Introduction to Ocean Power

The scientific community remains intrigued by the prospect of harnessing the energy of the ocean, if for no other reason than the fact that oceans covers over 70 percent of the earth’s surface. A few successes have been experienced in this area:

• Tidal power plant in France, designed to produce 240 MW, which is larger than a typical wind farm and about 30-50 percent of the capacity of a new coal-fired generation unit
• 20 MW plant in Nova Scotia
• 0.5 MW plant on the White Sea in Russia

Despite this relatively short list of successful implementations, there are a number of research projects underway to apply technology of converting the energy generated by waves, tides, and currents into electricity and there is general belief that ocean power can be competitive as compared to other sources of renewable energy.

The article provides a brief description of the basics of ocean power and some of the more promising technologies that may be applied in harnessing this source of energy.

Capturing Ocean Power

The more noticeable aspects of the ocean, and key to capturing ocean power, are waves and tides:

• The wind drives waves, traveling over long distances as a series of crests and troughs. This oscillating motion can be converted into electricity.
• Tides, on the other hand, are the result of the gravitational pull of the moon. All coastal areas experience two high and low tides every 24 hours, but only approximately 40 are suitable for generating electricity. There are requirements regarding the differences between high and low tide-must be at least 16 feet, to support the water flow necessary to produce electricity efficiently.

Harnessing the kinetic energy of all this moving water is the challenge:

• Building a dam whose sluice gates open when the tides produce sufficient difference in water level, facilitating the flow of water across turbines, very similar to the traditional hydroelectric process. In this case the turbines turn a generator which, in turn, produces electricity.
• Tapping into tidal currents provides an alternative approach, where turbines, similar to those used in wind farms but designed with the rotor underwater, are used. These rotors spin as the tidal currents go past the turbines, turning the generator and producing electricity.

An Alternative to Producing Ocean Power

The building of dams and use of turbine rotors to produce electricity represent the more traditional mechanical energy approaches. There is also tremendous potential in exploring how heat stored in the ocean can produce electricity. In fact, the ocean is the largest collector of solar energy, providing the equivalent of 250 billion barrels of oil in the form of thermal energy each day. Ocean Thermal Energy Conversion (OTEC) is the process used to convert this potential energy into usable electricity. There are three types of OTEC systems, each exploiting the difference in temperature between the warmer surface water and the colder deeper ocean water:

• Closed Cycle Systems: These systems contain a fluid with a low boiling point, such as ammonia, that, when exposed to the warm seawater immediately boils into a vapor that passes over the turbine, causing it to turn. This vapor is then exposed to the cold seawater and condenses back into its liquid form, ready for the next cycle.
• Open Cycle Systems: The warm surface water is pumped into a vacuum chamber and boils as the air is removed. The resulting steam is used to drive a turbine, which is then cooled by the cold seawater, and converted, back into water.
• Hybrid Systems: The warm surface water converts to steam in a LP container, which then vaporizes a low-boiling point fluid in a closed loop that drives the turbine.

OTEC systems current account for a small amount of electricity. But, given the amount of potential thermal energy that is collected in the ocean on a daily basis, many feel that the viability of ocean power as a key element of any energy diversification strategy will largely hinge on our ability to further develop and refine these systems.

Benefits of Ocean Power

Besides the obvious potential of the ocean as an enormous and seemingly endless source of clean energy, it is also fairly predictable:

• High and low tides can be calculated down to the minute,
• Dimensions and velocity of currents are well-documented

Additionally, seawater is three times the density of air, representing significantly more potential in transmitting energy to turbines placed in the ocean. The output of a tidal turbine in currents of only 5 miles per hour exceeds that of a wind turbine operating in strong winds.

Last, ocean power requires less space, far fewer units than wind farms or solar facilities, is quieter than wind power, and operates offshore and often below the surface,

Concerns Regarding Ocean Power

The primary hurdle to overcome the significant gap between ocean power’s current contribution to global electricity generation and that provided by fossil fuels is reliability. Marine environments are extremely harsh on mechanical systems, driving costs to the point where currently, the costs of wave energy (9 to 16 cents per kWh) and tidal energy (6 to 8 cents per kWh) is significantly higher than coal (3 cents per kWh) or natural gas (4.7 cents per kWh).

There are also environmental challenges to overcome, namely the effect of large structures in the ocean on marine life, normal flushing of silt and other pollutants, and water quality.

The following summarizes the specifics of these challenges by source of ocean power:

• Waves: The intensity can vary and equipment survivability is in question. Issues with navigation and sea space.
• Tides: The intervals can be extended and there are only a limited number of “qualified” sites; Major impact to environment.
• Tidal Currents: Equipment survivability and high operating costs.
• Ocean Currents: Limited number of sites and potentially impacts the circulation patterns of the ocean.
• Ocean Thermal Energy: High initial capital costs, remote sites and requirement to transmit electricity over long distances.

Despite these challenges, the ability to tap into a vast resource that produces carbon-free energy provides sufficient impetus to explore further ways to counter or mitigate these effects.