Phase control by latching.

In order to obtain the optimum oscillatory motion for maximising the absorbed energy or the converted useful energy it may be necessary to return some energy back into the sea during some small fractions of each oscillation cycle and profit from this during the remaining part of the cycle. For this reason ―optimum control‖ of WECs has also been termed ―reactive control‖. To achieve this in practice it is required to utilise a reversible energy-converting machinery with very low conversion losses. It could, for instance be a high-efficiency hydraulic machinery which can work either as a motor or as a pump.11 To realise the optimum control in practice, a computer with appropriate programme software, and with input signals from sensors measuring the wave1 and/or the WEC‘s oscillatory motion10, is required. It is also necessary to predict the wave some seconds into the future.

74 SETTING OF OTEC PLANTS

The Earth's oceans are continually heated by the sun, and cover nearly 70% of the earth‘s surface. The secret to harvesting the ocean‘s stored solar energy lies in exploiting the difference in temperature between the warmer water at the surface, and the colder water at greater depth.

If the extraction could be made cost-effective, it could provide two to three times more energy than other ocean-energy options, such as wave power. But the small magnitude of the temperature difference makes energy extraction, so far, relatively difficult and expensive.

How Does Ocean Thermal Energy Conversion Create Electrical Energy?

Perhaps the easiest way to understand ocean thermal energy conversion (OTEC) is by looking at the three primary types of OTEC plant: (1) open-cycle, (2) closed-cycle, and (3) hybrid.

All three plants make use of a ―heat engine‖ – a device placed between deep, cold ocean water and shallow, warmer water. As heat flows from the warm water to the cold water, the heat engine uses the energy of the transfer to drive a generator that creates electricity. Closed-cycle Ocean Thermal Energy Conversion

Warm surface seawater is pumped through a heat exchanger that vaporizes a fluid with a low boiling point (e.g., ammonia). The expanding vapor turns a turbo-generator to produce electricity.

Open-cycle Ocean Thermal Energy Conversion

Warm seawater is placed in a low-pressure container, where it boils. The expanding steam drives a turbine attached to an electrical generator. When the ocean water turns to steam, it leaves behind its salt and other contaminants. The steam is then exposed to cold ocean water, condensing it into fresh water for drinking or irrigation.

Hybrid Ocean Thermal Energy Conversion

Warm seawater enters a vacuum chamber, where it is flash-evaporated into steam (similar to the open-cycle process). The heat of the steam vaporizes ammonia in a separate container, and the vaporized ammonia drives a turbine to produce electricity (similar to the closed-cycle process). Vaporizing the seawater removes its salt and other impurities. When the steam condenses in the heat exchanger, it emerges as fresh, pure water for drinking or agriculture.

75 Where Are the Best Locations for OTC Plants?

OTEC plants can produce more power where the temperature difference between warm and cold ocean water is greatest. This generally occurs within 20° north and south of the equator, in the tropics.

What Is the Record Power Output From an OTEC Plant?

In May 1993, an experimental open-cycle OTEC plant at Keahole Point, Hawaii produced 50,000 watts of electricity, breaking the record of 40,000 watts set by a Japanese system in 1982.

Has Ocean Thermal Energy Conversion Been Tried in the Past?

In 1881, French physicist Jacques Arsene d‘Arsonval proposed tapping the thermal energy of the ocean. A student of d‘Arsonval‘s, Georges Claude, built the first OTEC plant in Cuba in 1930. The system generated 22 kW of electricity using a low-pressure turbine.

The Natural Energy Laboratory of Hawaii Authority, established in 1974, is one of the world's leading test facilities for OTEC technology. Hawaii is often said to be the best U.S. location for OTEC, because of warm surface water, excellent access to very deep, very cold water, and because Hawaii has the highest electricity costs in the U.S.

Japan has been a major contributor to the development of OTEC technology, primarily for export to other countries. In the 1970s, the Tokyo Electric Power Company built a 100 kW closed-cycle OTEC plant on the island of Nauru. The plant became operational in 1981 and produced about 120 kW of electricity (90 kW was used to power the plant, and the remaining electricity was used to power a school and several other facilities in Nauru). This set a world record for power output from an OTEC system where the power was sent to a real power grid.

What Share of the World’s Energy Needs Could OTEC Supply?

Some experts believe that if OTEC became cost-competitive, it could provide gigawatts of electrical power, and in conjunction with electrolysis, could produce enough hydrogen to completely replace all projected global fossil fuel consumption.

What Barriers Stand in the Way OTEC Power Production?

Managing costs remains a huge challenge. OTEC plants require expensive, large- diameter intake pipes, submerged at least a kilometer deep in the ocean to bring very cold water to the surface. Cold seawater is a requirement for all three types of OTEC

76 systems. The cold seawater can be brought to the surface by direct pumping, or by desalinating the seawater near the sea floor, lowering its density and causing it to ―float‖ through a pipe to the surface.

Has a Closed-cycle OTEC Plant Ever Been Built?

In 1979, the Natural Energy Laboratory and several private-sector partners developed a mini OTEC experiment that achieved the first successful at-sea production of net electrical power from closed-cycle OTEC. (Net power is that which remains after subtracting the power required to run the plant.) The mini OTEC vessel was moored 1.5 miles off the Hawaiian coast and produced enough net electricity to illuminate the ship's light bulbs and run its computers and televisions.

In 1999, the Natural Energy Laboratory tested a 250 kW pilot closed-cycle plant, the largest of its kind. Since then, no further tests of OTEC technology have been conducted in the U.S., largely because the costs of energy production today have delayed financing of a permanent, continuously operating plant.

What OTEC Projects are on the Drawing Board?

Planned OTEC projects include a small plant for the U.S. Navy base on the island of Diego Garcia in the Indian Ocean, to replace existing diesel generators. The plant would also provide 1,250 gallons of drinking water to the base per day.

A private firm has proposed building a 10-MW OTEC plant on Guam. And Lockheed Martin‘s Alternative Energy Development team is in the final design phases of a 10-MW closed cycle OTEC pilot system that will become operational in Hawaii in 2012 or 2013. The system will be designed to expand to 100-MW commercial systems in the near future.

Does OTEC Have Benefits Beyond Producing Power?

Yes, indeed. For example, the cold seawater from an OTEC system can provide air- conditioning for buildings. If such a system operated 8000 hours per year in a large building, and local electricity sold for 5¢-10¢ per kilowatt-hour, it could save $200,000- $400,000 in annual energy bills (U.S. Department of Energy, 1989). The InterContinental Resort and Thalasso-Spa on Bora Bora now uses OTEC technology to air-condition its buildings. The system passes cold seawater through a heat exchanger, where it cools fresh water in a closed-loop system. The cool freshwater is then pumped to buildings for cooling (no conversion to electricity takes place).

Another application is chilled-soil agriculture . When cold seawater flows through underground pipes, it chills the surrounding soil. The temperature difference between

77 plant roots in the cool soil and plant leaves in the warm air allows many plants that evolved in temperate climates to be grown in the subtropics.

Aquaculture, another viable OTEC offshoot, is considered one of the best ways to reduce the financial and energy costs of pumping large volumes of water from the deep ocean. Deep ocean water contains high concentrations of essential nutrients that are depleted in surface waters due to consumption by animal and plant life. This ―artificial upwelling‖ mimics natural upwellings responsible for fertilizing and supporting the largest marine ecosystems, and the largest densities of life on the planet. Cold-water delicacies such as salmon and lobster, and microalgae such as spirulina can also be cultivated in the nutrient-rich cold water from OTEC plants.

As described earlier, open-cycle and hybrid OTEC plants produce desalinated wate. System analysis indicates that a 2-megawatt (net) plant could produce about 4300 cubic meters of desalinated water per day (Block and Lalenzuela 1985).

OTEC plants can produce hydrogen via electrolysis, using electricity generated by the OTEC plant. Also, minerals can be extracted from seawater pumped by OTEC plants. Japanese researchers have recently found that developments in materials sciences and other technologies are improving the ability to extract minerals efficiently, using ocean energy.

What Barriers Stand in the Way of Ocean Thermal Energy Conversion?

The obstacles to OTEC as a viable power source are considerable, but probably not insurmountable. Political concerns include the legal status of OTEC facilities located in the open ocean. Costs, of course, also remain uncertain, because so few OTEC facilities have been deployed. One study estimated OTEC power generation costs as low as US $0.07 per kilowatt-hour, compared with $0.05 - $0.07 for subsidized wind systems.

How Positive is the Outlook for OTEC Power Generation?

Positive factors include the fact that OTEC is a renewable resource without waste products or limited fuel supplies; the vast area in which it is available (within 20° of the equator); freedom from dependence on petroleum; possible development of alternate ocean power sources, such as wave energy, tidal energy, and by extracting methane hydrates; and the possibility of combining OTEC with solar energy, aquaculture, air conditioning, and mineral extraction.

Do Technical Difficulties Stand in the Way of OTEC?

Unfortunately, yes. For example, OTEC plants often use direct contact heat exchangers, which generate gases that can degrade a plant‘s efficiency. Since the theoretical

78 maximum efficiency of OTEC plants is 6% to 7%, and present plants operate at slightly lower efficiencies, anything that degrades performance must be considered significant. Other problems include microbial fouling, which lowers thermal conductivity; improper sealing; and parasitic power consumption by exhaust compressors. However, these obstacles are the focus of ongoing research, and seem likely to be solved in the near future.

79 TIDAL ENERGY AND WAVE ENERGY

What is Wave and Tidal Energy?

In addition to its abundant solar, wind and geothermal resources, the Pacific Northwest is also uniquely situated to capture the renewable energy of the ocean. Special buoys, turbines, and other technologies can capture the power of waves and tides and convert it into clean, pollution-free electricity. Like other renewable resources, both wave and tidal energy are variable in nature. Waves are produced by winds blowing across the surface of the ocean. However, because waves travel across the ocean, their arrival time at the wave power facility may be more predictable than wind. In contrast, tidal energy, which is driven by the gravitational pull of the moon and sun, is predictable centuries in advance.

The technologies needed to generate electricity from wave and tidal energy are at a nascent stage, but the first commercial projects are currently under development, including some in the Pacific Northwest. Like most emerging energy technologies, wave and tidal technologies are currently more expensive than traditional generating resources, but with further experience in the field, adequate R&D funding, and proactive public policy support, the costs of wave and tidal technologies are expected to fol-low the same rapid decrease in price that wind energy has experienced.

Potential

Worldwide potential for wave and tidal power is enormous, however, local geography greatly influences the electricity generation potential of each technology. Wave energy resources are best between 30º and 60º latitude in both hemispheres,

and the potential tends to be the greatest on western coasts.

The United States receives 2,100 terawatt-hours of incident wave energy along its coastlines each year, and tapping just one quarter of this potential could produce as much energy as the entire U.S. hydropower system. Oregon and Washington have the strongest wave energy resource in the lower 48 states and could eventually generate several thousand megawatts of electricity using wave resources.2 _{Several sites in} Washington‘s Puget Sound with excellent tidal resources could be developed, potentially yielding several hundred megawatts of tidal power.3

While no commercial wave or tidal projects have yet been developed in the United States, several projects are planned for the near future, including projects in the Northwest. AquaEnergy Group, Ltd is currently designing and permitting a one- megawatt demonstration wave power plant at Makah Bay, Washington. Ocean Power Technologies has received a preliminary permit to explore construction of North America‘s first utility-scale wave energy facility off the coast of Reedsport, Oregon. With the support of the Oregon Department of Energy, Oregon State University is also seeking funding to build a national wave en-ergy research facility near Newport, Oregon. Several tidal power projects are also being explored in the region. Tacoma Power has secured a preliminary permit to explore a tidal power project at the Tacoma Narrows, one of the best locations for tidal power in the country, and Snohomish County Public

80 Utility District has received preliminary permits for seven other potential tidal power sites in the Puget Sound.4

Wave Energy Technologies

There are three main types of wave energy technologies. One type uses floats, buoys, or pitching devices to generate electricity using the rise and fall of ocean swells to drive hydraulic pumps. A second type uses oscillating water column (OWC)devices to generate electricity at the shore using the rise and fall of water within a cylindrical shaft. The rising water drives air out of the top of the shaft, powering an air-driven turbine. Third, a tapered channel, or overtoppingdevice can be located either on or offshore. They concentrate waves and drive them into an elevated reservoir, where power is then generated using hydropower turbines as the water is released. The vast majority of recently proposed wave energy projects would use offshore floats, buoys or pitching devices.

The world‘s first commercial offshore wave energy facility will begin operating by the end of 2007 off the Atlantic coast of Portugal. The first phase of the project, which Scottish company, Ocean Power Delivery (OPD) developed, features three ‗Pelamis‘ wave energy conversion devices and generates a combined 2.25 MW of electricity. OPD plans to expand the facility to produce 22.5 MW in 2007.5

Tidal Power Technologies

Until recently, the common model for tidal power facilities involved erecting a tidal dam, or barrage, with a sluice across a narrow bay or estuary. As the tide flows in or out, creating uneven water levels on either side of the barrage, the sluice is opened and water flows through low-head hydro turbines to generate electricity. For a tidal barrage to be feasible, the difference between high and low tides must be at least 16 feet. La Rance Station in France, the world‘s first and still largest tidal barrage, has a rated capacity of 260 MW and has operated since 1966. However, tidal barrages, have several environmental drawbacks, including changes to marine and shoreline ecosystems, most notably fish populations.6

Several other models for tidal facilities have emerged in recent years, including tidal lagoons, tidal fences, and underwater tidal turbines, but none are commercially operating. Perhaps the most promising is the underwater tidal turbine. Several tidal power companies have developed tidal turbines, which are similar in many ways to wind turbines. These turbines would be placed offshore or in estuaries in strong tidal currents where the tidal flow spins the turbines, which then generate electricity. Tidal turbines would be deployed in underwater ‗farms‘ in waters 60-120 feet deep with currents exceeding 5-6 mph. Because water is much denser than air, tidal turbines are smaller than wind turbines and can produce more electricity in a given area.7 _{A pilot-scale tidal turbine facility – the first in North America} – was installed in New York‘s East River in December 2006. The developer, Verdant Power, hopes to eventually install a 10 MW tidal farm at the site.8

81 Environmental Impacts

Unlike fossil-fueled power plants, wave and tidal energy facilities generate electricity without producing any pollutant emissions or greenhouse gases. Since the first wave and tidal energy facilities are currently being deployed, the full environmental impacts of wave and tidal power remain uncertain but are projected to be small. Concerns include impacts on marine ecosystems and fisheries. Environmental impact studies are currently underway and several pilot and commercial projects are undergoing environmental monitoring. The East River tidal turbine pilot project includes a $1.5 million sonar system to monitor impacts on fish populations, for example.9 _{Careful siting should} minimize impacts on marine ecosystems, fishing and other coastal economic activities. Wave and tidal facilities also have little or no visual impact, as they are either submerged or do not rise very far above the waterline.