Every piece of equipment on a ship can play its part in reducing emissions and boosting fuel efficiency, and freshwater generation systems are no exception, reports Riviera Maritime Media.
“Passenger ships have a very high freshwater demand,” says Wärtsilä sales manager for fresh water Jan-Willem von Drathen, noting that as well as passengers consuming water through sinks, toilets and showers, water is also needed for galley and laundry operations, and for technical purposes such as boiler feedwater.
“Every yard and shipowner has their own calculation for consumption but you can estimate a freshwater production capacity of approximately 300 litres per person per day,” Mr von Drathen adds.
Royal Caribbean Cruises Ltd
Royal Caribbean Cruises Ltd (RCL) report that their guests consume on average 250 litres per person each day.
This efficiency is achieved through shipboard use of low-flow showerheads and washbasin faucet aerators in both crew and guest cabins, reuse of condensed water from air conditioners to supplement freshwater demands for laundry, prioritising efficiency when selecting icemakers, dishwashers and laundry equipment, and other means.
RCL’s regulatory lead for environmental stewardship Nick Rose explains the three ways the company produces fresh water both for guests and for technical purposes: flash evaporation/steam desalination, reverse osmosis (RO) and bunkering.
Flash evaporation
“Flash evaporation or steam desalination brings in seawater and boils it to remove the salt, using waste heat from our engines,” he explains, adding “The steam generated is sent to a condenser where the steam then condenses back into distilled purified water.”
“From the condenser it is sent to a mineraliser that adds minerals back into it and finally it is treated with chlorine as a disinfectant.”
Reverse Osmosis
Moving on to RO, Mr Rose says “[This uses] seawater, but instead of using heat or high amounts of energy, you actually use a little bit of energy to push the saltwater through a semipermeable membrane.”
“The only thing that makes it through the membrane is purified, clean water,” he adds.
“All the salt and other possible contaminants remain blocked on the intake side of the membrane, while the water that passes through is sent to the mineraliser for flavour and chlorinated for added safety.”
RCL’s vessels can also bunker fresh water in local ports, Mr Rose says, adding “When we do this we comply with strict World Health Organisation [and] US Public Health standards, so water is taken from reputable sources.”
Flash evaporation for multiple engines
Comparing the use cases for the three technologies, Mr Rose says “Flash evaporation is best used when we are sailing with multiple engines that create plenty of waste heat so we are generating water without added costs.”
“Reverse osmosis is best for when we are not generating a lot of waste heat, so lower speed itineraries and little energy use [and] bunkering fresh water is our last option for ships that stay many days in port.”
Mr Rose adds that more than 80% of water on RCL vessels is produced on board rather than being bunkered.
He added that all RCL ships can supply water using any or all of the three methods outlined, noting “Being able to provide all of our ships with the means of producing it all three ways via waste heat recovery, reverse osmosis and bunkering is key to our operations and has allowed us to make it a sustainable operation.”
“Most cruise ships install evaporators and RO systems in parallel. Every technology has their particular benefits,” says Wärtsilä’s Mr von Drathen.
He noted that while RO systems can utilise waste heat to produce fresh water without the need for additional energy expenditure, evaporators may require additional heat but are able to produce higher-quality fresh water suitable for critical technical applications, and are less affected by changes in seawater temperature, salinity and impurities.
Fuel savings for smaller-scale applications
An innovative water generator designed for smaller-scale applications such as small cruise vessels can produce savings of up to a tonne of fuel per day.
Technology group Wärtsilä’s horizontal tube evaporator (HiTE) is designed for applications with freshwater demand between 30 and 150 tonnes per day.
Horizontal tube evaporator
It differs from existing technologies as it is a multistage evaporator, designed to function in a wide range of operational areas that prove challenging for technologies such as reverse osmosis or plate technology, such as in shallow waters with poor-quality seawater.
The HiTE produces clean water suitable for human consumption and technical applications. It can achieve energy savings of up to 75% compared to single-stage technologies, with a specific heat consumption of 200-280 kWh per tonne, Wärtsilä says.
Of further benefit is the unit’s automatic control system which allows it to operate flexibly when engines are operating at a low load. Automated smart processes adjust the system’s operations according to the amount of energy provided, an important factor for vessels that operate with varying engine profiles.
The first commercial application of the HiTE was a retrofit on board Allseas’ pipelay vessel Lorelay, which has accommodation for up to 225 people.
Allseas’ technical inspector Taco Straathof comments “This evaporator produces pure distillate, even in difficult seawater conditions, plus it does so reliably with low operating costs.”
“The possibility to operate the unit at part load gives it great flexibility to utilise available waste heat.”
Upstream flow results
The system functions by seawater being fed into tubes, which are then heated from the outside either by steam or hot water until the water inside reaches boiling temperature. The increasing temperature and decreasing pressure during upstream flow results in increasing amounts of vapour being released.
The resulting turbulent fluid and vapour mixture has good heat transfer, increasing the unit’s efficiency, and the vapour is condensed into high-quality distillate of ≤8 ppm.
Wärtsilä’s design evaporates seawater across multiple stages, using the heat generated from previous stages in follow-on stages, recovering energy and increasing total capacity for freshwater production.
“The system produces good quality water, and we are extremely happy with the outcome,” added Mr Straathof.
Osmosis-based freshwater generators
Wärtsilä also recently added reverse osmosis-based freshwater generators to its line of offerings, which generate fresh water by using a semi-permeable membrane to remove salts and ions from seawater.
The semi-permeable membrane is a porous sheet barrier, which halts the flow of salts and other solutes while permitting pure water to flow through.
Seawater under pressure is forced through microscopic pores in the sheet barrier, while larger dissolved solids and heavy molecular-weight contaminants are flushed away continuously.
The technology is capable of delivering between 10 and 1,500 m3 of fresh water per day, in a range of seawater temperatures from 0-35°C.
Discussing the company’s existing options for water generation, Wärtsilä’s sales manager for fresh water Jan-Willem von Drathen comments “Wärtsilä offers both reverse osmosis and evaporators, and hence is able to provide optimised freshwater solutions that make the best use of the specific characteristic of evaporators and reverse osmosis systems.”
Due to the high demand for freshwater systems, many owners opt to run both reverse osmosis (RO) and evaporator systems, as each technology offers different benefits.
RO systems are able to produce water without additional heat, while evaporators need heat but can produce higher-quality water suitable for technical operations, such as boiler feedwater, and are less affected by seawater quality.
Efficient way to meet hot water demand
Thermal storage of hot water to meet demand peaks during port stays could result in improved energy efficiency, according to a research paper.
Traditionally, cruise ships produce potable hot water with the help of recovered heat from the engine cooling lines. The paper notes that this requires no additional fuel consumption to heat the water.
Direct heating to tackle peak demand
However, in situations where demand for hot water outstrips what can be produced with this method, additional boilers may need to be brought online.
Using direct heating to address peak demand conditions increases both fuel consumption and greenhouse-gas emissions, the paper notes.
A typical situation in which demand for hot water exceeds supply in this way occurs during port stays. Vessels will often run on a single engine when in port, which will reduce the amount of heat that can be recovered from the cooling lines, and demand for hot water can spike before and after passengers depart the vessel.
A potential solution to this is using thermal storage to compensate when such mismatches between supply and demand occur.
Dynamic thermal simulations
The paper’s authors assessed the performance of different solutions using dynamic thermal simulations of hot water distribution systems, with different regimes and time-dependent heat requirements, and found that power required for preparing hot water could be reduced up to 39% with such a system, decreasing the need to bring additional heating sources online.
The paper, titled Potential of thermal storage for hot potable water distribution in cruise ships, was presented at the 73rd Conference of the Italian Thermal Machines Engineering Association in Pisa in September 2018.
The authors were the University of Trieste’s Marco Manzan and Ezio Zandegiacomo de Zorzi, the Italian National Council of Research’s Angelo Freni, Andrea Frazzica and Bianca Maria Vaglieco, and Fincantieri’s Deluca Claudio and Zentilomo Lucio.
Innovative filtration method
Water systems maker Case Marine has developed a self-cleaning prefiltration technology for use with its RO systems with Boll & Kirch Filterbrau.
The Bollfilter Automatic Type 6.18, which has been developed as a replacement for traditional sand filters, has a smaller footprint, a more precise grade of filtration, requires less maintenance and has a longer service life in comparison to its predecessor.
It comprises a compact housing with multi-filter chambers and is rated at 30 microns, meaning it has been designed to prevent particles larger than this size – 0.03mm – to pass through.
Unlike media-bed filtration, which must be taken offline for cleaning, the Bollfilter can use backwashing for cleaning while still online. It can also offer weight advantages of up to a fifth compared to a media-bed filter system.
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Source: Riviera Maritime Media
