel Cell Programmable Fuel Cell Load Temperature Controlled Water Circulator Cryo-Tracker Liquid Level and Temperature Probe 50 kg Liquid Oxygen Storage and Delivery System Figure 1 Phase II LOX Fuel Cell System Sierra Lobo, Inc. Page 5 of 12
A simplified schematic of the Phase III fully integrated LOX and LH 2 fuel cell system is shown in Figure 2a. The major components are shown in Figure 2b. Shown in Figure 2b are the following components: the LOX and LH 2 storage dewars, the vaporizer, pre-heater, humidification, water separation and DC power conversion. On/Off Valve Regulator System Purge Gas Ar/N 2 /He GO 2 LOX Vaporizer DC/DC Converter Pre- Heater Humidifier O 2 Water Separator VAC Port To Vent Fuel Cell Stack LH 2 Vaporizer DC/DC Converter GH2 Pre- Heater Humidifier H 2 Water Separator VAC Port To Vent Radiator Figure 2a Simplified Cryogenic UUV Power System Schematic Sierra Lobo, Inc. Page 6 of 12
Figure 2b Cryogenic UUV Power System Components The PEM fuel cell being used in the system is shown in Figure 3. Figure 3 PEM Fuel Cell for Cryogenic Power System Sierra Lobo, Inc. Page 7 of 12
The features, advantages, and benefits of the Sierra Lobo LOX-LH 2 System are given in Table 2. Table 2 - Features, Advantages, and Benefits of Sierra Lobo LOX LH 2 System Feature Advantage Benefit Liquid oxygen and Increased oxidizer and fuel Higher Specific Energy hydrogen storage density Autogenous pressurization No pressurant gas required Faster recharging for fast response to vehicle needs PEM Fuel Cell Low operating temperature Decrease thermal management issues Tube-in-tube reactant pre-heater heats cryogenic oxygen and hydrogen Fuel cell waste heat carried by water can be used for reactant pre-heating Uses less electric power and smaller ambient radiator for fuel cell Vacuum jacketed liquid oxygen and hydrogen vessels with multi-layer insulation system Patented Cryo-Tracker cryogenic liquid mass gauge Superior insulation system reduces heat in-leak to the cryogen, reducing boil-off Accurate mass knowledge of stored liquid cooling Longer liquid storage time resulting in longer sorties Less frequent refilling and autonomous refill capability The system has a uniquely designed pre-heater that effectively uses the fuel cell waste-heat water stream to warm the cold gaseous oxygen. All of the systems have been designed to established pressure vessel and oxygen safety standards, including MIL-STD- 882, NSS 1740.15, ASME B&PVC Section VIII, ASME B31.3, ASTM G-63, ASTM G- 88, ASTM G-93, ASTM G-94, and NFPA 53M. Any storage tank operating pressure can be accommodated and is usually based on the fuel cell operating pressure and UUV operational depth requirements. Sierra Lobo, Inc. Page 8 of 12
IV AUTONOMOUS FILLING AND NO VENT The LOX-LH 2 system has been designed to be filled with liquid oxygen or liquid hydrogen autonomously using the advanced Sierra Lobo Cryo-Tracker liquid-level sensor technology. Autonomously filling of the system reduces human exposure, improves system performance, increases accuracy, reduces human error, and has been proven reliable and repeatable [Ref. 3]. Sierra Lobo demonstrated autonomous filling techniques by controlling a nitrogen liquefier, while filling a liquid nitrogen storage system with a liquid-level accuracy of 0.05 inches. The UUV LOX System, when integrated with the Sierra Lobo Pulse Tube Cryocooler, can efficiently and reliably liquefy gaseous oxygen from any number of sources from onboard the host vehicle [Ref. 4]. Water electrolyzers onboard submarines provide a renewable oxygen source that, once dried, can be directly liquefied into the UUV LOX Storage System. This eliminates additional LOX storage requirements and the total quantity of LOX onboard the host vehicle. The multi-layer insulation system on the LOX and LH 2 storage dewars have been designed to minimize environmental heat leak. Testing conducted in Phase II shows the tank pressure climbs less than one psi per hour when the tank vent is closed enabling the UUV to idle while on station for a reasonable period of time without venting (48-72 hours). While the UUV is on the submarine and loaded with liquid oxygen or hydrogen, the Pulse Tube Cryocooler can be used to recondense any vapor generated by boil-off and eliminate any venting of oxygen into the submarine. The No-Vent System provides closed-loop control and monitoring of the oxygen within the system. Sierra Lobo, Inc. Page 9 of 12
V INTERFACE ISSUES/EQUIPMENT Several options are presented with regard to implementing a Cryogenic Fluid Management (CFM) system for loading an Unmanned Underwater Vehicle with cryogenic reactants from a host vehicle (see Figure 4). These options are derived based on general requirements of the host vehicle to support an Unmanned Underwater Vehicle. The requirements taken into consideration include: maintaining existing safety standards, minimizing the impact to crew responsibilities (high degree of autonomy), rapid turnaround times, and minimized footprints. The host vehicle either has an on-board, highpressure water electrolyzer that generates high-pressure gaseous oxygen for breathing air and gaseous hydrogen as a byproduct or an onboard chemically stored hydrogen and oxygen source that can be released for liquefaction. OPTION 1 DIRECT OXYGEN LIQUEFACTION OPTION 2 - Eliminates host vehicle dewar - 24 hrs loading time (50 kg) - Simplest LOX system - Cold head on host vehicle or UUV - Compressor on host vehicle 1-stage Cryocooler HV GO2 LIQUID OXYGEN TRANSFER - Quickest loading time ~ 30 minutes - Chilldown gases used to start fuel cell and prime oxygen loop - Liquid transfer line system required 1-stage Cryocooler HV GO2 LOX Dewar HV = Host Vehicle GO2 = Gaseous Oxygen GH2 = Gaseous Hydrogen OPTION 3 LOX AND LH2 LIQUEFACTION - Cryogenic storage of both oxygen and hydrogen - Accomodates a variety of hydrogen generation options on host vehicle - Direct liquefaction recommended - Eliminates host vehicle dewars - Utilizes Sierra Lobo/NIST patent pending dual cryogenic reactant cryocooler currently under development for NASA HV GO2 UUV UUV HV GH2 SLI 2-stage Cryocooler HOST VEHICLE HYDROGEN SOURCES - Diesel reformer allows generation of hydrogen using standard military fuel - Dedicated electrolizer for UUV located on host vehicle can produce both hydrogen and oxygen from water source - Host vehicle electrolizer used to generate breathing air generates waste hydrogen - Waste hydrogen can be managed internally to host vehicle using cryostorage thus eliminating noisy venting Diesel Reformer Dedicated Electrolizer Host Vehicle Electrolizer Figure 4 - Host Vehicle Cryogenic UUV Systems Support Options Including Oxygen and Hydrogen UUV HV GH2 Sierra Lobo, Inc. Page 10 of 12
VI CURRENT TECHNOLOGY DEVELOPMENT STATUS The Engineering Prototype LOX System design and fabrication was completed in August 2003 under the SBIR Phase II contract. The LOX system was integrated with a one kw PEM fuel cell and successfully tested at the Sierra Lobo Milan, Ohio, facility in December of 2003. A LOX system demonstration was conducted to investigate performance under start, stop, and transient conditions at the minimum and maximum LOX flow rates that would support a 10 kw fuel cell system. All design verifications and test objectives were accomplished through this successful test and demonstration program. Currently the Multifunctional Cryogenic Power System, which incorporates a three kw PEM fuel cell and will be integrated into a 21 UUV hull, is in development with testing to be completed in 2010. VII ACKNOWLEDGEMENT AND REFERENCES This work has been conducted under the Office of Naval Research contract N00014-08-C-062. The authors would like to acknowledge the program and technical support provided by Dr. Michele Anderson and Dr. Maria Medeiros at ONR and Mr. Eric Dow of the Naval Undersea Warfare Center, RI. In addition, we appreciate the hard work and dedication of the entire Sierra Lobo, Inc. team, especially Marty Roth and Lesa Wilder. The following technical references provide public domain information on technical work conducted by Sierra Lobo that is related to the development of the cryogenic storage system technologies. 1. Haberbusch, Mark S.; Stochl, Robert J.; Nguyen, Chinh T.; Culler, Adam J.; Wainright, Jesse S.; Moran, Matthew E.; Rechargeable Cryogenic Reactant Sierra Lobo, Inc. Page 11 of 12
Storage and Delivery System for Fuel Cell Powered Underwater Vehicles, Autonomous Underwater Vehicle Workshop, San Antonio, Texas, June 2002. 2. The Navy Unmanned Undersea Vehicle (UUV) Master Plan, November 9, 2004. 3. Culler, Adam J.; Haberbusch, Mark S.; Nguyen, Chinh T.; Autonomous Control of Cryogenic Liquid Replenishment and Liquefaction Systems, 49th International Instrumentation Symposium, ISA, 2003. 4. Nguyen, Chinh T.; Yeckley, Alexander J.; Culler, Adam J.; Haberbusch, Mark S.; Radebaugh, Ray; Hydrogen/Oxygen Propellant Densifier Using a Two- Stage Pulse Tube Cryocooler, Advances in Cryogenics, Volume 49, 2004. Sierra Lobo, Inc. Page 12 of 12