Using a Power Budget

**The following video is a duplicate video also found in section 5.9

Purpose

• Understand the role of the power subsystem in the context of spacecraft as a whole and between other subsystems
• Recognize possible sources of power in space and power generation technologies
• Analyze a power budget and profile
• Use Systems Tool Kit (STK) to assist in power budgeting and planning

Background and Key Concepts to Consider:

Application of: 5.4 Power Design Process and Drivers, 5.5 Power Generation, 5.6 Consumable Power Storage, 5.7 Rechargeable Power Sources, 5.9 Power Budget, and Profiling

Artemis CubeSat Kit Specific

• Power generation requirement of 2.5 W
• Solar cell selection of ANYSOLAR’s SolarBITs with 25% efficiency
  • Surface area of 23 x 8 [mm] or 184 mm2 (per cell)
  • Mass of 0.5 grams (per cell)
  • Total of 120 solar cells across 4 available faces
• Solar irradiance at Earth, I0 = 1360.8 W/m2
• Incidence angle across all solar arrays, qi= 54.7 degrees
  • Optimal Incidence angle from each solar panel: 45 degrees
  • Three panels partially pointing to the sun achieves 1.73 times the amount of power of a single head-on panel
• The total surface area across the 3 CubeSat faces is 30,000 mm2. The solar cells need to cover at least 42% of the CubeSat faces to satisfy the 2.5 W power generation requirement.
• Batteries
  • Rated below 50 Watt-hours, no pressurized containers within the kit
  • Each cycle is an interval between the charge (charge current 1,020mA) with 100mA cut-off and the discharge (discharge current 3,400mA) with 2.65V cut-off. Capacity after 500cycles.
  • Capacity ≥ 2,010mAh (60% of Standard Capacity)

Links to the Artemis CubeSat Kit Github: https://github.com/hsfl/artemis

Recall the basic procedure for designing the electrical power system:

1. Define the power consumption and electrical characteristics of the spacecraft bus components
2. Define the necessary power generation and energy storage required to fulfill the power consumption requirements
3. Select the power generation and energy storage methods
4. Analyze the system’s power budget and profile from the beginning of the mission to the end of the mission to ensure the selected components are sufficient to supply power
5. Design a power conversion, management, and distribution subsystem to interface the power sources and power consumers
6. Procure and fabricate components
7. Conduct tests on isolated components
8. Conduct tests on integrated components

The lab activity will walk through the process used in step 4.

Required Materials & Setup

• Reference Artemis CubeSat Kit Power Budget in Google Sheets/Excel
• Specifications and datasheets for spacecraft components
  • For power characteristics
  • Typically available from manufacturers’ websites or contacting sales/company representatives
• STK Basic/Free – Orbit Reporting
• STK Pro – Solar Panel Reporting (later)

Procedure

Preliminary Lab Procedures

Reference the Artemis Power Budget

The Artemis Power budget will be used as a reference for this lab. Make your own copy of the Artemis Power Budget. Find the link to the Artemis Power budget here. (https://docs.google.com/spreadsheets/d/1nS07D4-2hFsfBfmfiYS8sjpTHMR5pUvH-NhnE5spPVY/edit?usp=sharing). With the budget open in Google Sheets, click on “File” and either “Make a Copy” or “Download” as a Microsoft Excel spreadsheet (.xlsx). As needed, modify the spreadsheet accordingly to the spacecraft of interest or budgeting. Note that there are several sheets in the spreadsheet. Sheets are numbered according to the suggested order to follow/view the budget and analysis.

Spacecraft Components and Specifications

Note: Reference the sheet, “Power by Mode”

For the spacecraft, all components that use power should be listed in the power budget. Be sure to include all components per subsystem, and include the specific part name/number. Use this information to update columns A through C:

• Subsystem
  • E.g. Payload, communication subsystem (listed as COMMS), on-board computer subsystem (listed as OBC), attitude determination and control subsystem (listed as ADCS), thermal subsystem, electrical power subsystem (listed as EPS)
• Component
  • Hardware parts that comprise a subsystem. E.g. The thermal subsystem has heaters and thermal sensors.
• Part Name/Datasheet
  • Feel free to link the specifications or datasheet as needed.

Gather specification sheets detailing electrical characteristics for components of the spacecraft. Specifications to look out for when working on a power budget can include:

• Operating Conditions
• Voltage
• Current
• Consumption (peak and average power draw)
• Capacity

In some cases, if this information does not seem readily available on websites for parts, a sales/company representative of the manufacturer may be contacted for more information. Note: “Power by Mode” summarizes the entire power budget and also will be referenced later in the procedure for updated calculations.

Spacecraft Operations and Power Modes

Define and consider the mission, concept of operations, and operating modes of the spacecraft. Look at the beginning of the mission to the end of the mission. What kind of actions need to happen during operations? Are there different operating conditions the spacecraft must operate in? This will help organize the power budget into different power modes. See Step 1 in 5.9 Power Budget and Profiling for an example.

Main Lab Procedures

Consider the Power Usage by Mode

Subsystem (On/Off)

Note: Reference the sheet, “Subsystem (On/Off)”

The power budget will first be broken down first in simple terms – whether a component will be on or off during the modes of operation. Consider which components will be on/off to make sure operations proceed. Ensure the list of Parts (subsystems, components, and part name/datasheet in columns A through C) are updated. For the different power modes designated (as described in the pre-lab procedures), update the Power Usage by Mode section. The different power modes should be listed per column in row 3, starting from column E. For each mode, go down the list of the components on the spacecraft. Insert…

• “On” if the component will be used
• “Off” if the component does not need to be used

… in the mode of interest. Repeat as necessary for all the modes in each column. Note: It might help to consider first the primary subsystems needed for any active actions or passive actions (such as continuously running parts of the command and data handling subsystem to compute and relay commands) or vice versa. The sheet uses the color green and bolded, italicized text to indicate whether or not a component is “On” during modes.

See also Step 2 in 5.9 Power Budget and Profiling for an example.

Power Usage by Mode

Note: Reference the sheet, “Power by Mode”

Next, based on the On/Off schedule developed from the previous sheet, numbers will not be inputted to estimate the average power used per mode. Using the same information in the on/off schedule (copy and paste or duplicate the sheet, “1. Subsystem (On/Off)” as necessary)…

• Replace the “On” terms with the amount of wattage the component is expected to use based on specifications.
• Replace the “Off” terms with zero (0) wattage.
• For each mode, sum the total amount of power usage expected.

Optional: Plotting the average power per mode on a bar graph may help visualize the comparison of how much power each mode uses.

(See also Step 4 in 5.9 Power Budget and Profiling for an example.)

Mode Plot Data

Note: Reference the sheet, “Mode Plot Data”

“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.”

Data from STK Simulation

Note: Data from STK can be extracted via the Reports and Graphs Feature, to a .csv file.

• Refer to the Systems Tool Kit (STK) Lab Instructions for more help.
• Orbit Simulation – 1 orbit and 24 hours
  • Insert model into STK simulation, set up basic orbit
    • Factors like a position in space will affect power generation – how often are solar panels facing the sun?
• Solar Panel Simulation
  • (Later) Using a model with solar panels – need surface area of solar panels and report from STK
    • Calculate solar panel efficiency

Plots

• Power Usage in Each Mode vs. Time (24 hr)
• Power Usage in Each Mode vs. Time (One Orbit)
• Power Usage / Generation by Mode vs. Time (24 hr)
• Power Usage / Generation by Mode vs. Time (One Orbit)
• Power Generated over 24 hrs
• Power Generated over One Orbit

Typical Mode Sequence

Results

• Lowest Charge (Wh)
• Highest Charge (Wh)
• Average Charge (Wh)
• Highest Discharge Rate (W)
• Highest Charge Rate (W)
• Average Power Usage (W)
• Average Power Generated (W)

Typical Mode Sequence (Power)

Note: Reference the sheet, “3a. Typical Mode Sequence (Power)”

Typical Mode Sequence (Energy)

Note: Reference the sheet, “3b. Typical Mode Sequence (Energy)”

Power Allocation

Summarizing the power budget overall by:

• Peak Power Usage by component
  • Find the maximum value per component in the sheet, “2. Power by Mode”.
  • Use the “=MAX()” function to search by component/row.
• Duration per orbit by component
• Average Power Consumption per orbit (Wh) by component
• % of Power Budget

Lab Review and Deliverables

Goal(s):

• Modify the Artemis Power Budget starting with information on the spacecraft components, with a different payload selected. Work through the budget to integrate the payload components.
• Determine overall power allocation based on the developed power budget.
• Determine solar panel efficiency based on simulations via STK.
• Plot and characterize power/energy usage and generation for one orbit and over 24-hour periods.
• Ensure the selected components are sufficient to supply power.

References and Other Work

Artemis Power Requirements

3.1 The CubeSat power system shall generate power in LEO and provide sufficient power to all other bus components

• 3.1.1 The solar panels shall generate a minimum of 2.5W to charge the battery
• 3.1.2  The power distribution system shall supply sufficient power to all the other subsystems
• 3.1.3  The battery shall have a capacity of at least 10Wh

Design Tools

Section 5.10 Electrical Power System Design Tools