Morehead State University Morehead, KY Prof. Bob Twiggs RJTwiggs@gmail

Morehead State UniversityMorehead, KY

Prof. Bob [email protected]

Power Systems Design - 1

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Power System Design Considerations


System Requirements

Sources

Storage

Distribution

Control

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Operating regimes of spacecraft power sources


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Operating regimes of spacecraft power sources


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New Technology

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Sun spectral irradiance

Solar cell response

Peak sun irradiance

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Dual Junction Cell

Added by second junction

Efficiency

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Use of the Sun’s Spectrum

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Power Systems Design - 1 Triple Junction Cell

Added by second junction

Added by third junction

Efficiency

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Reduce Efficiency

Good Efficiency

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Power Systems Design –I Ended 10/21/10

Max Cell Voltage when open circuit

Max Cell Current when short circuit

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Peak Power

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Add cell voltages to get string voltage

String of cells

Parallel strings to cover panel

Solar Cell Strings

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ShadowingKills all power

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Some Solar Notes

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Sun

Approx Cosine

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Eclipse

Parallel Sun Rays

Sun

Earth

Satellite Orbit

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Gravity Gradient Stabilized

Sun

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Passive Magnetic Stabilized

N

S

SNSN

S N

S NS

N

S

N

S

N

S

N

S

N

S

N

S

N

SN

SN

SN

Sun

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Inertially StabilizedPower Systems Design - 1

Sun

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Questions?

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System Requirements

Sources

Storage

Distribution

Control

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Primary Secondary

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• Primary – non rechargeable batteries

• Secondary – rechargeable batteries

Electrical Power Battery Storage

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Energy Storage

Not Rechargeable

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Power Systems Design - 2 Not Rechargeable

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Not Rechargeable

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Power Systems Design - 2 Not Rechargeable

Not Good

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Power Systems Design - 2 Rechargeable

Old Technology

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Old Technology

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Old Technology

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New Technology

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• Use of NiCd batteries required reconditioning

• Reconditioning not required for Li Ion batteries.

Reconditioning battery system

Close sw to crowbar battery

Close sw to crowbar second battery

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How much Battery Charge Left?

Charging causes heating

Discharging causes heating

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Power Systems Design - 2 Batteries

Most common form of electrical storage for spacecraft

Battery terms:Ampere-hour capacity = total capacity of a battery (e.g. 40 A for

1 hr = 40 A-hrDepth of discharge (DOD) = percentage of battery capacity used in

discharge (75% DOD means 25% capacity remaining. DOD usually limited for long cycle life)Watt-hour capacity = stored energy of battery, equal to

A-hr capacity times average discharge voltage.Charge rate = rate at which battery can accept

charge (measured in A)Average discharge voltage = number of cells in series times

cell discharge voltage (1.25 v for

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Considerations for power calculations

We have a battery that has a power capacity of:

1000mA (1000mAHrs)@ 1.2vIt can supply 1000mA for 1 hour or 500mA for 2 hours or 250mA for 4 hours @ a voltage of 1.2 v.Power rating of 1000mA x 1.2 v = 1.2 watt hours

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Battery selection:

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Two batteries in series.

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Two batteries in parallel.

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Questions?

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System Requirements

Sources

Storage

Distribution

Control

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Power Systems Design - 3 or EPS

Solar Panels - source

Charge Control

Batteries

Voltage

Bus

Voltage

DC/DC

Voltage

DC/DC

Subsystem

Subsystem

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Radios

• Fixed voltage busses (5v, -5v, 7v, 3.3v, 12v, etc.)

• Quieter – generates less noise on voltage bus

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• DC/DC Converter/Regulators

• Regulate 2 Li Ion batteries - ~7.2v 5v

• “Buck Up” 1 Li Ion battery - ~3.6v 5v

Requires less circuitry, more efficient to regulate down

Requires more circuitry, less efficient to “buck up” voltage.

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Could be caused by arcing due to spacecraft charging

Failure in subsystem that causes a short

Feedback on voltage bus from some components

Multiple return paths for current to battery – don’t use grounded frame

Power cycling required to reset components that have latch up due to radiation

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What type of solar panel system does it take to generate 47.5 watts peak and 27.8 watts average?

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Questions?




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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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5.6.

1350

1350

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What do we need from the solar panel?

What are the attributes of a solar panel?

1. Total output power of solar panel.2. Voltage of solar panel.3. Maximum packing factor.4. Efficiency of the solar cells.5. Operating temperature of the panels.

Lets go back and look at the solar cell.

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This dual junction cell

1. Has an efficiency of ~ 22%2. Open circuit voltage ~ 2.2v3. Size – 76 x 37 mm

Lets go back and look at the solar cell.

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1. Has an efficiency of ~ 22%2. Open circuit voltage ~ 2.2v3. Size – 76 x 37 mm

Solar cell has an I-V curve like this

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1. Has an efficiency of ~ 22%

2. Open circuit voltage ~ 2.2v

3. Size – 76 x 37 mm

Looked at the solar cell.

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Need to select a battery to design forsolar panel voltage

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RechargeablePower Systems Design - 4

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Use a lithium ion batteryLi Ion batteries = 3.6 v nominal

Design Criteria for charging Li Ion battery:

1. Need 10-15% more voltage to charge than the nominal voltage.

2. Here we would need solar panel voltage of ~ 4.0 – 4.2v to charge this battery.

Design Criteria solar panel:

1. Number of cells = Max voltage/cell voltage.

2. 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 batteriesLi Ion batteries = 7.2 v nominal

Design Criteria for charging Li Ion battery:

1. Need 10-15% more voltage to charge than the nominal voltage.

2. Here we would need solar panel voltage of ~ 8.0 – 8.3v to charge this battery.Design Criteria solar panel:

1. Number of cells = Max voltage/cell voltage.

2. 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?

Got

Got

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Total Panel Area

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Packing Factor

Packing Factor = Total Cell Area/ Total Panel Area

Total Cell Area

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Packing Factor

What do you do if given a fixed size panel on which to put solar cells and you have these different size solar cells?

Fixed solar panel size

Cell type 3

Cell type 1 Cell type

2


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Packing Factor

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

11v

Total Panel Area

How do you mount these 5 cells on this panel?

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How do you mount these 5 cells on this panel?

NO!OK!

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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Can’t do. All cells for a single string must be on same face.

Got a cube? Put other cells on another face?

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Where are we now in the solar panel design?



Assume we could mount the 5 cells on a panel, what is total power for the cells selected?

Got

Got

Not got, but understand

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How much power from these cells?5 cells for

11v

11v

One cell area = 76 x 37 mm = 2812 mm^2Total cell area = 8*2812 = 22496 mm^2 = 2.25 x10-2 m^2

We have 1350 watts/m^2 from the sun in space

Direct power = (1350 w/m^2) x (2.25 x10-2 m^2) = 34.4 watts

Converted power = direct power x cell efficiency = 34.4 w x 0.22 eff

= 7.5 watts7.5 wattsFor this dual junction cell

1. Has an efficiency of ~ 22%

2. Open circuit voltage ~ 2.2v

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Where are we now in the solar panel design?



Now we can assume to start:1. panel is at 90 degrees with sun – max power2. operating temperature 20 degrees.. Centigrade –

22% eff

Got

Got

Not got, but understand

Got

Don’t 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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What is this?

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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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What is this?

R V

Measure current by measuring voltage across a low resistance precision resistor

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Power Systems Design - 4Power Systems or EPS

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Power Systems Design - 4Power Systems or EPS

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Power Systems Design - 4Expanded subsystem control

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What does a charge regulator do?

1. Controls voltage from PV to battery2. Controls rate of charge3. Prevents overcharging4. Can “boost” or “buck” PV voltage to match

battery needs.

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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 satellite’s attitude control for solar power generation.

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Eclipse

Parallel Sun Rays

Sun

Earth

Satellite Orbit


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Gravity Gradient StabilizedPower Systems Design - 4

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Passive Magnetic Stabilized

N

S

SNSN

S N

S NS

N

S

N

S

N

S

N

S

N

S

N

S

N

SN

SN

SN


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Inertially StabilizedPower Systems Design - 4

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• 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%

Some Solar Notes


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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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Documents

Morehead State University Morehead, KY Prof. Bob Twiggs RJTwiggs@gmail