The Simulation and Design of a High Efficiency, Lubricant Free, Linear Compressor for a Domestic Refrigerator

The first phase of a project at Sunpower, Inc. sponsored by the EPA is described. The program involves the design, computer simulation and fabrication of a high efficiency linear compressor for application in domestic refrigeration. The use of a high efficiency electric motor and non-contact operation can dramatically reduce the electrical and friction losses associated with crank compressors. This technology has already been proven at Sunpower in prototypes of Stirling cycle refrigerators. The computer simulation predicts stable operation of the design under all conceived operating conditions and indicates that a COP of 2.0 can be achieved for a heat lift of 250 Watts from -23 C to 32 C. Early tests conducted on the prototype suppon these findings. INTRODUCTION Rotary compressors for vapor compression cycles are highly developed but still have significant losses associated with them. The free piston linear compressor has the potential to reduce these losses substantially and achieve better efficiency. It also promises life and cost benefits and has the added advantage of being able to operate without lubrication or wear. This paper describes the first phase of a project at Sunpower, Inc. sponsored by the USA Enviromental Protection Agency. The program involves the design and computer simulation of a high efficiency linear compressor for application in a domestic refrigerator. At the completion of the project, a functional prototype will be tested by an independent third pany. The unit is designed for operation with Rl2 refrigerant so that direct comparison with existing units can be made. However, with minor modifications it could be used with any similar refrigerant such as R22 or HFC 134a. It could also be used for compressing any chemically compatible gas and, since it requires no lubrication, will not contaminate such a gas. This paper outlines a computer simulation used to study the dynamics of the system and presents preliminary test results .. Introduction to Free Piston Devices In a free piston compressor the piston is not rigidly attached to a driving mechanism such as a crank. Because of this its motion is not constrained by the geometry of the driver. Both the stroke and mean position of the piston can change and these are dictated by the mechanical, magnetic and pressure forces acting on it. This can be a disadvantage since the piston motion is not predefined making it necessary to have some mechanism to control its position, panicularly when fragile pans might collide. However, it makes the machine more versatile since the piston motion can be adjusted continually to achieve optimum performance. Another advantage of a free piston device is that because all the driving forces act along the line of motion, there is no sideways thrust on the piston. This reduces the high friction losses and wear rates associated with crank devices. A compact electromagnetic mechanism can't generate the high forces needed to drive the piston of a compressor. To approximate the sinusoidal piston motion desired the piston is sprung so that its natural frequency is close to the intended operating frequency. This springing can be provided mechanically, electrically or by pressure forces or any combination of the three. By driving the system close to resonance, a large amount of energy can be transferred to the system without the need for a large driving force. However, if the driving frequency or natural frequency change, the system will move away from resonance and its power capacity will decrease. Thus the tuning of the system has a significant effect on its response. IMPROVEMENTS OVER EXISTING COMPRESSORS