Big ocean swells could one day be as exciting to homeowners as they are to surfers, thanks to an innovative new device that harnesses the power of waves to produce cheap, clean electricity. Pioneered by scientists at Energetech, a small alternative-energy company in Randwick, Australia, a prototype of the $1.5-million device is now in testing off the Australian coast, and Energetech hopes to build another one near Rhode Island by 2007. Moored several miles offshore, Energetech's 40-foot-tall rig relies on the up-and-down motion of waves to force air in and out of a chamber, turning a turbine that produces electricity. The company's president, Tom Engelsman, says that a full-scale unit could power up to 5,000 homes; the output depends on the size and regularity of the swells. But recent theoretical work of two Chinese scientists on amplifying wave energy could soon make devices like Energetech's even more effective.