Spacecraft Power Systems

The Generation and Storage of Electrical Power

D. B. KanipeAero 401 February 9, 2016

Power Systemso Batteries Solar Cells + Batteries Fuel Cells

RTG Nuclear Reactors ?

o Functions of the Power System n Controls the generation, storage, and efficient use of power

n Provides protection against cascading failures

n Provides redundant paths or components in case of failure

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Power System Design Drivers (½)

o Customer/User requirementso Mission, ConOpso Spacecraft configuration

n Mass constraintsn Dimensional constraintsn Launch Vehicle constraintsn Thermal constraints

o Expected lifetime

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o Attitude control systemn Pointing requirementsn Viewing requirements

o Orbit or trajectoryn With respect to the sun

o Payload requirementsn Voltage, currentn Duty cycle, peak loadn Fault protection

o Mission constraintsn Maneuver ratesn G-loads create inertial loads

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Power System Design Drivers (2/2)

Power System Functional Block Diagram

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Power Source

SourceControl

Power Distribution, Main Bus Control & Main Bus Protection

Power Conditioning Load

Energy Storage

Energy Storage Control

MainBus

- Batteries- Solar- RTG- Fuel Cells- Nuclear- R Dynamic- Solar Dynamic

- Shunt Regulator- Series Regulator- Shorting Switch

Array

- DC-DC conversion- DC-AC conversion- Voltage regulator

- Battery charge control- Voltage regulator

Design Practice (1/2)

o Direct Current Switchingn Switches or relays: positive line to an element with a direct

connection to “ground’ on negative sideo Therefore, element is inert until commanded

o Arc Suppressionn Locate as close to the source of the arc as possiblen Current-carrying elements should not be exposed to the

ambient plasmao Conductive cables, connectors, solar array edges

o Modularityn Simplifies testingn Easier element replacementn Reduces “collateral” damage

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Design Practice (2/2)

o Groundingn Cause of some debate among EEsn Common ground preferable to individual component grounding

o Easier to maintain a common potentialo Less likely to disturb sensitive componentso Can be difficult to do in large spacecraft

n Sometimes it is necessary to completely isolate an element from other spacecraft noise

o Continuityn Avoid buildup of static potential; i.e., any voltage differencen Any shielding must have continuity and a common ground

o Complexityn KISS

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Battery Design Considerations

o Physicaln Size, mass, environmental requirements

o Electricaln Voltage n Current loading n Duty cyclesn Limits on depth-of-dischargen Fault recovery

o Programmatic n Cost, reliability, maintainability, safety

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Batteries: Definition of Terms (1/2)

o Charge Capacity, Cchgn Total electric charge stored in a battery; measured in amp-

hours (e.g., 40A for 1 hour = 40Ah)

o Average Discharge Voltage, Vavgn (Number of cells in series) * (Cell discharge voltage)

o Energy Capacity, Ebatn Total energy stored in a battery;

[Cchg* Vavg] (Joules or watt-hours)

o Depth of Discharge, DODn Percent of battery capacity used in discharge cyclen 75% DOD means 25% remainingn Try to limit DOD to promote longer cycle life

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Batteries: Definition of Terms (2/2)

o Charge Rate, Rchgn Rate at which the battery can accept charge

(amps/unit time)

o Energy Density, ebatn Energy per unit mass stored in batteryn Joules/kg or Watt-hours/kg

o Two categories of batteriesn Primaryn Secondary

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Primary Batteries

o Long storage capability (missile in a silo)o Dry (without electrolyte) until needed

n Activate by introducing electrolyte into dry batteryn Electrolyte may be solid at room temperature

Activate heater to melt electrolyte. (Thermal battery)

o Typically have a fairly large energy densityo Used for early major mission events

n Short durationn May be isolated from major power busn Usually non-rechargeablen Mass penalty

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Secondary Batteries (1/2)

o Lower energy density, but rechargeableo Requires DOD management

n LEO – eclipse is about 40% of the orbitn 12 – 16 discharge cycles per dayn Leads to battery degradation and lifetime reduction

o Maximum allowable DOD:Energy required during eclipse

Stored battery energy

PLtd PLtdCchgVavg Ebat

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DOD =

=

PL = load powertd = discharge timeCchg = charge capacityVavg = average discharge voltageEbat = total battery energy capacity

=

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Battery Type

Silver–zinc (Ag-Zn) ………………………………..

Silver-cadmium (Ag-Cd) ………………………..

Nickel-cadmium (Ni-Cd) …………………………

Nickel-hydrogen (Ni-H2) …………………………

Nickel-metal hydride (Ni-MH) ………………..

Lithium Thionyl Chloride (Li-SOCl2) ……….

Lithium Vanadium Pentoxide (Li-V2O5) …. Lithium Sulfur Dioxide (Li-SO2) ……………..

Energy Density

120 – 130 (W-hr)/kg

60 – 70 (W-hr)/kg

20 – 30 (W-hr)/kg

60 – 70 (W-hr)/kg

120 – 130 (W-hr)/kg

650 (W-hr)/kg

250 (W-hr)/kg

50 – 80 (W-hr)/kg

Secondary Batteries (2/2)

DOD Management

o Typically, a LEO spacecraft spends 40% of its time discharging and 60% charging

o DOD during eclipse is limited by the rate at which its batteries can be restored in sunlight by solar arrays

o All expended energy must be restored or net draino Driver is the charge rate, Rchg

n DOD limited to 7-8% per orbitn Battery temperature can affect charge rate

o Battery generally must be charged at a voltage > Vavg(~20% higher) to restore full charge

o This is a driver in the solar array design

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Solar Arrays

o Photoelectric Effectn Electrons are emitted from matter as a result of absorption of

short wavelength electromagnetic radiation such as visible light.

o Originally limited to spacecraft skin acreageo Deployable panels more flexible, but more complexo Solar Cell Characteristics

n 1st order: V decreases as T increases (and vice versa)n 2nd order: I increases as T increases, BUT

o Only about 10% relative to the voltage dropn Therefore, overall power output is reduced as temperature

increases. P = I*Vo May need radiators to remove excess heat

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Solar Cell Capability

o Delivered electrical power:Pe = ФeA(1- I) Ф = solar flux (W/m2)

e = cell efficiency (≈15% for silicon)A = area

I = parasitic losses (≈10%)

o Nominal solar flux density at earth: 1353 W/m2 at 1 AUФ = W(a/d)2cos(ө) W = Nominal solar flux

a = Mean earth-sun distanced = actual earth-sun distanceө = panel inclination

o Cell efficiency (t)eEOL = eBOLe-0.043T T = time in orbit years

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Maximum Power Point (MPP)

o Desirable to operate at the MPP if possiblen Minimize mass and maximize efficiency

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I

V

MPP

Maximum area rectangle under the IV curve

Sun Tracking

o Ideal situation: sun normal to the arrayo Cosine rule applies – up to a point

Optimum

Up to 45-60˚cosine functionworks, then falls off rapidly

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Beta and Alpha

o Beta, ßn Angle between a line from the sun to the

center of the earth, and spacecraft orbit

o Alpha, αn Apparent rotation of sun angle from

spacecraft pov during its orbit. α = 0-36019

Solar-to-Electric

o Efficiency of solar cellsn Gallium arsenide solar cells (Ga-As)

o More efficient (20%) and radiation toleranto More expensive

n Crystalline Silicon Cellso 11-16%, 18-20%, >20%?

n Multi-junction (multi-layer cells)o Top layer converts light in the visible rangeo Bottom layer(s) optimized for infraredo Up to 30% efficiencyo Not surprisingly, very expensive

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Radioisotope Thermoelectric Generator (RTG)

o Converts heat energy generated by radioisotope decay into DC energy via thermoelectric effectn Plutonium 238, 238Pun Strontium 90, 90Sr

o Complicated ground handlingo RTG radiation

n Alpha raysn Detrimental to spacecraft electronicsn Clothing (or paper) will stop alpha raysn Don’t inhale 238Pu dustn 238Pu pellets are a ceramic form – no dust if exposed

o Expensive but effective and reliable21

Fuel Cells

o Direct conversion of chemical energy into electricity

o More efficient than batterieso Oxidizer and fuel fed into a cello Electricity generated from oxidation

reaction in the cello Space applications use oxygen/hydrogeno By product: watero ~ 35% efficiency

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Power Conditioning & Control

o Voltage from power source, especially solar arrays, may fluctuate

o Power conditioning functionsn Control solar array outputn Control battery charge/discharge cyclen Regulate voltage supplied to spacecraft

systems

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Additional Power Sourceso Nuclear Reactorso Dynamic Isotope Systemso Alkali Metal Thermal-to-Electric

Conversion (AMTEC)o Solar Dynamic

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Backup

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Dissipative Systems

o Simplero Not in series with array output

Dissipates current in excess of instantaneous load requirement

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Solar Array

Shunt Battery Charge

Controller

Spacecraft Loads

Battery

Non-dissipative Systems (PPT)

o In series regulation of solar power

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o Usually reserved for large spacecraft

Solar Array

Battery Charge

Controller

Spacecraft Loads

Battery

Battery Discharge Controller

PPT