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A scheme in Arizona using solar energy to power a

Thu, 09/20/2012 - 5:24am
New Scientist

THE sun beats down on sand dunes and cracked red soil as we rumble through the desert in a heavy-duty pickup truck. It is monsoon season in the Navajo Nation but the scenery looks as if it never got the memo. Two decades of severe drought have left the land parched.

"It's hard to imagine that 30 years ago this was all covered in knee-high grass," says Kevin Black, a Native American affairs specialist for the US Bureau of Reclamation. "Clean water has become the Navajo's most precious commodity."

Arizona's largest aquifer lies 120 metres beneath us, but the water is not drinkable. It's half as salty as seawater, and a 2008 study found that it contains dangerous levels of uranium and arsenic.

Black says piping water from a tribe-owned purification facility to homes scattered across an area the size of West Virginia is an economic impossibility. For 80,000 Navajo, this means no running water at home.

Instead, Navajo families drive hundreds of kilometres every month to collect water. "It is called water hauling," Black says. "It is an expensive and time-consuming journey that has become part of the Navajo way of life."

The Bureau of Reclamation and engineers at the University of Arizona think they have come up with a way to help, by building a self-sufficient, solar-powered desalination plant.

Black points out the budding facility perched on a hill in the south-western corner of the reservation ( see map). Construction began in mid-August. On completion in 2013, it will produce close to 4000 litres of clean water a day, he says. With sufficient funding, the facility could be the first of a series across the reservation. That could halve the cost of hauling water.

Wendell Ela, the project's lead engineer, has been testing prototypes for the past year in Tucson. He says the process uses electricity from solar panels to pump contaminated water up from the aquifer and boil it.

The steam then passes through a series of membranes that filter out salt and other contaminants. As it cools, the difference in vapour pressure it creates draws more hot water vapour through the system. Purified water is then collected in an external condenser.

Although commercial desalination plants have been used in the Middle East and Australia since the early 1990s, they rely on reverse osmosis or multistage distillation - processes that are technically challenging and expensive to maintain.

By contrast, the team's membrane desalination system is ideal for an isolated population that does not have access to an electrical grid, Ela says. It is built using simple, low-tech, off-the-shelf components. The team is aiming to build a system lasting 30 to 40 years that would require only periodic maintenance. "We had to design the system to be within the capacity and budget of the water-users," he says.

Such reliability isn't guaranteed, says John Lienhard, an engineer at the Massachusetts Institute of Technology, who says the technology hasn't been around long enough to have much of a track record. And the systems cost up to $100,000 to build. But he says that should fall steeply as they gain wider adoption.

The impact on Navajo life would be profound. If the other planned desalination plants are built in the reservation, they could provide running water to tens of thousands of Navajo who have never had it.

"People always ask, 'If it is so bad why don't they leave?'" Black says. "My response is, 'Who are we to question them?' Their livelihood and culture has been intertwined with this place for thousands of years. We are trying to help them hold on to their identity in a world that is changing very fast."

Issue 2883 of New Scientist magazine

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