Flashcards
Power
Cards in this deck (32)
- 1What is the process for designing a power system?Identify requirements
Detailed power budget
Select and size power source
Size battery system
Select regulation and control - 2What is the purpose of a diode in a circuit?To only allow current to flow in one direction.
- 3Why are separation switches used?Safety system used during launch to cut off power the power system from the spacecraft
- 4What does a power block diagram look like?
- 5When power budgeting what do we design for?The highest demand required (worst case)
- 6What are the different types of power cells?Photovoltaic cells (solar)
Primary (irreversible) and secondary (reversible chemical reaction) batteries
Radioisotope thermoelectric generators RTG
Fuel Cells
Solar dynamic generators - 7Give a use for each of themShort missions such as eclipse - batteries
For week long fuel cells are suitable.
For longer photovoltaic cells are suitable for LEO, interplanetary or GEO satellites.
For ISS longer term higher demand will be made on the power, no new tech such as nuclear dynamic systems needed. - 8Which power type is high/low in power, specific power, cost, hardness to radiation and maneuverability? (don't need to know specific values just whether high or low)
- 9What is the seebeck effect?Current is produced when junctions of two dissimilar conducting materials are maintained at different temps.
- 10What are the disadvantages of RTG's?- Only 3-7% efficient
- Excess heat so need to kept away from some instruments such as IR cameras
- Cannot be switched off
-Must be robust to survive launch and launch failure so is often encapsulated.
- 11What does a fuel cell look like?
- 12Describe how it worksHydrogen pumped into anode side of cell where it meets platinum catalyst. This causes the H2 molecule to split into 2 H+ ions and 2 electrons. The electrons are conducted by the anode and go around the external circuit doing useful work and return at cathode side of fuel cell. At the cathode side, oxygen O2 is pumped in and becomes O- ions. H+ ions along with 2 electrons make their way through the membrane and create the waste product of H2O. These are very efficient 70% of chemical energy becomes electrical.
- 13What are the advantages and disadvantages of solar cells?Advantages - Long heritage, high reliability, high specific power, low cost
Disadvantages - Needs sun
High temps and radiation reduce performance
silicon only 14% efficient
Need deployment and pointing mechanisms - 14What are solar cells coated for?Anti reflection
- 15When do solar cells deliver max Power?When IV is maximum, this is the turning point on the curve.
- 16What alters the shape of the IV curve?Cell area, illumination, material used.
- 17What is the difference between short circuit and an open circuit?Open circuit has an effective resistance of infinity. Short circuit is the opposite i.e. no resistance, a cell generate no power in either if these circumstances.
- 18What advances have there been in solar tech?Gallium Arsenide - more efficient and more radiation hard but more costly
Multijunction - multiple band gaps allow response to a range of different frequencies of light
Thin films - low mass, 7-10% efficiency
Solar concentrations - Fresnel lenses concentrate light 10x onto cells. - 19How are solar arrays designed in order to obtain required power?Arrays are constructed in series in order to achieve required voltage. Then they are stacked up in parallel in order to create the current required and therefore power.
- 20What is the problem with non illuminated cells?They become effectively open circuits, so isolation diodes are need to prevent current from circulating uselessly whilst in shade.
- 21What is the array area equation?
- 22What is the difference in battery requirement for LEO and GEO orbits?LEO batteries needed 16x a day = 6000 recharge cycles per year. Max time =36 mins
GEO eclipse season 45 days = 90 cycles per year, max eclipse time 1.2 hours - 23What is the difference between primary and secondary batteries?Primary are non reversible chemical reaction batteries as secondary are reversible and therefore rechargeable.
- 24What does depth of discharge mean?% capacity used/available during discharge.
100% would mean all of the battery was useable.
Higher D.o.D leads to shorter cycle life - 25What are the different secondary battery types?
- 26What is the memory effect?the battery remembers the last level it was discharged too and can only be discharged to this level and so needs a full discharge regularly.
- 27What functions do power control electronics perform?Source control (array management)
Storage control (battery management)
Output control (power distribution and regulation) - 28How many solar cells and strings are needed for a bus voltage of 28V and power of 112W, if each cell is 0.5V and 0.05A? (Assume no losses).No. cells in a string=28/0.5=56 and current=112W/28V=4A, so no. of strings required is 4/0.05=80 strings.
- 29How do solar dynamic generators work?Use suns heat to work a thermodynamic cycle.
Collector focuses sunlight onto a boiler to heat a working fluid which is then used in a heat engine.
Rankine and Brayton (turbine cycles) and Stirling Cycle (pistons) are used.
Helium is heated by plutonium radioisotope then drives piston, then alternator which covers motion into electricity - 30What are the disadvantages of solar dynamic generators?Dynamic generators such as pistons are heavy and mechanically complex but suitable for high power demands.
Any mechanisms require lubrication which is a huge issue in space. - 31Is an 80% or 50% DoD better80% better as it will last for longer
- 32How can we improve efficiency of photovoltaics?Galium Arsenide, solar concentration lens, multifunction cells, thin films, tracking sun with solar array deployment mechanism.
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- Degree Aerospace Engineering Flashcards on Power, created by Luke Hansford on 29/12/2015.
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- Luke Hansford
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