power systems Flashcards
why do we need power systems?
store energy and provide a conversion system to provide power to various systems
what power system is the most commonly used for earth-orbiting systems?
solar array photo-voltaic
describe how a photo-voltaic system works
use semi conductors to convert solar EM radiation to electrical energy
describe how the solar dynamics system works
use solar thermal energy to convert to kinetic energy which can subsequently be converted to electrical energy
describe how the chemical dynamic system works
use chemical potential energy released in an exothermic or combustion reaction to power a turbine
how do nuclear power sources work
use energy available from a nuclear fission chain reaction in a nuclear reactor - this can then be converted to electrical energy
what does ‘RTG’ stand for?
radioisotope thermal generator
how does an RTG power system work?
use thermal energy released from thermal decay of radioactive material - thermal energy converted to electrical energy using thermoelectric effects
describe how flywheel storage power systems work
use kinetic energy stored in a flywheel - subsequently converted to electrical energy
describe how fuel cell power systems work
use chemical potential energy which is converted to electrical energy in an electrochemical cell
describe the structure of a solar array
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
what is a p-n junction?
boundary interface between two types of semiconductors, p and n
what is a p type semiconductor?
electron deficient
what is an n type semiconductor?
electron excess
describe the differences between a p-n junction with no illumination compared to with illumination
- 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
what is meant by the band gap?
gap between valence band and conduction band in the junction
fill in the missing word: a large band gap requires a ____ energetic wave
more
what are the effects on power due to sun angle?
current output decreases as the angle of incidence between the incoming illumination and the surface normal increases
what are the effects due to temperature on power?
solar cells are sensitive to their operating temperature with peak powers reducing with increasing temperature, for a fixed illumination level
when may an array provide maximum power?
coming out of an eclipse - when it is at its coldest operating temperature
what are the effects on power due to radiation? and what can be done to reduce these effects?
- 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.
what can be said about solar cells arranged in series?
used to achieve a desired voltage, results in power loss
what can be said about solar cells arranged in parallel?
used to achieve a desired current, increase current flow and provide some redundancy in case of failure
describe how RTG’s work, and where they are mounted on the spacecraft
- 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
why does the area of a solar array need to increase if a satellite is orbiting further away from the sun?
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
list the advantages and disadvantages of RTGs
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
describe how a fuel cell works
generate electrical power by the reaction a fuel (hydrogen) and an oxidiser (oxygen) through an electrolyte
describe the advantages and disadvantages of fuel cells
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
when is a power storage system required?
required as part of the power system on a spacecraft in particular where intermitten power sources such as solar arrays are used
define depth of discharge DOD
percentage of battery capacity used in discharge
define hysteresis
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