Recent developments open the way for cruise ships to enhance energy efficiency using waste heat energy, energy storage and fuel cells, says independent design consultant Foreship.
The 2020 global 0.5 percent cap on fuel sulfur content is not the only reason cruise shipping is newly open-minded when it comes to the energy sources that can improve vessel efficiency, says Jan-Erik Räsänen, Head of New Technology. Rapid advances in battery technology mean that the cruise sector could emulate the car industry in exploiting hybrid power, albeit differently packaged.
Formerly with ABB, Räsänen is an acknowledged expert in shipboard energy optimization. Efficiency improvements could come from considering new energy storage techniques - either electrical or thermal - but they might also result from reclaiming efficiencies from existing systems. What’s important is the complete energy flow of the vessel, he says.
“An overall understanding of the energy balance between production and consumption is needed, so that insight can be offered into where the energy should be used and where it is typically wasted. We need to establish what the energy sources of the vessel are/will be, and where efficiency gains might be available.
“You could, for example, describe waste heat energy as an alternative energy source which can be re-used by being fed through absorption chillers, organic Rankine cycle or steam turbines,” says Räsänen.
In the past, container shipping companies have used steam turbines to reclaim the plentiful waste heat generated by 60-70 MW two stroke engines. Until recently, cruise ship engines generating on average 9-16 MW have not been large enough to justify sizeable steam turbines, especially when considering the typical operational profile of a cruise ship. “We are seeing a change, because now it is becoming possible to fit small steam turbines on these ships, partly because of lower heat demand in traditional steam processes with LNG fuel and fresh water production, as well as improved waste heat energy recovery systems,” he says.
Foreship is undertaking several feasibility studies evaluating next generation waste heat recovery systems for cruise ship owners. “One goal is to minimize the use of oil fired boilers for, saving the fossil fuel that would otherwise be burned while also serving the combined needs of the galley heating process, fuel heating, the reverse osmosis plant producing fresh water and laundry services, for example,” Räsänen says.
New potential is also fast-emerging for cruise ships to exploit battery power, where the energy stored can be derived from a variety of sources.
There are ship types that have been natural candidates for battery power, Räsänen explains – typically ferries and shortsea vessels requiring power in short bursts, or vessels used in the oil and gas sector that spend a lot of time idling. Others have not been such an easy fit, even though a hybrid solution with batteries and conventional engine can improve fuel efficiency with up to 15 percent.
The objection to battery technology in the cruise market has always been based on space and cost; now both are coming into place, Räsänen says. Foreship has already been involved in a feasibility study to evaluate the use of battery power for a leading cruise operator.
Furthermore, shipboard battery options are fast-developing. In late 2016, one battery maker was offering a 6.5 kWh battery with dimensions of approximate 36cm by 30cm by 30cm. Six months later, the same supplier is offering a 9.7 kWh battery of the same size. “Typically, a high charge and discharge rate has been one of the major criteria to minimize the size of the batteries due to the big physical size in space and weight,” Räsänen says. “But, with increased density and lower price/kWh, we see a step away from this. With moderate charge and discharge rate, we expect a longer life time of the batteries as well.”
Early cruise adopters of battery technology include ship operators calling in particularly sensitive areas, and Räsänen acknowledges that it can be difficult to move from case study to generalization in terms of uptake. However, he cites recent moves by Statoil to contract seven more supply vessels with hybrid battery operation and invest in a lithium ion battery manufacturer to develop energy storage for dynamic positioning (DP) duties as part of a groundswell opinion heralding change.
“It’s not likely that we will see large numbers of ships operating solely on batteries, but I believe 40-60 percent of all vessels could benefit from auxiliary battery load exploitation to support peak load shaving,” says Räsänen. “A small number of ships – say, five percent of the fleet – will also use them for specific duties: in the cruise sector, for example, battery power could be useful during port entry, where the environmental gains would be strong.”
Recently Foreship has also seen an increased interest in fuel cells in parallel to combustion engines to improve fuel efficiency. For the time being, Räsänen believes the most promising of the various technologies available is PEM – Polymer Electrolyte Membrane, whose use in the automotive industry has brought a lower per energy unit price than the less mature Solid Oxide Fuel Cell (SOFC). However, he adds that SOFC efficiency can be raised up to 65-70 percent, while PEM has a lower efficiency of around 45 percent.
He adds that while both technologies use hydrogen as fuel, other types of fuel can be used such as LNG and methanol. However, both LNG and methanol would emit pollutants and need a reforming process to produce hydrogen. After the reforming process, carbon monoxide and hydrogen remain, and Räsänen notes that while SOFC plants can utilize both carbon monoxide and hydrogen as fuel, the carbon monoxide needs to be removed in the case of PEM, thus lowering the overall efficiency and complicating the fuel treatment process.
For the moment, both technologies have a relatively high cost per energy output, Räsänen concludes.