Turning the tide to energy  

NASA JPL has an article on a scheme to generate tidal power using a "hydrokinetic energy system" - Turning the tide to energy.

NASA researchers who developed a new way to power robotic underwater vehicles believe a spin-off technology could help convert ocean energy into electrical energy on a much larger scale. The researchers hope that clean, renewable energy produced from the motion of the ocean and rivers could potentially meet an important part of the world's demand for electricity.

Many different methods already exist for using moving water to create power. Hydroelectric plants, for example, are among the most established and least expensive sources of electricity. They benefit from the large hydrostatic pressure difference between the water surface behind the dam and the turbines that can be harnessed to produce power. But the power that can be produced in this manner is limited, because most of the suitable rivers already have hydroelectric dams.

Other technologies have been designed -- and are being developed -- to turn the energy of ocean currents, tides, and flowing rivers into another kind of power, called hydrokinetic energy. Many of these hydrokinetic energy systems use underwater turbines, similar to those in wind farms. Ocean currents or tides turn the turbines, which generate electricity that can be transferred by cable to shore.

Jack Jones, an engineer at NASA's Jet Propulsion Laboratory, and Yi Chao, a JPL scientist, have designed a new kind of underwater hydrokinetic energy system. It uses water motion to generate a high-pressure liquid rather than electricity. That liquid is then transported to shore and used to produce electricity on land. Caltech, which operates JPL for NASA, holds the patent on this innovative energy technology.

The JPL/Caltech hydrokinetic energy system is a spin-off from a research project to find a new way to power robotic underwater vehicles. Most robotic underwater vehicles run on batteries and have to be recovered by ship to have their batteries recharged or replaced.

In this project, initiated by Pat Beauchamp of JPL's Center for In Situ Exploration and Sample Return, Jones was asked to develop a way to use temperature differences in the ocean to power submersibles. He had previously developed thermally controlled balloons for Venus, Mars and Titan. Jones was teamed up with Chao, who uses underwater gliders in his oceanographic research. "I saw we could extend the lives of these vehicles significantly by harvesting energy from the ocean environment," Chao says.

Jones and Chao designed a system that takes advantages of changes in ocean temperature to create a high-pressure fluid that can be used to generate power. "The trick was to find a special substance known as a phase change material that changes from a solid to a liquid as the temperature in the environment changes from cold to warm," Chao says. "When the material melts, it expands, compressing a central tube in which another liquid is stored. This liquid, now under high pressure, is used to generate electricity to charge the battery underwater."

Working with colleagues from JPL and the Scripps Institution of Oceanography at the University of California at San Diego, and funded by the Office of Naval Research, Jones and Chao are developing a prototype underwater vehicle powered by this new energy system. They will conduct field tests in the Pacific Ocean this fall.

While they were working on this project, the researchers realized that they could employ the same concept -- using an environmental pump to generate a high-pressure liquid -- to produce electricity from the world's ocean.