Spacecraft Power Systems - MIT OpenCourseWare

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Spacecraft Power SystemsDavid W. MillerJohn Keesee

Electrical Power Power Regulationand Control

Power SourcesPrimary BatteriesRadioisotopeSecondary BatteryFuel cellThermionic converterThermoelectric converterRegenerative fuel cellChemical dynamicNuclearPhotovoltaicSolar dynamicFlywheel StorageElectrodynamics Tethers Propulsion-charged tether

Power Source Applicability100LOAD POWER (kW)FUEL CELL10CHEMICALDYNAMIC(APUs)NUCLEAR THERMIONICSSOLAR DYNAMIC ANDPHOTOVOLTAICNUCLEARNUCLEAR THERMIONICOR SOLAR DYNAMICPHOTOVOLTAIC ORISTOTOPE - THERMOELECTRIC1PRIMARYBATTERIES10 DAYS 11 DAY0.10.11YEARSMONTHS101002 3612103HOURSApproximate ranges of application of different power sources.10424 6 810105

Design Space for RTGs107105% of Original PowerElectric - Power Level (kW)106Nuclear reactors1041031021015-Year DesignLife500ChemicalSolar1 HOUR1Years1087The 87-year half-life of Pu-238 results in 96% of the original heatoutput even after five years10010-110MIN1001 DAY1 MONTHDuration of UseRadioisotopes1 YEAR10 YEARS

Primary Battery TypesSilver zincLithium sulfur rideEnergy density(W h/kg)130220210275Energy density(W h/dm3)360300320340Op Temp(deg C)0-40-50 – 75? – 82-40 – 70Storage Temp(deg C)0 – 300 – 500 – 100 – 30Storage Life30-90 dayswet, 5 yr dry10 yr2 yr5 yrOpen age(V/cell)2.72.53.2ManufacturersHoneywell,Power ConverEagle PitcherDuracell,Altus, ITTEagle Pitcher,Yardley

Silver Zinc Cells Wide use in industry High energy density, high discharge ratecapability, fast response Short lifetime Vent gas during discharge Potentially rechargeable but few cycles

Lithium cells Higher energy density than silver zinc Wide temperature range Low discharge rate (high internalimpedance)– Rapid discharge may cause rupture Slow response

Secondary Battery TypesSilver zincNickel cadmiumNickel hydrogenEnergy density(W h/kg)903575Energy density(W h/dm3)2459060Oper Temp (deg C)0 – 200 – 200 – 40Storage Temp (C)0 – 300 – 300 – 30Dry Storage life5 yr5 yr5 yrWet Storage life30 – 90 days2 yr2 yrMax cycle life20020,00020,000Open circuit(V/cell)1.91.351.55Discharge (V/cell)1.8 – 1.51.251.25Charge eyTechnical ProdEagle-Pitcher,Gates AerospaceBatteriesEagle-Pitcher,Yardney, Gates,Hughes

Nickel Cadmium Cells Long space heritageHigh cycle life, high specific energyRelatively simple charge control systemsBattery reconditioning necessary tocounteract reduction in output voltage after3000 cycles

Nickel Hydrogen Cells Potentially longer life thanNiCads– Hydrogen gas negativeelectrode eliminatessome failure modes Highly tolerant of highovercharge rates andreversal Individual, common andsingle pressure vessel types

Lithium Ion Cells Recently developed system, may providedistinct advantages over NiCd and NiH2 Operating voltage is 3.6 to 3.9 v whichreduces the number of cells 65% volume advantage and 50% massadvantage over state of the art systems

Depth of Discharge(Image removed due to copyright considerations.)

Fuel CellsLoadH2CathodeHYDROGEN Anode 2e2H Electrolyte 30% KOH2e1/2 O2OXYGENH2OWastewater

Fuel Cell Characteristics Output voltage per cell 0.8 volts in practice Consumes hydrogen and oxygen, produceswater 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 fuelcells are possible

Radioisotope ThermoelectricGenerators Used in some interplanetary missions Natural decay of radioactive material provideshigh temperature source Temperature gradient between the p-n junctionprovides 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 AdvantagesDo not require sunlight to operateLong lasting and relatively insensitiveto the chilling cold of space andvirtually invulnerable to highradiation fields.RTGs provide longer missionlifetimes than solar power systems.– Supplied with RTGs, the Vikinglanders operated on Mars for four andsix years, respectively.– By comparison, the 1997 MarsPathfinder spacecraft, which usedonly solar and battery power,operated only three months. They are lightweight and compact. Inthe kilowatt range, RTGs providemore power for less mass (whencompared to solar arrays andbatteries). No moving parts or fluids,conventional RTGs highly reliable.RTGs are safe and flight-proven.They are designed to withstand anylaunch and re-entry accidents.RTGs are maintenance free.DisadvantagesThe nuclear decay process cannot beturned on and off. An RTG is activefrom the moment when theradioisotopes are inserted into theassembly, and the power outputdecreases exponentially with time.An RTG must be cooled and shieldedconstantly.The conversion efficiency is normallyonly 5 %.Radioisotopes, and hence the RTGsthemselves, are expensive

Subsystem: Power (RTG) Modeling, Assumptions and Resources:– RTG database– 3 RTG types used for modeling– General PurposeHeat Source (GPHS)– Batteries– Combinations of different types of RTGsPow e r SourcePBOL [We ] PEOL[We ] M as s [k g]Dim e ns ions [m ]Life [yrs ]Pu[k g]Cos t [M ]TRLNote sD 0.41,L 0.610425.0079 GPHSSRG 1.01149427D 0.27,L 0.8930.920.0042 GPHS1402282542802851 Cassini 1143423683994204565605706847005 MMRTG326 SRG1232 Cassinior 5 SRG1404 MMRTGNew MMRTG4 SRG18 GPHS3 MMRTG91 SRG 1 Cassini35.002 SRG 1 MMRTG83 SRG10.752 MMRTGD 0.41,L 1.121 SRG 1 MMRTG55.52 SRG2101 MMRTG2851 SRGCassini RTGWattsKKG

Subsystem: Power (RTG) Validation of model:Hundreds of millions of – 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.KKG

Heat FlowThermoelectric GeneratorThermal source ThotElectrical insulationConnecting straps ElectricalinsulationP N-P N-Thermal sink TcoldLoadP N-

Flywheel Energy Storage Modules (FESM) couldreplace batteries on Earth-orbit satellites. While in sunlit orbit, the motor will spin theflywheel to a fully charged speed– generator mode will take over to discharge theflywheel and power the satellite during the eclipsephase– present flywheel technology is about four times betterthan present battery technology on a power stored vs.weight comparison. Weighing less than 130 lbs, the FESM is 18.4-in. indiameter by 15.9-in. in length– Delivers 2 kW-hr of useful energy for a typical 37minute LEO eclipse cycle– high speeds of up to 60,000 rpm the current average for commercial GSO storage is2,400 lbs of batteries, which is decreased to 720 lbswith an equivalent FESM.Honeywell has developed an integrated flywheelenergy storage and attitude control reaction wheel– Energy stored in non-angular momentum changemode

Solar Cell Long heritage, high reliability power source High specific power, low specific cost Elevated temperature reduce cellperformance Radiation reduces performance and lifetime Most orbits will require energy storagesystems to accommodate eclipses

Solar Cell PhysicsCovalentbondPhotons -- Electrons- - -Holes npLoad Flow ofelectronsPhotonsSi molecule

Solar Cell OperatingCharacteristicsIscMaximumpower pointI-V curveP constantImpOutput currentPmpIncreasingpowerArea maximumpower outputumit mOpadloencatsiserVmpVoc

Solar Cell OperatingCharacteristicsP-VcuVmprveOutput powerPmpOutput voltage

Temperature Effects160140CURRENT 100806040200 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0VOLTAGE (volts)Voltage - current characteristics vs cell temperaturefor 2 x 2 cm 10 ohm cm N/P solar cellSilicon thickness 0.012 inch, active area 3.9 cm2Spectrosun solar simulator AMOBalloon calibration

Radiation EffectsRELATIVE OUTPUT (%)10012 mil thick904 mil thick80706050401013101410151016FLUENCE, 1 Mev electrons/cm21017

Alternate Solar CellTechnologiesCell typePlanar tionBest laboratoryEquivalent time ingeosynchronousorbit for 15%degradation- 1 MeV electrons- 10 McV electronsSiliconThin sheetamorphous 5%21.8%18%19.9%22.0%25.7%10 yr4 yr10 yr4 yr33 yr6 yr155 yr89 yr33 yr6 yr

Solar Array Construction Construct arrays with cells in series to provide therequired voltage Parallel strings provide required current Must plan for minimum performance requirements– Radiation affects at end of life, eclipse seasonsand warm cells Shadowing can cause cell hot spots and potentiallycascading failure

Cell ShadowingAffected portion ofmodule with openor shadowed solarcell VA lAA Total cells sxpVUTotal cells (s - 1) x plU VBUSAffectedsolar cellUnaffected portionof module of s-1cells in series Bl1

Cell Shadowing1.04 Parallel Cells0.9OP2Q3CURRENT (A)0.8HighLeakageLow(3 cells)Q40.70.60.52 Parallel Cells0.4OP1Q10.3HighLeakageLow(one cell)Q20.20.10VBUS1020 304050V

Solar Array ConstructionMulti-layer bluereflecting filterMg Fl AR coatingCoverglass (0211 microsheet orCorning 7940 fused silica)SiO AR coatingGlass/Cell AdhesiveSolar CellSolderCell/Substrate AdhesiveFiberglass InsulatorSubstrate Aluminum FacesheetFacesheet/Core AdhesiveAluminum Honeycomb CoreFacesheet/Core AdhesiveSubstrate Aluminum FacesheetThermal Control Coating

Power Supply-Demand Profiling Solar array:SiliconGaAsMulti junction Batteries:Secondary BatteryNickel-CadmiumNickel hydrogenLithium-IonSodium-SulfurSpecific energydensity (W-hr/kg)25-353070140Ld(1 deg radation Rover 'slifetime)yearRN

Power Distribution Systems Power switching usually accomplished withmechanical or solid-state FET relays Load profiles drive PDS design DC-DC converters isolate systems on the powerbus Centralized power conversion used on smallspacecraft Fault detection, isolation and correction

DET Power Regulation Systems Direct Energy Transfer (DET) systemsdissipates unneeded power– Typically use shunt resistors to maintain busvoltage at a predetermined level– Shunt resistors are usually at the array orexternal banks of resistors to avoid internalheating Typical for systems less than 100 W

PPT Power Distribution Systems Peak Power Trackers (PPT) extract theexact power required from the solar array– Uses DC to DC converter in series with thearray– Dynamically changes the solar array’soperating point– Requires 4 - 7% of the solar array power tooperate

Other Topics Lenses are sometimes used to concentrate solarenergy on cells– Higher efficiency– Some recent evidence of premature degradation Tethers– Felectron e(vxB), decay orbital energy to produceelectricity– Use high Isp propulsion to spin up tethers over manyorbits– Discharge tether rapidly using it as a slingshot to boostpayloads into higher orbits or Earth escape

Shuttle fuel cells produce 16 kW peak Reaction is reversible so regenerative fuel cells are possible. Radioisotope 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 .