Satellite Electrical Power Systems, design considerations

Pratheek Manjunath7th Semester, TC

RVCEBangalore

Contents

1. Power Budget Design Considerations

2. Primary Source – Solar Energy

3. Secondary Source - Batteries

4. Power distribution and management

1. Eclipse duration2. Altitude3. Orbit duration4. Battery charge/discharge

cycles5. Mission lifetime6. Operating duration of Loads7. Losses in the system

GEOLEO

N

Factors which determine the Power Budget

Factor LEO GSO

Altitude Upto 2,000 km > 35,000 km

Orbit Duration 88 – 127 min 24 hours

Eclipse Duration 20 – 35 min 0 – 72 minutes

Mission Lifetime 7 years 15 years

Average Battery cycles 300 1,350

Discharge depth 20% 80%

Application Remote sensing Telecommunication, Weather forecast

Comparison between design factors of LEO and GSO

PRIMARY SOURCES – ENERGY GENERATION

1.SOLAR PANELS2.FUEL-CELLS3.NUCLEAR REACTORS

SECONDARY SOURCES – ENERGY STORAGE AND REUSE

4.BATTERIES

Power Budget

Solar array size is determined by:

1. Average power need for 1 orbit2. Sun/eclipse ratio3. Losses in the system

Battery sizing is determined by:

1. Eclipse power need

2. Eclipse duration

3. Capacity fading (due to mission lifetime and charge/disch. cycles

4. Losses in the system

Primary Power Source - Solar Arrays

• A solar cell is composed of a semiconductor material and converts photons to electrons.

• The solar cells work on the principle of photovoltaic effect.

Solar Arrays

• The solar flux is reflected, absorbed by the solar cell or crosses it• Every absorbed photon whose energy is greater than

semiconductor gap is going to release an electron and to create a positive « hole » (lack of electron). This electron is part of the crystalline network

• Photons with excess energy dissipate it as heat in the cell, leading to reduced efficiency

• An electrical field is introduced in the cell in order to separate this pair of opposite charges

• The efficiency of Silicon semiconductor is 17-19%, whereas GaAs is 26-29%.

Solar Cells

Solar Arrays

Solar Array types

• Rotating wings– Satellite can be earth-pointing

Provides 1 degree of freedom

• Fixed wings– Satellite must be sun-pointing as the

panels are attached rigidly to the body

• Body-mounted

Satellite can be tumblingSolar cells are glued on the structure of the satelliteThe power is limited by the surface of the satellite

Secondary power source - BatteriesThe number and type of batteries depends on:

• Expected life of satellite.• Type of Orbit – LEO/MEO/GEO• Backup power required.• Charging cycles.• Extent of discharge or load.• Temperatures of operation.

Space batteries, Ni technologies• Nickel-Hydrogen (Ni-H2)

––

Heritage on GEO satellitesNo longer used for new designs

• Nickel-Cadmium (Ni-Cd)––

Heritage on LEO satellitesNo longer used for new designs

Space batteries, Li technologies• Lithium-ion

– Higher energy density than theNickel-based batteriesHeritage on LEO and GEO satellites, widely used for new designs

–

• Lithium Polymer– Even higher energy density than

Li-ionNot yet qualified for use on satellite power subsystems

–

Comparison of typical Battery typesCharacteristics Ni-Cd Ni-H2 Ag-Zn

Energy Density 25 70 120

Life Cycle High Highest Low

Optimum Operating Temp

-10 to 25 10 to 15 Better

Voltage Constant Discharge

Not as good as NiCd

Poor

Charge Discharge Cycles

1000 900 200

Suitability Up to 7 years Up to 10 years Short life satellites

Renowned Battery Manufacturers

• SAFT–––––

Dedicated development for space useCells placed in parallel/series configurationLarge single cell: 1.1 kgBattery cell balancing performedRelatively low shelf-life capacity fading

• ABSL–––––

Battery cells from commercial origin (Sony 18650HC)Cells placed in series/parallel configurationSmall single cell: 42 g

No Battery cellRelatively high

balancing performedshelf-life capacity fading

Payload5%

T&C5%

Control4%

Receiver5%

HPA80%

Others1%

Power Distribution

Regulators

To keep bus voltage constant in both fromthe point of view of the seasonal

variationsat the output of solar panel as well asbackup batteries, it is essential to useregulators.

The types of voltage regulators 1. Series regulator.2. Shunt regulator.3. Switch mode regulator.

For constant loads - shunt regulators can be used

large fluctuations of input supply - switch mode varying loads - series regulators

Power distribution and management units

BAPTA (Bearing and Power transfer assembly)

- Solar Array sun pointing by stepper motor- Peak power generation

Power Control Unit–––

Directs power to the loadsBattery charge/discharge regulationCommunication to On-board Computer

Power distribution Unit––

Circuit protectionOn/Off switching

Small PCU

Large PCU

SA BAPTA

DriveElectr

Architecture

Loads.

Satellite bus

SA pointing and Circuit protectionand power distributionpower transfer

BAT

OBC

SA BAPTA PCU PDU