Introduction to Space Systems and Spacecraft Design

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Power Systems Design -II
Introduction to Space Systems and Spacecraft
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Power Systems Design II
Power Systems or EPS
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Look at the parts of the EPS
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Take Solar Panel
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What do we need from the solar panel?

• What are the attributes of a solar panel?
• Total output power of solar panel.
• Voltage of solar panel.
• Maximum packing factor.
• Efficiency of the solar cells.
• Operating temperature of the panels.

Lets go back and look at the solar cell.
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Lets go back and look at the solar cell.

• This dual junction cell
• Has an efficiency of 22
• Open circuit voltage 2.2v
• Size 76 x 37 mm

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Solar cell has an I-V curve like this

• This dual junction cell
• Has an efficiency of 22
• Open circuit voltage 2.2v
• Size 76 x 37 mm

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• This dual junction cell
• Has an efficiency of 22
• Open circuit voltage 2.2v
• Size 76 x 37 mm

• What are the attributes of a solar panel?
• Total output power of solar panel.
• Voltage of solar panel.
• Maximum packing factor.
• Efficiency of the solar cells.
• Operating temperature of the panels.

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Need to select a battery to design for solar
panel voltage

• What are the attributes of a solar panel?
• Total output power of solar panel.
• Voltage of solar panel.
• Maximum packing factor.
• Efficiency of the solar cells.
• Operating temperature of the panels.

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Rechargeable
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Use a lithium ion battery
Li Ion batteries 3.6 v nominal

• Design Criteria for charging Li Ion battery
• Need 10-15 more voltage to charge than the
  nominal voltage.
• Here we would need solar panel voltage of 4.0
  4.2v to charge this battery.

• Design Criteria solar panel
• Number of cells Max voltage/cell voltage.
• Take minimum number of whole cells.
• cells (4.2v/string)/(2.2v/cell)
• 1.9 or 2 cell for a string voltage
  of 4.4v

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Use two lithium ion batteries
Li Ion batteries 7.2 v nominal

• Design Criteria for charging Li Ion battery
• Need 10-15 more voltage to charge than the
  nominal voltage.
• Here we would need solar panel voltage of 8.0
  8.3v to charge this battery.

• Design Criteria solar panel
• Number of cells Max voltage/cell voltage.
• Take minimum number of whole cells.
• cells (8.3v/string)/(2.2v/cell)
• 3.77 or 4 cell for a string voltage
  of 8.8v
• Lets be conservative and use 5 cells for 11v.

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Now we have
Two Li Ion batteries 7.2 v nominal
5 cells for 11v to charge with.
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What is packing factor?

• What are the attributes of a solar panel?
• Total output power of solar panel.
• Voltage of solar panel.
• Maximum packing factor.
• Efficiency of the solar cells.
• Operating temperature of the panels.

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Packing Factor
Packing Factor Total Cell Area/ Total Panel Area
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Packing Factor
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Cell type 1
Cell type 2
Fixed solar panel size
Cell type 3
What do you do if given a fixed size panel on
which to put solar cells and you have these
different size solar cells?
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Packing Factor
Power Systems Design II
What do you do if given a fixed size panel on
which to put solar cells and you have these
different size solar cells?
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Now we have
5 cells for 11v where the string has all of the
cells hooked in series
How do you mount these 5 cells on this panel?
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How do you mount these 5 cells on this panel?
OK!
NO!
Visually we can see a very poor packing factor.
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What if the cells were bigger?
Oh Oh!
Now you have only 4.4v in the string.
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Got a cube? Put other cells on another face?
Cant do. All cells for a single string must be
on same face.
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Where are we now in the solar panel design?

• What are the attributes of a solar panel?
• Total output power of solar panel.
• Voltage of solar panel.
• Maximum packing factor.
• Efficiency of the solar cells.
• Operating temperature of the panels.

Got
Not got, but understand
Got
Assume we could mount the 5 cells on a panel,
what is total power for the cells selected?
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How much power from these cells?
5 cells for 11v
One cell area 76 x 37 mm 2812 mm2 Total cell
area 82812 22496 mm2 2.25 x10-2 m2 We
have 1350 watts/m2 from the sun in
space Direct power (1350 w/m2) x (2.25 x10-2
m2) 34.4
watts Converted power direct power x cell
efficiency 34.4 w x 0.22 eff 7.5
watts
11v

• For this dual junction cell
• Has an efficiency of 22
• Open circuit voltage 2.2v
• Size 76 x 37 mm

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Where are we now in the solar panel design?

• What are the attributes of a solar panel?
• Total output power of solar panel.
• Voltage of solar panel.
• Maximum packing factor.
• Efficiency of the solar cells.
• Operating temperature of the panels.

Got
Got
Not got, but understand
Got

• Now we can assume to start
• panel is at 90 degrees with sun max power
• operating temperature 20 degrees.. Centigrade
  22 eff

Dont forget, temperature counts a lot.
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Start here Tuesday for Idaho
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Now that we have beat our way through the solar
panel design ----- lets go look at the some more
parts of the EPS.
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Power Systems or EPS
What is this?
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Power Systems or EPS
Back bias diode
When panel 1 is shaded, the back bias diode keeps
the current from flowing backwards through panel
1, when panel 2 is generating a voltage across it.
Panel 1
Panel 2
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Power Systems or EPS
What is this?
R
V
Measure current by measuring voltage across a low
resistance precision resistor
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Power Systems or EPS
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Power Systems or EPS
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Expanded subsystem control
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Expanded subsystem control
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• What does a charge regulator do?
• Controls voltage from PV to battery
• Controls rate of charge
• Prevents overcharging
• Can boost or buck PV voltage to match battery
  needs.

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Expanded subsystem control
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Consider When high current occurs in a
subsystem, it could be from latch-up. What to
do? Cycle power. Where do you do this
hardware controlled in the EPS.
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Consider the satellites attitude control for
solar power generation.
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Satellite Orbit
Parallel Sun Rays
Eclipse
Sun
Earth
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Gravity Gradient Stabilized
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Passive Magnetic Stabilized
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N
S
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Inertially Stabilized
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Some Solar Notes

• Power from sun in orbit 1350 watts/meter2
• Power from cells on ground 35 less than in
  space
• Can get some power form albedo earth shine
  35

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Need to consider the power requirements of all of
the subsystems and when they are used to build a
power budget.
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Questions?
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