May 15, 2008

A new wave: Ocean power

By Sonal Patel

Ocean current energy

Oceanic bodies are constantly in motion, propelled by a variety of factors, including winds, water temperature, salinity levels, and the Coriolis force. Surface currents—which constitute about 10% of all the water in the oceans—are restricted to the upper 1,300 feet of the oceans and predominantly derive their energy from the wind and the sun.

According to the U.S. Minerals Management Service, the world’s oceans harbor about 5,000 GW of power, with densities of up to 1.5 kW/f². By some estimates, this means that capturing just 0.1% of the available energy from the Gulf Stream—which has 21,000 times more energy than Niagara Falls and a flow 50 times that of the world’s freshwater rivers collectively—could generate enough power to supply Florida with 35% of its electrical needs.

Compared with other ocean energy technologies, the harnessing of ocean current power is still in its infancy. A handful of prototypes and demonstration units have been developed, including Hammerfest Strøm’s submerged wind-like turbines that capture energy with hydrodynamic, rather than aerodynamic, lift or drag. However, developers of this technology continue to grapple with a long list of potential problems, from complex and costly maintenance to the potential disruption of delicate marine ecosystems.

Open-center turbine. The stand-alone open-center turbine that features a fixed outer disk and a rotating inner disk is thought to suit both tidal stream and ocean current applications. Gulfstream Energy Inc.’s device is designed to be anchored to the seafloor, 200 feet below the surface. Installed 5 miles offshore in Florida’s Gulfstream, where the current flows at an average sustained speed of 3 knots, the turbine could produce an estimated 2.5 MW of power.

OpenHydro’s open-center turbine, on the other hand, has been applied to the extraction of tidal stream energy with such promising results that the company will engage in a demonstration project in the Bay of Fundy. The device has a self-contained rotor fitted with a solid-state permanent magnet generator, all encapsulated within an outer rim (Figure 9).

9. Goes with the flow. OpenHydro’s open-center turbine extracts tidal stream energy and will be used in a demonstration project in the Bay of Fundy. Courtesy: OpenHydro

Salinity gradient energy

It has been estimated that 2.6 TW may be derived from exploiting the salinity gradient—or salt differential—between the world’s seawater and freshwater. The process, called pressure-retarded osmosis (PRO), basically involves pumping seawater at 60% to 85% of the osmotic pressure against one side of semipermeable membranes whose other side is exposed to freshwater.

When freshwater, compelled by osmosis, flows across the membranes, it dilutes the saltwater and increases its volume—and consequently, the pressure within the saltwater chamber. A generator-driving turbine is spun as the pressure is compensated. (For details of the PRO process see "Osmotic power from the ocean" in POWER, November/December 2006.) PRO can be thought of as the reverse osmosis process (used for desalination and water treatment) running backward and producing power from the flow of freshwater.

A decade of collaborative research and development by the Norwegian University of Science and Technology and Statkraft, a Norwegian power company, has yielded promising results, including the development of a high-performance membrane. In 2007, Statkraft initiated construction on the world’s first osmotic plant prototype. The plant, at Tofte on the Oslo fjord, scheduled for completion by the end of 2008, will produce between 2 kW and 4 kW of power.

Ocean thermal gradient energy

Tides and currents aren’t the only potential energy source we can harness from the oceans. Oceans also absorb and store tremendous amounts of solar energy. According to the National Renewable Energy Laboratory, 23 million square miles of tropical seas absorb an amount of solar radiation equal in heat content to about 250 billion barrels of oil—a tenth of which could supply 20 times the power needs of the entire U.S. on any given day. The technology for converting this solar radiation into electrical power is ocean thermal energy conversion (OTEC), which exploits the ocean’s thermal gradients—temperature differences of 36 degrees F or more between warm surface water and cold deep seawater—to drive a power-producing cycle.

Besides sourcing a clean, renewable reserve of energy, OTEC has the potential to provide many useful by-products such as freshwater, hydrogen via electrolytic processing of freshwater, and lithium and uranium, which may be extracted from deep seawater. Despite these potential payoffs, OTEC development has been gradual, primarily because of competitive operation costs. The technology was first proposed as far back as 1881 by a French physicist, and several prototypes have been tested intermittently since the first experimental 22-kW low-pressure turbine was deployed in 1930.

Closed-cycle OTEC. In the closed-cycle version of OTEC, warm seawater from the ocean’s surface vaporizes a working fluid with a low boiling point, such as ammonia, which then flows through an evaporator. The vapor expands and turns a generator-driving turbine. The vapor is then condensed using cold seawater pumped from deep within the ocean. The working fluid is continuously recycled within this closed system.

Open-cycle OTEC. In the open-cycle variant of this technology, warm seawater becomes the working fluid and is flash-evaporated in a vacuum chamber, producing pressurized steam. The steam then expands through a low-pressure turbine. Cold seawater condenses the steam, and, if it remains separated, could supply desalinized water as a by-product.

Hybrid-cycle OTEC. A hybrid-cycle OTEC employs features of both the closed and open cycles (Figure 10). Warm seawater is flash-evaporated, the steam is used to vaporize the working fluid, and that fluid drives a turbine. Finally, steam condenses and provides desalinized water.

10. Hybrid model. Ocean thermal energy conversion (OTEC) takes advantage of ocean temperature gradients to generate power. In the hybrid OTEC process, warm seawater is flash-evaporated, the steam is used to vaporize the working fluid, and that fluid drives a turbine. Source: National Renewable Energy Laboratory

Although components to test the technology are widely available, no commercial-scale plants—or even pilot plants connected to a grid—exist. The most ambitious prototype to date was an Indian research vessel that carried a 1-MW OTEC plant in 2002. That effort, a collaboration with the Japanese company Xenesys Inc. and Saga University in Japan, was unsuccessful due to a failure of the deep sea cold water pipe.

Xenesys is determined to power on, however. It opened a research and development center dedicated to OTEC last November to meet what it sees as increased demand as a result of renewed interest. According to its web site, the Indian government plans to construct 1,000 OTEC power plants, each 50,000 kW, throughout the country. And the island nation of Palau is planning to launch a 3,000-kW OTEC plant; it hopes to make a complete switch from diesel oil for power generation to OTEC in the next 10 years. Xenesys said that it has been receiving offers for research support and technical collaboration from more than 50 countries, including South Korea, the Philippines, Indonesia, Sri Lanka, Maldives, Cook Islands, and the United States.

Companies like U.S.-based Sea Solar Power Inc. (SSP) have also continued to test and develop key elements of the hybrid Rankine cycle OTEC plant. SSP President James H. Anderson Jr. penned articles for POWER as far back as 1965 with his father, J. Hilbert Anderson, regarding the feasibility and economic viability of power harvested from the ocean’s thermal gradient. The younger Anderson asserts that the technology continues to have tremendous potential—that the only setback it may experience is a lack of interest from the government.

The Andersons had estimated in 1965 that it would cost $5.54 million to set up a floating 20-MW “sea thermal” plant, including the installation of major equipment and auxiliaries plus the cost of services such as engineering and supervision. Today, a small but commercial-sized floating OTEC plant of 20 MW—which could be built in as little as 39 months—could cost from $120 million to $190 million.