Sunpower’s linear alternator is a unique device designed by Sunpower specifically for our free-piston technology. It is a key component that converts the linear motion of the engine’s piston to electricity, or vice versa.

In a Stirling engine, a heat source drives the Stirling cycle that moves the piston which generates electricity through the alternator. In this application the alternator is commonly called a generator. In a Stirling cryocooler an electrical source drives the piston which creates cooling through the Stirling cycle. When power is provided to a linear alternator to drive the piston the alternator becomes, or is called a motor.

The Sunpower linear alternator operates, like all generators and motors, through the interaction between a coil of wire and a magnetic field. A changing magnetic field (caused by the motion of magnets attached to the piston) induces a voltage in the coil causing an electrical current to flow. Conversely, in a motor, flowing current through a coil induces a change in the magnetic field causing the piston to move.

The illustration represents Sunpower’s linear alternator. The alternator has four main components; the coil (orange), the iron (dark blue), the magnet (purple), and the structure (gray) connecting the magnets to the piston. As the piston reciprocates, the magnets are forced to have the same motion since they are connected to the piston. The magnets, with their radially directed magnetic field, cause a sinusoidally varying magnetic field to flow around the iron. This varying magnetic field then induces a sinusoidal voltage into the coil.