Watermakers
Watermakers Produce Fresh Water Every Day
Before the invention of devices to desalinate seawater, sailors had to carry enough water on board for an entire voyage between land-falls. Water stored for long periods attracts bacteria, making it taste sour. Modern watermakers remove this burden, producing fresh water every day. Although there are several means of desalinating seawater, the only practical marine application is a reverse-osmosis watermaker.
For example, a relatively new watermaker technology uses a type of dehumidifier to condense water from the air. Manufacturers include companies such as Air Eater Proprietary Limited, Hyflux and Vapaire Water Purifier. But the capacity of such systems is limited, around 1 litre/hour (6.25 gal/day), in conditions of 45 to 60%percent humidity. These systems, while interesting, clearly do not meet the daily output requirements of a boat.
Reverse Osmosis
Reverse osmosis is the technology best suited to watermakers. Osmosis is the process whereby a solution passes through a semi-permeable membrane to seek equilibrium. Clear enough? For example, given a solution on one side of a membrane, and a lesser solution on the other, e.ggram., pure water, the denser solution will pass through the membrane into the less dense one, until both have the same density.
Reverse osmosis is the process of causing pure water to move out of the solution and through the membrane to the other side. To accomplish this, a high-pressure pump must be used, around 800 psiPounds per square inch. The process is not very efficient, either. About 90% of the seawater and energy are wasted. In some designs, notably Spectra and Livol, this energy is partially recovered by applying it on the backside of the piston in the pump.
Pre-Filtration
To prevent clogging the membranes, a pre-filtration system can remove particles of sand and organic matter down to 1 micron in size. Install a back-washed sand filter at the inlet. Use a series of filters (e.g., 50-25-5-1 micron) to remove other particles. If you put the finer filter first, it will clog immediately. Install an oil-water separator to remove oil. An absorption/reactive carbon filter will remove taste, odour, chlorine and organic chemicals.
Post-Filtration
AAmpere (amp), SI unit of electrical current second carbon filter can capture chemicals not removed by the reverse osmosis membrane. An ultraviolet lamp can sterilize any microbes that may escape filtering by the reverse osmosis membrane.
Right Sizing the Capacity
Right sizing the electrical system, heating, air conditioning, water capacity and related watermaker are the most critical lifestyle decisions you will have to make. Getting it wrong will lead to deeply felt dissatisfaction, and costly retrofitting. Yet getting it right is very difficult. You won’t know whether it’sSecond right until you start living the experience. And sizing the water-maker is more difficult than the HVACHeating, ventilation, air conditioning. Any system you select should have automatic TDSTotal dissolved solids (total dissolved solids) testing and automatic flush and back-wash.
In general, the watermaker should have a capacity to meet daily use plus a safety factor of 20-25%. For example, at 25 gal/person/day, for four to six people, you would requires a watermaker that can generate 125-189 galGallon a day. At 60 gal/person/day, a watermaker throughput of 240-360 gal is needed.
The following discussion assumes 25 gal/person/day consumption and and an electrical design that only requires the main engine (or genset) to run 1.3 hours/day for charging the house bank of batteries.
The difficult part of the equation is how fast can the watermaker generate water. At one extreme is a very large watermaker generating all the water in one or two hours, typically while the engine is charging the house bank. At the other extreme is a small watermaker running 24 hours a day. Using the worse case of 189 gal/day, the large watermaker would have to produce 145 GPHGallons per hour during the charging period while the small one would have to produce 7.9 GPH continuously.
Selecting an in-between operational mode is impractical. You would have to design for a peak load during x indeterminate hours.
Most watermakers have an energy efficiency between 1 and 3 gallons per hour produced by one amp-hour (GPHAGallons per hour per ampere). Given typical energy efficiency, e.g., 1.6 GPHA, the total daily energy requirements would be the same, 118 AHAmpere-hours (A*H), for either approach. The difference lies in the instantaneous electrical load. The larger unit would draw around 91 A for a short period, and the smaller one around 5 A for a continuous period.
The small unit would cost a lot less, too. So, the immediate temptation is to go with a smaller unit running 24 hours/day. But most watermakers are not rated for continuous duty, and even if they are, running any equipment continuously will wear it out sooner. Plus there is the factor of noise.
At the other extreme, very large watermakers are not available in DCDirect current models and require ACAlternating current to keep the amperage and wire size manageable. But on a boat AC is not as efficient as DC because DC electricity from the house bank has to be converted to AC using an inverter. The inverter also has to be larger, and more expensive, if we run the watermaker on AC.
Alternatively, a large watermaker could be run directly off the engine during its daily run to charge the batteries. The drive for the watermaker would have to be clutched, to disengage it on long engine runs.
Somewhere between these two extremes is probably best, but exactly where? A lower duty cycle increases longevity; while we would expect larger capacity gives greater efficiency, but this doesn’t hold true (see Table below). So within reason bigger is better.
For boats other than superyachts, the optimum choice is a large DC watermaker running for several hours a day, rather than an AC model running for a shorter time but requiring an over-sized inverter. The optimum, considering efficiency, backwashing, wear and tear, etc., could be running a medium-size watermaker for two or three hours every three days.
This will probably necessitate designing the electrical system for 24 VDCVolts direct current; although two 12-VDC units could be put in parallel.
Once you have determined the capacity required in GPH, pick a model close to this which has the best GPHA (GPH/AH) rating. Next, analyse the impact on the electrical system. If that doesn’t calculate-out the way you would like, repeat the exercise with a modified scenario.
Installation and Maintenance
For cold water expeditions, the water intake to the watermaker should be preheated.
