Lecture 7:Lecture 7:Power Systems and Thermal Power Systems and Thermal

ManagementManagement

Power System Structure and RequirementsPower System Structure and Requirements

Electrical Power Subsystem

Energy Storage

Power Source

Power Distribution

Regulation & Control

Typical Requirements

Supply continuous electrical power to s/c loads during mission

Control and distribute electrical power

Handle average and peak electrical load

Provide ac, dc power converters

Protect against failures in the EPS

Suppress transient voltages and protect against faults

Power System Design ProcessPower System Design Process

StepStep Info. RequiredInfo. Required Derived Derived RequirementsRequirements

1. Identify requirements1. Identify requirements Top-level requirements, s/c Top-level requirements, s/c configuration, mission life, configuration, mission life, payload definitionpayload definition

Design requirements, Design requirements, average and peak poweraverage and peak power

2. Select power source2. Select power source S/c configuration, average S/c configuration, average load requirementsload requirements

EOL power required, EOL power required, type of solar cell, mass type of solar cell, mass and area of solar array, and area of solar array, solar array configurationsolar array configuration

3. Select energy storage3. Select energy storage Orbital parameters, Orbital parameters, average and peak load average and peak load

Battery capacity Battery capacity required, battery mass, required, battery mass, volume and typevolume and type

4 Identify power 4 Identify power regulation and controlregulation and control

Power source selection, Power source selection, mission life, regulation and mission life, regulation and thermal control thermal control requirementsrequirements

Peak power tracker or Peak power tracker or direct energy-transfer direct energy-transfer system, thermal control system, thermal control requirements, bus-requirements, bus-voltage quality, power voltage quality, power control algorithmscontrol algorithms

Power SourcesPower Sources

Power sources

StaticPhotovoltaic Dynamic

Planar Concentrators Thermionics Thermoelectrics Brayton Stirling Rankine

Photovoltaic solar cells convert incident solar radiation directly to electrical energy

Static power sources uses a heat source, typically plutonium- 238 or uranium-235 for direct thermal-to-electrical conversion

Dynamic sources also use a heat source – concentrated solar, plutonium-238, or enriched uranium – to produce power via Brayton, Stirling or Rankine cycles

Comparison of Power SourcesComparison of Power Sources

Design Design ParametersParameters

Solar Solar PhotovoltaicPhotovoltaic

Solar Thermal Solar Thermal DynamicDynamic

Radio-Radio-

isotopeisotopeNuclear Nuclear ReactorReactor

Power range (kW)Power range (kW) 0.2 - 250.2 - 25 1 - 3001 - 300 0.2 - 100.2 - 10 25 - 10025 - 100

Specific power (W/kg)Specific power (W/kg) 26 - 10026 - 100 9 - 159 - 15 8 - 108 - 10 15 - 2215 - 22

Specific cost ($/W)Specific cost ($/W) 2500 - 30002500 - 3000 800 - 1200800 - 1200 16K – 18K16K – 18K 400 - 700400 - 700

Hardness to natural Hardness to natural radiationradiation

MediumMedium HighHigh Very highVery high Very highVery high

Stability and Stability and maneuverabilitymaneuverability

LowLow MediumMedium HighHigh HighHigh

Degradation over lifeDegradation over life MediumMedium MediumMedium LowLow LowLow

Storage required for Storage required for eclipse?eclipse?

YesYes YesYes NoNo NoNo

Sun angle sensitivitySun angle sensitivity MediumMedium HighHigh NoneNone NoneNone

Sensitivity to Sensitivity to shadowingshadowing

Low (with Low (with bypass diodes)bypass diodes)

HighHigh NoneNone NoneNone

Fuel availabilityFuel availability UnlimitedUnlimited UnlimitedUnlimited Very lowVery low Very lowVery low

Solar Array Design ProcessSolar Array Design Process

1.1. Determine requirements and constraintsDetermine requirements and constraints Av. Power needed during daylight and eclipseAv. Power needed during daylight and eclipse Eclipse durationsEclipse durations Design lifetimeDesign lifetime

2.2. Calculate power that must be produced, Calculate power that must be produced, PPsasa

& Power requirements during eclipse and daylight, resp.

& Times spent in eclipse and daylight, resp.

Efficiency of paths from th

e e d d

e dsa

d

e d

e d

e

PT P TX X

PT

P P

T T

X

e solar arrays through the batteries to the loads

0.65,direct energy transfer

0.60, peak-power tracking

Efficiencies of paths directly from the arrays through to the loads

0.85,direct ene

dX

rgy transfer

0.80, peak-power tracking

Solar Array Design ProcessSolar Array Design Process

3. Select type of solar cell and estimate power output, 3. Select type of solar cell and estimate power output, PP0 0 , , with the sun normal to the surface of the cellswith the sun normal to the surface of the cells

4. Determine BOL power production per unit area, taking 4. Determine BOL power production per unit area, taking account of inherent degradation:account of inherent degradation:

And the cosine loss and life degradation: And the cosine loss and life degradation:

2 20

2 20

2 20

Si: 0.148 1,367 202

GaAs: 0.185 1,367 253

Multijunction: 0.22 1,367 301

P W m W m

P W m W m

P W m W m

Elements of inherent degradationElements of inherent degradation NominalNominal RangeRange

Design and assemblyDesign and assembly 0.850.85 0.77-0.900.77-0.90

Temperature of arrayTemperature of array 0.850.85 0.80-0.980.80-0.98

Shadowing of cellsShadowing of cells 1.001.00 0.80-1.000.80-1.00

Inherent degradation, Inherent degradation, IIdd 0.770.77 0.49-0.880.49-0.88

0

s/ c life

cos ,

, 1 degradation/ yr

BOL d

EOL BOL d d

P P I

P P L L

Energy StorageEnergy Storage Primary batteries have higher specific energy densities Primary batteries have higher specific energy densities

but cannot be recharged. Thus, they typically apply to but cannot be recharged. Thus, they typically apply to short missions.short missions.

Characteristics of some secondary batteries:Characteristics of some secondary batteries:

Secondary Battery CoupleSecondary Battery Couple Specific Energy Specific Energy Density Density

(W-Hr/Kg)(W-Hr/Kg)

StatusStatus

Nickel-CadmiumNickel-Cadmium 25-3025-30 Space-qualified, Space-qualified, extensive databaseextensive database

Nickel-HydrogenNickel-Hydrogen

(individual pressure vessel)(individual pressure vessel)35-4335-43 Space-qualified. Good Space-qualified. Good

databasedatabase

Nickel-HydrogenNickel-Hydrogen

(common pressure vessel)(common pressure vessel)40-5640-56 Space qualified for GEO Space qualified for GEO

and planetaryand planetary

Nickel-HydrogenNickel-Hydrogen

(single pressure vessel)(single pressure vessel)43-5743-57 Space-qualifiedSpace-qualified

Lithium-IonLithium-Ion 70-11070-110 Space-qualifiedSpace-qualified

Sodium-SulfurSodium-Sulfur 140-210140-210 Under developmentUnder development

Energy StorageEnergy Storage

2

Needed battery capacity:

40 60% for NiH Depth of discharge

10 20% for NiCd

Battery-to-load transmission efficiency 90%

Number of batteries

e er

PTC W hr

DOD Nn

DOD

n

N

Black Body Radiation ModelBlack Body Radiation Modelor

photons are modelled as a gas of bosons

The gas interacts with atoms that randomly emit or absorb photons

The interacting atoms form the walls of a c

Thermal radiation blackbody radiation model :

avity containing the gas

The most likely distribution of photons among energy levels is the one that is

"most random" - i.e. maximizes the statistical mechanical entropy.

A sea of photons is surrounded on all sides by high temperature atoms. These particles randomly absorb or emit photons, permitting all possible energy transitions compatible with conservation of overall energy

Black Body Radiation ModelBlack Body Radiation Model

2

5

2 1

exp 1

energy per unit wavelength, per unit

hcE

ch kT

E

Planck's Law:

spectral irradiance

2 1

34 2

surface area ( )

wavelength

Planck's constant 6.626 10

Absolute te

W m m

h W s

T

23

mperature

speed of light

Boltzmann's constant 1.3807 10 /

c

k W s K

Black Body Radiation ModelBlack Body Radiation Model

UV & Vis Infrared Microwave

Wien’s law

COBE (Cosmic Background Explorer) satellite data COBE (Cosmic Background Explorer) satellite data precisely verifies Planck’s radiation lawprecisely verifies Planck’s radiation law

Black Body Radiation ModelBlack Body Radiation Model

2

4

8 2 4

for the / :

Stefan-Boltzmann constant 5.6705 10

Wavelength for which the spec

b

b

W W m

W T

W m K

Stefan - Boltzmann Law total radiant emittance,

max

trum has the maximum value =

2,898m T K

Wien's

Displacement Law:

Thermal Equilibrium of an Isolated Body in SpaceThermal Equilibrium of an Isolated Body in Space

absorbed source absorb peak

source

absorb

peak

Energy flux from source

(Sun, Earth or Moon)

Projected area of object

that absorbs the radiation

absorbtivity of t

q G A

G

A

he

material at the wavelength

of peak source emission

4emitted

Temperature of body

Stefan-Boltzmann constant

Emissivity of the body

in the IR range of wave-

lengths

Area of radiating surface

IR r

IR

r

q T A

T

A

Electronics

dissipated Wq Q

absorbed dissipated emitted

4source absorb peak dissipated

14

source absorb peak W

IR r

IR r

q q q

G A q T A

G A QT

A

Spherical Spacecraft EquationsSpherical Spacecraft Equations

2 2

2

Solar flux 1418 to 1326

4

solar absorbtivity of the sphere

S S C S

S

C

S

Q G A

G W m W m

A D

WQ

SQS EQ

WQ

EQMQ

S MQ

2

2

solar absorbtivity of the sphere

albedo of the Moon

0.664 0.521 0.203

1 cos 2

Angular radius of the Moon

accounts fo

S M M S M S M

S

M

M M M

M M

M

M

Q AF G a K

A D

a

K

F viewfactor

K

r reflection of sun-

light from a spherical Moon

Analogous expression for S EQ

2

Moon IR emission

IR emissivity of the sphere

1 cos 2

Angular radius of the Moon

Analogous expression for

M M M IR

M

IR

M M

M

E

Q AF q

A D

q

F

Q

Spherical Spacecraft EquationsSpherical Spacecraft Equations

WQ

SQ

S EQ

WQ

EQMQ

S MQ

4

Power flow balance:

IR S S M S E M E W

S C S M S M S M E S E

A T Q Q Q Q Q Q

T G A AF G a K AF G a

14

S S M M IR E E IR W IRK AF q AF q Q A

Putting the Equations to Work: Putting the Equations to Work: The Preliminary Design ProcessThe Preliminary Design Process

StepStep NotesNotes1. Determine requirements and constraints1. Determine requirements and constraints Identify temperature limits – see Table 11Identify temperature limits – see Table 11

43, L&W43, L&W Estimate electrical power dissipationEstimate electrical power dissipation

2. Find the diameter of a sphere with the 2. Find the diameter of a sphere with the

same surface area as the spacecraftsame surface area as the spacecraft

Make first-order estimates assuming an Make first-order estimates assuming an isothermal, spherical spacecraft (using the isothermal, spherical spacecraft (using the above equations).above equations).

3. Select radiation surface property values3. Select radiation surface property values Initially assume white paint with Initially assume white paint with SS=0.6 and =0.6 and IRIR=0.8=0.8

4. Compute worst-case hot and cold temp.s 4. Compute worst-case hot and cold temp.s

for the spacecraftfor the spacecraft

Upper limit: Use high-side values of all power Upper limit: Use high-side values of all power input termsinput terms

Lower limit: Include only the IR emissions.Lower limit: Include only the IR emissions.

5. Compare worst-case hot and cold temp.s 5. Compare worst-case hot and cold temp.s

with temp. limits found in step 1.with temp. limits found in step 1.

If worst-case hot temperature is > required If worst-case hot temperature is > required upper limit, use a deployed radiator with a upper limit, use a deployed radiator with a pumped-looped system. Otherwise, use body-pumped-looped system. Otherwise, use body-mounted radiatorsmounted radiators

6. Estimate required area for body-mounted 6. Estimate required area for body-mounted

radiator.radiator.

Use upper temp. limit for radiator temp., assume Use upper temp. limit for radiator temp., assume no heat inputs and max. heat dissipation – see no heat inputs and max. heat dissipation – see equation 11.23, L&Wequation 11.23, L&W

7. Estimate radiator temp. for worst-case cold 7. Estimate radiator temp. for worst-case cold

conditionsconditions

Use the area from step 6 and min. heat Use the area from step 6 and min. heat dissipationdissipation

The Preliminary Design Process - ContinuedThe Preliminary Design Process - Continued

StepStep NotesNotes

8. If temp. in step 7 is less than the lower 8. If temp. in step 7 is less than the lower

limit, determine heater power required to limit, determine heater power required to

maintain radiator at lower temp. limitmaintain radiator at lower temp. limit

Assume radiator temp. is at the lower limitAssume radiator temp. is at the lower limit

9. Determine if there are special thermal 9. Determine if there are special thermal

control problemscontrol problemsIdentify components with narrow temp. Identify components with narrow temp. ranges, high power dissipation or low temp. ranges, high power dissipation or low temp. requirements. See thermal control options in requirements. See thermal control options in section 11.5.2, L&W.section 11.5.2, L&W.

10. Estimate subsystem weight, cost and 10. Estimate subsystem weight, cost and

power.power.I f no special problems, use 4.5% of I f no special problems, use 4.5% of spacecraft dry weight, 4% of the total spacecraft dry weight, 4% of the total spacecraft cost, and heater power from step spacecraft cost, and heater power from step 8.8.

Thermal Control Devices and Strategies Thermal Control Devices and Strategies - If special thermal control problems are encountered in step 9- If special thermal control problems are encountered in step 9

Materials and CoatingsMaterials and Coatings Optical Solar reflectorsOptical Solar reflectors Silver-Coated TeflonSilver-Coated Teflon MultiLayer InsulationMultiLayer Insulation Electrical HeatersElectrical Heaters ThermostatsThermostats Space radiatorsSpace radiators Cold-PlatesCold-Plates DoublersDoublers Phase Change DevicesPhase Change Devices Heat PipesHeat Pipes LouversLouvers Temp. SensorsTemp. Sensors Adhesive TapesAdhesive Tapes FillersFillers Thermal isolatorsThermal isolators Thermoelectric CoolersThermoelectric Coolers Cryogenic SystemsCryogenic Systems Active Refrigeration SystemsActive Refrigeration Systems Expendable Cooling SystemsExpendable Cooling Systems

Thermal Control Devices and Strategies Thermal Control Devices and Strategies Materials and Coatings: paints, silverized plastics, special coatings – Materials and Coatings: paints, silverized plastics, special coatings –

all with special absorptivity & emissivity values– See Table 11-44all with special absorptivity & emissivity values– See Table 11-44 Optical Solar Reflectors (OSRs): Optical Solar Reflectors (OSRs):

– Highly reflective surface mounted on a substrate and overlaid Highly reflective surface mounted on a substrate and overlaid with a transparent coating. with a transparent coating.

– Reflects most incoming radiation back to space, IR emissivity = Reflects most incoming radiation back to space, IR emissivity = 0.8, solar absorptivity = 0.150.8, solar absorptivity = 0.15

– Expensive and fragile.Expensive and fragile. Silver-Coated Teflon - Cheaper alternative to OSRs. Silver-Coated Teflon - Cheaper alternative to OSRs. MultiLayer Insulation (MLI):MultiLayer Insulation (MLI):

– The primary spacecraft insulation device. The primary spacecraft insulation device. – Alternate layers of aluminized Mylar or Kapton, separated by net Alternate layers of aluminized Mylar or Kapton, separated by net

material, e.g. nylon, Dacron or Nomexmaterial, e.g. nylon, Dacron or Nomex– See Fig. 11-22 for the effective emmitance of MLISee Fig. 11-22 for the effective emmitance of MLI

Electrical HeatersElectrical Heaters– Used in cold-biased systems to bring selected components up to Used in cold-biased systems to bring selected components up to

proper temp.proper temp.– Thin electrical resister between two Kapton sheetsThin electrical resister between two Kapton sheets– Typical power densities Typical power densities 1 W/cm 1 W/cm22

ThermostatsThermostats– Switches to turn heaters on/offSwitches to turn heaters on/off– Typical operating range: -50 to 160Typical operating range: -50 to 16000CC

Thermal Control Devices and Strategies Thermal Control Devices and Strategies

Space radiatorsSpace radiators– Heat exchanger on the outer surface of the spacecraft that radiates waste heatHeat exchanger on the outer surface of the spacecraft that radiates waste heat– Can be structural panels or flate plates mounted on the spacecraftCan be structural panels or flate plates mounted on the spacecraft

Cold-PlatesCold-Plates– Heat dissipated by electrical equipment is conducted across the interface to the cold plate. Fluid Heat dissipated by electrical equipment is conducted across the interface to the cold plate. Fluid

circulating through the cold plate Carries the heat to a space radiator.circulating through the cold plate Carries the heat to a space radiator.

Heat PipesHeat Pipes– Lightweight devices used to transfer heat from one location to another, e.g. from an electrical Lightweight devices used to transfer heat from one location to another, e.g. from an electrical

component to a space radiatorcomponent to a space radiator

Temp. SensorsTemp. Sensors– Thermisters: Semiconductor materials that vary their resistance with temperature. They operate Thermisters: Semiconductor materials that vary their resistance with temperature. They operate

around -50 to +300 around -50 to +300 00C.C.– Resistance Thermisters: Uses a pure platinum conductor. Very accurate and expensiveResistance Thermisters: Uses a pure platinum conductor. Very accurate and expensive

4cos 0

radiator area

waste heat

s W R

R

W

G Q A T

A

Q

Heat in - evaporation

Wicking material

Gas

Heat out - condensationLiquid flow via wick