Work in progress

Commercialising energy storage systems in the maritime sector is a challenging proposition. Sean Puchalski discusses the evolution of battery-powered vessels and explains how Corvus Energy is working to scale up the technology for use on larger vessel types

Currently, the adoption of an energy storage system (ESS) such as lithium ion batteries (LiB) in marine vessels is limited to battery hybrid vessels with highly variable load profiles and all-electric vessels on short routes. In the case of battery hybrid vessels this is true because the economic benefit of the ESS is derived mostly from load-leveling, peak-shaving and spinning reserve functionality.

The first two functions in particular are more pronounced in a vessel that must respond to high variation in propulsion loads. It is in these vessels that the economic benefit of the ESS likely provides a sufficient return on the cost of the ESS at today’s LiB pricing.

Examples of vessels with highly variable load profiles are tug boats, offshore supply vessels and short haul ferries where the generators are typically sized to handle a certain peak load that is significantly higher than the average load required. A harbour tug, for instance, may have generators that can produce over 7,000 HP for an 80-ton bollard pull in ship assist mode, yet the tug spends less than 10% of its time at or near this full bollard pull rating.

A similar situation exists in offshore vessels in dynamic positioning mode where thrusters must respond rapidly to maintain vessel position against wind and waves. In both cases, with a hybrid ESS arrangement, the generators can be sized smaller or be fewer in number to handle the base load and let the ESS shave the peaks or level the load on the generators. This reduces fuel consumption, emissions, and wear and tear on the engines. As a vessel’s propulsion load profile becomes flatter, the cost savings diminish relative to the cost of the ESS. Think of a long haul cargo vessel; once it leaves the harbour, it steadily steams across a vast sea for days at a relatively constant speed. It can therefore be seen that for hybrid vessel adoption, the cost of the ESS is a constraining factor.

As mentioned above, all-electric vessel adoption is currently limited to short routes with modest energy requirements. As in electric cars, the range in electric ships is limited by the capacity (typically measured in kWh or MWh) of energy storage that is feasible to install onboard. Feasibility is currently limited by ESS cost or physical size. That is, beyond a certain battery capacity, either the costs exceed the savings to such a degree that it is not a financially viable solution, or the volume or weight of the battery system becomes too large to accommodate in the vessel design.

Once these challenges are resolved, there is also a secondary issue of recharging such a large battery quickly enough to maintain a useful schedule for passengers or for other operational requirements.

Some very smart engineers are working on scaling up charging infrastructure to handle repeated large power transfers, but this is also currently a limitation of the battery cells themselves. In addition, electrical grid infrastructure in the ports of call will need to be able to deliver the required energy many times throughout the day.

Therefore, for all-electric vessels to become commonplace we need not only have  improvements in battery cost but also performance with respect to energy density and recharge rates. These improvements are mostly required at the battery cell level. At Corvus, our job is to create energy storage systems from these cells that do not de-rate the performance of the cells, but instead, harnesses their full potential.

Energy density, charge rates, cell cost and many other factors have all improved dramatically in the five years Corvus has been marketing maritime ESS products, and these trends are continuing, if not quickening. As improvements in density, performance and cost are commercialised, we will see adoption grow to include electric ferries (and other vessels) of ever increasing route length. Compare the first electric ferries in Norway, the MF Ampere and MF Folgefonn, owned by Norled AS, with crossings of 5.6 km and 6 km, respectively, to the more recently announced projects, Aero Ferry (up to 24 km) and HH Ferries’ MF Tycho Brahe and MF Aurora (24-hour operation, 50,000 passengers/9,000 cars per day) to see this is already happening. We will also see hybridisation extend into longer haul vessels such as coastal cargo vessels and eventually freighters.

‘For all-electric vessels to become commonplace we need not only improvements in battery cost but also performance with respect to energy density and recharge rates. These improvements are mostly required at the battery cell level’

Corvus’ approach to managing the complexities of the required technology improvements in energy storage whilst turning a profit in the here-and-now was to create an extensible technology platform comprised of the safest possible battery pack mechanical and electrical design, combined with a battery management system (BMS) refined over seven years.

The term ‘battery cell agnostic’ is overused and so we avoid it; what we do is incorporate the best available cell technology for a given application into our mechanical, electrical and software backbone. We have just come out with two new products and have two more on our roadmap, each using a different cell. The extensible platform concept serves us well and is a good way of looking at adapting to the needs of the market longitudinally in time.

We also look at it laterally with the goal of optimising total cost for a given application by offering enough breadth in our product line. Even amongst the vessel types that are installing ESS today, there are differing requirements for what the products must do. These differences are mostly in terms of whether the vessel needs an ESS that can discharge evenly over an extended period of time – think all electric ferry – or whether the vessel needs an ESS that can absorb/support large fleeting transient power loads – think offshore supply vessel dynamically positioning next to a drill rig in rough weather. These needs can be arranged along a power to energy continuum and we build a product for each case along the continuum. In some applications, like mobile drilling rigs, which are like floating cities, a number of different products are required to optimally serve the multiple use cases onboard.

In the future, as adoption extends to vessels with longer range, there will be a need to scale up the maritime ESS from current levels to something two to three times as large. The largest marine ESS installation in the works so far is approximately 4 MWh. The challenges of scaling up to the 10 MWh-plus level is not so much a matter of BMS scalability or system architecture, as these sizes have been achieved already in stationary power (i.e. electricity grid) ESS applications. For maritime ESS, there is a higher standard of care around safety, especially with respect to thermal runaway (TR) .

Thermal runaway is an extremely rare situation that can occur with batteries, especially LiB, where a faulty or damaged cell can catch fire resulting in a cascading thermal reaction, if uncontrolled. The difference between a containerised battery system in a remote wind farm going into TR and the same thing happening on a ship with passengers and crew many miles offshore underscores the need. It is challenging to scale up with respect to TR safety. Therefore, Corvus puts a lot of resources into leading the market with respect to TR prevention and mitigation. In the extremely unlikely event it becomes necessary, our new products actually limit TR to a single cell, isolating the reaction before it can cascade. The only sensible strategy for TR safety is to limit as much as possible the amount of energy in the reaction.

Commercialising energy storage technology is not for the faint of heart and many challenges remain to be solved. Our belief is that with the right technology and product strategy, as well as sound relationships with the customer base and regulatory bodies, we put ourselves in good stead. We look forward to the future and are excited to see the development of smart energy usage aboard the world’s fleets.

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