Flemming Hansen

1
Satellite Technology CourseElectrical Power
Subsystem
Flemming Hansen MScEE, PhD Technology
Manager Danish Small Satellite Programme Danish
Space Research Institute Phone [phone] E-mail
fh_at_dsri.dk
Downloads available from http//www.dsri.dk/roeme
r/pub/sat_tech
RØMER 3D Model by Jan Erik Rasmussen, DSRI
2
Electrical Power Subsystem
The job of the Electrical Power Subsystem (EPS)
is to provide uninterrupted power to on-board
electronics both in sunlight and in eclipse There
are five types of power sources in use today

• Solar cells also denoted Photovoltaics (Silicon,
  Gallium-Arsenide etc.)
• Secondary batteries (rechargeable)Used as energy
  storage medium to suply power during eclipse or
  adverse pointing of the solar arrays.
• Primary batteries (non-rechargeable)Used only on
  launchers and on small experimental missions with
  a lifetime of a few days.Not considered further
  in this lecture.
• Fuel cells ... producing electricity by
  electrochemically burning oxygen and hydrogen
  to water.Used presently only on the Space
  Shuttle. Not considered further in this lecture.
• Radioisotope Thermal Generators (RTG) ... using
  the heat produced by radioactive decay of
  Plutonium-238 to produce electricity via
  thermo-electrical cells. Used only on
  interplanetary missions to the outer planets.Not
  considered further in this lecture.

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Solar Cells
A solar cell is a PN-junction formed in silicon
(Si) or Gallium-Arsenide (GaAs) or other
material When photons penetrate into the
junction, electron-hole pairs are created. The
electrons travel to the N-side of the junction
round the external circuit and back to the P-Side
of the junction where they recombine with the
holes and close the current path.
N-type semiconductor
PN-junction
P-type semiconductor
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Space Environment for Solar Cells

• Solar Energy FluxFor solar array design, the
  value of the solar constant is S0 1353 W/m2,
  AM0 (Air Mass Zero), seasonal average

• Thermal ConsiderationsSolar arrays are usually
  thermally decoupled from the spacecraft body,
  either with MLI blankets or if deployable solar
  array wings are used. For this reason and because
  solar panels have little thermal capacity, the
  temperature of the solar cells exhibit large
  excursions, up to around ?100 ?C (sunlight to
  eclipse)

• RadiationThe solar cells are exposed to the
  ionizing radiation from the Earth radiation belts
  and cosmic rays with only a thin microsheet glass
  cover. Solar cell performance degrade with
  accumulated dose

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Current-Voltage Characteristics of Solar Cells
Silicon cell
Conversion efficiency ? 14.8
Conversion efficiency ? 19.0
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The Latest and Greatest in Solar Cells
Triple-Junction GaAs
Conversion efficiency ? 27.4
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Data for Spectrolab GaAs/Ge Single Junction Solar
Cells
See http//www.spectrolab.com
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Data for Spectrolab InGaP2/GaAs/Ge Triple
Junction Solar Cells
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Future InN/GaN Multi-Junction Solar Cells
Wladek Walukiewicz
Perfect matching of band gap of In1-xGaxN to
solar spectrum
LBNL, 2002
Literature
GaInP
GaAs
Ge
Solar Flux (1021 photons/sec/m2/mm)
Ga content in In1-x GaxN alloy
Maximum, theoretically predicted efficiencies
increase to 50, 56, and 72 for stacks of 2, 3,
and 36 junctions with appropriately optimized
energy gaps, respectively.
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Secondary Batteries - 1

• Types of secondary batteries in use in satellites
  today
• Nickel-Cadmium (NiCd) batteries
• Nickel Hydrogen (NiH2) batteries
• Lithium-Ion (Li-Ion) batteries
• Other interesting types of batteries include
• Nickel-Metal Hydride (NiMH) batteries Used as a
  replacement for NiCd batteries in many consumer
  and professional ground-based applications. Does
  not suffer the memory effect (see later). More
  environmentally friendly (No Cadmium). Not used
  to any significant extent in satellites.
• Lithium-Polymer batteriesThe latest member of
  the Lithium battery family. Planar battery by
  nature of its design. Usually packed in a heavy
  plastic foil bag like a bag of vacuum-packed
  coffee. Swells in vacuum due to impefect vacuum
  in the bag. Not yet used in satellites.

Danionics Li-Polymer battery
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Secondary Batteries - 2

• Nickel-Cadmium (NiCd) batteries
• Used in all kinds of spacecraft Pico-, nano-,
  micro- and large satellites
• Advantages Proven through hundreds of missions
  in all kinds of orbit
• Drawbacks Suffers from memory effect, the loss
  of capacity after prolonged trickle charging to
  maintain the fully chaged state
• Energy density 30 - 40 Wh/kg
• Depth of Discharge Max. ?20 for LEO, Max. ?50
  for GEO
• NiCd batteries are best operated at cool
  temperatures around 10 15 C and suffers
  degradation at elevated temperatures
• The Cadmium content makes this battery type very
  environmentally unfriendly

Eagle-Picher Super-NiCd cells
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Secondary Batteries - 3

• Nickel-Hydrogen (NiH2) batteries
• Used mainly in large telecom satellites
• Advantages Allows more cycles for a given Depth
  of Discharge
• Drawbacks NiH2 batteries are rather voluminous
  due to the pressure vessel required to hold the
  gaseous hydrogen released during charging
  (consumed during discharge)
• Energy density 40 - 60 Wh/kg
• Depth of Discharge Max. 60
• Not considered further in this lecture

Eagle-Picher NiH2 cell and battery
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Secondary Batteries - 4

• Lithium-Ion (Li-Ion) batteries
• The new standard in spacecraft batteries. Flight
  qualified. Flight proven in 2000 (Duracell DR202
  on board Swedish MUNIN satellite launched 21
  Nov. 2000). Many more to come.
• Advantages Very high energy density, high cell
  voltage (3 x NiCd)
• Drawbacks Sophisticated individual cell charge
  control required (Li-Ion cells will not tolerate
  overcharge)
• Energy density 120 - 140 Wh/kg (Generation 2
  devices)
• Depth of Discharge Max. 25 - 30 for LEO, Max.
  60 - 80 for GEO
• Li-Ion batteries are best operated at room
  temperatures around 20 25 C and suffers
  degradation at low temperatures
• AAU Cubesat will fly Danionics DLP 443573, 700
  mAh Li-Polymer battery. To be launched 30 June
  2003

Danionics Li-Polymer Battery
MUNIN Battery Pack
Duracell DR202
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Battery Characteristics - NiCd

• Typical NiCd Cell
• Capacity 4000 mAh (available from lt 1 Ah to
  about 40 Ah)
• Charge-discharge (Coulomb) efficiency 95
• Charge voltage 1.65 V
• Discharge voltage 1.25 V (average)
• End of discharge voltage 1.0 V
• Size ø30 x 60 mm

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Battery Characteristics - Li-Ion

• SONY Li-Ion US18650 Cells
• Capacity 1500 mAh
• Charge-discharge (Coulomb) efficiency ?100
• Charge voltage 4.2 V
• Discharge voltage 3.6 V (average)
• End of discharge voltage 3.0 V
• Size ø18 x 65 mm

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Battery Characteristics Capacity vs.
Charge/Discharge Cycles
Danionics Li-ion battery tested at room
temperature. Depth of discharge ???
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Battery Characteristics - Cycles versus Depth of
Discharge
Li-Ion target for ground applications
Danionics Li-Polymer
ESA Li-Ion
SONY Li-Ion
Li-Ion Target for GEO
SONY Li-Ion
Mature Space Grade Li-Ion Batteries ???
Li-Ion target for LEO
SAFT Li-Ion
5 years in LEO
One year in GEO
One year in LEO
10 years in GEO
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Future Battery Li-Ion Technology Performance
From Danionics presentation
G2
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Electrical Power Subsystem Architecture - 1
Generic EPS Architectures
Peak Power Point Tracker (PPT) Systems
Direct Energy Transfer (DET) Systems
PPT Peak Power Point Tracker SA Solar
Array SR Shunt Regulator
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Electrical Power Subsystem Architecture - 2
Sequential Switching Shunt Regulator SSSR or S3R
Main Bus Filter
Blocking Diodes
Batteries
Solar Array Strings
Main Bus Voltage Sense Line
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Electrical Power Subsystem Architecture - 3
Ørsted Electrical Power Subsystem
Sunlight battery being charged Only top panel
illuminated insufficient power, battery in
discharge Ørsted experienced a peak ?92 solar
eclipse at 103215 UT on 11 August 1999 while
passing East of Nürnberg, Germany Eclipse -
battery being discharged
Solar panel peak output power 66 W Orbit
average power con-sumption 31 W
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Electrical Power Subsystem Architecture - 4
Solar cell strings
EPS for STRV Satellite
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Electrical Power Subsystem Architecture - 5
EPS for Meteosat Second Generation
Satellite Launced 27 August 2002