Spacecraft Power Systems

• Supply electrical power to spacecraft

• Condition, convert, control and distribute electrical power

• Meet average and peak electrical loads

• Protect spacecraft against EPS failure

• Provide energy storage for eclipse and peak demands

• Provide specialized power for specific functions such as firing ordinance for mechanism deployment

Power Subsystem Functions

Design• Identify requirements

– mission lifetime– spacecraft electrical power profile, especially average power

• Select and size power source– usually solar arrays for Earth-orbiting s/c– EOL requirement, type of solar cell, configuration, all drive

size• Select and size energy storage

– eclipse and load-leveling requirement– battery type

• Identify power regulation and control– peak-power tracker or direct-energy-transfer– thermal control– bus-voltage quality, conversion

Requirements

• Average electrical power - sizes power source(solar array size)

• Peak electrical power - sizes energy storage(battery capacity)

• Mission life - degradation affects sizing ofbatteries and solar arrays

• Orbit - defines achievable solar energy, eclipseperiods, radiation environment

• Spacecraft configuration - spinner impliesbody-mounted solar cells; 3-axis implies solarpanels

Batteries

•Inside the battery a chemical reaction produces the electrons. •The speed of electron production by this chemical reaction •this internal resistance controls how many electrons can flow between the terminals. •the chemical reaction does not take place unless a flow path is in place.

Parallel and Series

Battery Chemistry

• The acid molecules break up into three ions: two H+ ions and one SO4-- ion.

• The zinc atoms on the surface of the zinc rod lose two electrons (2e-) to become Zn++ ions.

• The Zn++ ions combine with the SO4-- ion to create ZnSO4, which dissolves in the acid.

• The electrons from the zinc atoms combine with the hydrogen ions in the acid to create H2 molecules (hydrogen gas).

• We see the hydrogen gas as bubbles forming on the zinc rod

Sulfuric Acid

H2SO4

Zinc

Battery Chemistry• Insert Carbon rod. 2 things happen:• The electrons flow through the wire

and combine with hydrogen on the carbon rod

• hydrogen gas begins bubbling off the carbon rod.

• Electrons flow through the wire, and you can measure a voltage and current in the wire. Some of the heat energy is turned into electron motion. There is less heat generated.

• The electrons move to the carbon rod because they find it easier to combine with hydrogen there.

• There is a characteristic voltage in the cell of 0.76 volts.

• Eventually, the zinc rod dissolves completely or the hydrogen ions in the acid get used up and the battery "dies."

Sulfuric Acid

H2SO4

Zinc Carbon

• Zinc-carbon battery - Also known as a standard carbon battery, zinc-carbon chemistry is used in all inexpensive AA, C and D dry-cell batteries. The electrodes are zinc and carbon, with an acidic paste between them that serves as the electrolyte.

• Alkaline battery - Used in common Duracell and Energizer batteries, the electrodes are zinc and manganese-oxide, with an alkaline electrolyte.

• Lithium photo battery - Lithium, lithium-iodide and lead-iodide are used in cameras because of their ability to supply power surges.

• Lead-acid battery - Used in automobiles, the electrodes are made of lead and lead-oxide with a strong acidic electrolyte (rechargeable).

• Nickel-cadmium battery - The electrodes are nickel-hydroxide and cadmium, with potassium-hydroxide as the electrolyte (rechargeable).

• Nickel-metal hydride battery - This battery is rapidly replacing nickel-cadmium because it does not suffer from the memory effect that nickel-cadmiums do (rechargeable).

• Lithium-ion battery - With a very good power-to-weight ratio, this is often found in high-end laptop computers and cell phones (rechargeable).

• Zinc-air battery - This battery is lightweight and rechargeable. • Zinc-mercury oxide battery - This is often used in hearing-aids. • Silver-zinc battery - This is used in aeronautical applications because the power-to-weight ratio

is good. • Nickel Hydrogen – pressure vessels: spacecraft – no on-orbit failures• Metal-chloride battery - This is used in electric vehicles.

Nickel Cadmium Battery

Nickel Hydrogen NiH2

Solar Cells

Photons to Electrons

• Photovoltaic (PV) cells convert sunlight directly into electricity.

• Made of N and P type semiconductors • Light strikes the cell, a certain portion of spectrum is

absorbed within the semiconductor • The energy of the absorbed light knocks electrons loose,

allowing them to flow freely. • PV cells also all have one or more electric fields that act

to force electrons freed by light absorption to flow in a certain direction.

• This current, together with the cell's voltage (which is a result of its built-in electric field or fields), defines the power (or wattage) that the solar cell can produce.

Materials

•Carbon, silicon and germanium •4 electrons in its outer orbital. •Form crystals. •Covalent bonds with four neighboring atoms, creating a lattice.•In carbon: diamond. •In silicon: silvery, metallic-looking material

N-Type DopingSemiconductors

Semiconductors

N-type

•Impurities of phosphorus or arsenic

•Each has five outer electrons

•The fifth electron has nothing to bond to

•Small quantity of the impurity

• N-type for negative charge.

P-Type Doping Semiconductors

Semiconductors

P-type

P-type

•P-type - boron or gallium

•Three outer electrons

•They form "holes" in the lattice where a silicon electron has nothing to bond to.

•Absence of an electron creates the effect of a positive charge – P type

•Holes can conduct current by accepting electrons

Semiconductor Junction

Add light with the correct energy

Cell Structure

Band Gap Energies

• Silicon Bandgap 1.1 Ev• Lower energy photons

pass through material• Higher energy photons

move one electron; excess energy lost (heat)

• Multilayer technology: high energy layer first, lower energy layers below.

• Number of layers limited by strain of materials.

• Indium Gallium Nitride

Doping• N-type - phosphorus or arsenic• each have five outer electrons• The fifth electron has nothing to bond to• small quantity of the impurity • N-type for negative charge. • P-type - boron or gallium • each have only three outer electrons. • they form "holes" in the lattice where a silicon electron has nothing

to bond to. • The absence of an electron creates the effect of a positive charge,

hence the name P-type. Holes can conduct current. A hole happily accepts an electron from a neighbor, moving the hole over a space. P-type silicon is a good conductor.

• N-type or P-type doping turns a silicon crystal from a good insulator into a viable conductor -- hence the name "semiconductor."

Orbital Considerations

Production efficiency, η, of solar cells ranges from 14-22%– silicon: 14% gallium arsenide: 19% indium phosphide: 18%

multijunction GaAs: 22%• Path efficiency is from solar array through batteries to loads– direct energy transfer: Xe =0.65, Xd = 0.85– peak-power tracking: Xe =0.60, Xd = 0.80• Inherent degradation– design & assembly losses, temperature-related losses, shadowing due

to appendages ≈ 0.77 (0.49-0.88)• Cosine loss, factor of cos θ

– incidence angle between array normal and Sun vector– typically use worst-case Sun angle

• Life degradation– thermal cycling, micrometeoroids, plume impingement, material outgassing, radiation: degradation/year = 2-4%/year

– LIFE DEGRADATION = (1-degradation/year)^^satellite life