power systems

Question 1

why do we need power systems?

Answer 1

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

Question 2

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

Answer 2

solar array photo-voltaic

Question 3

describe how a photo-voltaic system works

Answer 3

use semi conductors to convert solar EM radiation to electrical energy

Question 4

describe how the solar dynamics system works

Answer 4

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

Question 5

describe how the chemical dynamic system works

Answer 5

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

Question 6

how do nuclear power sources work

Answer 6

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

Question 7

what does ‘RTG’ stand for?

Answer 7

radioisotope thermal generator

Question 8

how does an RTG power system work?

Answer 8

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

Question 9

describe how flywheel storage power systems work

Answer 9

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

Question 10

describe how fuel cell power systems work

Answer 10

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

Question 11

describe the structure of a solar array

Answer 11

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

Question 12

what is a p-n junction?

Answer 12

boundary interface between two types of semiconductors, p and n

Question 13

what is a p type semiconductor?

Answer 13

electron deficient

Question 14

what is an n type semiconductor?

Answer 14

electron excess

Question 15

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

Answer 15

• 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

Question 16

what is meant by the band gap?

Answer 16

gap between valence band and conduction band in the junction

Question 17

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

Answer 17

more

Question 18

what are the effects on power due to sun angle?

Answer 18

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

Question 19

what are the effects due to temperature on power?

Answer 19

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

Question 20

when may an array provide maximum power?

Answer 20

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

Question 21

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

Answer 21

• 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.

Question 22

what can be said about solar cells arranged in series?

Answer 22

used to achieve a desired voltage, results in power loss

Question 23

what can be said about solar cells arranged in parallel?

Answer 23

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

Question 24

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

Answer 24

* 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

Question 25

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

Answer 25

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

Question 26

list the advantages and disadvantages of RTGs

Answer 26

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

Question 27

describe how a fuel cell works

Answer 27

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

Question 28

describe the advantages and disadvantages of fuel cells

Answer 28

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

Question 29

when is a power storage system required?

Answer 29

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

Question 30

define depth of discharge DOD

Answer 30

percentage of battery capacity used in discharge

Question 31

define hysteresis

Answer 31

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