Hybridization (plug-in or conventional)

On a full-electric ship, all the power, for both propulsion and auxiliaries, comes from batteries. A plug-in hybrid ship, similar to a plug-in hybrid car, is able to charge its batteries using shore power and has a conventional engine in addition. The ship can operate on batteries alone on specific parts of the route, when manoeuvring in port, during stand-by operations. A conventional hybrid ship uses batteries to increase its engine performance and does not use shore power to charge its batteries.

The introduction of batteries enables selection of smaller engine sizes that can operate at optimal loads for a larger portion of the time, due to additional power being obtained from the batteries when required (peak shaving). When power requirements are low, the batteries can be charged using the excess energy generated by running the engine still at the optimal load. Also, for vessels with electric cranes and other cargo equipment with transient peak loads and options for regenerating power, batteries can introduce significant benefits. Alternatively, in operating conditions requiring very low loads, the ship may be able to operate on battery power alone saving engine running hours, fuel- and maintenance cost.

Applicability and assumptions

Hybridization has a wide range of applications, but is particularly relevant to ferries, offshore vessels and tugs. Hybrid solutions are in general suitable where there are large fluctuations in power output, where the battery bank can stand for power spikes while engines constantly operating smoothly within optimal range. Hybridization with batteries can be used on ships with either diesel, LNG and biofuels.

The specific fuel oil consumption of, and emissions from, an internal combustion engine depend on the engine load. Typically, engines are calibrated for optimum performance at high loads. For ship types that experience large load variations during operation, the introduction of batteries may allow the engines to operate optimally with respect to fuel oil consumption and/or emissions. An example of this is dynamic positioning (DP) vessels often experiencing high transient power demands operating frequently at unfavourable low engine loads.

For the cases when operation solely on battery power is feasible, like for the initially very low engine load operations, maintenance costs can be reduced from avoiding incomplete fuel combustion. Incomplete fuel combustion can lead to contamination of the lubrication oil and the build-up of carbon residue on vital engine parts, inflicting amongst others increased maintenance cost and fuel consumption.

Peak shaving is applicable for all ship types and size categories.

Hybridization of a vessel is also favourable for use in conjunction with direct current (DC) grid system onboard. A DC grid system can amongst others disable restrictions for speed on power producing engines, enable these engines to be more optimum at energy storage and production for hybrid vessels resulting in overall fuel savings.

Hybridization requires by itself no charging infrastructure on land. However, utilizing the main engines to produce charging power would cause a somewhat loss in available onboard energy due to mechanical and electric efficiency losses along the power production and storage chain. However, for vessels with the attributes mentioned above and with the proper power management, battery hybridization would still reduce the total fuel consumption even if the batteries are to be charged with energy from onboard engines.

Cost of implementation

Experience shows that the total equipment and installation costs are between $500,000 – $5,000,000 per ship for the most common battery pack installations (200 – 2000 kWh) that contribute to propulsion purposes. The average value is between $1,500,000 – $2,500,000 per vessel. The battery equipment then contains the batteries themselves and other electrical installations such as control systems, converters, switchboards, cabling, fire safety equipment and adapted cooling and ventilation systems, etc. In addition, there are costs associated with the planning and design work, and the work related to installation, testing and approval (class costs).

The cost per kilowatt hour for the battery itself is today about $500 to $900/kWh, while the total additional costs would be approximately $1,300-$5,000/kWh. In other words, the cost varies significantly depending on the battery size, configuration, etc. How the costs are distributed among the various components varies, but the table below gives an indication, based on an average new build with an installation of 500 kWh. This example indicates a total cost of $2,300/kWh, which corresponds to three times the unit cost of the battery itself, which in this case is set at $700 / kWh.

Table: Example of total cost and cost distribution of a battery hybrid plant

The measure in itself does not require any extra maintenance, on the contrary it can contribute to less wear and tear on the machinery on board due to improved machine operation and reduced running hours. Thus, longer maintenance intervals may be followed. For existing ships, an additional installation cost is estimated, this is very dependent on the specific configuration and could vary greatly from project to project.

Reduction potential

Fuel consumption and following emissions are potentially reduced by an estimated 2-15% of annual ship consumption. The reduction potential is highest for larger battery installations, where the stored energy contributes to propulsion support and optimizes main machinery operations. Vessels with varying power requirements, a significant amount of low-load operation and a large proportion of the operation with requirements for power redundancy are assumed to be able to expect the greatest savings. Furthermore, smaller reductions shall be expected for concepts where the battery is used exclusively as support for the auxiliary machinery. There are big variations in possible savings between the smallest and the largest battery installation.

Other References

1. ABB (2024) Example of fully battery powered ferry
2. DNV (2024) In focus – The future is hybrid: A guide to use batteries in shipping
3. Nuchturee, I. et al. (2020) Energy efficiency of integrated electric propulsion for ships – A review
4. Øverleir, K. (2015) Hybridization of General Cargo Ships to meet the Required Energy Efficiency Design Index.
5. Reusser, L. et al. (2021) Challenges for Zero-Emissions Ship