“Greener” yellow ferries

A recent feasibility study on CO2 and energy reduction performed by SSPA on behalf of the Swedish Transport Administration (STA) and the department for road ferries found that the reduction needs to focus on the five to ten ferry routes with the largest emissions if the goals are to be successfully met. The overall environmental goals to be met were as follows: by 2020, a 15% reduction in CO2 emissions; by 2025, a further 15% reduction in CO2 emissions to a total of 30%; and by 2025, a 10% reduction in energy consumption. All of the above reductions are to take 2015 as the base year. The goals were “trickled down” as part of the STA’s overall goals set by the Ministry of Enterprise and Innovation and the Ministry of the Environment and Energy. The project owner at the STA was the national coordinator for climate mitigation. The reduction was to be evaluated for the operation of STA Road Ferries as a whole.

The task of the research study

The task given to SSPA can be summarised as follows:

  • Analyse the ferry routes to identify and describe trends and similarities so as to provide a base for further analysis.
  • Identify and cost various technical and operational measures to reduce the CO2 emissions while keeping the quality of service intact.
  • Devise and test different strategies of how to combine the measures identified so that the goals are reached for 2020 and 2025, respectively.
  • Cost the successful strategies in both terms of initial investment cost but also as a lifecycle cost (LCC) evaluated over a longer period of time.

Among the technical measures evaluated were the following: the propulsion system changed to an electric hybrid variant, free-sailing ferries becoming cable ferries, the addition of heat recovery systems and the use of auto mooring systems. With regard to operational measures, different fuels were evaluated – methanol, ethanol, shore supplied electricity, HVO and natural gas were among the fuels studied.

Differences between routes and ferries

Initial efforts to analyse the various ferry routes found that these could be divided into four major types with respect to energy consumption, namely: cable ferries powered from shore with electricity, cable ferries powered with on-board diesel engines, larger free sailing ferries and smaller free sailing ferries. When plotted against the amount of car deck lane metres on each ferry, four distinctive groups of ferries emerged. These were as follows: free-sailing ferries with a capacity of over 250 lane metres, free sailing ferries with a capacity of under 250 lane metres, cable ferries with on-board power generation and cable ferries driven with electrical energy from fixed shore cables. In the next step, each of the four groups was analysed once again, but this time the calculated fuel consumption in relation to the transport work produced was plotted as a histogram. The figure shows this for one of the identified groups.

As can be seen, a typical relative fuel consumption for the free sailing smaller ferries is in the range of 105 to 175 millilitres/(lane metre x nautical mile). For the cable ferries, the range is 40 to 75 millilitres/(lane metre x nautical mile). “Typical” in this case was evaluated as the range between 30% and 70% of the value of the cumulative distribution.

This large variation is an expression of the highly varying ferry sizes, power needs and large differences in the length of the ferry routes. In effect, it means that any evaluation of the measures has to be made for each combination of ferry and route – no generally valid assumptions can be made as to which is the most cost-effective measure on a specific route or ferry. This means that each of the suggested measures would need to be tested on each ferry route – with roughly 65 ferries this would be several hundred combinations at the very least. A rather daunting task. Luckily, as is shown in the next figure that plots the histogram for the sorted energy consumption for all of the ferry routes it is apparent that the energy consumption of the different routes varies greatly. This simplifies the task greatly.

From the graph it can be seen that the route with the largest singular emission contributes 17% to the total emissions, and that the six largest routes contribute 53% to the emissions between them. This means that a successful reduction to the set goals needs to focus on the routes with the largest emissions. It also means that any measures taken on the routes with smaller emissions should focus on those that result in cost savings since these savings could be used to finance measures taken on the routes with larger emissions.

Strategies of reduction and overview of findings

Using the above described data as well as knowledge of the possible technical and operational measures needed to reach the environmental goals, three strategies were tried:

  • purely technical solutions;
  • different fuels;
  • combinations of technical measures and changes to the fuel used.

Only strategies 1 and 3 turned out to fulfil both the requirements of energy saving and CO2 emission reduction. Changing the fuel cannot in itself save the required 10% on energy consumption. It was found that under the used parametrisation of costs (fuels, battery prices, etc.), strategy 3 was the most economical in LCC terms. If the initial investment cost was the most important factor, strategy 2 of solely changing the fuel was the better alternative – but this cannot give the required reduction in energy use.

STA Road Ferries in short

STA Road Ferries is the largest ship owner in Sweden. Its operations for the year 2014 can be summarised as follows:

  • 41 ferry routes operated by 69 ferries (some of which are backup ferries).
  • Roughly 1,118,000 crossings sailing a distance of 682,000 nautical miles, or just over 45 times the circumreference of the earth.
  • 22 million passengers and total number of vehicles transported equivalent to 14.7 million cars.
  • 12.3 million litres of diesel fuel MK1 were consumed to produce this transport work, which in turn produced 37 million tonnes of CO2 emissions.
  • 5 million kWh of electrical energy were also consumed, producing 482 tonnes of CO2 emissions.
  • 1 ship yard in operation.

Photos and illustrations

One of STA Road Ferries’ “yellow ferries”, M/S Saturnus, on the Gullmarsleden route on the west coast of Sweden. Photo: David Eckerdal.

Fuel consumption relative to the transport work performed.

Cumulative distribution of CO2 emissions for the 40 ferry routes for the year 2014.