Power

Common Power Sources

Feature	Solar Photovoltaic	Radioisotope Thermoelectric Generator (RTG)	Fuel Cell
Power range (kW)	0.2-300	0.2-10	0.2-50
Specific power (W/kg)	25-200	5-20	275
Specific cost ($/kg)	800-3000	16K-200K	50K-100K
Power storage required for eclipse	Yes	No	No
Principal applications	Earth-orbiting spacecrafts and missions as far as Mars	Interplanetary	Interplanetary, short missions
Concept	Convert solar radiation into electrical energy	Radioactive source emits heat as it decays. Temperature difference between hot source and cold radiators converted into electricity.	Hydrogen + oxygen flow into cell, creating an oxidation reaction, which generates a current.
Examples	OneWeb	Viking 1 Lander	Space Shuttle

Energy Budget

Define the components power consumption
Estimate the total spacecraft power consumption for each activity
Size the solar arrays
Size the battery
Size the PCDU
Create an energy budget with the selected components
Simulate different mission phase and specific scenarios
Iterate!

Components Power Consumption

Get the power requirement from the selected component, this can be different for different modes the component is in (example: Radio receives or transmits).

Example

Mode	Power consumption
Off	0 W
Receiving	2 W
Receiving + Transmitting	8 W

Set the margin on the power requirement based on the maturity level of the component:

Commercial Off The Shelf (COTS) -> 5%
COTS with minor modifications -> 10%
New design or major modifications -> 20%

P_{margin} = P \cdot (1 + M)

Component Duty Cycle

This is the percent that the component is in a certain mode. This needs to be included to get an estimation for the average power consumption.

Example

| Camera Mode | Off | Start-up | Picture | Compress |
| :--- | :--- | :--- | :--- | :--- |
| Power consumption | 0 W | 1 W | 3 W | 5 W |
| Duration | 46 sec | 3 sec | 1 sec | 10 sec |
| Duty cycle | 76.7% | 5% | 1.7% | 16.7% |

Estimate total spacecraft power consumption for each activity

Sum over all components for the same activity, for example the radio is not linking down while the camera system is acquiring images, but the captured images have to be compressed by the computing module.

This can give you a power requirement over the full mission timeline, and also estimations for peak power consumptions.

Size the Solar Arrays

There are multiple kinds of solar panel and also different methods to configurations how there are attached to the spacecraft.

Power Subsystem-1.png
Power Subsystem-2.png

Feature	Body-Mounted	Deployable	Steerable
Power generation	Low	Medium	High
Cost	Low	Medium	High
Design complexity & reliability	No mechanism	Deployment mechanism	Deployment + actuating mechanism
Cell efficiency	Lower because of s/c high body temperature	Nominal	Nominal
Attitude control required torque	+	+++ because of increased external disturbance & inertia	+++ because of increased external disturbance & inertia
Structural stiffness	Good	Potential flexing	Potential flexing
Orbital decay	Nominal	Faster because of increased drag area	Faster because of increased drag area

Efficiency

Cell Type	Silicon	GaAs	Triple Junction GaAs (most common)	Power plants on Earth
Efficiency at 28°C	22%	18.5%	30%	~ 20-24%
Degradation in LEO	3.75% per year	2.75% per year	0.5% per year	-
Cost	Low	Medium	High	-

Calculate Power Generation

P = \frac{L_{s u n} \cdot η_{c e l l} \cdot A_{c e l l}}{4 \cdot π \cdot d^{2}} \cdot \cos (θ)

$L_{s u n}$ the Sun luminosity (3.83 * 10^26 W)
$η_{c e l l}$ the solar cell efficiency
$A_{c e l l}$ the cell area (the total illuminated area, not the total area)
$θ$ the angle between the solar array normal and the Sun vector
$d$ the distance between the Sun and the satellite

On average, in one orbit, there should be more energy generated than used.

Calculating $θ$

Power Subsystem-3.png
This leads to a formula to $θ$

\cos θ = - \sin (Ω) \cos (i) \sin (ρ) + \cos (Ω) \cos (ρ)

To account for other solar array angles or pointing errors, there can be added more transformations to the coordinate system.
Power Subsystem-4.png

Power generation over one orbit dependent on solar array orientation

Power Subsystem-5.png

Size the battery

Batteries have a rated energy capacity, but if we would use the full capacity all the time the lifetime of battery will be greatly reduced. For this there is an allowed Depth of Discharge (DoD) corresponding to a required amount of charge cycles. With the DoD the energy requirements between phases of power generation (eclipse) have to be covered.

Batteries also loose capacity over time, so they have a limited shelf life.

Size the Power Conditioning and Distribution Unit (PCDU)

A board that regulates, converts and switches power. It interfaces with a lot of the other modules in the power system.

PCDU and Solar Arrays

Solar Arrays have a voltage with peak efficiency, a Peak Power Tracker (PPT) tries to keep the solar array on this peak but it requires some overhead. Direct Energy Transfer (DET) uses the power directly from the solar cells, this means that there is no overhead but the solar cells are maybe not running on peak efficiency.

	DET	PPT
Concept	Direct power transfer from solar arrays to the bus	DC-DC converter between the solar array and the bus
Voltage	Same as battery -> When the battery has a low voltage, the solar array operates below its capacity	Adjusted to reach Maximum Power Point
Excess power	Shunted through resistors -> excess power dissipated as heat	Voltage adjusted to produce less power
Pro	Robust and simple. Better conversion efficiency	More power generated
Con	Power generation is not maximized	Added weight, complexity and cost. Can become less efficient than DET at EOL

PCDU and Battery

The Battery has to be charged, managed and balanced. This also requires measuring the battery temperature and limit charge and discharge rates.

PCDU and other Components

OBC
- Send telemetry (current voltage readings, battery temperature, etc.)
- Command and control
Umbilical interface
- External interface to supply power during tests
- Can also be used by the launcher to supply power during long trips
Separation switches
- Detect separation from the launcher
- Triggers spacecraft activation

Create an energy budget with the selected components

For each point in time, compute the power generated
Remove the solar array harness losses (about 1-3%)
Remove the PCDU solar power conversion losses
Define the spacecraft activity
Assign the corresponding total power consumption
Add harness losses (about 1-3%)
Add the PCDU distribution losses
Add the PCDU consumption
For each point in time, add the battery losses
Add the system margin to get the total power need
Compute the battery energy
Compute the battery State-of-Charge

Power Subsystem-6.png

Simulate different mission phases and specific scenarios

Different phases to consider are:

Begin of Life (BoL) vs End of Life (EoL)
Launch and Early Operations (LEOPS)
Nominal
Maneuver and Transfer
Safe Mode
Failure Scenarios

Iterate

If something is not correct make some changes and see what happens.

Power

Common Power Sources

Energy Budget

Components Power Consumption

Component Duty Cycle

Estimate total spacecraft power consumption for each activity

Size the Solar Arrays

Efficiency

Calculate Power Generation

Calculating θ

Power generation over one orbit dependent on solar array orientation

Size the battery

Size the Power Conditioning and Distribution Unit (PCDU)

PCDU and Solar Arrays

PCDU and Battery

PCDU and other Components

Create an energy budget with the selected components

Simulate different mission phases and specific scenarios

Iterate

Calculating $θ$