The study compared the hybrid “dolphin class” tug to a conventional version powered by two 1,902 kW CAT 3512C main engines and two 195 kW John Deere 6081 auxiliary engines.

The research project was conducted in three stages:

• The first stage involved development of a data-logging system capable of simultaneously monitoring and reporting the status of the power sources on each tug. This system was installed for a period of one month on each tug. Gigabytes of data were analyzed to determine the weighing factors, i.e., the fraction of time spent by the tug in the six discrete operating modes shore power, dock, transit, ship assist and barge move. Further engine histograms for all eight engines at these operating modes were established.

  A small sample of activity data (~1.5 days) was collected on the hybrid tug operating without batteries to quantify the effects of the diesel electric drive train versus batteries on the total emission reductions.

• The second stage of the research was a two-phase emissions testing program that focused on establishing an emissions profiles of the diesel engines. Emissions of criteria pollutants—nitrogen oxide, carbon monoxide, particulate matter—and the greenhouse gas carbon dioxide were measured based on the ISO 8178 protocols.

• The final stage of the research involved combining the activity and emissions data to calculate the overall in-use emissions from each tug and the emission reductions with the hybrid technology.

The hybrid tug Carolyn Dorothy. Click to enlarge.

The study found that the diesel electric drive train on the hybrid tug that allows the use of auxiliary power for propulsion was the primary cause for the overall in-use emission reductions as opposed to the energy storage device (batteries). The transit operating mode—in which the hybrid tug was powered by one or two auxiliary engines and batteries while the conventional tug used one auxiliary and two main engines—was the most significant contributor to the overall emission reductions.

Among the other findings of the report was that the hybrid system increased the average operating load on the auxiliary engine from 12% to 34%. However, the average load on the main engines was found to be only 12% of the maximum rating. These engines are still operating in inefficient zone suggesting the need for a larger energy storage system and smaller main engines in the next generation of hybrid tugs.

The study also found that further improvements will result when the plug-in version is operative.

ARB called the findings significant due to the heavy impact port pollution—caused largely by diesel-powered ship engines and, to a lesser extent, smaller harbor craft such as tugboats—has on regional air quality. The twin Ports of Los Angeles and Long Beach are the largest contributors to air pollution in the South Coast Basin. Diesel pollution in particular can have devastating health impacts, including cancer and a host of respiratory and cardiovascular ailments.

Resources

• Evaluating Emission Benefits of a Hybrid Tug Boat