MIT Technology Review

Thursday, April 08, 2010

Solar-Powered Desalination

Saudi Arabia's newest purification plant will use state-of-the-art solar technology.

Saudi Arabia meets much of its drinking water needs by removing salt and other minerals from seawater. Now the country plans to use one of its most abundant resources to counter its fresh-water shortage: sunshine. Saudi Arabia's national research agency, King Abdulaziz City for Science and Technology (KACST), is building what will be the world's largest solar-powered desalination plant in the city of Al-Khafji.

The plant will use a new kind of concentrated solar photovoltaic (PV) technology and new water-filtration technology, which KACST developed with IBM. When completed at the end of 2012, the plant will produce 30,000 cubic meters of desalinated water per day to meet the needs of 100,000 people.

KACST's main goal is to reduce the cost of desalinating water. Half of the operating cost of a desalination plant currently comes from energy use, and most current plants run on fossil fuels. Depending on the price of fuel, producing a cubic meter now takes between 40 and 90 cents.

Reducing cost isn't the only reason that people have dreamed of coupling renewable energy with desalination for decades, says Lisa Henthorne, a director at the International Desalination Association. "Anything we can do to lower this cost over time or reduce the greenhouse gas emissions associated with that power is a good thing," Henthorne says. "This is truly a demonstration in order to work out the bugs, to see if the technologies can work well together."

While the new concentrated PV technology might generate affordable electricity, solar power still costs more than fossil fuels in many parts of the world. But even with those high costs, using it to power desalination makes sense, Henthorne says. "You're not doing it because it's the cheaper thing to do right now, but it would be the cheapest thing down the road."

Desalination plants typically use distillation. Most upcoming plants, including the one in Al-Khafji, will use a process called reverse osmosis, which forces seawater through a polymer membrane using pressure to filter out salt. Both these methods are energy-intensive. Saudi Arabia, the top desalinated water producer in the world, uses 1.5 million barrels of oil per day at its plants, according to Arab News.

The new plant's concentrated PV and reverse-osmosis systems will use advanced materials developed by IBM for making computer chips.

In a concentrated PV system, lenses or mirrors focus sunlight on ultra-efficient solar cells that convert the light into electricity. The idea is to cut costs by using fewer semiconductor solar cell materials. But multiplying the sun's power by hundreds of times creates a lot of heat. "If you don't cool [the device], you end up overheating the circuits and killing them," says Sharon Nunes, vice president of IBM Big Green Innovations. IBM's solution is to use a highly conducting liquid metal--an indium gallium alloy--on the underside of silicon computer chips to ferry heat away. Using this liquid metal, the researchers have been able to concentrate 2,300 times the sun's power onto a one-square-centimeter solar device. That is three times higher than what's possible with current concentrator systems, says Nunes.

For desalination, IBM has worked with researchers at the University of Texas at Austin to develop a robust membrane that makes reverse osmosis more energy-efficient. Desalination is done today with polyamide membranes that get clogged with oil and organisms in seawater. The chlorine used to pretreat seawater also breaks down the membranes over time.

The new polymer membrane contains hexafluoro alcohols, a material IBM uses to pattern copper circuits on computer chips. At high pH, the fluorine groups become charged and protect the membrane from chlorine and clogging. As a result, water flows through it 25 to 50 percent faster than through currently used reverse-osmosis membranes, according to IBM.

The new membrane removes 99.5 percent of the salt in seawater. This is comparable with conventional polyamide membranes, says Menachem Elimelech, chair of chemical engineering at Yale University. "You need to achieve this high rejection, otherwise you can't get good water quality by one pass, you have to desalinate again."

The Al-Khafji desalination plant is the first of three steps in a solar-energy program launched by KACST to reduce desalination costs. The second step will be a 300,000-cubic-meter facility, and the third phase will involve several more solar-power desalination plants at various locations.