Solar desalination

Solar desalination sees the light of dawn

New solar technologies promise a more sustainable way for water-starved communities to squeeze drinkable water out of the salty seas.

The take-away

  • Nearly half the global population are living in areas with potentially no freshwater at least one month per year.
  • Of the 16,000 desalination facilities worldwide, only 130 draw their power from renewable sources. New solar technologies offer hope of cheap and sustainable desalinated water for all.

Tormented by thirst, the sailors in Samuel Taylor Coleridge’s poem The Rime of the Ancient Mariner wail “Water, water, every where, Nor any drop to drink.” The same plight faces many parched communities in arid regions of the world today – they may have access to the ocean but have little water to quench their thirst.

Climate change and population growth are making fresh, drinkable water an increasingly valuable and critical commodity. According to a 2018 UNESCO report, 3.6 billion people worldwide (nearly half the global population) are living in areas potentially bereft of freshwater at least one month per year, and this number will only grow. To tackle this global challenge, countries are making their agricultural sectors more energy efficient, building their reservoirs bigger, and plugging leaks in domestic and industrial supply. But is this enough?

The Middle East is oil-rich but one of the most water-poor regions on the planet. For the likes of Saudi Arabia, the United Arab Emirates, Bahrain and many more, the situation had become so critical that bolder solutions were needed. They therefore used their oil wealth to make undrinkable ocean water drinkable. 53% of the estimated 16,000 desalination facilities active worldwide – which extract salt from sea (or other saline) water to transform it into freshwater – are concentrated in the region.

Currently, Saudi Arabia leads the world in the production and consumption of desalinated water. The Saudi Saline Water Conversion Corporation (SWCC)’s 31 desalination plants produce half of the country’s drinking water, with a world-record daily production capacity of 5 million cubic metres. And they are not stopping there, investing € 21 billion through 2020 in the construction of several more desalination plants.

However, desalination comes with a variety of environmental concerns. Alongside the worry that brine – a byproduct of desalination typically pumped back into the water source – can hurt marine ecosystems, one of the biggest is the carbon footprint of desalination, which relies on fossil fuel power plants for its electricity supply in most cases; only about 130 of these plants around the world are powered by renewable energy sources.

According to World Bank figures, if current trends continue and no improvements are made, by 2050 desalination around the globe will account for around 400 million tons of greenhouse gas emission carbon equivalents per year – roughly the same as consuming 1 billion barrels of oil, or the emissions produced from 100 coal-fired power plants.

Running desalination on renewables

Desalination techniques can be split into two broad classes: thermal and electrical desalination. The former is a process whereby energy is used to evaporate water and subsequently condense it again. Meanwhile, there are various electrical desalination techniques. The most prominent is reverse osmosis – accounting for 69% of the volume of desalinated water produced – which pressurises the salty water so that it flows through a water-permeable membrane, separating the water from the dissolved salts. Though reverse osmosis is nearly 10 times more energy efficient that thermal desalination, the energy to power desalination is still prohibitively expensive and unsustainable. This is why attention has turned to renewables, particularly harnessing the power of the Sun.

Many attempts have been made over the past two decades at developing commercial solar desalination systems, but only limited small-scale success has been achieved. Why? “My speculation: fouling [i.e. clogging of membranes] is still an issue, which requires energy and chemicals to overcome, energy requirements are still high, and fossil fuel-based energy is too cheap,” says Martin Rygaard, an associate professor at DTU Environment (Technical University of Denmark).

However, this picture is starting to change. For instance, as part of an EU-backed project that aims to provide clean drinking water to the population of the Gaza Strip, in August last year the largest solar photovoltaic power plant in the region was unveiled. It will generate 12.5% of the power needed for the Southern Gaza Desalination Plant, which uses reverse osmosis and serves 75,000 people. Meanwhile, in California in the US WaterFX is taking a different tack, building a solar thermal desalination facility whose parabolic trough mirrors direct 100 percent of solar radiation into heat to evaporate salty water.

Yet despite these and other worthy attempts at making desalination sustainable, issues remain, and most revolve around money. Solar power may be thought of as free energy, but the land solar farms require and the manufacturing process behind solar energy collectors are not.

Various groups are tackling this problem from different angles. In the US, for example, international aerospace and defence company Lockheed Martin has produced an incredibly thin and strong graphene-based material called Perforene as an alternative to the filters used in reverse osmosis. With Perforene, reverse osmosis will require ~100 times less energy, making desalination more energy efficient and therefore requiring drastically fewer solar cells.

Across the Atlantic, EPFL’s Mohammad Khaja Nazeeruddin is tackling another problem – how to make the manufacturing process for solar cells used in desalination more environmentally friendly. “With silicon solar cells you have to melt the ingredients at 1400 degrees,” he says. “It’s very energy intensive and technologically challenging.”

To solve this issue, Nazeeruddin’s team is developing a new type of solar cell to power conventional reverse osmosis desalination, based on perovskites (compounds that can be used to form photovoltaic cells): “The new technology requires two ingredients that are commercially available, and these can form the solar cells at 120 degrees.”

Though the technology’s stability and efficiency at scale still need to be proven, once they overcome these hurdles the prospects are enormous: “The vision is for people to inkjet print their own perovskite solar cells, not only for desalination, but for producing cheap electricity.”