Spacecraft Power SystemsDavid W. MillerJohn KeeseeElectrical Power SystemPowerSourceEnergyStoragePowerDistributionPower Regulationand ControlEPSPower SourcesPrimary Batteries RadioisotopeSecondary Battery Thermionic converterFuel cell Thermoelectric converterRegenerative fuel cell PhotovoltaicChemical dynamic Solar dynamicNuclear Flywheel StorageElectrodynamics Tethers Propulsion-charged tetherPower Source ApplicabilityFUEL CELLCHEMICALDYNAMIC(APUs)HOURSLOAD POWER (kW)NUCLEARNUCLEAR THERMIONICSSOLAR DYNAMIC ANDPHOTOVOLTAICNUCLEAR THERMIONICOR SOLAR DYNAMICPHOTOVOLTAIC ORISTOTOPE - THERMOELECTRICMONTHSYEARSPRIMARYBATTERIES1DAY0.10.1110100112 246810361210 100 10310410510 DAYSApproximate ranges of application of different power sources.Design Space for RTGs5-Year Design Life% of Original PowerYears50010011087The 87-year half-life of Pu-238 results in 96% of the original heat output even after five yearsElectric - Power Level (kW)Duration of Use10MIN1 HOUR 1 DAY 1 MONTH 1 YEAR 10 YEARSChemical10-1100101102103104105106107RadioisotopesNuclear reactorsSolarPrimary Battery TypesSilver zinc Lithium sulfur dioxideLithiumcarbonmonofluorideLithiumthionylchlorideEnergy density (W h/kg)130 220 210 275Energy density (W h/dm3)360 300 320 340Op Temp(deg C)0-40 -50 – 75 ? – 82 -40 – 70Storage Temp (deg C)0 –30 0 –50 0 –10 0 –30Storage Life 30-90 days wet, 5 yr dry10 yr 2 yr 5 yrOpen circuit voltage(V/cell)1.6 3.0 3.0 3.6Dischargevoltage(V/cell)1.5 2.7 2.5 3.2Manufacturers Eagle Pitcher, YardleyHoneywell,Power ConverEagle Pitcher Duracell,Altus, ITTSilver Zinc Cells• Wide use in industry• High energy density, high discharge rate capability, fast response• Short lifetime• Vent gas during discharge• Potentially rechargeable but few cyclesLithium cells• Higher energy density than silver zinc• Wide temperature range• Low discharge rate (high internal impedance)– Rapid discharge may cause rupture• Slow responseSecondary Battery TypesSilver zinc Nickel cadmium Nickel hydrogenEnergy density(W h/kg)90 35 75Energy density(W h/dm3)245 90 60OperTemp (deg C) 0 –20 0 –20 0 –40Storage Temp (C) 0 – 30 0 – 30 0 – 30Dry Storage life 5 yr 5 yr 5 yrWet Storage life 30 – 90 days 2 yr 2 yrMax cycle life 200 20,000 20,000Open circuit (V/cell)1.9 1.35 1.55Discharge (V/cell) 1.8 – 1.5 1.25 1.25Charge (V/cell) 2.0 1.45 1.50Manufacturers Eagle-Pitcher,YardneyTechnical ProdEagle-Pitcher,Gates Aerospace BatteriesEagle-Pitcher,Yardney, Gates, HughesNickel Cadmium Cells• Long space heritage• High cycle life, high specific energy• Relatively simple charge control systems• Battery reconditioning necessary to counteract reduction in output voltage after 3000 cyclesNickel Hydrogen Cells• Potentially longer life than NiCads– Hydrogen gas negative electrode eliminates some failure modes• Highly tolerant of high overcharge rates and reversal• Individual, common and single pressure vessel typesLithium Ion Cells• Recently developed system, may provide distinct advantages over NiCd and NiH2• Operating voltage is 3.6 to 3.9 v which reduces the number of cells• 65% volume advantage and 50% mass advantage over state of the art systemsDepth of Discharge(Image removed due to copyright considerations.)Fuel Cells−+LoadWastewaterElectrolyte =30% KOHAnodeCathode2e-2H+1/2O2H2H2O2e-HYDROGENOXYGENFuel Cell Characteristics• Output voltage per cell 0.8 volts in practice• Consumes hydrogen and oxygen, produces water as by-product (1 Pint/kW h)• High specific power (275 W/kg)• Shuttle fuel cells produce 16 kW peak• Reaction is reversible so regenerative fuel cells are possibleRadioisotope Thermoelectric Generators• Used in some interplanetary missions• Natural decay of radioactive material provides high temperature source• Temperature gradient between the p-n junction provides the electrical output• High temperatures– Lead telluride (300 – 500 deg C, silicon germanium >600 deg C• Excess heat must be removed from the spacecraft(Dis) advantages of RTGs• Advantages• Do not require sunlight to operate• Long lasting and relatively insensitive to the chilling cold of space and virtually invulnerable to high radiation fields.• RTGs provide longer mission lifetimes than solar power systems.– Supplied with RTGs, the Viking landers operated on Mars for four and six years, respectively.– By comparison, the 1997 Mars Pathfinder spacecraft, which used only solar and battery power, operated only three months.• They are lightweight and compact. In the kilowatt range, RTGs providemore power for less mass (when compared to solar arrays and batteries).• No moving parts or fluids, conventional RTGs highly reliable. • RTGs are safe and flight-proven.They are designed to withstand any launch and re-entry accidents. • RTGs are maintenance free..• Disadvantages• The nuclear decay process cannot be turned on and off. An RTG is active from the moment when the radioisotopes are inserted into the assembly, and the power output decreases exponentially with time.• An RTG must be cooled and shielded constantly.• The conversion efficiency is normally only 5 %.• Radioisotopes, and hence the RTGsthemselves, are expensiveSubsystem: Power (RTG)• Modeling, Assumptions and Resources:– RTG database– 3 RTG types used for modeling– General Purpose Heat Source (GPHS)–Batteries– Combinations of different types of RTGsPow er Source PBOL [We] PEOL[We] Mass [kg] Dim ensions [m] Life[yrs] Pu[kg] Cost [M$] TRL NotesCassini RTG 285 210 55.5 D = 0.41,L=1.12 10.75 8 35.00 9 18 GPHSNew MMRTG 140 123 32 D = 0.41,L = 0.6 10 4 25.00 7 9 GPHSSRG 1.0 114 94 27 D = 0.27,L = 0.89 3 0.9 20.00 4 2 GPHS<114 140 228 280254Watts285 342 368 399 420 560 684570456 7001 SRG1 MMRTG2 MMRTG1 Cassini2 SRG1 SRG + 1 MMRTG1 SRG + 1 Cassini2 SRG + 1 MMRTG3 SRG4 SRG3 MMRTG4 MMRTG2 Cassinior 5 SRG6 SRG5 MMRTGKKGSubsystem: Power (RTG)• Validation of model:– Confirmation of data by multiple sources.– Tested ranges of variables:• Power required (< 0 to > 1.37 kW)• Mission lifetime (< 0 to > 3.5548e4 sols)– No discrepancies found.Hundreds of millions of $KKGThermoelectric GeneratorThermal sink TcoldLoadElectricalinsulationThermal source ThotHeat FlowElectrical insulationConnecting strapsPN P N NP+++++-----++-Flywheel Energy Storage Modules (FESM) could replace batteries on Earth-orbit satellites.• While in sunlit orbit, the motor will spin the