power systems Flashcards

1
Q

why do we need power systems?

A

store energy and provide a conversion system to provide power to various systems

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2
Q

what power system is the most commonly used for earth-orbiting systems?

A

solar array photo-voltaic

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3
Q

describe how a photo-voltaic system works

A

use semi conductors to convert solar EM radiation to electrical energy

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4
Q

describe how the solar dynamics system works

A

use solar thermal energy to convert to kinetic energy which can subsequently be converted to electrical energy

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5
Q

describe how the chemical dynamic system works

A

use chemical potential energy released in an exothermic or combustion reaction to power a turbine

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6
Q

how do nuclear power sources work

A

use energy available from a nuclear fission chain reaction in a nuclear reactor - this can then be converted to electrical energy

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7
Q

what does ‘RTG’ stand for?

A

radioisotope thermal generator

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8
Q

how does an RTG power system work?

A

use thermal energy released from thermal decay of radioactive material - thermal energy converted to electrical energy using thermoelectric effects

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9
Q

describe how flywheel storage power systems work

A

use kinetic energy stored in a flywheel - subsequently converted to electrical energy

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10
Q

describe how fuel cell power systems work

A

use chemical potential energy which is converted to electrical energy in an electrochemical cell

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11
Q

describe the structure of a solar array

A

assembly of thousands of individual solar cells, each consisting of a semiconductor p-n junction that produces an electron flow when illuminated by photons with sufficient energy

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12
Q

what is a p-n junction?

A

boundary interface between two types of semiconductors, p and n

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13
Q

what is a p type semiconductor?

A

electron deficient

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14
Q

what is an n type semiconductor?

A

electron excess

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15
Q

describe the differences between a p-n junction with no illumination compared to with illumination

A
  • no illumination: junction in equilibrium
  • with illumination: electrical potential difference, electrons in the valence band of the junction able to move into the conduction band which enables current flow
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16
Q

what is meant by the band gap?

A

gap between valence band and conduction band in the junction

17
Q

fill in the missing word: a large band gap requires a ____ energetic wave

A

more

18
Q
A
18
Q

what are the effects on power due to sun angle?

A

current output decreases as the angle of incidence between the incoming illumination and the surface normal increases

19
Q

what are the effects due to temperature on power?

A

solar cells are sensitive to their operating temperature with peak powers reducing with increasing temperature, for a fixed illumination level

20
Q

when may an array provide maximum power?

A

coming out of an eclipse - when it is at its coldest operating temperature

21
Q

what are the effects on power due to radiation? and what can be done to reduce these effects?

A
  • radiation damages solar cell material over time, resulting in reduced performance
  • general effects include reduced open circuit voltage, and short circuit current over time
  • solar cells have a cover glass which is used to provide some protection, the degree of protection depends on the thickness of the cover glass.
22
Q

what can be said about solar cells arranged in series?

A

used to achieve a desired voltage, results in power loss

23
Q

what can be said about solar cells arranged in parallel?

A

used to achieve a desired current, increase current flow and provide some redundancy in case of failure

24
Q

describe how RTG’s work, and where they are mounted on the spacecraft

A
  • material A and B form a junction at the hot end and a potential difference between the two material drives a current through the external load of the circuit converting thermal energy into electrical energy
  • the heat source in an RTG is provided by the natural radioactive decay of certain materials, resulting in the emission of high energy particles
  • the absorption of these particles by the material causes heat
  • when RTGs are used, they are usually mounted on booms far from the spacecraft
25
Q

why does the area of a solar array need to increase if a satellite is orbiting further away from the sun?

A

due to solar insolation inverse square law - area must increase. this cause mass to increase, but there is also some effect of improved performance due to lower temperatures

26
Q

list the advantages and disadvantages of RTGs

A

adv:
* power production independent of spacecraft orientation and distance from the sun
* low power levels for long durations
* not susceptible to radiation damage in van Allen belt

disadv
* adversely affect the radiation environment of the spacecraft
* require careful handling procedures during integration
* high temperature operation impacts on thermal environment
* interfere with sensitive payloads

27
Q

describe how a fuel cell works

A

generate electrical power by the reaction a fuel (hydrogen) and an oxidiser (oxygen) through an electrolyte

28
Q

describe the advantages and disadvantages of fuel cells

A

adv:
* constant power - no attitude or illumination constraints
* compact
* water as by product - good for manned missions
disadv:
* life is limited by fuel storage capacity
* water as by product - not good for unmanned missions

29
Q

when is a power storage system required?

A

required as part of the power system on a spacecraft in particular where intermitten power sources such as solar arrays are used

30
Q

define depth of discharge DOD

A

percentage of battery capacity used in discharge

31
Q

define hysteresis

A

if the battery is not fully discharged, there is a loss of capacity over time, the battery can be reconditioned by underoging a full discharge. a higher DOD on each cycle leads to a quicker capacity loss