4259680
Description
Flashcards by Luke Hansford, updated more than 1 year ago
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Created by Luke Hansford
almost 9 years ago
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| Question | Answer |
| What 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 |
| What is the purpose of a diode in a circuit? | To only allow current to flow in one direction. |
| Why are separation switches used? | Safety system used during launch to cut off power the power system from the spacecraft |
| What does a power block diagram look like? | |
| When power budgeting what do we design for? | The highest demand required (worst case) |
| What 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 |
| Give a use for each of them | Short 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. |
| Which 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) | |
| What is the seebeck effect? | Current is produced when junctions of two dissimilar conducting materials are maintained at different temps. |
| What 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. |
| What does a fuel cell look like? | |
| Describe how it works | Hydrogen 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. |
| What 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 |
| What are solar cells coated for? | Anti reflection |
| When do solar cells deliver max Power? | When IV is maximum, this is the turning point on the curve. |
| What alters the shape of the IV curve? | Cell area, illumination, material used. |
| What 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. |
| What 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. |
| How 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. |
| What 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. |
| What is the array area equation? | |
| What 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 |
| What is the difference between primary and secondary batteries? | Primary are non reversible chemical reaction batteries as secondary are reversible and therefore rechargeable. |
| What 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 |
| What are the different secondary battery types? | |
| What 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. |
| What functions do power control electronics perform? | Source control (array management) Storage control (battery management) Output control (power distribution and regulation) |
| How 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. |
| How 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 |
| What 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. |
| Is an 80% or 50% DoD better | 80% better as it will last for longer |
| How 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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