5. Power System

5.9 Power Budget and Profiling

 

To keep track of power consumption and generation, the EPS subsystem lead can generate a power budget and mission profile. This analysis will evaluate if the power generation and power storage are sufficient to support the mission. If not, a new power generation and/or storage system is selected and reanalyzed to verify requirement satisfaction.

  1. Define the mission’s concept of operations and time in each phase,
Operational Mode Time Since Deployment Time in Phase
i. Initial Checkouts 30 minutes 60 minutes
ii. Initial Deployment 1.5 hours 30 minutes
iii. Sun Pointing/ Spacecraft Charging 2 hours 90 minutes
iv. Science Operations 3.5 hours 30 minutes
v. Radio Pointing 4 hours 30 minutes
vi. Sun Pointing/ Spacecraft Charging 4.5 hours 120 minutes
Return to iv and iterate science operations loop until the mission is fulfilled

2. Define discrete modes of operations where different subsets of avionics are on or off,

Subsystems >

Mode v

 Payload Attitude Sensing and Control Communications Command and Data Handling Thermal Control Structures and Mechanisms Propulsion
Initial Checkouts Off On On On Off Off Off
Initial Deployment Off Off Off On Off On Off
Science Operations On On Off On Off Off Off
Radio pointing/ data uplink and downlink Off On On On Off Off Off
Sun pointing/ spacecraft charging Off On Off On On Off Off
Stationkeeping/ Orbit adjustment Off On Off On Off Off On

3. Define the subsystem components, which subsequently defines each component’s voltage, current, peak power draw, and average power draw

Subsystem Component Voltage Current Peak power draw Average power draw
Payload
Structure and Mechanisms Deployment Mechanisms
Thermal Control Temperature Sensor
Heater
Power (including harness) Battery
Telemetry and Control Receiver
Transmitter
Command and Data Handling On-Board Computer
Daughter Boards
Attitude Determination and Control Inertial Measurement Unit
Torque Coils

4. Summarize the average power draw for each mode by summing the individual power draw of each component in each mode

Subsystem Component Initial Checkouts Initial Deployment Science Operations Radio pointing/ data uplink and downlink Sun pointing/ spacecraft charging Stationkeeping/ Orbit adjustment
Payload
Structure and Mechanisms Deployment Mechanisms
Thermal Control Temperature Sensor
Heater
Power (including harness) Battery
Telemetry and Control Receiver
Transmitter
Command and Data Handling On-Board Computer
Daughter Boards
Attitude Determination and Control Inertial Measurement Unit
Torque Coils
Average Power during Mode

5. Derive the power generation profile over time of an orbit using your solar cell specifications, orbit-defined solar irradiance, and incident angles over time or STK. The following power profile is an example profile taken from STK with default solar cell specification for a 1U CubeSat surface area in ISS orbit.

Time (min) Power Generated (W)
0 3.004166667
1 2.9665
2 2.728
3 3.142666667
4 2.627333333
5 2.941833333
6 2.902833333
7 2.665333333
8 3.159833333
9 2.6665
10 3.014833333
11 2.9365
12 2.726
13 3.009833333
14 2.641166667
15 2.978833333
16 2.870833333
17 2.7025
18 3.244333333
19 2.641333333
20 2.897
21 2.921333333
22 2.6825
23 3.0465
24 2.675333333
25 3.018166667
26 2.967166667
27 2.726333333
28 3.126666667
29 2.623333333
30 2.940166667
31 2.8825
32 0.3241666667
33 0
34 0
35 0
36 0
37 0
38 0
39 0
40 0
41 0
42 0
43 0
44 0
45 0
46 0
47 0
48 0
49 0
50 0
51 0
52 0
53 0
54 0
55 0
56 0
57 0
58 0
59 0
60 0
61 0
62 0
63 0
64 0
65 0
66 1.005166667
67 2.711333333
68 3.2025
69 2.631833333
70 2.9205
71 2.8985
72 2.6745
73 3.104
74 2.691333333
75 3.042333333
76 2.942
77 2.7195
78 3.071666667
79 2.634
80 2.976
81 2.8585
82 2.690333333
83 3.237666667
84 2.669
85 2.923166667
86 2.914833333
87 2.700333333
88 2.9925
89 2.678666667
90 3.003166667

6. Fill in the power profile with the modes throughout mission time, power consumption associated with each mode, and the integrated energy storage

Orbit Number Time (min) Mode Energy Used (Wh) Energy Generated (Wh) Energy Balance (Wh) Battery Charge (Wh)
1 0 Power Generation 0 0.05006944444 0.05006944444 29.6
1 1 Power Generation 0 0.04944166667 0.04944166667 29.6
1 2 Power Generation 0 0.04546666667 0.04546666667 29.6
1 3 Power Generation 0 0.05237777778 0.05237777778 29.6
1 4 Power Generation 0 0.04378888889 0.04378888889 29.6
1 5 Power Generation 0 0.04903055556 0.04903055556 29.6
1 6 Power Generation 0 0.04838055556 0.04838055556 29.6
1 7 Power Generation 0 0.04442222222 0.04442222222 29.6
1 8 Power Generation 0 0.05266388889 0.05266388889 29.6
1 9 Power Generation 0 0.04444166667 0.04444166667 29.6
1 10 Power Generation 0 0.05024722222 0.05024722222 29.6
1 11 Power Generation 0 0.04894166667 0.04894166667 29.6
1 12 Power Generation 0 0.04543333333 0.04543333333 29.6
1 13 Power Generation 0 0.05016388889 0.05016388889 29.6
1 14 Power Generation 0 0.04401944444 0.04401944444 29.6
1 15 Power Generation 0 0.04964722222 0.04964722222 29.6
1 16 Power Generation 0 0.04784722222 0.04784722222 29.6
1 17 Power Generation 0 0.04504166667 0.04504166667 29.6
1 18 Power Generation 0 0.05407222222 0.05407222222 29.6
1 19 Power Generation 0 0.04402222222 0.04402222222 29.6
1 20 Power Generation 0 0.04828333333 0.04828333333 29.6
1 21 Power Generation 0 0.04868888889 0.04868888889 29.6
1 22 Power Generation 0 0.04470833333 0.04470833333 29.6
1 23 Power Generation 0 0.050775 0.050775 29.6
1 24 Power Generation 0 0.04458888889 0.04458888889 29.6
1 25 Power Generation 0 0.05030277778 0.05030277778 29.6
1 26 Power Generation 0 0.04945277778 0.04945277778 29.6
1 27 Power Generation 0 0.04543888889 0.04543888889 29.6
1 28 Power Generation 0 0.05211111111 0.05211111111 29.6
1 29 Power Generation 0 0.04372222222 0.04372222222 29.6
1 30 Pointing 0 0.04900277778 0.04900277778 29.6
1 31 Data Collect 0.04762060333 0.04804166667 0.000421063333 29.6
1 32 Data Received 0.04896559333 0.005402777778 -0.04356281556 29.55643718
1 33 Data Transmit 0.05491559333 0 -0.05491559333 29.50152159
1 34 Data Transmit 0.05491559333 0 -0.05491559333 29.446606
1 35 Data Transmit 0.05491559333 0 -0.05491559333 29.3916904
1 36 Data Transmit 0.05491559333 0 -0.05491559333 29.33677481
1 37 Nominal 0.01072893667 0 -0.01072893667 29.32604587
1 38 Nominal 0.01072893667 0 -0.01072893667 29.31531694
1 39 Nominal 0.01072893667 0 -0.01072893667 29.304588
1 40 Nominal 0.01072893667 0 -0.01072893667 29.29385906
1 41 Nominal 0.01072893667 0 -0.01072893667 29.28313013
1 42 Nominal 0.01072893667 0 -0.01072893667 29.27240119
1 43 Nominal 0.01072893667 0 -0.01072893667 29.26167225
1 44 Nominal 0.01072893667 0 -0.01072893667 29.25094332
1 45 Nominal 0.01072893667 0 -0.01072893667 29.24021438
1 46 Nominal 0.01072893667 0 -0.01072893667 29.22948544
1 47 Nominal 0.01072893667 0 -0.01072893667 29.21875651
1 48 Nominal 0.01072893667 0 -0.01072893667 29.20802757
1 49 Nominal 0.01072893667 0 -0.01072893667 29.19729863
1 50 Nominal 0.01072893667 0 -0.01072893667 29.1865697
1 51 Nominal 0.01072893667 0 -0.01072893667 29.17584076
1 52 Nominal 0.01072893667 0 -0.01072893667 29.16511182
1 53 Nominal 0.01072893667 0 -0.01072893667 29.15438289
1 54 Nominal 0.01072893667 0 -0.01072893667 29.14365395
1 55 Nominal 0.01072893667 0 -0.01072893667 29.13292501
1 56 Nominal 0.01072893667 0 -0.01072893667 29.12219608
1 57 Nominal 0.01072893667 0 -0.01072893667 29.11146714
1 58 Nominal 0.01072893667 0 -0.01072893667 29.1007382
1 59 Nominal 0.01072893667 0 -0.01072893667 29.09000927
1 60 Nominal 0.01072893667 0 -0.01072893667 29.07928033
1 61 Nominal 0.01072893667 0 -0.01072893667 29.06855139
1 62 Nominal 0.01072893667 0 -0.01072893667 29.05782246
1 63 Nominal 0.01072893667 0 -0.01072893667 29.04709352
1 64 Nominal 0.01072893667 0 -0.01072893667 29.03636458
1 65 Nominal 0.01072893667 0 -0.01072893667 29.02563565
1 66 Power Generation 0 0.01675277778 0.01675277778 29.04238843
1 67 Power Generation 0 0.04518888889 0.04518888889 29.08757731
1 68 Power Generation 0 0.053375 0.053375 29.14095231
1 69 Power Generation 0 0.04386388889 0.04386388889 29.1848162
1 70 Power Generation 0 0.048675 0.048675 29.2334912
1 71 Power Generation 0 0.04830833333 0.04830833333 29.28179954
1 72 Power Generation 0 0.044575 0.044575 29.32637454
1 73 Power Generation 0 0.05173333333 0.05173333333 29.37810787
1 74 Power Generation 0 0.04485555556 0.04485555556 29.42296343
1 75 Power Generation 0 0.05070555556 0.05070555556 29.47366898
1 76 Power Generation 0 0.04903333333 0.04903333333 29.52270231
1 77 Power Generation 0 0.045325 0.045325 29.56802731
1 78 Power Generation 0 0.05119444444 0.05119444444 29.6
1 79 Power Generation 0 0.0439 0.0439 29.6
1 80 Power Generation 0 0.0496 0.0496 29.6
1 81 Power Generation 0 0.04764166667 0.04764166667 29.6
1 82 Power Generation 0 0.04483888889 0.04483888889 29.6
1 83 Power Generation 0 0.05396111111 0.05396111111 29.6
1 84 Power Generation 0 0.04448333333 0.04448333333 29.6
1 85 Power Generation 0 0.04871944444 0.04871944444 29.6
1 86 Power Generation 0 0.04858055556 0.04858055556 29.6
1 87 Power Generation 0 0.04500555556 0.04500555556 29.6
1 88 Power Generation 0 0.049875 0.049875 29.6
1 89 Power Generation 0 0.04464444444 0.04464444444 29.6

From this analysis, we see that the battery capacity over the orbit returns back to full charge, verifying that the spacecraft is sufficiently supported by the electrical power system. If the battery charge slowly drains every orbit, the EPS specialist must find a power generation technology that produces more power, or the systems engineer must modify the mission operations timeline to spend longer periods in power generation mode. If the power budget reveals a large surplus of power that’s just being shunted or risks overcharging the battery, the systems engineer may decide to utilize that surplus energy by powering other operations or scaling down the power generation technology to save on mass.

Suggested Activity
  • Produce a power budget and profile. The Artemis CubeSat Kit power budget and profile lives here.

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A Guide to CubeSat Mission and Bus Design Copyright © by Frances Zhu is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.

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