DIY Design for Solar Seawater Desalinator

In our recent article on methods of desalinating water using solar power, some videos were included which showed simple home built devices. They used the sun’s power to distill pure drinking water from contaminated water sources, such as sea water, using evaporation. While those devices were simple, cheap and easy to build, it seems that they yield only small amounts of water each day, and were thus somewhat impractical.

Given the crisis brewing in the Western Cape, we have been researching this subject, to perhaps help people who are faced with a very difficult situation.

We decided to develop our own design, which has the target of producing at least 5-10 litres of drinking water on a hot sunny day. This article discusses that design, at a limited conceptual level.

Our design uses low cost materials, which are available at most hardware shops, and should be reasonably easy to build.

Basic Approach

We intend to use the sun to boil sea water, and then condense the steam produced, to yield drinking water. We will be using a flat plate solar water heater to boil the sea water.

Flat Plate Solar Water Heater

While you can probably use any suitably sized flat plate solar water heater, we have based this discussion on our own low cost design (posted in a separate article, please read through that for more details). To work out what size collector you need, please look up the energy available from the sun at your location on earth from  Global Solar Atlas website (instructions on how to do that are provided in more detail in a previous article), and then use that number in the calculator below.

Overview of the Proposed System

The picture above shows the main components of the proposed system.

A large plastic bin is filled with seawater. Our calculator above should help estimate what bin size you need for a given target yield of distilled water per day. A pipe inserted through the bottom of the plastic bin, allows the seawater to flow into the bottom of the solar water heater (which is inclined at about 30 degrees). The sun heats up the water until it boils. Steam flows out of the top of the solar water heater through a  copper pipe, which is then immersed below the level of the sea water in the bin. The large volume of seawater cools the steam, which condenses back into pure water, which then flows out via a pipe that penetrates the bottom of the bin.

What is not shown in the above diagram is some sort of frame to hold the components in position. The system should be positioned above ground, perhaps on a table or something, so that the distilled water draining from the bottom of the bin can run into a collection container below. The bin height should also be precisely positioned relative to the solar heater (discussed in more detail below). You may want to add a valve (which is not shown) to the pipe feeding the seawater into the solar water heater, for maintenance purposes.

The picture below, viewed from the back, where the plastic bin has been “sliced open” to show the pipe work inside the bin, reveals more details of the system.

Steam piped from the solar heater is submerged below the surface of the water in the plastic bin. The water in the bin cools the steam, so that it condenses inside the submerged pipes. The submerged pipes are shaped basically in a large “U”, so that the maximum length of pipe is submerged in the bin. The open end of the “U” rises back up above the level of the sea water in the bin, to vent the system to atmospheric pressure. This ensures that steam does not pressurise the pipe work. It also allows excess steam to escape (although it is hoped that all the steam produced will condense before then – there should be more than enough cooling available in this configuration). A drain pipe is also included, extending down from the the bottom of the main steam down-pipe, to allow the condensed water to run out, where it can be collected for drinking.

Another view of the system, from below, shows the way in which seawater is piped from the bottom of the bin into the bottom of the solar water heater.

Plastic Bin Size and Water Level

An important aspect of this design is the water level in the plastic bin, and the size of the bin itself.

The water level in the bin is important for a number of reasons:

• The water level in the bin will determine the water level in the solar water heater. The level of the water in the solar water heater should be about 250mm (measured along the riser pipes in the heater), from the top cross-pipe. This gap is needed to ensure that agitated splashes from the water boiling in the riser pipes, do not escape and then contaminate the steam leaving the solar water heater (for this same reason, the pipe leaving the heater is first directed upwards for a short length before it transverses across to the bin).
• A relatively large amount of seawater is needed to cool the steam. The length of submerged pipework is obviously dependent on the water level in the bin. The heat absorbed by the seawater, as it cools the steam, will cause that water temperature to rise. This adds to the efficiency of the system, on one hand, because some of the heat gained by the system is retained. Seawater flowing into the solar water heater is thus already partially heated. But on the other hand, the lower the temperature of the seawater in the bin, the more effective the cooling of the steam. A larger volume of water enables heat to be absorbed from the steam by the water in the bin, without too large a temperature rise, which is beneficial for the system operation. Thus it is quite important to keep the bin full, and to have a large enough bin. Our calculator above should help estimate what bin size you need for a given target yield of distilled water per day.
• The target production volume of distilled water produced from sea water, in this discussion, is 5-10 litres a day. If a large bin is used, then a daily reduction of 5-10 litres will have less of an effect on the water level in the bin, than if a smaller bin was used. This assumes that the seawater in the bin is topped up daily. Although for ideal operation, it would be better to install a float valve (similar to what is used in a toilet cistern), to automatically top up the bin from a separate feeder tank.

A frame must be built to hold the bin at the right height with respect to the solar water heater.

System Operation

The system is designed to operate completely passively. All you need to do is keep the plastic bin topped up with seawater, and then as long as there is sufficient heat from the sun to boil water in the solar water heater, the system should produce distilled water. Your location on earth, and the size of the solar water heater, will determine how effective the system will be at various times of the year. Obviously, during overcast weather, the system will probably be relatively ineffective.

System Maintenance and Other Potential Issues

Hot seawater is a corrosive, aggressive medium. In addition, the distillation process will result in an accumulation of salt and other contaminates within the solar water heater pipes. For this reason, it is recommended that you periodically flush the solar water heater with distilled water (depending on the size of the solar water heater, the volume of water needed is probably not much more than a litre). Demineralised distilled water will tend to absorb salt and other deposited minerals quite well if you leave it to soak for a while, and then rinse and repeat.

As far as possible, materials that are resistant to the expected harsh environment have been suggested, such as copper pipes, brass or copper connectors and a plastic tank. But some corrosion and degradation is to be expected over time.

It is recommended that you filter the sea water before pouring it into the bin. Seawater contains microscopic lifeforms such as plankton and other suspended biological solids which might make the bin smell bad when that dies and decomposes, so it is better to try remove that stuff beforehand, as far as possible.

When unused, it is recommended that you cover the solar water heater to stop the system heating up to very high temperatures.

This design discussion is at a conceptual level only. It is possible that unforeseen issues arise which are not discussed here. Some trial and error may be necessary.

Disclaimer:  This article is meant to provide no more than a conceptual design for discussion purposes.  This article should thus not be  relied upon as detailed instructions for any such project. We make no claims about how effective this design might be, or whether it will work at all. Anything you attempt as a result of reading this article is done entirely at your own risk.