Evaluating Performance of Energy Saving Devices (ESDs) for Energy Efficiency Existing Ship Index (EEXI)

With increasingly stringent regulations for greenhouse gas emissions imposed by the International Maritime Organization (IMO), the shipowners are looking for performance improvements under every rock. A question that eventually comes to mind to every ship owner is: How do I assess different Energy Saving Devices (ESDs) available on the market? There are too many to choose from and not every ESD is suitable for every type of ship and hull form. In this article, we give an overview of what exactly is the Energy Efficiency Existing Ship Index (EEXI), what kind of Energy Saving Devices (ESDs) are there in the market and how can Computational Fluid Dynamics (CFD) help assessing their performance before they are installed.

What is the Energy Efficiency Existing Ship Index (EEXI)?

The Energy Efficiency Existing Ship Index (EEXI) is the most important technical measure to increase the fuel efficiency of the existing merchant fleet that International Maritime Organization (IMO) introduced in June 2021. The EEXI can be thought of as an extenstion of the Energy Efficiency Design Index (EEDI), which only affected the newly built ships and was introduced to promote the use of more energy efficient technologies in the shipping industry. The EEDI, and therefore EEXI, are defined as the ratio of CO₂ emissions and ship’s cargo capacity multiplied with the reference speed. Such definition aims at quantifying the negative impact of the shipping industry on the environment with respect to direct benefits to the whole society, according to an IMO publication.

How to improve the Energy Efficiency Existing Ship Index (EEXI)?

There are a couple of options for improving the Energy Efficiency Existing Ship Index (EEXI):

• Using alternative fuels with lower carbon content,
• Installing active systems that decrease ship’s resistance, such as the air lubrication system,
• Applying high-performance coatings that decrease the viscous resistance of the ship,
• Appending the hull with different Energy Saving Devices (ESDs) that most often work by increasing the propulsive efficiency.

Althought there are clear technological trends towards alternative fuels such as hydrogen, ammonia, ethanol and others, the practical and economical issues when installing such systems on existing fleet are tremendous. On the other hand, installing active air lubrication systems does not rely on global infrastructure. In one example, such a system from Silverstream had yielded 5% decrease in fuel consumption on Grimaldi’s Eco Valencia. However, installing an air lubrication system on an existing ship is much more challenging than installing it on a newly built ship. Applying a high-performance coating and appending the hull with ESDs is the most technically and commercially viable solution because the ship is periodically dry-docked anyhow and their installation does not require modification of the general arrangement. All the ESDs aim to improve the efficiency of the hull-propeller interaction by equalizing the propeller wake field and/or adding a swirl component to the propeller inflow. There are a dozen of such devices in the market, such as the Schneekluth Duct, Mevis Duct, Pre-Swirl Stator, Mitsubishi Reactor Fin, Rudder Thrust Fins, Propeller Boss Cap Fins (PBCF), just to name a few. A good overview of some of these devices can be found here. Since there are dozens of ESDs in the market, they often leave shipowners puzzled on how to choose the right one for their ship.

Using Computational Fluid Dynamics (CFD) to choose the right Energy Saving Device (ESD)

With the increasing cost effectiveness and availability of large computational resources in modern day, Computational Fluid Dynamics (CFD) gained significant traction and recognition in the marine industry. The most advanced method for solving practical ship hydrodynamics problems is the Reynolds Averaged Navier-Stokes (RANS) method combined with an appropriate free surface model. A CFD simulation starts with the meshing process, where the space in the vicinity of the ship is usually described by finite volumes (i.e. cells). The non-linear fluid flow equations are then solved, usually on a High Performance Computing (HPC) server with dozens or thousands of cores, depending on the problem at hand.

To correctly capture the hydrodynamics of the Energy Saving Device (ESD), the simulations must be performed in full-scale and they must include a rotating, discretized propeller. Only then will the CFD simulations produce accurate results of different kinds of ESDs appended to the same hull. Such virtual testing helps shipowners reduce risk when deciding which ESD to install.

Evaluating different Energy Saving Devices (ESDs) for a Panamax bulk carrier

In collaboration with Atlantska Plovidba Inc. and Ocean Pro Marine Engineers Ltd., Cloud Towing Tank performed a study for 4 Panamax bulk carriers. The study included two types of Energy Saving Devices (ESDs). First, a set of self-propulsion simulations had been carried out to find the propulsion point for the design speed and load condition of the bare hull that represents the current state of the vessel. The procedure has been repeated two times for the two ESDs. Figure below shows a snapshot of streamlines and the dynamic pressure field for one of the ESDs.

Appending the hull with the first ESD resulted in 0.8% increase in the propeller RPM and 5.2% increase in the required power. The second ESD yielded 1.8% decrease in the propeller RPM and 2.2% decrease in the required power*. The results translate to increased fuel consumption of 1.3 metric tonnes per day for the first ESD, and decreased fuel consumption of 0.5 metric tonnes per day for the second ESD.

Conclusion

The study presented here shows how  shipowners can benefit from a preliminary Computational Fluid Dynamics (CFD) analysis before deciding to install a particular Energy Saving Device (ESD) to improve the Energy Efficiency Existing Ship Index (EEXI). CFD simulations offer a cost-effective way to reduce the risk when deciding on which ESD to install. Since not all hull forms are the same, it is rational that shipowners want to evaluate different solutions for different ships. The fact that one of the ESDs evaluated in this study increased the required power and  fuel consumption, emphasizes the importance of CFD in the preliminary feasibility study.

*In the study, no attempt to optimize the position and the shape of the ESD has been made. The geometry of each ESD represents an approximation based on the practical experience with such devices and it is reasonable to assume that the distributors of these ESDs can achieve better results based on their experience and proprietary information.