| Diesel powered electric generators supply power to isolated grids around the world, often operating at low average load. The addition of renewable power to save fuel reduces engine load even further. This low load operation results in poor fuel efficiency and increased maintenance due to engine fouling, especially in cold ambient conditions. Variable speed operation of these diesel generator sets has been studied, in general, and as a solution to efficiency and fouling problems. Variable speed operation requires the addition of a supervisory controller and an electronic power converter to provide constant frequency power.; Conclusions are based on testing of diesel generators and thermodynamic and dynamic modeling. Tests included steady state tests of two diesel generators (5 kW and 15 kW systems) and tests of a prototype 5 kW variable speed system. Heat release, combustion duration, and cylinder gas temperatures were derived from cylinder pressure data. System component design and the effects of component limitations on variable speed operation are discussed. Once a variable speed operating strategy is determined for a sample 15 kW system, test data are used to determine fuel savings and to estimate the potential for improvements in emissions, cold weather fouling, and engine wear.; Results show that variable speed systems are most applicable where the peak power system capability is large compared to the average load. In existing, isolated power systems fuel savings could be as high as 40% of annual fuel. A comparison of combustion conditions between constant speed and variable speed operation of diesel generator sets shows that unburned hydrocarbon emissions from local overleaning, wall quenching, and bulk quenching of combustion should be reduced with variable speed operation. Likewise, particulate emissions and low load, cold weather engine fouling should be reduced. The effects on other sources of unburned hydrocarbon emissions and on NOx emissions are ambiguous. An analysis of corrosive and abrasive wear mechanisms shows that engine wear need not be a concern with variable speed operation. The analysis also discusses factors affecting variable speed operation of diesel generator sets larger than those tested in this study. |
