Energy consumption is an important issue, especially among sailors, and often determines the choice of system. Accordingly, we would like to go into this topic in more detail here.
The energy consumption of a Watermaker is almost entirely caused by the high-pressure pump. The required power is closely related to the volume flow and the corresponding pressure. Here systems can only differ due to small differences in the efficiency of the motor and pump.
Two variables are of interest for the efficiency of conventional systems – the delivery volume of the pump and the surface of the membrane.
The relationship between the volume flow of seawater and the amount of fresh water produced is not linear. Pumping more seawater through the system increases the amount of fresh water, but to a lesser extent. Ergo, the energy efficiency related to the amount of fresh water produced decreases as the volume flow of seawater increases.
The ratio of volume flow to membrane surface is therefore crucial for energy efficiency. This ratio determines the proportion of fresh water that is obtained from the pumped amount of seawater.
The more water you have already removed from the lake water, the saltier this concentrate becomes. For this reason, an additional membrane will always deliver significantly less water than the previous ones. So there is a limit beyond which an additional membrane on an existing system no longer provides much added value, but does reduce the product water quality.
Another aspect, especially for small systems, is that the membrane requires a minimum volume flow to flush out the retained salt. This minimum dimension is determined by the diameter of the membrane and the operating conditions of the system. If the amount of seawater is too low, the lifespan of the membrane suffers.
With the conventional watermaker, the operating pressure is set at a pressure control valve where a large part of the energy is lost. Apart from the option described above of pumping less water or increasing the membrane surface, the only option left is energy recovery.
The so-called energy transfer principle has become established among watermakers for yachts to achieve this. Here, energy from wastewater that is no longer needed is used to increase the pressure on the inlet side.
Instead of a conventional high-pressure pump, a so-called Clark pump is used here. With this previously patented pump design, the pressure on the wastewater side is used directly to increase the pressure on the inlet side. This requires a pump body that is specifically tailored to the respective product and contains different pistons and valves.
In addition to the efficiency of the Watermaker, how much energy is available determines the amount of water that can be produced. Basically it can be said that a watermaker with energy recovery consumes around 4-4.5 Wh/liter of energy based on the amount of water produced. For a conventional watermaker, this consumption is in the order of 6.5 – 12 Wh / liter depending on the configuration (membrane) and water temperatures – we assume 8 Wh / l below.
The efficiency does not take into account the amount of fresh water that is used again to flush the system after operation has ended. If the Watermaker is not used every day, but produces more water with each run, this becomes less and less important.
Let’s look at two examples:
For comparison: the refrigerator compartment on board a standard 40ft yacht typically uses around 50-60 Ah per day at 12V.
The most effective way to save energy is through water consumption. The needs of an economical crew can be met with negligible electricity consumption, even without energy recovery. Energy recovery has a non-negligible effect, but even with a conventional watermaker, a comfortable water requirement can easily be met by most boats and hardly exceeds the requirement of a refrigerator compartment.
If you compare different conventional systems in terms of the specified amount of fresh water and energy consumption, remember that magic cannot be achieved without energy recovery. Our systems are designed in such a way that, despite taking energy efficiency into account, worry-free operation over a long period of time is not neglected.
Anyone who values minimal energy consumption more highly, i.e. the robustness of a conventional system, is best advised to go with a system with energy recovery.
Learn more on the pros and cons of the two styles of watermakers.
