In many areas of the world, and here in particular in the hotter countries, water supply is a big problem. When we talk about water, many people think of drinking water only but, think about it: we use more water for washing, cooking, cleaning than for drinking. In addition, there’s the huge field of irrigation.
Now, desalination is one possibility to address (and I choose the word “address” here rather than “solve”, since I believe that there is no simple solution) these problems. Again, most people would think that desalination means crating fresh water from sea water but there are many variations of the theme. In the Niger delta for example, even the ground water has an iron content too high to be healthy, so that this water would need to be filtered. River water practically always has undesirable organic content, so it needs to be treated. And so on.
There are three main techniques for the desalination, each of which has advantages and disadvantages.
Distillation: a classic technique, Water is evaporated and condensed, resulting not just in fresh but in distilled water. Check out the price difference and you’ll see what I mean. The advantages are obvious, the product is biologically clean, and does not contain any residual ions. In many countries of the world distilled water is used as drinking water (the assumption that it affects your body’s mineralogical balance is not true). It is also used for fruit juices, and in technical and medical applications. A very versatile, high value product. It has only one real disadvantage: the price. It takes an energy of 2550 kJ for evaporate on litre of water. Assuming a cost of 12 c per kWhr, and a boiler efficiency of 0.8, the distillation of one litre costs 10.4 cent in energy alone. Compare this to your water bill..
Reverse Osmosis (RO): a filtering technique, the sea water is pressed through a semi-permeable membrane. The osmotic pressure for sea water is around 5.5 bar, so that fairly high pressures are required. In addition, the filters have to be cleaned or replaced every so often. Finally, the product still contains some impurities making it taste a bit like Spa water, and – more importantly – it still contains biological content so that additional treatment stages are required.
Multi-Stage Flash Distillation (MSF): slightly more complicated to explain. From your school physics, you may remember that when the pressure is lowered, water boils at a lower temperature. Temperature. MSF uses this principle. Water is heated up to a temperature below 100C. The water is then passed into a heat exchanger with a lower pressure, and some of the water evaporates flash-like until an equilibrium is reached. The steam is then condensed. The water is then pumped into the next stage with a still lower pressure, some of it flash evaporates and so on. Around 60% of the world’s desalinated water comes from such a process. It does not require filters which need to be cleaned and changed, and impurities are reduced when compared with RO. However, biological content may still be present, and again further treatment is required.
There are of course several other desalination techniques available. But, apart from freezing, they all employ similar principles.
Why am I telling you all this: well, our Condensing Engine of course evaporates and condenses water, i.e. it acts like a distillation plant only it generates energy AND water. The CE operates at low pressure, so that it can employ materials such as brass or even plastic where lubrication is provided by the water itself – i.e. there is no contamination and the condensed water could be used as distilled water.
We see in particular the combination of CE and flat plate solar thermal collectors as a very interesting option to produce energy as well as drinking water..