Diesel Generator Set Optimization for Marine Transportation
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It is well known that for a given use profile, the number of specific motors and generators on a ship can be optimized. The formal optimization can consider such factors as purchase cost, fuel economy, maintenance logistics, size, and weight. In addition, it has been shown that the choice of granularity, i.e., the number and power rating of the component systems, affects the maximum fuel efficiency that can be obtained for a given use profile. Once the granularity has been selected, the control system can then be set to achieve maximum efficiency with the set of installed units.
A fundamental reason for this dependence on granularity is summarized in Figure 1, which is provided by Caterpillar on their website.
This figure summarizes the specific fuel consumption as a fraction of its full power for diesel generator sets from a particular manufacturer, Caterpillar. The general nature of the curves is driven by fundamental physics and so they are not particularly different for other manufacturers. The key feature for this discussion is that lightly loaded machines are much less efficient than are more heavily loaded units. Moreover, the increase in specific fuel consumption with a decrease in load tends to be much larger below about half load than above half load. Thus, it is obvious that some granularity should provide a benefit in fuel economy.
Specific Investigation for 100 m Class Vessel
To present the effects of diesel generator set selection, a 100 m class passenger vessel was selected. A typical mission profile was identified which included varying power levels needed for the vessel’s anticipated modes of operation. Since operating modes and power levels will vary with ship type and use, diesel generator set granularity must be assessed on a case-by-case basis.
This analysis was limited to available Caterpillar data and focused on the seven diesel generator systems shown in Table 1. With this set of choices, there are 470 combinations that will yield power levels between 5200 kW and 6200 kW, which is the power range of interest. For each of these possible combinations, the resultant power, volume, and mass were tabulated. In addition, for each combination, the annual fuel consumption was estimated based on an anticipated ship use profile.
It is likely that the most important observation from this data is that for the 470 cases, the maximum fuel consumption is about 8.5% larger than the lowest fuel consumption. While this difference may be large enough to be significant in driving decisions, it must be recognized that the annual use profile is estimated, and not likely known to this precision. Variations in the use profile could change the annual fuel consumption of many combinations by several percent leading to a different order.
Table 2 highlights the fact that the power, volume, mass, and annual fuel consumption are not minimized simultaneously. The table shows which combinations of diesel generator sets yield the lowest value of:
- 1. annual fuel consumption
- 2. volume
- 3. mass
- 4. installed power