How the Slingshot Water Purifier Works

Only 42 percent of Nigerians have access to drinking water. The rest of the population must go directly to the source in rivers or natural storm-water reserves, which could be purified by the Slingshot. See more green science pictures.
Getty Images/ Pascal Parrot/Stringer

For plenty of people, getting a drink of water on the hottest of days is not as easy as filling a glass at the tap. Instead, they walk miles to the nearest well. Or they pay big fractions of their incomes -- more than the average person in a developed country pays -- to have a distributor truck in water, which may not even be clean. About one in six people, as of 2005, couldn't access clean water [source: United Nations]. But no one can live without water, so it's often that people without access to a clean water supply end up drinking water laced with chemicals or populated by disease-causing organisms, which can kill children and shorten adults' lives.

With this in mind, it's easy to see why the World Health Organization put access to safe drinking water on its list of Millennium Development Goals, or targets to meet by 2015 [source: WHO]. But can it be done? Each pocket of people suffering water stress needs an affordable method that fits the local conditions and lifestyle. Chlorine tablets and clay pots, boiling and cloth filters, sun barrels and rain barrels, and filter-equipped straws that can be worn on a necklace have all been tried, but some people still lack a method that works for them [sources: EPA, IDE, EAWAG, Vestergaard].

A certain well-known engineer has a product to add to the mix. Here's a hint: He rides a Segway. Dean Kamen, who invented the Segway and several groundbreaking medical devices, has put a decade of work into a water purifier that he calls the "Slingshot." The name is a reference to the story of David and Goliath -- to Kamen, waterborne disease is a Goliath of a problem, and technology is the slingshot [source: Richardson]. Read on to learn how the purifier works.

Slingshot Purification

From the outside, the water purifier looks like a black box. It's about the size of a dormitory refrigerator. Inside, there's a system for purifying water that's actually quite old and common. Drug companies use the same method to purify water for use in medicines [source: MECO]. The U.S. Navy has used the method to desalinate drinking water [source: MECO].

Drug company and submarine versions aren't practical for developing countries, though. They're too big to move and need technicians on call. The Slingshot is simpler and more portable.

All of these purifiers work by vapor compression distillation. Kamen once ran down a partial list of what this process can purify: the ocean; water laced with arsenic, poison, heavy metals, viruses and bacteria; liquid at a chemical waste site; or the contents of a latrine [source: Comedy Partners]. Remarkably, all it takes is boiling and re-liquefying water at precise temperatures. Let's see how it works.

Kamen's black box first connects to an electricity source. Next, you hook it up to a water source by dropping the hose in some water. The dirty water gets sucked into the system, where it warms to its boiling point (212 degrees Fahrenheit or 100 degrees Celsius). Then, it enters an evaporator, where it's heated a little more and boils [source: Pacella]. Already, some contaminants are lost. Anything that boils at hotter than 212 degrees F (100 degrees C) -- stones, dirt, salt -- stays in the evaporator and is drained out. Bacteria, viruses, eggs and spores get hit twice: They don't rise with the steam in the evaporator and are pasteurized by the heat in the purifier.

Steam rises from the evaporator into a compressor. The compressor squeezes the steam a little, raising its temperature a bit above 212 degrees F. The steam flows into an outer chamber whose walls are about 212 degrees F, creating another filtering step [source: Pacella]. Any contaminant that boils at colder than 212 degrees F, such as benzene, remains a gas and is vented out. Only pure water condenses on the walls.

The clean water drips into a final chamber, ready to be spouted out. But there's a problem: The water is still hot. Since hot water would be awful on a hot day, the machine cools it using a clever method. It flows incoming and outgoing streams of water past one another, so dirty water heats to 212 degrees F and outgoing water cools to the outside temperature. This heat recycling trick is called a counter-flow heat exchanger.

While the Slingshot is a powerful purifier, there's a catch. Read on to find out what it is.

The Stirling Generator

Dean Kamen, left, sits on an energy-efficient Stirling engine that he used in Bangladesh to create electricity with methane gas generated by cow dung.
Dean Kamen, left, sits on an energy-efficient Stirling engine that he used in Bangladesh to create electricity with methane gas generated by cow dung.
Associated Press/Jessica Hill

The Slingshot doesn't scrub contaminants -- from the culprits causing cholera to typhoid fever -- from water for free. It needs electricity. It's not much electricity, though -- about 1 kilowatt, which is a mere coffee-maker's requirement.

But electricity can be hard to find in places that need clean water. In cities with an electricity grid, the purifier can plug in to a wall outlet. If there's no electricity grid, the purifier can plug in to a diesel generator, which is what many off-grid hospitals use to power equipment. But in the remote desert or bush, where diesel is impractical, Kamen might suggest his Stirling engine.

Stirling engines need only a source of heat and cold to work. The heat and cold expand and compress a gas to make pistons pump. You can see how they work in the article How Stirling Engines Work. Designs that require heat, as Kamen's does, can burn almost anything, from kerosene to methane from decomposing cow dung. As a source of cold, they can use air. So the materials to run these Stirling engines can be found almost anywhere.

Kamen's Stirling is more than an engine -- it's also a generator. (Engines convert fuel to motion, and generators convert motion into electricity.) An extra part allows Kamen's Stirling engine to produce electricity. When the engine's pistons pump, they turn a magnetic rotor. The rotor rotates inside a metal coil, which creates a current [source: Van Arsdell].

When running on the Stirling generator, the Slingshot plugs in to it with a power cord [source: Kamen]. One version of Kamen's Stirling produces 1 kilowatt -- enough to run the water purifier [source: Kamen].

But the two devices may fit together even more harmoniously. The generator happens to make a lot of extra heat as it burns fuel -- up to 85 percent of it doesn't get used. But when the generator and purifier are connected by a tube, hot air can blow into the purifier. There, it can do work, heating the incoming water and surrounding the purifier like a jacket, trapping heat inside [source: Kamen]. With help from the generator, the purifier can be even more efficient.

Stirling engines are hard to make because some of the concepts are tricky to execute, and it's challenging to mass-produce them affordably. But some companies are marketing Stirling engines, and DEKA (a research and development corporation founded by Kamen) hopes its design will make it even easier [source: WhisperGen].

Now that we know what the Slingshot is and one way it can be powered, let's look at why it might be appealing to a village in need of clean water.

Slingshot Cost

Let's say you live in a rural village of 100 people. Nearby, there is a stream. Unfortunately, your and your neighbors' outhouses empty into it. Whenever you need potable water, you must walk six miles (10 kilometers) to a well and lug a small supply of water home in jugs or you drink the stream water and take your chances. How would the Slingshot help you and your community?

In one day of running stream water through the Slingshot, you'd be able to make 264.2 gallons (1,000 liters) of clean water [source: Schonfeld]. Since each villager uses about 5.3 gallons (20 liters) of water a day for drinking, cooking, and bathing, which is typical in a developing village, one Slingshot could supply enough water to support the needs of half of the village [source: United Nations]. This sounds great -- but could the village afford it?

Assuming the community has electricity to run the Slingshot, it would need $1,000 to $2,000 to buy one [source: Schonfeld]. Every villager could chip in $10 to $20, but that's more than a week's salary in plenty of places [source: United Nations]. More realistically, some community members might get a loan, buy the Slingshot and then sell clean water to the rest of the village at an affordable price (perhaps three cents per gallon or one cent per liter) until the machine is paid for [source: Schonfeld].

OK, so you know what it will cost financially, but what else is there to consider? What are the pros and cons of using the Slingshot?

One convenience of the Slingshot system is that the village would not need an expert to run the purifier. The instructions are super simple -- you stick the hose in dirty water and press a button. This simplicity makes the system safe to operate with little room for human error or mishaps. Another plus is that the water should have no chemical aftertaste thanks to the distillation process.

While operating the system requires the simple push of a button, you would still need to get water to the purifier. Typically, that means either carrying dirty water to the purifier or putting the purifier near the dirty water supply. The purifier is too heavy for one person to carry, so moving it would require a little bit of man- (or woman-) power. And finally, the machine's moving parts could eventually break and require servicing or replacing, which would cost money.

Read on to find out what's on the horizon for the Slingshot.

Plans for the Slingshot

With the help of new technology like the Slingshot, maybe some day everyone will have ready access to clean drinking water.
With the help of new technology like the Slingshot, maybe some day everyone will have ready access to clean drinking water.
Digital Vision/Getty Images

Kamen's company tested the Slingshot in Honduras. By one account, the results were excellent [source: Richardson]. The next step is production. DEKA Research and Development is looking for a financer and a manufacturer to help it make Slingshots. According to one report, Kamen approached several large companies and private foundations for financing, with no success [source: Richardson].

The company is rethinking how to market Slingshots, in one scenario, first selling it to industries for commercial distilling to get it into production. Kamen has also mentioned bodegas in Mexico, imagining regions that can plug the purifier in to a wall outlet but need an inexpensive way to make and distribute clean water [source: Richardson]. The primary goal however remains -- to get the Slingshot to anyone who needs safe drinking water.

Thinking about that goal brings to mind an exhibition that recently ran at the National Design Museum. The exhibition, which is titled "Design for the Other 90 Percent," covers the topic of design for poor populations [source: Smithsonian]. Martin Fisher, a mechanical engineer who worked on development projects in Kenya for more than 17 years, contributed an essay describing his design principles for the poor. Here are the first few. Does the Slingshot meet them?

  • The top need of people who are poor is to make money. The device should help someone make money on the local market.
  • People who are poor don't lack time and labor, so unless they can make money from the saved time and labor, they won't buy the device.
  • A device should pay for itself in "farm time" -- three to six months.
  • Successful devices address people's true needs rather than what "we" think "they" need.

Fisher adds that if a device won't make someone an immediate profit but will save money, it shouldn't sell for more than the cost of a chicken at the local market. A chicken, like this device, is an affordable, occasional luxury for poor families. But if the device costs more, only the middle class will buy it, and this group already has money for its basic needs [source: Fisher].

While Fisher's principles are reasonable for many product designs developed for poorer populations -- the LifeStraw certainly fits some of these criteria -- they don't seem applicable to the Slingshot in that it's a system that will provide for a large population, rather than an individual. And, while it's easy to agree with Fisher's statement that poor people need to make money, there's one caveat to consider; is it truly their "top" need? It's likely many would argue that access to clean drinking water demands top billing.


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