Transcript eps

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 of
batteries and solar arrays
• Orbit - defines achievable solar energy, eclipse
periods, radiation environment
• Spacecraft configuration - spinner implies
body-mounted solar cells; 3-axis implies solar
panels
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
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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
Zinc
Sulfuric
Acid
H2SO4
Battery Chemistry
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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."
Zinc
Carbon
Sulfuric
Acid
H2SO4
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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 potassiumhydroxide 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 Doping
Semiconductors
•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.
N-type
Semiconductors
P-Type Doping
Semiconductors
•P-type - boron or gallium
P-type
•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
P-type
•Holes can conduct current by accepting
electrons
Semiconductors
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
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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