When he isn't snowboarding or volunteering for Engineers Without Borders, Dave Williams spends his days thinking about something most of us take for granted: ice. As he discovered on a volunteer trip to Haiti in 2002, ice can be a godsend to a poor village, keeping fish fresh on a journey to market or preserving vaccines. But how do you make it without electricity, without access to coolants like Freon or fuels like propane? Williams, 26, knew that forcing compressed air through a hole in the middle of a pipe causes hot and cold air to flow from opposite ends, a phenomenon known as the Ranque-Hilsch vortex-tube effect. No one is quite sure how the separation works, but feed the cold air into a container, he reasoned, and you would have an icemaker and a freezer, which would have zero operating costs and would be environmentally friendly, since it wouldn't require chemicals and the jet of air could be generated via a compressor powered by wind, water, man or animal.

At least that was the idea. Tinkering with heat-transfer equations, Williams tried to determine how much energy it would take to yield a block of ice. "It had been a while since I'd done real math problems. I had to break out the old textbook," says Williams, a product-development consultant with his own firm, Dissigno, in San Francisco. After eons of number crunching, he hit on the right formula and built a prototype. It isn't very efficient; his device uses 35 times as much energy as an electric fridge to make 1 kg of ice. But its simplicity could yield a killer app in Third World villages, where Williams hopes aid groups will distribute his icemaker as an economic-development tool. He aims to field-test it in Haiti later this year. --By Daren Fonda. Reported by Matt Smith/ New York

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