Satellite Power Systems

The Basics

Satellite power modules are widely known as electrical power systems, or EPS. If you are purchasing Cubesat components as a kit, the EPS will come as a self contained unit, with outputs for voltage bus distribution and battery connnections. The EPS can be broken down conceptually into four smaller blocks. They are the power sources, the energy storage, the power distribution, and the Power Regulation and Control modules.

For those interested in designing their own power system, a table of the design steps can be found below courtesy of Space Mission Analysis by Wiley Larson.

Power Sources

There are many power sources available to the budding satellite designer. One might find useful applications for nuclear generators (RTGs), or regenerative fuel cells. That being said, most all satellites use photovoltaic cells. When it comes to power generation, this is the one important component you should know. The solar cells are managed via a peak power tracker or PPT unit which is discussed in the [Power Distribution] section.

Energy Storage

Most satellites store their power through a set of primary and secondary batteries. One can find a table of industry standard batteries and their characteristics below.

Power Distribution

It is typical to single a single power distribution system (PDS) in smaller satellites. In larger satellites it is not uncommon to have multiple distribution systems for collections of components. Typically a distribution bus will contain rails of 5v, and 3.3v for Cubesats. For special applications and different desired rail voltages, a modified EPS may be required. This is the critical module that handles voltage protection, isolation, and fault detection within the power block.

The PDS also typically contains the Peak Power Tracking, PPT module. The PPT monitors the solar panels and places the appropriate load resistance across the solar panel terminals in order to draw the maximum amount of power from them. Solar panels have a non-linear IV curve, which means their power output changes across loads and voltages. As the sunlight changes the voltage, the PPT adjusts the load to match.

Solar cell operating characteristics graph. Caption: the PPT keeps the solar pannels operating along the center line of peak performance.

Common Power values for Cubesats in Industry

Power Consumption

Storage:
	Pumpkin battery pack	40Wh
Optical Payload:
	Nasa's Lunar Cubesat	10W
	MIT Free-Space	15W
Solar panel Production
	Nasa's Lunar Cubesat	50W
	CubeSatShop	2.3W per 1U (translates to 48W for similar size of Lunar Cubesat)

Resources to Learn More

MIT open courseware. Class on how to build a satellite. Very useful information

Pumpkin talk on Power Systems