Solar water desalination: decentralized desalination systems

The lack of safe, potable water is one of the major issues for the decades to come. Just like energy, clean drinking water is a very precious good – and all efforts to produce it where needed are key to a self-supporting micro-economy¹.

According to the WHO, over 1 billion people lack access to safe water. That’s 1 out of 7 people!

The majority of people that are lacking safe drinking water are living in rural areas and in geographic terms overwhelmingly in the southern hemisphere of the globe.

Such areas are sparsely populated and remote, which makes it non-economical to install traditional clean water solutions.

Why does water desalination make sense?

Of all the water on earth, 97% is salt water, leaving us with only 3% of freshwater… and even less than that! Two-thirds of fresh water on earth is frozen as snow and ice in glaciers and ice caps. 

Only 1% of all Earth’s water is actually liquid and non-salty, making it a very precious and scarce resource.

Tapping into the large reserve of 97% salty water makes sense in regions where there is limited freshwater availability, but an abundance of salty water. These regions, major parts of the Middle East and Africa, often have an abundance of renewable energy sources, such as sunshine. And why not use sunshine, one of the most abundant renewable energy resources to counter the fresh-water shortage?

With solar water desalination we use the most abundant resource on earth: sunshine to remove unwanted minerals from saltwater and turn it into safe, drinking water or water suitable for irrigation.

Why use solar energy to power desalination?

 Cost reduction and reduction of greenhouse gas emissions

1. What’s the main way to obtain drinkable water in areas lacking a fresh water supply? Right.. Over 300 million people boil water for purification. A necessity, which unfortunately creates millions of tonnes of CO2 every year.
2. Water purification often runs on generators powered by diesel, generating CO2
3. Bottles with drinking water are often trucked to remote places, increasing cost and carbon footprint

Decentralized solar powered water desalination systems offer independence and help to avoid being taken hostage by price raises from the utility- or water company.

 

With this major challenge in mind, in this article, we’ll explore solar water desalination and how it can contribute to providing safe drinking water where it’s needed the most.

We’ll start with an introduction to the different solar water desalination technologies. After we discuss several equipment suppliers and their latest technological solutions.

Solar water desalination technologies

There are two primary means of achieving desalination using solar energy: distillation or by mechanical separation, with the use of a membrane.

Therefore we categorize solar-powered desalination technologies into Thermal and Membrane technologies. 

Thermal desalination

Examples of thermal desalination technologies are:

• Simple stills 
• MEH (Multi Effect Humidification)
• MED (Multi-Effect Distillation)
• MES (Multi-Effect Solar Desalination) 
• MSF (Multi Stage Flash) 

Membrane desalination

• RO (Reverse Osmosis)
• EDR (Electrodialysis Reversal)
• MD (Membran Distillation)

In this article, we’ll introduce one example of thermal desalination: solar stills, and introduce two membrane technologies: RO and EDR. 

Thermal desalination

The principle of conventional solar distillation is similar to the hydrologic cycle we have on earth..

With conventional solar water desalination, a basin is filled with seawater; this basin is covered by a transparent curved or sloping surface, which allows solar radiation to pass through. The water is heated up and evaporates, leaving any toxins and salt behind. The water condenses on the covering and is collected in a separate container.

An example of a solar still that allows distilling brackish water on a small scale is the Watercone:

Advantages and disadvantages of solar distillation

The advantage of solar distillation is the simplicity of the process which reflects in the limited capital requirements for an installation. For very small-scale freshwater production, solar distillation is competitive compared to the indirect desalination methods.

Disadvantages are the relatively large land area requirements when it’s scaled up and its low efficiency per /m².

Membrane solar water desalination

In this section, we will now have a closer look at two membrane solar water desalination technologies.

Reverse Osmosis (RO)

One of the simplest forms of indirect solar desalination is Reverse Osmosis. 

The basic principle is: that the brackish or salty water is pressed through a fine membrane, filtering out the salt.

Reverse Osmosis (RO) works as follows:

Brakish or salty water is pumped from a source (sea, groundwater, etc) and first, the water runs through a filter to remove sand, and an activated carbon filter to remove chemicals.

After the brackish or salty water is pumped under pressure through a membrane. This membrane filters on a molecular level the salt from the water. Essentially the salt is filtered with the use of an ultra-fine membrane. The membrane pores are extremely small (usually around 0.0001 microns), and only allow water molecules to pass through.

The leftover water is called Brine. The Brine contains high levels of salt and is often pumped into an evaporation pond, where the brine evaporates and only the salt will be left. For large-scale Reverse Osmosis, the large amount of Brine is an issue, as it can not be pumped back into the sea without disturbing sea life.

 RO Advantages:

• It’s a proven technology that has been around for over 20 years
• RO process is straightforward and can be applied on any scale.
• Equipment comes in different price levels, generally affordable

 RO Disadvantages:

• Material costs for the (pre-treatment) membranes (replacement depending on raw water quality)
• The operation of the plant needs a trained operator.

Portable or movable RO systems are available from 50 up to 3,500 liters of clean drinking water production per day. Have a  look at a movable solar-powered RO system that can produce up to 120 liters/hour of drinking water using less than 1 kW 220VAC, as explained by Oliver Kopsch:

Two major trends make small-scale desalination systems based on Reverse Osmosis incredible attractive:

1. Cheaper and more powerful solar (PV) components
2. Proven small-scale pressure recovery system reducing the energy demand of pumps dramatically

Reverse Osmosis and solar technologies have been around for so many years now, that they do not need to prove that they work. Technically, different solutions from different market players are readily available. It is now up to the market to understand its full potential and benefit from massive cost savings compare to current grid-based solutions which are often costly, unreliable, and CO2 intensive.  

Solar Powered Electrodialysis Reversal

A relatively new and less popular water desalination technology that lends itself to being powered by solar energy is Electrodialysis Reversal. Solar-powered electrodialysis reversal can be used to desalinate water with low levels of salinity. Therefore it’s not suitable to be used to desalinate seawater.

 How salty is seawater?

Average seawater contains about 35,000 mg/L of salt. Low-level salinity is considered in the range of 500 to 3,000 milligrams per liter. 

Perhaps these levels of low salinity don’t seem significant at first glance, however, even low levels of salinity can often be too high for drinking water, and too high for irrigation purposes as well.

 How does Solar Powered Electrodialysis Reversal work?

Salts and minerals naturally dissolve in water. When salt dissolves in water, it’s broken up into individual ions. There are positively and negatively charged ions. The positively charged ones, like sodium, are called cations. The negatively charged ions, such as chloride, are called anions.

Ions, just like individual atoms, can move freely through the water.

Electrodialysis works as follows:

water flows between two electrodes with opposite charges. Ions have the ability to move in water when Direct Current is applied. What happens next is the negatively charged Ions (the Chloride ones) move to the positively charged Anode, and the positively charged Ions (Sodium), move to the negatively charged Cathode.

Electrodialysis is a membrane technology that uses Ion movement to desalinate water

The salt that has been dissolved in water consists of positive- and negatively charged ions. When electrodialysis is applied, the electrodes attract the ions, creating two separate streams of water: a desalinated water layer and a salty water layer.

After the membranes separate the desalinated water stream from the increasingly salty one.

Advantages Electrodialysis Reversal

Durability membranes

1. Similar to a Reverse Osmosis system, Electrodialysis requires the use of a membrane. However, an electrodialysis system works under much lower pressure compared to a RO system, and therefore the membranes last longer.

2. On top of that, the membranes used in an electrodialysis system can be easily cleared simply by reversing the Direct Current flow. As a result, the salt is driven back into the water, and the membrane surface is cleaned off.

This means that the expensive membranes can be used a lot longer!

3. Another advantage of electrodialysis systems is their water recovery rate: these systems recover more than 90 percent, compared to only 35-65% from RO systems. This is obviously an advantage in areas where water is scarce. 

For this blog post, we interviewed Oliver Kopsch. Oliver is a top water expert and has extensive experience with solar-powered desalination systems.  

His company is called DWC, which stands for “Decentralised and Renewable” based water consultancy and offers integrated, product-independent solutions to solve any water-related problem based on renewable energies and on a decentralized basis.  Visit the DWC website here.

Watch out for a detailed interview with Oliver Kopsch in our next blog post!