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Power Systems Design - 1 Morehead State University Morehead, KY Prof. Bob Twiggs [email protected] 1 Power Systems Design - 1 Power System Design Considerations System Requirements Sources Storage Distribution Control SSE -122 2 Power Systems Design - 1 SSE -122 3 Power Systems Design - 1 SSE -122 4 Power Systems Design - 1 Operating regimes of spacecraft power sources SSE -122 5 Power Systems Design - 1 Operating regimes of spacecraft power sources SSE -122 6 Power Systems Design - 1 SSE -122 7 Power Systems Design - 1 New Technology SSE -122 8 Power Systems Design - 1 Solar cell response Peak sun irradiance Sun spectral irradiance SSE -122 9 Power Systems Design - 1 SSE -122 10 Power Systems Design - 1 Dual Junction Cell Efficiency Added by second junction SSE -122 11 Power Systems Design - 1 Use of the Sun’s Spectrum SSE -122 12 Power Systems Design - 1 SSE -122 13 Power Systems Design 1 Triple Junction Cell Efficiency Added by second junction Added by third junction SSE -122 14 Power Systems Design - 1 Good Efficiency SSE -122 Reduce Efficiency 15 Power Systems Design 1 SSE -122 16 Power Systems Design –I Ended 10/21/10 Max Cell Current when short circuit Max Cell Voltage when open circuit SSE -122 17 Power Systems Design - 1 Peak Power SSE -122 18 Power Systems Design - 1 String of cells Solar Cell Strings Parallel strings to cover panel Add cell voltages to get string voltage SSE -122 19 Power Systems Design - 1 SSE -122 20 Power Systems Design - 1 Shadowing Kills all power SSE -122 Power Systems Design - 1 Some Solar Notes SSE -122 22 Power Systems Design - 1 Approx Cosine Sun 23 SSE -122 23 Power Systems Design - 1 Satellite Orbit Parallel Sun Rays Eclipse Sun Earth SSE -122 24 Power Systems Design - 1 Gravity Gradient Stabilized Sun 25 SSE -122 Power Systems Design - 1 Passive Magnetic Stabilized N S N S S N N Sun S SSE -122 26 Power Systems Design - 1 Inertially Stabilized Sun SSE -122 27 Power Systems Design - 1 Questions? SSE -122 28 Power Systems Design - 2 Morehead State University Morehead, KY Prof. Bob Twiggs [email protected] 29 Power Systems Design - 2 Power System Design Considerations System Requirements Sources Storage Distribution Control SSE -122 30 Power Systems Design - 2 SSE -122 31 Power Systems Design - 2 SSE -122 32 Power Systems Design - 2 Primary SSE -122 Secondary 33 Power Systems Design - 2 Electrical Power Battery Storage • Primary – non rechargeable batteries • Secondary – rechargeable batteries SSE -122 34 Power Systems Design - 2 Not Rechargeable Energy Storage SSE -122 35 Power Systems Design - 2 Not Rechargeable Not Rechargeable SSE -122 36 Power Systems Design - 2 Not Rechargeable Not Good SSE -122 37 Power Systems Design - 2 Rechargeable Old Technology SSE -122 38 Power Systems Design - 2 Rechargeable Old Technology SSE -122 39 Power Systems Design - 2 Rechargeable Old Technology SSE -122 40 Power Systems Design - 2 SSE -122 Rechargeable 41 Power Systems Design 2 Rechargeable New Technology SSE -122 42 Power Systems Design - 2 Close sw to Close sw to crowbar • Use of NiCd batteries required reconditioning crowbar battery second battery • Reconditioning not required for Li Ion batteries. Reconditioning battery system SSE -122 43 Power Systems Design - 2 How much Battery Charge Left? Discharging causes heating Charging causes heating SSE -122 44 Power Systems Design - 2 Batteries Most common form of electrical storage for spacecraft Battery terms: Ampere-hour capacity = total capacity of a battery (e.g. 40 A for 1 hr = 40 A-hr Depth of discharge (DOD) = percentage of battery capacity used in discharge (75% DOD means 25% capacity remaining. DOD usually limited for long cycle life) stored energy of battery, equal to A-hr capacity times average discharge voltage. Watt-hour capacity = Charge rate = Average discharge voltage = SSE -122 rate at which battery can accept charge (measured in A) number of cells in series times cell discharge voltage (1.25 v for most commonly used cells) 45 Power Systems Design - 2 Considerations for power calculations We have a battery that has a power capacity of: 1000mA (1000mAHrs)@ 1.2v It can supply 1000mA for 1 hour or 500mA for 2 hours or 250mA for 4 hours @ a voltage of 1.2 v. Power rating of 1000mA x 1.2 v = 1.2 watt hours SSE -122 46 Power Systems Design - 2 Battery selection: SSE -122 47 Power Systems Design - 2 Considerations for power calculations Two batteries in series. SSE -122 48 Power Systems Design - 2 Considerations for power calculations Two batteries in parallel. SSE -122 49 Power Systems Design - 2 SSE -122 Rechargeable 50 Power Systems Design - 2 Questions? SSE -122 51 Power Systems Design - 3 Morehead State University Morehead, KY Prof. Bob Twiggs [email protected] SSE -122 52 Power Systems Design - 3 Power System Design Considerations System Requirements Sources Storage Distribution Control SSE -122 53 Power Systems Design - 3 SSE -122 54 Power Systems Design - 3 Power Systems Design 3 or EPS Charge Control Solar Panels - source Subsystem Voltage DC/DC Voltage Bus Voltage Subsystem DC/DC Batteries SSE -122 55 Power Systems Design - 3 Radios • Fixed voltage busses (5v, -5v, 7v, 3.3v, 12v, etc.) • Quieter – generates less noise on voltage bus SSE -122 56 Power Systems Design - 3 • DC/DC Converter/Regulators • Regulate 2 Li Ion batteries - ~7.2v 5v Requires less circuitry, more efficient to regulate down • “Buck Up” 1 Li Ion battery - ~3.6v 5v Requires more circuitry, less efficient to “buck up” voltage. SSE -122 57 Power Systems Design - 3 Could be caused by arcing due to spacecraft charging Failure in subsystem that causes a short Feedback on voltage bus from some components Multiple return paths for current to battery – don’t use grounded frame Power cycling required to reset components that have latch up due to radiation SSE -122 58 Power Systems Design - 3 SSE -122 59 Power Systems Design - 3 SSE -122 60 Power Systems Design - 3 61 Power Systems Design - 3 What type of solar panel system does it take to generate 47.5 watts peak and 27.8 watts average? 62 Power Systems Design - 3 63 Power Systems Design - 3 Questions? 64 Power Systems Design - 4 Morehead State University Morehead, KY Prof. Bob Twiggs [email protected] 65 SSE-122 Power Systems Design - 4 Power Systems or EPS 66 SSE-122 Power Systems Design - 4 67 SSE-122 Power Systems Design - 4 Look at the parts of the EPS 68 SSE-122 Power Systems Design - 4 Take Solar Panel 69 SSE-122 Power Systems Design - 4 1350 1350 5. 6. 70 SSE-122 Power Systems Design - 4 What do we need from the solar panel? What are the attributes of a solar panel? 1. 2. 3. 4. 5. Total output power of solar panel. Voltage of solar panel. Maximum packing factor. Efficiency of the solar cells. Operating temperature of the panels. Lets go back and look at the solar cell. 71 SSE-122 Power Systems Design - 4 Lets go back and look at the solar cell. This dual junction cell 1. 2. 3. Has an efficiency of ~ 22% Open circuit voltage ~ 2.2v Size – 76 x 37 mm 72 SSE-122 Power Systems Design - 4 Solar cell has an I-V curve like this This dual junction cell 1. 2. 3. Has an efficiency of ~ 22% Open circuit voltage ~ 2.2v Size – 76 x 37 mm 73 SSE-122 Power Systems Design - 4 Looked at the solar cell. This dual junction cell 1. 2. 3. Has an efficiency of ~ 22% Open circuit voltage ~ 2.2v Size – 76 x 37 mm What are the attributes of a solar panel? 1. 2. 3. 4. 5. Total output power of solar panel. Voltage of solar panel. Maximum packing factor. Efficiency of the solar cells. Operating temperature of the panels. 74 SSE-122 Power Systems Design - 4 Need to select a battery to design for solar panel voltage What are the attributes of a solar panel? 1. 2. 3. 4. 5. Total output power of solar panel. Voltage of solar panel. Maximum packing factor. Efficiency of the solar cells. Operating temperature of the panels. 75 SSE-122 Power Systems Design - 4 Rechargeable 76 SSE-122 Power Systems Design - 4 Use a lithium ion battery Li Ion batteries = 3.6 v nominal Design Criteria for charging Li Ion battery: 1. 2. Need 10-15% more voltage to charge than the nominal voltage. Here we would need solar panel voltage of ~ 4.0 – 4.2v to charge this battery. Design Criteria solar panel: 1. Number of cells = Max voltage/cell voltage. 2. Take minimum number of whole cells. # cells = (4.2v/string)/(2.2v/cell) = 1.9 or 2 cell for a string voltage of 4.4v 77 SSE-122 Power Systems Design - 4 78 SSE-122 Power Systems Design - 4 Use two lithium ion batteries Li Ion batteries = 7.2 v nominal Design Criteria for charging Li Ion battery: 1. 2. Need 10-15% more voltage to charge than the nominal voltage. Here we would need solar panel voltage of ~ 8.0 – 8.3v to charge this battery. Design Criteria solar panel: 1. Number of cells = Max voltage/cell voltage. 2. Take minimum number of whole cells. # cells = (8.3v/string)/(2.2v/cell) = 3.77 or 4 cell for a string voltage of 8.8v Lets be conservative and use 5 cells for 11v. 79 SSE-122 Power Systems Design - 4 Now we have: Two Li Ion batteries = 7.2 v nominal 5 cells for 11v to charge with. 80 SSE-122 Power Systems Design - 4 What is packing factor? What are the attributes of a solar panel? 1. 2. 3. 4. 5. Total output power of solar panel. Got Voltage of solar panel. Maximum packing factor. Got Efficiency of the solar cells. Operating temperature of the panels. 81 SSE-122 Power Systems Design - 4 Packing Factor Total Cell Area Total Panel Area Packing Factor = Total Cell Area/ Total Panel Area 82 SSE-122 Power Systems Design - 4 Packing Factor Cell type 1 Cell type 2 Fixed solar panel size Cell type 3 What do you do if given a fixed size panel on which to put solar cells and you have these different size solar cells? 83 SSE-122 Power Systems Design - 4 Packing Factor What do you do if given a fixed size panel on which to put solar cells and you have these different size solar cells? 84 SSE-122 Power Systems Design - 4 Now we have: 5 cells for 11v where the string has all of the cells hooked in series Total Panel Area 11v How do you mount these 5 cells on this panel? 85 SSE-122 Power Systems Design - 4 How do you mount these 5 cells on this panel? NO! OK! Visually we can see a very poor packing factor. 86 SSE-122 Power Systems Design - 4 What if the cells were bigger? Oh Oh! Now you have only 4.4v in the string. 87 SSE-122 Power Systems Design - 4 Got a cube? Put other cells on another face? Can’t do. All cells for a single string must be on same face. 88 SSE-122 Power Systems Design - 4 Where are we now in the solar panel design? What are the attributes of a solar panel? 1. 2. 3. 4. 5. Total output power of solar panel. Got Voltage of solar panel. Not got, but Maximum packing factor. understand Got Efficiency of the solar cells. Operating temperature of the panels. Assume we could mount the 5 cells on a panel, what is total power for the cells selected? 89 SSE-122 Power Systems Design - 4 How much power from these cells? 5 cells for 11v One cell area = 76 x 37 mm = 2812 mm^2 Total cell area = 8*2812 = 22496 mm^2 = 2.25 x10-2 m^2 We have 1350 watts/m^2 from the sun in space Direct power = (1350 w/m^2) x (2.25 x10-2 m^2) = 34.4 watts 11v Converted power = direct power x cell efficiency = 34.4 w x 0.22 eff = 7.5 watts For this dual junction cell 1. 2. 3. SSE-122 Has an efficiency of ~ 22% Open circuit voltage ~ 2.2v Size – 76 x 37 mm 90 Power Systems Design - 4 Where are we now in the solar panel design? What are the attributes of a solar panel? Got 1. Total output power of solar panel. Got 2. Voltage of solar panel. Not got, but 3. Maximum packing factor. understand Got 4. Efficiency of the solar cells. 5. Operating temperature of the panels. Now we can assume to start: 1. panel is at 90 degrees with sun – max power 2. operating temperature 20 degrees.. Centigrade – 22% eff Don’t forget, temperature counts a lot. 91 SSE-122 Power Systems Design - 4 Start here Tuesday for Idaho 92 SSE-122 Power Systems Design - 4 Now that we have beat our way through the solar panel design ----- lets go look at the some more parts of the EPS. 93 SSE-122 Power Systems Design - 4 Power Systems or EPS What is this? 94 SSE-122 Power Systems Design - 4 Power Systems or EPS Back bias diode Panel 1 When panel 1 is shaded, the back bias diode keeps the current from flowing backwards through panel 1, when panel 2 is generating a voltage across it. Panel 2 95 SSE-122 Power Systems Design - 4 Power Systems or EPS What is this? R V Measure current by measuring voltage across a low resistance precision resistor 96 SSE-122 Power Systems Design - 4 Power Systems or EPS 97 SSE-122 Power Systems Design - 4 Power Systems or EPS 98 SSE-122 Power Systems Design - 4 99 SSE-122 Power Systems Design - 4 100 SSE-122 Power Systems Design - 4 Expanded subsystem control 101 SSE-122 Power Systems Design - 4 Expanded subsystem control 102 SSE-122 Power Systems Design - 4 What does a charge regulator do? 1. 2. 3. 4. Controls voltage from PV to battery Controls rate of charge Prevents overcharging Can “boost” or “buck” PV voltage to match battery needs. 103 SSE-122 Power Systems Design - 4 Expanded subsystem control 104 SSE-122 Power Systems Design - 4 Consider: When high current occurs in a subsystem, it could be from latch-up. What to do? Cycle power. Where do you do this – hardware controlled in the EPS. 105 SSE-122 Power Systems Design - 4 Consider the satellite’s attitude control for solar power generation. 106 SSE-122 Power Systems Design - 4 Satellite Orbit Parallel Sun Rays Eclipse Sun Earth 107 SSE-122 Power Systems Design - 4 Gravity Gradient Stabilized 108 SSE-122 Power Systems Design - 4 Passive Magnetic Stabilized N S N S S N N S 109 SSE-122 Power Systems Design - 4 Inertially Stabilized 110 SSE-122 Power Systems Design - 4 111 SSE-122 Power Systems Design - 4 112 SSE-122 Power Systems Design - 4 Some Solar Notes • Power from sun in orbit ~ 1350 watts/meter2 • Power from cells on ground ~ 35% less than in space • Can get some power form albedo – earth shine ~ 35% 113 SSE-122 Power Systems Design - 4 114 SSE-122 Power Systems Design - 4 Need to consider the power requirements of all of the subsystems and when they are used to build a power budget. 115 SSE-122 Power Systems Design - 4 Questions? 116 SSE-122