Here Comes The Sun: Engineers Develop Solar Desalination System

September 16, 2003

GAINESVILLE, Fla. — A solution to the growing need for fresh water in many parts of the world may come from a natural source: the sun.

Engineers at the University of Florida have developed a system that uses a gravity-induced vacuum and solar energy instead of electricity or fossil fuels to desalinate water. The system is significantly more efficient than previous solar “stills” for removing salt, yet is simple and inexpensive enough to be built in remote locations where conventionally powered technologies would be either too expensive or impractical.

“We know that nature uses solar energy to get fresh water from salt water,” said Yogi Goswami, a professor of mechanical engineering and director of UF’s Solar Energy and Energy Conversion Laboratory. “We use the same process as nature, except we enhance the process.”

A paper about the system by Goswami and a May UF doctoral graduate in mechanical engineering recently appeared in the Proceedings of the 2003 International Solar Energy Conference.

A lack of potable water is a growing problem worldwide, according to a United Nations report released in March. The World Water Development Report says many countries in the Middle East, Africa and Asia currently face severe water crises, and the number is likely to grow in coming years as increasing populations and worsening pollution chip away at fresh-water supplies. In less than half a century, 7 billion people in 60 countries could face water scarcity, the report says.

Seawater desalination has long been seen as an attractive solution since the world’s oceans, which contain 97.5 percent of the Earth’s water, are inexhaustible. But current desalination systems are heavy energy users, which makes them impractical for poor countries facing water shortages, Goswami said.

Nature has its own process for desalination. Fresh water evaporates from the ocean, forms clouds, condenses and falls to the ground as rain. Goswami sought to recreate and enhance this process by exploiting solar energy and natural barometric pressure.

The guts of their system consist of a U-shaped pipe placed upside down, with one end of the pipe suspended in a tank of salt water and one in fresh water. When the 32-foot-high pipe is filled with water, some of it drops into the tanks, leaving behind a vacuum.

The area of the pipe where the vacuum occurs is surrounded by an evaporator that circulates water heated in a solar collector. The heat forces the salt water to begin evaporating – a process facilitated by the vacuum because it significantly lowers the boiling or evaporating temperature of the water. The resulting steam then enters a condenser. The fresh water drips from the condenser down the pipe into the tank.

Tests on a small, experimental version of the system revealed it is 90 percent efficient, which means 90 percent of the solar energy piped into the evaporator was used in the desalination. Previous “flat basin” solar stills were only 50 percent efficient, according to Goswami. Although the system produced only about a half cup of fresh water an hour, it can be scaled up to provide more, Goswami said.

The system, which consists of off-the-shelf pipes and other readily available parts, offers a potentially low-cost solution to desalinating seawater in remote or inaccessible areas, said E. Delyannis, a scientist and solar desalination expert at the National Center for Scientific Research’s Solar & Other Energy Research Laboratory in Athens, Greece.

“Dr. Goswami’s system is promising (as) a new method that will improve solar distillation efficiency,” Delyannis said. “But you have to keep in mind that all these systems are not compatible to conventional desalination plants of large capacity. They can be applied in small communities lacking fresh or good-quality water or to remote places to provide the people leaving there with small quantities of water.”