The desalination challenge
Bassem Halabi discusses strategic approaches to desalination
Water desalination is considered as one of the most viable water supply solutions in many areas around the world, where fresh water is not easily accessible. This much needed commodity is essential for economic development and social stability.
According to the 2011 Global Food and Water Crises Research Programme report, there are about 14,500 desalination facilities across the globe, with the highest desalination capacity located in the Middle East.
It is projected that the global demand for desalination projects will in fact triple in the next six years resulting in a possible global desalination market that tops US $30 billion by 2016 (The Global Water Intelligence (GWI) 2012 report).
With the above in mind, it is critical that we look at the cost of such projects – as desalination could be a costly solution that also consumes considerable amounts of energy. To put this into perspective, it is safe to say that energy consumption is the most costly aspect of running a desalination plant; often amounting to one-third to more than half of the cost.
The advanced reverse osmosis systems, for instance, apply energy recovery or pressure conversion devices and report higher energy consumption of above 2 kW hour/m3. The ratio of the desalinated water volume to the seawater volume used to produce it is called the recovery ratio.
High recovery ratio saves on the cost of seawater preparation prior to the osmosis process, and low recovery ratio saves on the energy cost of desalination. The optimal recovery ratio depends on the relative costs of these operations and may vary under different conditions.
That being said, developing desalination technologies that are energy efficient is therefore vital to making desalination an economically viable and sustainable solution.
Nowadays, the most applied desalination technologies across the globe remain to be reverse osmosis (RO) and thermal distillation. In recent years, RO seawater desalination plants have become even more popular, because of their cost effectiveness and simplicity. It is worth mentioning that Metito was the first company to design and install a reverse osmosis plant outside the United States in 1972.
Another advanced solution that is commonly used to help monitor and make energy consumption more efficient in thermal distillation systems is the Gained Output Ratio (GOR).
This ratio measures the amount of thermal energy being consumed in a desalination process; typically the value of GOR ranges from 1 to 10 kg of distillate/kg of steam (kg/kg), with the lower values atypical in applications where there is a high availability of low value thermal energy.
Higher GOR values, up to 18 kg/kg, are associated with situations where local energy values are high. Higher GOR systems cost more but consume less energy, reflecting on much lower operating costs.
To overcome the energy cost challenge and its associated environmental footprint, alternative sources of energy are increasingly being used. A good example of this is the “Osmotic Energy” - an environmentally friendly energy that is created by the transportation of solutions or salinity gradient energy.
The water in this process aspires to decrease the salt-concentration on the side of the membrane that contains most salt, and in doing so the water streams through the membranes creating pressure on the opposite side. This pressure can be utilised to create energy using a turbine and a generator.
We still have a long way to go in tackling the power and water consensus and in further exploring energy efficient /green technologies for use in desalination - a true challenge that needs to be prioritised.
The fact is; investing in such technologies will not only solve a dire need for more desalination projects where water is most needed but it will also ensure sustainable sources of water supply that can be relied on for generations to come.