Measuring Ships’ Energy Efficiency

During December 2012 Transport & Environment NGO released a paper investigating opportunities to establish an energy efficiency benchmark measurement. The paper also makes references to different metrics and proposals already under consideration at the IMO or in the EU and identifies possible options in the current EU discussions on monitoring, reporting and verification.

Fuel consumption. The most direct measure of the energy use of a ship is its fuel consumption which can be established by different methods:

by calculating consumption on the basis of oil record books / bunker delivery notes,
by using on-board fuel flow meters for the main and the auxiliary engines,
by sounding the tanks, etc. Accuracy depends highly on the type of equipment used.

While modern fuel flow meters and tank sounding systems (radar/electric) can be highly precise even given the wide range of fuel characteristics (e.g. fuel density, viscosity, etc.), methods relying solely on written documentation (oil record books and bunker delivery notes) are more questionable.

CO2 emissions. CO2 emissions are directly proportional to fuel consumption. The amount of CO2 emitted by a ship is generally calculated on the basis of its fuel consumption by applying an emission factor. Fuel mass to CO2 mass conversion factors have been established at the IMO for marine diesel, light and heavy fuel oils, liquefied petroleum and natural gas. As a result, the formula to calculate CO2 emissions is very simple: fuel consumption multiplied by carbon conversion.

Energy efficiency. The energy efficiency of a ship depends not only on its fuel consumption but also on the amount of transport work undertaken and the level and intensity of activities, etc. Indeed, efficiency is defined as being “the difference between the amount of energy that is put into a machine in the form of fuel, effort, etc. and the amount that comes out of it in the form of movement” (Cambridge Dictionary). Limiting the monitoring requirements to fuel consumption only would inform one part of the equation (the ‘input’, i.e. the amount of energy used) without considering the output produced by the combustion, which can be calculated in the case of shipping in terms of distance sailed, available capacity, cargo carried, ship speed, etc.

Technical efficiency – the EEDI. The EEDI calculation reflects the theoretical design efficiency of a newbuild ship and provides an estimate of CO2 emissions per capacity-mile. Its calculation is based on assumptions regarding the specific fuel consumption of the engines (in g/kWh) compared to the power installed on the ship. For new ships, the EEDI represents a measure of the “design” efficiency of the ship, but it does not give any indication concerning its operational efficiency. In this respect, two sister ships with the same EEDI may have different emissions depending on their load factor, sea conditions and the way the ship is operated. The EEDI is a static figure, unless the ship undergoes a major conversion. The full EEDI equation is contained in the IMO circular MEPC.1/Circ.681.

Operational efficiency – the EEOI. The IMO has also developed the Energy Efficiency Operational Indicator (EEOI), an indicator that provides information concerning the efficiency of the ship in operations. The calculation is based on an individual vessel’s fuel consumption and data on the achieved transport work (e.g. cargo mass, number of passengers carried, etc.) resulting in a figure of CO2 emissions per ton nautical mile. The full EEOI equation is contained in the IMO circular MEPC.1/Circ.684. The application of the EEOI remains nonmandatory but the EEOI has also now been included in the Ship Energy Efficiency Management Plan (SEEMP), as a possible index to verify and measure the SEEMP effectiveness.

Existing Vessel Design Index (EVDI). In an attempt to develop a single efficiency metric, the Carbon War Room, together with Rightship, developed the Existing Vessel Design Index (EVDI). The EVDI is based on IMO’s EEDI methodology and can be calculated directly from the IHS Fairplay database (the IMO’s EEDI reference line input database). The EVDI formula therefore replicates the EEDI formula (although Rightship has not divulged certain key elements of its calculations). The main and significant difference between the EEDI and the EVDI concerns data collection. EEDI data is collected from the design data of new ships via classification societies at the time of certification, while EVDI data for existing ships is extracted from whatever data is available (such as IHS Fairplay, ship yards, classification societies) and may eventually be verified/corrected by ship-owners or ship operators if they so wish.

It is interesting to note that the EEDI, EEOI and EVDI metrics are expressed in grams of CO2 per ton nautical mile (g/tnm). They all refer to fuel consumption (or CO2 emissions) and in one way or the other also relate to distance and capacity. However, the similarities stop here and there are major differences in the way EEDI, EEOI or EVDI estimate or calculate fuel consumption, distance, and capacity.

As a result, EEDI and EVDI metrics calculate average / theoretical CO2 emissions per ton capacity per nautical mile, while the EEOI measures real CO2 emissions per ton transported per nautical mile. The equivalent comparison for road vehicles would be the rated fuel efficiency of new cars in gCO2/km (test cycle) versus actual on-road performance.

EU MRV. The European Commission recently decided to step back from introducing a proposal to directly regulate CO2 from European shipping and will instead, as a first step, establish an emissions monitoring, reporting and verification scheme (MRV). The precise requirements to be contained in the EU MRV scheme are not yet known and the legislative proposal is not expected before the first quarter of 2013. The following table explores 5 possible efficiency metrics that the EU could adopt and assesses each of them as well as the type and amount of data that would be required.