For vessel owners interested in reducing fossil fuel consumption to meet IMO 2020 international marine emission standards, hybridization and electrification technologies complement or offer an effective alternative with many benefits.
A basic definition of hybridization is any system with two or more sources of energy acting together to accomplish a task. In the automotive world, a common example of hybridization technology is the hybrid car where a conventional internal combustion engine is combined with an electric propulsion system to create a ‘hybrid’ powertrain. The benefits of hybridization, in this instance, are fuel savings, performance improvements and reduced emissions. Similar solutions and benefits are available and becoming more widely implemented in marine applications.
Many long-haul vessels are still operating with direct diesel propulsion and no electric propulsion system. These vessels can improve efficiency and optimize main engine load power and emissions by adding a shaft generator/motor between the propeller and the main engine. This solution, called Power Take Out (implementing this system also enables Power Take In, which can be employed, for example, in “take me home mode”) is an electrical add-on that makes vessels more efficient and ready for hybridization. In hybrid vessels, a shaft generator/motor with inverter technology allows the optimum control of propulsion machinery at various speeds to save energy.
For short-haul vessels where electricity is available via a shore supply, ships can use drive technology and energy storage to source clean energy from local grids. The ship’s main generators can be switched off completely to prevent unnecessary NOx and carbon emissions and noise pollution while the vessel is docked.
Benefits of hybrid and electric-propulsion drives
Given that compliance with the International Maritime Organization’s “IMO 2020 Rule” can be achieved solely with engine design and technology enhancements, why should vessel owners consider a hybrid or an electric propulsion solution?
A hybrid propulsion system uses diesel engine generators to charge batteries that can be delivered by drives to electric motors. Compared to a 100% fossil fuel power plant subject to pricing variations, or a 100% battery-powered electric drive that requires extensive dock time to charge batteries from a shore power connection, a hybrid system has several advantages:
- Reduced engine size and complexity: Because diesel engines are used to drive generators instead of propellers, the size of engines and SCR components can all be reduced. This reduces complexity and maintenance costs as well as engine noise and emissions. It also improves the ship design, because the diesel generators can be placed freely as the engine no longer has to be physically connected to the propulsion system. The drive technology creates a smooth drive train with less current spikes, and if peak loads are present, the energy will be handled from the energy storage.
- Improved engine efficiency: The engines can operate at their most efficient point. The gensets charge the batteries when the vessels are at a standstill or running at low speed. When the vessel accelerates, the peak power is drawn from the batteries. In other words, the electric motors are used to do what they do best—provide high torque—and the batteries are used to shave peak power and offload the horsepower demands to improve engine efficiency and downsizing.
- Retrofit to augment or replace engines: When a higher tier engine is retrofitted into a ship (e.g. a Tier 4 engine retrofitted into a tug), horsepower may suffer. In this case, batteries can be used to provide supplemental horsepower to the original rating to secure the vessel’s earning power and profitability. In other retrofit situations, it may be possible to replace one engine with a battery and electric drive. This option is feasible when vessels are replacing their backup generator and auxiliary engine.
- Meet port operation requirements: Using batteries and electric-motor drives enable emission-free idling in port or at an offshore rig. This option meets demanding port standards, already a primary consideration for European ports.
Concerns about hybrid and electric-propulsion drives
As with the application of any advanced technology, there are several potential drawbacks or risks to be considered.
First costs of hybrid/electric solutions are higher than traditional systems due to the addition of energy storage systems. However, battery costs are coming down as electric vehicles gain market adoption, helping to improve ROI. In many cases, the benefits of hybridization result in a payback of less than four years.
Fire safety is a consideration anywhere a lithium-ion battery is deployed. One early marine li-ion battery pilot project experienced a battery fire in 2012, which highlighted the potential risks. However, a subsequent risk assessment concluded that a properly designed and installed battery energy storage system would enable the vessel to achieve performance, emissions and fuel reduction objectives safely.
To address fire-safety concerns, solution providers can employ water mist, CO2, foam, or dry chemical powder systems—such as those offered by Danfoss Semco marine fire protection systems—that can be customized to fit particular needs.
To meet IMO 2020 international marine emission standards, commercial vessel operators can take one of three different options: 1) purchase higher cost low sulphur fuel oil (LSFO) or marine gas oil (MGO); 2) retrofit their engine systems with a selective catalytic reduction (SCR) scrubbers that entail a high initial capital cost; or, 3) convert to alternative propulsion systems like liquefied natural gas (LNG) or electric solutions. For each of these options, hybrid/electric drive technologies make it possible to optimize power generation and propulsion on board. To make the most advantageous decision, vessel owners and operators can compare these solutions in their specific applications, and plan ahead to avoid fuel cost and supply uncertainties looming on the horizon.
