Technologies
Since the invention of the free-piston Stirling engine in the early 1960s, Sunpower’s extensive research and development has been applied to the following Sunpower core technologies
Technologies
Gas Bearings
Gas bearings are a key component of Sunpower’s technology portfolio. They provide non-contacting operation between critical components, which allows for designs with exceedingly long life.
Gas bearings function in much the same way as an air hockey table. High pressure gas is forced through small holes creating a cushion. In an air hockey table, the cushion of gas allows the puck and paddles to float and slide across the table easily. In Sunpower machines, the same principle allows for non-contacting operation.
Technologies
Linear Alternators and Motors
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.
The Stirling Engine and Cycle
A Scottish minister named Robert Stirling invented the Stirling engine around 1816. Various manufacturers built Stirling engines over many years, but those engines usually had low power compared to their weight (low specific power). One of the major technical limitations with kinematic Stirling engines was the difficulty in maintaining low friction sealing surfaces and the reliability of the mechanical linkages. Although pressurizing the cycle can achieve a higher specific power, sealing of the gas often proved problematic.
In 1964, while teaching a mechanical engineering class at Ohio University in Athens Ohio, William Beale conceptualized what would come to be known as the Free-Piston Stirling Engine (FPSE.) He realized that with a proper design, the engine did not require any mechanism and could be easily hermetically sealed. This new design eliminated the driving shaft sealing surface and most mechanical linkages, as well as opened the door for other technical innovations which would improve the efficiency and reliability of the Stirling engine. In the early 1970s, Beale founded Sunpower, Inc. to continue the work he started at Ohio University.
The theoretical Stirling cycle has the following four stages in its cycle as shown in the graph:
- (1-2) Isothermal Expansion
- (2-3) Constant Volume Cooling
- (3-4) Isothermal Compression
- (4-1) Constant Volume Heating
Practical engines however have smooth continuous motions represented by the ellipse within these bounds.
One could build an engine which operates as shown in the diagram with alternate heating and cooling within a single space, but losses would exist caused by the alternate heating and cooling of the metal walls. There would also be significant stress and material issues with such an engine.