December 14, 2015 — Marine hybrid and electric propulsion comes of age.
(Extended coverage from Marine Log’s December 2015 edition).
By Allan Grant, Vice President, Business Development, Corvus Energy
Call me Ishmael. Melville’s famous opening line in his novel Moby Dick refers to an outcast and could once have been the motto of the hybrid and electric marine industry. There were a number of projects where hybrid and full electric drivetrains were prototyped and they worked well…for a while. The limitation was always the battery technology, where lead acid batteries were just not suited to this type of application. As a result, these projects and technologies were “interesting” with “lots of potential” but not a serious technology for the mainstream marine industry.
The advent of lithium ion batteries has fundamentally changed the value proposition for the hybrid and electric drivetrain. These batteries are smart, durable and have a lifetime that will ensure the customer is able to achieve their targeted Return on Investment (ROI). As a result, the hybrid and electric drivetrain has come of age.
To date, Corvus Energy has deployed over 30 MWh of its Lithium Polymer battery-based Energy Storage Systems (ESS) on over 45 projects. These vessels include large and small ferries, offshore supply vessels (OSVs), tugboats, research vessels, luxury superyachts and port cranes. Only two of these applications are for full electric ferries, the remainder are all hybrid vessels, but a common element is that both types of drivetrains make the vessel more efficient, as well as more environmentally sustainable.
Of course an ESS cannot create energy, but only stores it. The ESS therefore enables the vessel to use the cheapest energy available and shift it in time to the point where it is most cost effective. For a full electric this means the ESS is charged from shore using low cost energy and eliminates completely energy generated by costlier, on-board generators. For hybrids, it moves energy from the efficient portion of the generator’s load curve and replaces energy that would have been generated on the less efficient portion of the generator’s load curve.
Full Electric Drivetrains
At one time the round trip efficiency of an ESS was at 70% or less and the energy density was also not very high. This meant an ESS required a LOT of space on a vessel and did not actually store that much energy. The round trip efficiency on the Corvus ESS is now over 95% and the energy density is high enough that a space the size of a 20-foot container can store 650 kWh. This means that an ESS can now deliver enough power and energy to make a full electric drivetrain cost effective. Even with these improvements, full electric drivetrains still work best for applications where the duty cycle is relatively short such as a ferry with a route that is less than an hour between charging points. The 120-car Ampere ferry in Norway, for example, has a route which crosses Sognefjord between Lavik and Oppedal in 20 minutes and then recharges for 8 minutes from a shore-based charging station which uses the grid and another, smaller Corvus ESS to deliver a large amount of power into the vessel’s ESS in a short period of time. The shore-based charging station then recharges from the grid for about 48 minutes until the ferry returns.
The ferry operator, Norled, has other, diesel-fueled vessels on this route and based on a comparison has estimated that the Ampere is saving over 1 million liters of expensive, ultra-low sulfur diesel fuel per year. It is also saving 90% of the maintenance cost of the diesel engines and 100% of the cost of the NOx emissions, which are taxed in Norway. Norled is now looking at expanding the operating hours of the Ampere each day and is examining other routes it operates to determine if they could benefit from a fully electric ferry.
A hybrid drivetrain consists of a either a diesel or LNG engine as the prime generator for the vessel and supplements this engine with an ESS. The ESS is able to charge with energy produced when the engine generator is operating in its most efficient range and then, when the engine generator would be operating in its inefficient range, the engine is shut off and energy is extracted from the ESS to operate the vessel.
A harbor tug is a good application for a hybrid drivetrain. It will typically spend up to 25% of its time sitting and waiting for a call to push a ship. While in this mode, the vessel needs to maintain its position and run its hotel load. The engine is too large to idle and provide power just to the hotel load and so the engine drives its thrusters in opposite directions, thereby keeping the vessel in position but “boiling water” and wasting fuel. With a hybrid system, the engine is turned off and the ESS is used to periodically operate the thrusters to keep the vessel in position, and also serving the hotel load. When the ESS gets to the minimum state-of-charge, the engine is started to charge the batteries, serve the hotel load and run the thrusters. Depending on the hybrid drivetrain configuration, the ESS can also be used to supplement the engine when the tug is pushing or pulling a vessel to get it into position. When the batteries are charged the engine is turned off. An ESS on a tug can save up to 25% on fuel and emissions, as well as reducing maintenance on the engine(s).
An ESS is also very effective to replace the operation of a second engine that is running in spinning reserve to provide emergency power in the event the prime mover generator fails during a critical operation. On the Scandlines ferries in Denmark and Germany they only operate one generator during crossing and use their ESS for spinning reserve. Scandlines have identified 13% reduction in fuel and emissions and a 45% reduction in generator maintenance.
Likewise, Offshore Supply Vessels (OSVs) in the North Sea can use an ESS for spinning reserve during Dynamic Positioning (DP) operation, where the vessels have to be kept stationary when servicing floating oil and gas platforms. DNV-GL has now approved ESS for DP operations. Eliminating the operation of an additional generator used in spinning reserve has the potential to save fuel, emissions, maintenance and even the capital cost of the additional generator.
The key benefits from an ESS include:
- Reduced generator hours/generators running in efficient range
- Reduced fuel consumption
- Reduced emissions
- Reduced maintenance
- Improved capital management
- Reduced number of generators
- Reduced size of generators
- Generator life extension – operation in efficient range provides longer lifetime
- Emergency backup
- Spinning reserve
- Emergency and auxiliary backup
- Passenger/crew comfort
- Reduced noise
- Reduced vibration
- Reduced fumes
- Reduced emissions
- Reference air emissions – NOx , SOx , PM
- Improved performance
- Better ramping on LNG engines
- Instant response to throttle
- Better transient load management
No longer the elusive white whale, cost effective and proven hybrid electric propulsion has arrived and is here to stay with early adopters now reaping the benefits. An ESS helps owners make their vessels more efficient by helping them manage the operating and capital costs of their vessels, while also making these vessels more reliable and environmentally sustainable. No longer outcasts like Ishmael, the electric and hybrid marine industry has proven the value proposition for electric drivetrains and energy storage. Corvus Energy’s ESS is a key piece of these innovative hybridization projects for commercial vessels, which are fundamentally changing these marine markets. Where once there was only a potential for these benefits, Corvus Energy has the ESS technology, people and experience to make them real.