At the edge of the Atacama Desert

, one of the driest places on earth, Lima, Peru, receives almost no rainfall. About 700,000 people have no access to clean water for drinking or bathing. Another 600,000 of the city's 7.5 million residents rely on cisterns for their water, which must be filled by pumps or by hand and cleaned regularly.

But Lima's Pacific Coast location experiences humidity of more than 90 percent on summer days, from December to February. So engineers from Peru's University of Engineering and Technology (UTEC) have devised a way to turn that humid air into usable water. Last December, they erected a billboard in the Bujama District of Lima that by early March had produced 9450 liters (about 2500 gallons) of water.

The idea came about because UTEC was facing a slump in enrollment as the new semester approached; the engineering department wanted a way to attract more engineering students to the university. They went to Peruvian ad agency Mayo Publicidad, and the partnership of engineers and marketers crafted an advertisement that would provide a very visible demonstration of the university's engineering projects. The water-collecting billboard was born.

Electricity from the city's power lines runs the five condensers inside the billboard. Like the condenser in your home air conditioner, the ones in the UTEC billboard are cooler than the air outside. When air contacts the cooled surfaces of the condensers, the air also cools, and the water vapor in the air condenses into liquid water. After reverse-osmosis purification, the water flows down into a 20-liter storage tank at the base of the billboard. The billboard generates about 96 liters of water each day, and a simple faucet gives local residents access to the water. UTEC has not yet announced whether the water will be available for free, but the billboard reportedly cost only about $1200 to install.

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This is not the first attempt to pull clean water out of thin air. In 2011, French company Eole installed a wind turbine in Abu Dhabi, which the company claims generates more than 1400 liters of water each day. The WMS1000 is 24 meters (about 78 feet) tall, and its 13-meter rotor turns at up to 100 rpm to run a 30-kilowatt generator. This in turn powers a cooling compressor inside the turbine. An intake pulls air into the compressor, and moisture condenses out as the air cools. The water runs down into a purification and storage tank at the base of the turbine.

The turbine needs winds of at least 15 mph to generate enough power for the compressor. In a desert climate with an average temperate of 95 degrees Fahrenheit and average relative humidity of about 30 percent, the WMS1000 generates about 350 liters of water a day. In humid coastal climates, production increases to about 1200 liters a day. Adding a solar power unit to the turbine could increase output by a few hundred liters more.

Eole designed the turbine for remote communities of fewer than 5000 people, but when it launched the WMS1000 commercially, in 2012, the price tag for a single turbine was about $660,000, well beyond the budget of most small communities in developing countries.

Back in Lima, the UTEC engineering department and Mayo Publicidad may have found a way to offset the cost barrier: advertising.

Since the billboard's installation, UTEC reports a 28 percent increase in enrollment. Results like that may attract the attention of private companies looking for new ways to advertise. The city of Lima and other urban areas, such as Cairo, Egypt, suffer the same lack of potable water as remote villages, and an advertising-funded solution that taps into an existing electrical infrastructure may work well there. UTEC has not yet announced plans to install more billboards in Lima or to make the technology commercially available elsewhere, but the project has started new discussions about how to provide access to clean water.

Worldwide, the World Health Organization estimates, about a billion people lack access to safe drinking water. Lack of clean water is a leading cause of cholera and other diseases that cause diarrhea. Perhaps UTEC's idea can make the situation a little better, one sign at a time.

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