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FUEL CELLS ON THE NASA SPACE SHUTTLE.
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Reviews the fuel cells used on the orbiter vehicles.... More...
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Paper Abstract:
Reviews the fuel cells used on the orbiter vehicles. Table comparing fuel cell types. Fuel cell structure. Fuel cell components; the three fuel cell system on a space shuttle. Fuel cell reactants, storage and distribution. Fuel cell operation including consideration of parameter and processing. 3 Figures.
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FUEL CELLS ON THE NASA SPACE SHUTTLE ORBITER
Introduction
This research reviews the fuel cells used on the NASA space shuttle orbiter vehicles. The presentation of the review focuses on (1) fuel cell structure, (2) fuel cell components, (3) fuel cell reactants, (4) fuel cell storage and distribution, and (5) fuel cell operation. The review of fuel cell operation includes consideration of (a) parameter and (b) processing.
Fuel Cell Structure
The concept of the fuel cell is not a new technology. The fuel cell dates to 1839 and British scientist Sir William Grove. The first fuel cell appeared in 1842 (Connelley 1). Practical applications of the fuel cell concept proved elusive, however, until the National Aeronautics and Space Administration (NASA) funded the
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Electrical power is producedthroughout the purge sequence, although no more than 1 kilowatts should berequired from a fuel cell being purged because of the increased reactantflow and pre-heater limitations (Dicks and Larminie, 62 ). The electrochemical reaction produces electrons (electricalpower), water, and heat. A fuel cell continuously converts chemicals into direct-currentelectricity through electrochemical reactions. The fuel cells containan electrolyte consisting of potassium hydroxide and water, an oxygenelectrode (cathode), and a hydrogen electrode (anode). Reaching back in history,NASA selected the fuel cell. The hydrogen-water vapor mixturethen combines with the regulated hydrogen from the dual gas generatormodule, and the operation of loop begins anew (Dicks and Larminie, 617). At this point, theelectrons react with hydrogen and water, thereby producing additional OH-ions that replenish the ions used at the anode (Hoogers 174). Batteries were both too heavy and too toxic.Further, batteries had a finite life. A water purity sensor (pH) at the common product water outlet of thewater relief panel provides a redundant measurement of water purity. With an operating load of approximately sevenkilowatts, it requires only a few minutes to flood the fuel cell withproduced water, thus effectively halting power generation (Dicks andLarminie, 62 ). As an electromechanicaldevise, the fuel cell converts chemical energy caused by the oxidation of agas in to electrical energy in a continuous process. This outcome occurs because hydrogen flows over oneelectrode while oxygen flows over a second electrode. If there are no internal or external loads on the fuel cell, noreactant consumption occurs. Water and electricity are the products of the chemical reaction ofoxygen and hydrogen that takes place in the fuel cells. "Tomorrow's Engine." Discover 19 (February 1998): 26-27. Fuel Cell Storage and Distribution Fuel cells essentially are batteries with a fuel source thatcontinually recharges a battery (Winters 26). The thermostatically controlled heatersmaintain the water relief valve's temperature when in use within a range of21 -to-38 . Figure 2 [pic] (Source: Dicks and Larminie 2 ) Oxygen is routed to the fuel cell's oxygen electrode. As the current output of the fuel cell increases, thereactant flow rates increase, and the pre-heaters raise the temperature ofthe reactants to a minimum of minus 4 C to prevent the seals in the dualgas regulator from freezing (Dicks and Larminie, 623). The first stage of the regulator reduces thepressure of the hydrogen and oxygen to 135-to-15 psia (atmosphericpressure). Internal control of the circulatingfluid maintains the cell stack at a normal operating temperature ofapproximately 93 C (Dicks and Larminie, 616). Theremaining circulating hydrogen flows back to the fuel cell stack (Dicks andLarminie, 614). When a purge isinitiated by opening the purge valves, the oxygen and hydrogen systemsbecome open-loop systems. Fuel Cell Reactants Fuel cells typically contain expensive metal catalysts. The accessory section consists of the hydrogen and oxygen flowsystem, the coolant loop, and the electrical control unit (Hoogers 181). The rest pfthe hydrogen flows through the fuel cell stack, removing the product watervapor formed at the hydrogen electrode. Fuel cells can theoretically convertfuel to electricity with nearly 1 percent efficiency (Hoogers 164). A hydrogen pump/water separator thenseparates the liquid water from the hydrogen-water mixture (Dicks andLarminie, 617). Fuel Cell Systems Explained. The fuel cellconsists of a power section, where the chemical reaction occurs, and anaccessory section that controls and monitors the power section'sperformance. Fuel cell purge can be activated automatically or manually by the useof fuel cell switches. Manifolds extendthrough the length of the substacks (Hoogers 181). The first fuel cellappeared in 1842 (Connelley 1). Improvements made following each space flight, however, led toincreasingly reliable fuel cell functioning. Hydrogen is routed to the fuel cell'shydrogen electrode, where it reacts with the hydroxyl ions from theelectrolyte. In the alkaline fuel cell, there exists an excess ofOH- ions. Figure 3 Fuel Cell Used on NASA Space Shuttle Orbiter [pic] (Source: Dicks and Larminie 2 ) The location of the fuel cell units on the space shuttle orbiter isunder the payload bay and aft of the crew compartment. London: John Wiley & Sons, Ltd., 2 2.Morrison, G. NASA, however, selected the alkalinefuel cell system for use in Apollo 13 and later for the space shuttleorbiter vehicles. The 1,1 -wattsustaining heaters normally are used during low power periods to maintainthe fuel cells at their operational temperature (Dicks and Larminie, 624). The importantcharacteristics of fuel cells for space vehicle applications areefficiency, modular design, and, reliability (stemming from an absence ofmoving parts). 2. Asingle measurement of water purity is a part of each fuel cell. On early space flights, NASA experienced problems with fuel cellsystems. The water can be directed from thesingle panel to a potable water tank or to the fuel cell power plant waterrelief nozzle. When the reactants enter the fuel cells, they flow through a pre-heater that raise their temperature to a minimum of 4.5 C from cryogenictemperature levels. Figure 2 below illustratesthis process. Reactant consumption is commensurate to the electrical currentproduced. Each fuel cell system supplies aseparate, isolated, simultaneously operating 28-volt dc bus. The review of fuelcell operation includes consideration of (a) parameter and (b) processing. The power section is where hydrogen and oxygen aretransformed into electrical power, water, and heat. "The Perils of Partnership." Science 275 (24 January 1997): 468-471.Winters, J. The coolant is circulated through the fuel cell stack to absorb thewaste heat from the hydrogen/oxygen reaction occurring in the individualcells. In theNASA space shuttle orbiter vehicles, 96 fuel cells in three stacks converthydrogen and oxygen into electrical power, water, and heat via an alkalineelectrolyte (Morrison 54). The coolant exits the fuel cells to the fuel cell heat exchanger,where it transfers its excess heat to be dissipated through the Freon-21coolant loop systems in the mid-fuselage of the space shuttle orbiter.Further, by means of the coolant loop, the fuel cell has internal startupand sustaining heaters. Both hydrogen and methane are difficult to store and dangerous tohandle (Winters 26). The differential pressure sensor senses a pressuredifferential across the pump to determine the status of the pump. Table 1 below compares some of the characteristics of several fuel celltypes.|Table 1 - Comparing Fuel Cell Types ||Fuel Cell Type |Operating |Applications: Current & Potential || |Temperature | ||Alkali |5 -25 C |Current: NASA Space Vehicles || | |Potential: Land Vehicles || | |Submarines ||Solid Polymer |5 -1 C |Current: Industrial Use || | |Potential: Housing || | |Offices ||Phosphoric Acid |-22 C |Current: Industrial Use || | |Commercial Use ||Molten Carbonate |-6 C |Current: Medium-Level Combined || | |Heat & Power (CHP) || | |Potential: Large CHP Systems ||Solid Oxide |5 -1 C |Potential: All Levels CHP Systems ||Source: Dicks and Larminie 2 | Each of the 96 fuel cells on a NASA space shuttle orbiter is a self-contained unit measuring 14 x 15 x 45 inches. The accessory section monitors the reactant flow, removes waste heatand water from the chemical reaction, and controls the temperature of thestack. Theresulting saturated gas mixture moves through a condenser, where thetemperature of the mixture is reduced, condensing a portion of the watervapor to form liquid water droplets. Direct methanol fuel cells are similar to PEM fuel cells.Direct methanol fuel cells (DMFC) also use a polymer membrane as anelectrolyte. If the oxygen's and hydrogen's pressuredecreases, the coolant's pressure also decreases to prevent a largedifferential pressure inside the stack that could deform the cell stackstructural elements (Dicks and Larminie, 616). The location of the coolant loop system in the mid-fuselage of the space shuttle orbiter. An internal circulating hydrogen system removes excess water vapor.Hydrogen and water vapor from the reaction exits the cell stack, and thenmixes with replenishing hydrogen from the storage and distribution system.Once mixed, the hydrogen enters a condenser, where waste heat from thehydrogen and water vapor is transferred to the fuel cell coolant system.The process lowers the temperature, which in turn condenses some of thewater vapor to water droplets. The fuel cell coolant system circulates a liquid fluorinatedhydrocarbon and transfers the waste heat from the cell stack through thefuel cell heat exchanger of the fuel cell power plant to the Freon-21coolant loop system. Figure4 below illustrates the location of fuel cells within a NASA space shuttleorbiter. Practical applications of the fuel cellconcept proved elusive, however, until the National Aeronautics and SpaceAdministration (NASA) funded the development of fuel cell applications forearly space flights. "Fuelling the Future." New Scientist 17 (16 June 2 1): 1-5.Hoogers, G. Thisaction permits waste heat from each fuel cell power plant to be used tomaintain a uniform temperature profile for each fuel cell power plant(Dicks and Larminie, 624). Thisreaction produces water and releases electrons. Then, increased flows allow the reactants tocirculate through the stack, pick up the contaminants and blow them outoverboard through the purge lines and vents. Thisreaction is as follows: O2 + 2H2 +4e > 4 OH. The manifolds distribute hydrogen,oxygen and coolant to the fuel cells (Hoogers 186). Fuel Cell Components Fuel cells are relatively uncomplicated devices. Anappreciable amount of excess reactants used indicates a probable leak(Dicks and Larminie, 619). Figure 3below presents a photograph of a single fuel cell unit of the type used onNASA space shuttle orbiters. The hydrogen pump and water separator of each fuel cell power plantwere also improved. Each of the various types of fuel cells operates somewhat differently. Depending on the orbit trajectory and vehicle orientation,the heaters may require 27 minutes to heat the lines to the requiredtemperatures. In the event of the single water relief panel freeze-up, pressurewould build up and relieve through the redundant paths to the second watertank. In reactions in nature, hydrogen burns whenit encounters oxygen. NASA's current space shuttleorbiters rely entirely on fuel cells for electrical power and drinkingwater while in orbit (Hoogers 168).Processing Fuel cells use electrochemical processes, mediated by electrolytes, toproduce electricity from light fossil fuels (Bakker and Cohn 42). Today, NASA uses advanced fuel cell technology in its spaceshuttle orbiter vehicles (Hart 1; Stone, Allakhverdov, and Lawler 468). Regenerative fuel cells separate water into hydrogen andoxygen through the application of a solar-powered electrolyzer. The electrons move through the electricalcircuit attached to the fuel cell to reach the cathode. As an operational improvement, the end-cell electrical heaters oneach fuel cell power plant were deleted due to potential electricalfailures and were replaced by fuel cell power plant coolant passages. The stack inlet control valve and flow control valve havebypass orifices to allow coolant flow through the coolant pump and tomaintain some coolant flow through the condenser for water condensation,even when the valves are fully closed due to the requirements of thermalconditioning (Dicks and Larminie, 623). Table 1 belowillustrates the essential components required to support the fuel cellprocess of producing electricity. An additional benefit associated with the use offuel cell technology was the fuel cell byproduct of water (Hoogers 157). Fuel cell performance is independent of system size andoperational noise is exceptionally low. Any inert gases or other contaminants accumulate in andaround the porous electrodes in the fuel cells and reduce the reactionefficiency and electrical load support capability. At the anode, hydrogen gas reacts with the OH- ions. The primary deficiency of fuel celltechnology was its cost of application. Waterfrom the potable water storage tanks provides water for the space shuttleorbiter crew consumption and cools the Freon-21 coolant loops. Upon leaving the dual gas regulator module, the incoming hydrogenmixes with the hydrogen-water vapor exhaust from the fuel cell stack. Cost, however, was not an issue inthe race to the moon. A proposed modification of the fuel cell technology now inuse by NASA would operate at reduced reactant temperatures that lead togreater resistance to corrosion and to improved reliability (Morrison 54). London: John Wiley & Sons, Ltd., 2 .Hart, D. Fuel cell maintenance on NASA's space shuttle orbiter vehicles fuelcost approximately $15 million per year. The oxygen from the dual gas regulator module flows directly throughtwo ports into a closed-end manifold in the fuel cell stack, achievingoptimum oxygen distribution in the cells. There are three fuel cell systems on each space shuttle orbiter. The normal temperature of productwater ranges from 6 -to-66 C. The emissions of fuel cells -carbon dioxide and clean water - are useful to space vehicle personnel(Bakker and Cohn 43). The entire reaction occurs within the fuel cell (Hoogers186).Parameter NASA needed to solve mission critical problems related to extendedspace flights. Because of this direct proportion, leaks maybe detected by comparing reactant consumption and current produced. Fuel Cell Operation Fuel cells produce energy. At this point,the oxygen reacts with the water and returning electrons to producehydroxyl ions. A centrifugal water separator then extractsthe liquid water and pressure-feeds the water to potable tanks in the lowerdeck of the pressurized crew cabin of the space shuttle orbiter. The purge line heaters are turned on to heat thepurge lines to ensure that the reactants will not freeze in the lines. Fuel Cell Technology Handbook. The overall reaction is hydrogen (g) +oxygen (g) [right arrow] 4 water (I). The 2,4 -watt startup heater is used only duringstartup to warm the fuel cell to its operational level. "Briefing on solid-Oxide Fuel Cells." Futuretech (3 June 2 ): 1-9.Dicks, A., and Larminie, J. After the coolant leaves the stack, its temperature is sensed andthe data transmitted to the fuel cell stack temperature meter and to theCRT display. The product water from all three fuel cell power plants flows to asingle water relief control panel. The electrons are routed through the spaceshuttle orbiter's electrical power distribution and control (EPDC)subsystem to provide electrical power to the spacecraft. The first fuel cells used by NASA in space vehicles were protonexchange membrane (PEM) fuel cells. One of the mission critical problems was the provision ofelectrical power on spacecraft. In a typical fuel cell,hydrogen gas combines with hydroxyl ions at one electrode to produce waterelectrons. During normal fuel cell operation, the reactants are present in aclosed-loop system and are 1 percent consumed in the production ofelectricity. A number of power source alternativesproved to be unsatisfactory. Thehydrogen reactant is the more likely to freeze because it is saturated withwater vapor. All oxygen that flows into thestack is consumed, except during purge operations (Dicks and Larminie,618). Thetemperature-actuated flow control valve downstream from the pump adjuststhe coolant flow to maintain the fuel cell coolant exit temperature between88 C and 99 C. The location of thecryogenic tanks containing the hydrogen and oxygen processed in the fuelcell units is adjacent to the fuel cells (Dicks and Larminie 618). The reactants then enter a two-stage, integrated dualgas regulator module. fuel cells on the nasa space shuttle orbiter Introduction This research reviews the fuel cells used on the NASA space shuttleorbiter vehicles. If the potassium hydroxide electrolyte in the fuel cell migrates intothe product water, a pH sensor located downstream of the hydrogenpump/water separator senses the presence of the electrolyte, and alerts thecrew of the space shuttle orbiter (Dicks and Larminie, 62 ). Thermostatically controlled heaters will maintain the water linetemperature above 12 C, when required. Recirculation of the water to theelectrolyzer renews the entire process. The second stage reduces the oxygen pressure to a range of 62-to-65 psia and maintains the hydrogen pressure at 4.5-to-6 psiadifferential below the oxygen pressure. Othertypes of fuel cells that NASA is considering include the following(Connelley 2): 1. Figure 1 [pic] (Source: Dicks and Larminie 2 ) Each of the three fuel cell systems on a NASA space shuttle orbiterconsists of 96 fuel cells contained in three substacks. Fuel Cell Structure The concept of the fuel cell is not a new technology. Saturation decreases theefficiency of the reactions. The product water lines from all three fuel cell power plantsincorporate a parallel (redundant) path of product water to a secondpotable water tank in the event of a freeze-up of the single water reliefpanel. Figure 3 Fuel Cell Within A NASA Space Shuttle Orbiter [pic] (Source: Dicks and Larminie 2 ) Works CitedBakker, W., and Cohn, A. Alerts occur if temperatures fall below 78 C or above 118 C. Thechemical reaction is as follows: 2H2 + 2 > 2H2 . Pitot pressure then expels the hydrogen gas into the hydrogen pump's inlethousing though a bleed orifice (Dicks and Larminie, 625). The fuel cell current is checked to ensure a load of lessthan 35 amps, due to limitations on the hydrogen and oxygen pre-heaters inthe fuel cells. The fuel celldates to 1839 and British scientist Sir William Grove. Thehydrogen and oxygen flow into the fuel cell, which then generateselectricity, heat, and water. The PEM fuel cells used a moistpolymer membrane as the electrolyte. An electrolytesubstance separates the two electrodes, which facilitates the movement ofcharged ions. Each fuel cell unit weighs26 pounds. "Shuttle Diplomacy." Mechanical Engineering - CIME 121 (March 1999): 52-55.Stone, R., Allakhverdov, A., and Lawler, A. The risks associated with the hydrogen and oxygenrequired for the application of fuel cell technology were relativelyinsignificant because the spacecraft engines used far greater quantities ofboth hydrogen and oxygen. Further, the fuel cells do notperform effectively over the planned operational time between repairs andmaintenance. The consumptionof oxygen and hydrogen is proportional to the electrical power demand ofthe space shuttle orbiter (Dicks and Larminie, 614). Specifically, a fuelcell combines a fuel source containing hydrogen with oxygen to produceelectric power, heat, and water (Connelley 1) The three fuel cell systems on a NASA space shuttle orbiter operate asindependent electrical power sources. The electrons perform electrical work by flowing through anexternal circuit to the other electrode, where they recombine with oxygenand water to produce hydroxyl ions. In DMFC technology, an anode catalyst draws hydrogen liquidmethanol, thereby eliminating the need for a fuel reformer. The functionof the catalysts is to release electrons from hydrogen or other reactants.Contaminants, however, can compromise the effectiveness of the process.Fuel cell technology today works effective with only with a few fuelsources - reactants, among which the more prominent in use are hydrogen andmethane. A revaluation of PEM fuel cell technology could lead totheir use in future space shuttle orbiter flights (Morrison 54). A second problem was the provision ofwater to personnel on space flights. The centrifugal force of high-velocity waterflowing around the pitot tube's bends separates the hydrogen gas and water. The hydrogen pump circulates the hydrogen gas back to the fuel cellstack, where some of the hydrogen is consumed in the reaction. The reaction produces water, light, and heat. Temperature sensors located on the fuel cell water dischargeline, relief valve, relief line, and vent nozzle are displayed on a CRTmonitor (Dicks and Larminie, 62 ). The reaction is asfollows: 2H2 + 4 OH > 4H2 + 4e. The water must beremoved or the fuel cells become saturated. The hydroxyl ions then flow to the hydrogen electrode,where a hydrogen reaction begins. Temperature sensors and a pressure sensor installed on each of theredundant water line paths transmit data via telemetry for groundmonitoring (Dicks and Larminie, 627). The hot coolant from the stack flows through the oxygen and hydrogenpre-heaters, where it warms the cryogenic reactants before they enter thestack (Dicks and Larminie, 623). If thefuel cell power plant pH sensor failed, the space shuttle orbiter crewreceives an alert to sample the potable water (Dicks and Larminie, 627).Application The characteristics of fuel cells cause their use to be feasible for"unforgiving space applications" (Bakker and Cohn 42). The presentation of the review focuses on (1) fuel cellstructure, (2) fuel cell components, (3) fuel cell reactants, (4) fuel cellstorage and distribution, and (5) fuel cell operation. The electrolyte in an alkaline fuel cell typically is a KOH (potassiumhydroxide) solution. The Soviet space program also used fuel celltechnology. In a fuel cell, thereaction between hydrogen and oxygen produces electrical energy in placelight and heat. Photovoltaic devices required solarpanels that were both too large and too heavy. The regulated oxygen lines connectto an accumulator, which maintains an equalized pressure between the oxygenand the fuel cell coolant. Normally, the water is directed to the potable water tank.In the event of a line rupture in the vicinity of the single water reliefpanel, water could spray on all three water relief panel lines, causingthem to freeze and prevent fuel cell power plant water discharge (Dicks andLarminie, 625). To minimize excessive hydrogen gas entrained in eachfuel cell power plant's product water, modifications were made to the waterpickup (pitot) system. Manifolds extendthrough the length of the substacks. Each fuel cell system of 96 fuel cell units can provide 12 kwhof electrical power continuously or 16 kwh for short periods (Hoogers 17 ). "Advances in Solid-Oxide Fuel Cells." EPRI Journal 21 (September-October 1996): 42-45.Connelley, T. Two 16 -watt end-cell electrical heaters on each fuel cell power plantwere used to maintain a uniform temperature throughout the fuel cell powersection. The fuel cells used in NASA space shuttlevehicles produce energy through the creation of a chemical reactioninvolving hydrogen and oxygen. The coolant pump uses three-phase AC power to circulate the coolantthrough the loop. Purging, therefore, isrequired at least twice daily to cleanse the cells.
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