Response to Global Greenhouse Gas Emission Regulations: Strategic Choices and Future Prospects of Ship Energy-Saving Technologies

Current climate change is progressing much more rapidly than previously predicted by the international community, affecting various industries worldwide. The maritime industry, in particular, is undergoing significant changes to meet the growing societal demand for decarbonization. As a result, international regulations on greenhouse gas emissions and air pollutants are tightening, with active discussions focused on the development of eco-friendly ship design technologies, the establishment of integrated digital platforms to enhance operational efficiency, and the use of alternative eco-friendly fuels.

Recently, the International Maritime Organization (IMO) announced a firm target to achieve 'Net-Zero' greenhouse gas emissions by 2050 for the international shipping sector. This sets an important benchmark for the sustainable development of the shipping industry and indicates that international carbon emission regulations will likely become even stricter in the future. These changes provide shipping companies with the opportunity to reconsider their existing operational practices and adopt new technologies.

Additionally, as part of the outcome of the 83rd MEPC session in April 2025, the IMO adopted the 'Greenhouse Gas Fuel Intensity (GHG Fuel Intensity, GFI)' regulation as a mid-term measure for greenhouse gas reduction. This provides shipping companies with the opportunity to devise swift and effective responses. At the same time, there is a deep focus on introducing energy-saving technologies that improve actual fuel efficiency during operation. While there is an extensive range of vessel energy-saving technologies currently in development, the practical response strategies to regulatory frameworks like EEDI (Applicable to new ships) and EEXI (Applicable to existing ships), enforced by the MEPC for international navigational vessels, remain quite limited. These technologies are outlined in MEPC.1/Circ.896, and shipping companies need to consider various constraints when applying them to their fleets.

This article aims to provide useful information that can help shipping companies strategically adopt energy-saving technologies to achieve successful outcomes. It will present valuable insights from various perspectives, including the types of energy-saving technologies, energy-saving mechanisms, the effectiveness of responding to greenhouse gas regulations through technology , and future economic viability projections.

Categorization of Energy-Saving Technologies under MEPC

At its 77th session in November 2021, the MEPC approved the 2021 Guidelines (“2021 Guidance on Treatment of Innovative Energy Efficiency Technologies for Calculation and Verification of the attained EEDI and EEXI”) for the calculation and verification of the attained values for EEDI/EEXI. Consequently, the MEPC distributed document MEPC.1/Circ.896 to the administrations of member states, industry stakeholders, relevant shipping organizations, shipping companies, and other interested parties.

The major roles of the innovative energy-saving technologies categorized under MEPC.1/Circ.896 for the calculation of EEDI/EEXI are summarized as follows:

· Category A: Technologies that shift the power-speed curve, altering the combination of propulsion power (Pp) and
reference speed (Vref)
-This category primarily includes Propulsion Improvement Devices (PID), which achieve energy savings mainly through flow control. It also encompasses technologies that directly reduce viscous resistance, such as fins, low-friction coatings, and air resistance reduction techniques through superstructure optimization

· Category B: Technologies that reduce Pp at a fixed Vref without generating electricity
-Category B-1: Technologies that can be used regardless of weather conditions during vessel operation, with an availability factor (feff) set at 1.0 (applicable at all times)
-Category B-2: Technologies that can only be utilized at maximum output under limited wind conditions, with the availability factor (feff) applied at less than 1.0 (weather-dependent)

· Category C: Technologies that generate electricity, reducing energy consumption from auxiliary engines, with the reduced energy being calculated independently.
-Category C-1: Technologies that can be used regardless of weather conditions during vessel operation, with an availability factor (feff) set at 1.0 (applicable at all times)
-Category C-2: Technologies that can only be utilized at maximum output under limited conditions (e.g., sunlight), with the availability factor (feff) applied at less than 1.0 (weather-dependent)

Energy-saving technologies classified as PID (Propulsion Improvement Device) in Category A can be further categorized based on their installation location. They can be classified into: energy-saving technologies installed forward of the propeller (Pre-EET), high-efficiency propellers, and energy-saving technologies installed aft of the propeller (Post-EET).

Energy-Saving Technologies and EEDI/EEXI

The IMO agreed to distribute MEPC.1/Circ.815 (“2021 Guidance on Treatment of Innovative Energy Efficiency Technologies for Calculation and Verification of the attained EEDI”) through the MEPC 65th session (2013).

Subsequently, prior to the enforcement of the Energy Efficiency Existing Ship Index (EEXI), which became effective on January 1, 2023, as a short-term measure to reduce greenhouse gas emissions from existing vessels, the International Maritime Organization (IMO) amended MEPC.1/Circ.815 during the 77th session of the Marine Environment Protection Committee (MEPC 77th, 2021) to include provisions related to EEXI. Additionally, IMO distributed MEPC.1/Circ.896 (“2021 Guidance on Treatment of Innovative Energy Efficiency Technologies for Calculation and Verification of the Attained Energy Efficiency Design Index (EEDI) and EEXI”) to member states and stakeholders to facilitate implementation.

For the EEDI, to apply energy-saving technologies and obtain approval from a Recognized Organization (RO), the guidelines of IACS Procedural Requirement No.38-Rev.4 must be followed. This requirement details the procedures for all activities in which the classification society is involved during the inspection and certification of EEDI, as per the regulations 5, 6, 7, 8, and 9 of MARPOL Annex VI. Conversely, it is important to note that the application of EEXI is governed differently by Resolution MEPC.335(76), MEPC.350(78), MEPC.351(78), IACS Recommendation No.172-Rev.1 (revised in April 2024), and IACS Recommendation No.173.

Considerations for the Adoption of Category A Technologies

Energy-saving technologies falling under Category A primarily refer to those that reduce the vessel's resistance or improve propulsion efficiency, thereby increasing the reference speed (Vref) from the power (PME) set by EEDI/EEXI regulations.

These technologies theoretically reduce the actual fuel consumption based on the power reduction derived from model tests, sea trials, or Computational Fluid Dynamics (CFD). However, the calculation of the attained EEDI/EEXI is based on a predefined formula that incorporates power (PME) and the increased reference speed (Vref). As a result, the power reduction effect of energy-saving technologies is not directly reflected in the improvements of the attained EEDI/EEXI, which is a limitation. Generally, the reference speed (Vref) is approximated to be proportional to the cube root of power. For example, if Category A energy-saving technologies are applied to reduce the brake power by 5% on a vessel, the theoretical improvement effect on the attained EEDI/EEXI may be calculated to be approximately 1.7%.

The figure below briefly illustrates the principles and energy-saving mechanisms of the representative Propulsion Improvement Device (PID) energy-saving technologies that shipping companies most commonly selected during recent voluntary re-verifications of EEXI.

Notably, the technologies most commonly selected by shipping companies during recent retrofits for various vessel types, including Bulk Carriers, Tankers, LNG Carriers, and Container Carriers, are as follows: (A1) Vortex Generating Fins, (A2) Duct with Fins/Pre-swirling Stators, (A3) Hub with Fins, (A4) Twisted Rudder with Bulb, and (A5) Wind/Air Resistance Reduction Cap (primarily applied to Container Carriers).

The anticipated approximate power savings (%), EEXI improvement (%), CAPEX (USD), and retrofit duration (from design to installation) for each individual technology are estimated and illustrated in the following figure. Furthermore, the projected overall power reduction and EEXI enhancement when integrating the energy efficiency technologies corresponding to (A1) + (A2) + (A3) + (A4) across four representative vessels are also depicted.

It is important to recognize that the specific numerical values are inherently variable, depending on the actual ship speed-power curve characteristics and the manufacturer of the energy-saving technologies. Analysis of the trend indicates that lower-speed, fuller vessels tend to realize greater power savings from energy efficiency measures, while, conversely, the effects diminish for high-speed, slender, and longer ships.

Considerations for the Adoption of Category B Technologies

Energy-saving technologies that fall under Category B are positioned as independent terms on the far-right side of the numerator in the EEDI/EEXI calculation formula, as can be observed below.

The technology serves to reduce the vessel's fuel consumption based on the following principles and energy-saving mechanisms.

In contrast to energy-saving technologies categorized under Category A, the reductions in main engine power attributable to the air lubrication system (B-1) and wind-assisted propulsion system (B-2), as specified in the guidelines of MEPC.1/Circ.896, can be directly integrated into the respective separate components in the numerator of the EEDI/EEXI calculation. This method ensures that the estimated reduction in main engine power accurately corresponds to the actual improvement in EEDI/EEXI, thereby providing a precise representation of these technologies' effectiveness.

The air lubrication system (B-1) is typically applied to vessels with a flat hull bottom and low draft (e.g., large container ships, LNG carriers). This is advantageous as the flat bottom helps maintain an air lubrication layer formed by fine air bubbles injected from the compressor at high pressure. Additionally, a lower draft allows for a reduction in the capacity of the air compressor, leading to greater net fuel savings (the figure representing fuel savings from the main engine due to the air lubrication system is calculated excluding the fuel consumption of the auxiliary engine that drives the air lubrication system).

Wind-assisted propulsion systems (B-2), such as Flettner Rotors and Sails, utilize wind energy as a renewable resource to achieve energy savings. These systems are primarily applied to vessels with simple superstructure geometries and smaller surface areas, such as bulk carriers and tankers. They function by converting aerodynamic forces from the wind into auxiliary propulsion, thereby reducing fuel consumption of the main engine. The anticipated fuel savings from main engine reductions through both B-1 and B-2 systems are approximately 5–9%. As a result, these systems are garnering attention as an effective and prominent strategy within the current newbuilding market for meeting EEDI Phase III requirements.

For vessel types such as bulk carriers, tankers, LNG carriers, and container carriers, the approximate expected power savings (%), EEDI/EEXI improvement effects (%), CAPEX (USD), and retrofit duration (from design to installation) when applying the Air Lubrication System (B1) and Flettner Rotor(s) (B2) technologies have been assessed, as these technologies have been frequently selected by shipping companies in recent newbuilding or retrofit projects. However, it is important to note that specific figures may vary based on actual operating conditions and the manufacturers involved.

Considerations for the Adoption of Category C Technologies

Energy-saving technologies in Category C refer to innovative systems capable of generating electricity, which the vessel's generators are responsible for supplying. In the EEDI/EEXI calculation formula.

These technologies are represented as independent terms in the numerator and serve to reduce the vessel's fuel consumption based on the following principles and energy-saving mechanisms.

Energy-saving technologies classified under Category C can determine their power generation in accordance with the guidelines of MEPC.1/Circ.896 and incorporate this value into a separate term in the numerator of the EEDI/EEXI calculation formula. This effectively allows the technology to be reflected as a direct reduction in auxiliary engine power, thereby providing the advantage of accurately capturing the power-saving effects in EEDI/EEXI improvements, similar to the approach used for technologies classified under Category B.

The waste heat recovery system (C-1) and solar cells (C-2) are known to have significantly high capital expenditures (CAPEX), with the market indicating that the CAPEX for the waste heat recovery system accounts for approximately 10% of the newbuilding costs. However, both C-1 and C-2 have seen very few applications on large vessels, leading some stakeholders to raise concerns about economic viability and operational safety issues.

Despite these challenges, technologies within Category C hold significant potential to enhance the overall energy efficiency of vessel operations. As the shipbuilding industry continues to evolve and regulatory pressures for decarbonization from the IMO increase, advancements in waste heat recovery and solar technology could provide substantial long-term savings for shipping companies. Furthermore, ongoing research and development efforts may address current limitations, ultimately improving both the operational feasibility and economic justification for broader adoption in the maritime sector.

Energy-Saving Technologies and CII

The Carbon Intensity Indicator (CII) is a short-term measure implemented by the IMO to reduce greenhouse gas emissions from existing ships, complementing the EEXI as a technical measure. Following the approval of amendments to MARPOL Annex VI at MEPC 75th session, the final amendments including technical guidelines were adopted at MEPC 76th session, establishing both EEXI and CII as mandatory requirements effective from January 1, 2023.

The technical guidelines and key considerations established to facilitate the effective implementation of CII, adopted during the 76th and 78th sessions of the MEPC, are summarized as follows:

· Resolution MEPC.336(76)
CII Reference Lines Guidelines (G1) → Application and calculation methods for individual vessels' attained CII (AER or cgDIST)

· Resolution MEPC.337(76)
CII Reference Lines Guidelines (G2) → Methods for establishing reference line calculations and reference line calculations by vessel type

· Resolution MEPC.338(76)
CII Reduction Factor Guidelines (G3) → Methods for determining CII reduction factors and reduction rates for 2023~2030

· Resolution MEPC.339(76)
CII Rating Guidelines (G4) → Methods for assigning CII ratings to existing vessels

· Resolution MEPC.355(78):
Guidelines for CII Correction Factors and Voyage Adjustments for CII Calculations (G5) → Methods for applying specific correction factors (such as cargo retention, handling systems, etc.) and voyage exclusions (navigating in ice-covered areas, severe sea conditions, or long-term anchorage for safety reasons) in the determination of the attained CII.

These technical guidelines are set to be revised after decisions or agreements reached at the MEPC 83rd session, with most revisions expected to take place after 2026.

· Determined CII reduction rates for 2027~2030 through the MEPC 83rd session

· Fuel consumption during port waiting time and idle time is usually incurred regardless of the owner's intent, so it has been agreed to exclude the fuel consumption used while at anchor from the calculation of the attained CII and the CII reference lines.

· It has been agreed to further discuss the review of IMO DCS data, the review of CII indicator units (the scope of fuel consumption excluding anchoring, port waiting, and docking), the recalculation of reference lines (amendment of Guidelines G2), and the potential for amendments to other IMO documents in the second phase review to be conducted after 2026.

It is predicted that the timing for implementing the agreed amendments related to CII through the MEPC 83rd session (2025) will be reflected after the second phase review to be conducted after 2026, with revisions to G1~G5 guidelines. However, to date, the attained CII has been calculated based on the guidelines established by MEPC 76th session (2021) and the existing IMO DCS data collection format. The calculation formula is as follows:

When calculating the attained CII using the vessel's capacity based on deadweight (DWT), it is referred to as AER. When calculated using gross tonnage (GT), it is referred to as cgDIST.

To calculate the required annual operational CII, the reference CII for the target vessel must be calculated first, and the formula for determining the reference CII is as follows:

Here, “a” and “c” are parameters derived from the attained CII and the capacities of individual vessels based on the IMO DCS statistics data collected in 2019, with values categorized by vessel type and size as follows:

Once the reference CII is determined, the required annual operational CII is calculated by finally incorporating the reduction rate (G3) as follows:

“Z” is the annual CII reduction factor compared to the 2019 baseline, applied uniformly to all vessels regardless of type and size. The finally confirmed reduction rates for the period from 2023 to 2030 at the MEPC 83rd session are as follows:

CII ratings range from A to E, with a total of five categories, and the boundary for each rating is determined based on the required CII as follows:

“exp(dx)” represents the exponent of the dd vector, indicating the direction and distance from the required CII, with the values categorized by vessel type and size as follows:

Recently, shipping companies have adopted slow steaming strategies based on engine power (or shaft power) limitations as the most practical and cost-effective means of complying with the EEXI and CII regulations for existing vessels. The figure below presents two years of recorded IMO DCS data collected from the same vessel (with the same EEXI) operating on identical routes, illustrating the effectiveness of slow steaming in enhancing the attained CII.

However, limiting engine power (or shaft power) faces several challenges, including reduced speed under charter agreements, a consequent decrease in competitiveness in the charter market, and inherent limitations that may not be sustainable in light of the anticipated strengthening of international greenhouse gas emission regulations. Therefore, shipping companies may consider adopting energy-saving technologies as a cost-effective short-term alternative, serving as another technical measure to address international greenhouse gas emission regulations.

Historical cases of EEDI/EEXI approval demonstrate that energy-saving technologies can directly reduce a vessel's resistance and propulsion losses through various methods, including simple appendage-type devices or more complex electric control systems. Additionally, these technologies can leverage renewable energy sources like wind to generate additional thrust, thereby decreasing the vessel's fuel consumption (CO2 emissions) and garnering attention as effective responses to technical regulations such as EEDI/EEXI.

In contrast, CII is determined based on various data collected from the actual annual operations of existing vessels, unlike EEDI/EEXI. To improve the attained values and ratings of CII, it is necessary to integrate optimal operational strategies and appropriate maintenance methods alongside design-oriented responses like energy-saving technologies. Therefore, a comprehensive effort is required to minimize fuel consumption (CO2 emissions) as measured and reported within the IMO DCS framework by utilizing all these approaches.

For example, as illustrated in the previous figure, even when the same energy-saving technology is implemented and the vessel has attained the same EEXI, it can be observed that the attained CII significantly decreased in 2020 compared to 2019. The primary reason is that while the number of operational days and mooring periods were similar in both periods, the total operational distance in 2020 was reduced by approximately 3.5% due to slow steaming, while the total fuel consumption (CO2 emissions) decreased by about 6.6%.

As shown in this case, to reduce the fuel consumption (CO2 emissions) measured within the IMO DCS framework, a thorough analysis of the vessel's historical operational pattern data must precede. Additionally, effective data analysis requires systematic collection of external environmental data, including the vessel’s mechanical operating state, weather, and sea state. This underscores the necessity for a comprehensive measurement and monitoring system (smart platform) capable of managing this data.

Such a measurement system must be able to record real-time changes in the mechanical states of the vessel, including environmental factors that significantly affect the vessel's resistance characteristics, such as wind, waves, and currents during actual operations, as well as parameters like draft, trim, main engine output, RPM, and speed. Based on this information, shipping companies will be able to accurately diagnose the necessary measures to improve the targeted CII rating (e.g., operational aspects, design-oriented aspects like the introduction of energy-saving technologies, and maintenance aspects), which will be an indispensable prerequisite for establishing customized CII response strategies.

The Role of Energy-Saving Technologies in Response to Greenhouse Gas Fuel Intensity (GFI)

One of the critical issues in the international community's response to greenhouse gas regulations is the transition from fossil fuels to low carbon fuels for shipping. This transition to low carbon fuels is expected to gain momentum after the implementation of the IMO’s mid-term measures in 2027.

The GFI regulation, which is a key component of the IMO's mid-term measures aimed at reducing greenhouse gases and approved during the 83rd MEPC session (April 2025), is set to be newly added to the MARPOL Annex VI amendment. It is scheduled to be adopted during the special session of the MEPC set for October 2025 and will enter into force internationally on March 1, 2027.

Additionally, the MEPC 83 session agreed to hold an Inter-Session Working Group (ISWG) meeting immediately after the special session and just before the 84th MEPC session to develop various guidelines to support the implementation of the IMO’s mid-term measures (including calculation guidelines for attained GFI determination, compliance measures, and guidelines for compensating ships using Zero or Near-Zero fuels or technologies).

The main elements^* of the Greenhouse Gas Fuel Intensity (GFI) regulation are outlined below:

* KR IMO News Flash (MEPC 83)