A Pragmatic Approach for Comparative Analysis of Linear and Rotary Generators
This paper identifies the need for consolidating analysis techniques for the purpose of designing linear electrical generation systems. Additionally, it identifies a need for concise system development tools that help designers make practical comparisons between linear and rotary machines.
The basic principles for converting mechanical energy to electrical energy have been known for over a century. It would seem that after this much time, advancements in electrical generation should have reached their limits; however, up to the late 1980’s and early 90’s the use of linear ...view middle of the document...
During this decision process, many opinions and assumptions between these competing systems were made, but finding hard data that compared the two methods was not readily available. Taking the time to thoroughly evaluate the electrical and mechanical technical aspects of this choice (rotary vs. linear) conflicted with the tight scheduling requirements of the project. This situation forced the team to make a best approximation based on limited experience instead of a deliberate and thoroughly evaluated approach.
WHY LINEAR GENERATORS ARE USED
Electrical energy generation is being expanded by creative innovation, and linear generators are a large part of this trend. In Livermore, California at Sandia National Laboratories, an innovative and practical use for linear generators is being developed. The system being studied is an internal combustion engine with a single free piston inside of a double-ended two-stroke cylinder. The free piston is embedded with permanent magnets and the outside of the aluminum cylinder has armature coils wrapped around it. As each cylinder alternates firing, the piston is driven back and forth causing the permanent magnet’s field to cross the alternator coils and generate electricity. In this application the generator’s electric demand is controlled by electronics in order to develop controlled loading to the piston and controlled back-pressure to the cylinder. This refined combustion process results in compression ratios of about 30:1 and thermal efficiencies as high as 56%, depending on the type of fuel used. In this study eight different fuels were tested with very low emissions (NOx emissions <10ppm). The application of this device is to eliminate drive train losses and reduce emissions in vehicles by efficiently generating electrical energy directly from the piston’s motion. This electricity is then stored and used to drive electric motors at the vehicle’s wheels. An important consideration when evaluating linear systems is the translational speed of the driving force, in this case the driven piston speed was approximately 1.1 meters per second. 
Furthermore, research to find the best generator design for low speed and high thrust applications is occurring. One noteworthy study “Linear Generators for direct-drive wave energy conversion” is intended to select the best type of generator to be used in the Archimedes Wave Swing (AWS) ocean energy pilot plant . The AWS is a very large underwater power plant; it has a 9m diameter upper floater that is driven by the varying wave mass above it. This research compared five different types of linear electric generators. Each generator was specified to operate under the AWS’s driving thrust of 1MN (That’s 10^6 N!) at translational speeds of approximately 2.2m/sec. These numbers suggest a peak mechanical driving power of 2.2MW to the AWS’s electric generator . Predicted material costs for these generators ranged from 138,000 to 287,000 Euros ($167,000...