The race is on to find the most cost-effective, energy-efficient, and low carbon desalination plants: urgently needed sustainable technology for ensuring drinking water supplies globally.
Even in the wettest of nations, access to fresh water can be unreliable. England has had its driest summer for more than a century, raising questions over the resilience of its water supplies in the face of more regular extreme weather events. In August, 64 percent of the continent of Europe was either flagged as under hazardous ‘warning’ or ‘alert’ conditions for drought. In 2021, the western counties of the USA experienced drought emergencies. At the same time, global water demand has increased by 600 percent over the past 100 years and is rising exponentially (UN, World Water Development report 2018).
Finding the most sustainable — and financially viable — means of re-using seawater has become imperative. An infrastructure of desalination technologies will be essential for the wellbeing of large populations, alongside economic prosperity, and in some cases, social and political stability. The current output of desalination plants globally is more than 90 million cubic metres per day, and growing populations and their demands for higher standards of living mean this figure will need to grow exponentially to meet needs - and wants. In the Middle East and North Africa, the gap between demand for water and actual supply is said to be around 42 cubic kilometres per year, according to the World Bank figures, and expected to increase by five times that by 2050.
Desalination plants — converting seawater into drinking water and water for agriculture and other purposes — have been used since the 1950s in the Middle East and tropical regions of the world. But while the technology has met urgent needs for clean water supplies, the environmental cost has been heavy, dependent on oil and gas-powered plants. The World Bank has said it is “critical” that the desalination industry makes the shift away from fossil fuels. As much as the fuels involved, there have also been the environmental risks presented by the large-scale traffic in oil tankers.
Solar-powered desalination plants, in theory and practice, have been under development since the start of the 21st century. The challenge continues to be around gaining hard evidence of viability in terms of costs and scale of implementation.
The new generation of solar desalination may be based on multi-effect distillation (MED) technology. In MED, feed water is evaporated in several stages under reduced temperature and pressure. In the first stage, seawater is heated and evaporated by the heat from the external source — in this case either solar, geothermal or waste heat from any industrial plants. For the remaining stages, feed water is evaporated by using the latent heat of freshwater vapours flowing from the previous stage. In this way, the latent energy of the vapours is used to evaporate the seawater, and freshwater condensation is collected from each stage. But in the last stage, vapours produced are condensed by the incoming seawater. In the case of solar-driven-MED units, energy to evaporate feedwater in the first stage is supplied by using a solar-thermal technology such as a parabolic trough collector or evacuated tube collector etc.
Research has demonstrated that solar-powered MED technology is a viable option involving lower temperature and energy requirements by comparison with other commercially available thermal technologies. As the first tech of its kind though, the water production costs, though, are still relatively high. The integration of the MED with low temperature (60 °C–95 °C) solar collectors means a cost of between 2$/m3 and 3.6$/m3; for medium temperature (165 °C–200 °C) it is 1.4$/m3–3.1$/m3; and high temperature (370 °C–530 °C): 1.8$/m3−2.2$/m3. In general, that would mean a payback period in terms of investment into the infrastructure of four to 16 years.
Reverse osmosis (RO) technology is more energy efficient and widely implemented than MED technology due to the low cost of water production, although carbon emissions are higher. Also, solar-driven MED plants are important for those regions with severe water shortage but a higher incidence of solar energy. In the case of Gulf countries, where seawater can have harsh characteristics, such as high temperature, residuals of boron, high TDS and bromides, and severe fluctuations in seawater intake quality, the RO technology faces operational challenges. Also, with thermal storage technology becoming more mature, the challenge of operating solar-driven MED plants for 24 hours a day can be easily addressed.
The challenge with this — and all other forms of desalination technology — is finding the working balance between an effective low-carbon process and one that is economically viable for industry and investors.
One important way of improving viability is the deployment of renewable energy driven desalination systems at a faster pace. For example, Morocco via NOORo project with CSP technology and DEWA is using PV technology at Mohammed bin Rashid Al Maktoum Solar Park. As the costs reduce, the pace of deployment will inevitably accelerate. Also, capturing and re-using the waste heat available from industries and power plants could be an alternative to respond towards the decarbonisation of this sector.
Other approaches being adopted include forward osmosis (FO), where a concentrated salt or other solute solution is used to draw out the pure water from the seawater using the physical principle of osmotic pressure (both a low energy and low cost solution); and the combination of desalination with generating electricity: extra heat generated from photovoltaic panels is used as a power source for the membrane distillation process. Latent heat from vapour condensation is captured and re-used to power cycles of water evaporation, creating a circular system of use and re-use.
Minerals can be recovered from the desalination process, providing another revenue stream for sustainable desalination operators (zero liquid discharge) — and a way to avoid returning brine into the sea, a traditional by-product from plants. A group at Cranfield is researching the production of Lithium from waste brines to meet the global Lithium shortage.
With an infrastructure of sustainable desalination technology in place, the global water supply system will have much-improved resilience to extreme weather events and shocks. But major investment is needed: a will in the industry to support innovation.