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Johns Hopkins Magazine

A new pump rocks 'n' rolls the salt out of water

By Elise Hancock

In the developing world, water can be scarce and low-tech is best tech - - and oh, laddie, do we have some low-tech for you! Think of seawater, made good for either drinking or irrigation, for about a penny a gallon.

That is the promise of a new mechanical device, the McCabe Wave Pump, recently placed in the Shannon Estuary off the Irish coast. As it rocks on the waves, the motion drives a pair of pumps, up down, up down, which forces seawater through a desalination system, then up to land.

If this prototype succeeds, says Michael McCormick, research professor in civil engineering, the device "will revolutionize water use on the world's 100,000 islands." That's important, because in some places, potable water can cost as much as $4 a gallon.

And McCormick, a specialist in wave energy since 1972, thinks it will work. Of all the devices he has ever studied, he says the McCabe pump is "the simplest, cheapest, easiest to use, and most effective." As for its effect on the environment, says McCormick, it is "positive - - like all wave energy systems. Fish like to congregate and procreate around these things. And if you have several of them deployed offshore, they act as breakwaters; they can actually decrease beach erosion."

The pump was invented by Irish engineer Peter McCabe, of Hydam Inc., to generate electricity, which in turn could power desalination. But the pump was just an idea when McCabe first met McCormick at a wave energy conference in Cambridge, England, in 1980. At that time McCormick was a department chairman at the U.S. Naval Academy. He liked McCabe's idea, and offered to have some midshipmen build a model, then test it in a tank - - good experience for the students.

The middies did, the pump worked, and so was born an international collaboration, with McCormick supplying the theoretical work. "I'm not being paid for this," he says. "It's just for the interest of it." (Besides, McCormick is fervently Irish.) McCormick concluded, among other things, that while electricity was (and is) a possible product, energy is lost by generating electricity and then desalinating. It would be more efficient to desalinate directly, by using the pump to force seawater through reverse osmosis units.

"It takes a lot of power to produce the high pressures we need for reverse osmosis," explains McCormick. Desalinating seawater directly "gives you that power for free." In the configuration used in the Irish prototype, the device generates only enough electricity to work its own filtering and reverse osmosis system.

The pump being tested is about half the length of a North American football field, and has three parts. In the center rides the inertial barge: basically, a large box, with stabilizer, that houses the pump. (Adjusting the stabilizer will "tune" the pump for any local wave conditions.) Hinged to the inertial barge are the power pontoons. These rock and roll freely with the waves, their motion powering two-stroke pumps in the barge. "The upstroke draws water up and in," says McCormick. "The downstroke pumps it into the RO system at a very high pressure."

The water is first given a pre-treatment that is "basically a sanitary precaution," says McCormick. Then the water is filtered, desalinated, and piped to shore for use in irrigation.

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