By Mattie DeDoes
The world’s oceans provide a seemingly limitless, clean, and relatively untapped source of energy. Harvesting energy from waves, currents, temperature changes, and chemical differences would help our society’s constant quest to limit our environmental impact.
Widespread implementation of technologies that exploit oceanic energy sources is hope for the future. However, the potential that exists is extremely exciting, and many important approaches, prototypes, and energy-producing installations are being developed worldwide.
In recent years, a few methods have been tested and implemented to collect the energy available from the ocean. The majority of these attempt to harness the mechanical energy from waves, tides, and ocean currents, and convert it to electricity. Another viable approach utilizes the temperature difference between surface waters and deep ocean waters to generate steam, which can power a turbine.People have been using power from moving water to perform work since the Ancient Greeks used water wheels to grind grains into flour. Hydroelectric power has been commonplace since the beginning of electricity transmission in the late 1800’s. However, the vast potential of the ocean’s energy remains widely unused. By comparison, solar and wind energy technologies are much more advanced in both their efficiency and their affordability for large-scale installations.
Conceptually, the most straightforward method for extracting the ocean’s energy is to capture the power of water in motion. In standard hydroelectric plants situated on rivers, the direction of water flow is clear and constant; therefore, a device that converts this energy to electricity can be engineered fairly simply. Unfortunately, the direction of ocean waves is much more difficult to predict than the direction of a river. In order to combat this uncertainty, many wave energy converters have been built to move up and down with the wave, rather than side-to-side. This eliminates the need to predict the direction in which the waves are moving in order to produce power.
One type of device for capturing wave power consists of a floating or submerged buoy connected by a cable to a hydraulic pump. This pump is fastened to the sea floor. The force from the wave lifting the buoy pulls up on a piston inside the pump, pressurizing a fluid contained inside. This pressurized fluid can be pumped back onshore and used to drive a generator. In 2014, three of these buoy systems were installed by Carnegie Wave Energy off the western coast of Australia, marking the first ever grid-connected wave energy installation. According to the Australia Renewable Energy Association (ARENA), the system expects to have a peak capacity of 240 kilowatts.
Pelamis Wave Power in the United Kingdom took a slightly different approach. Their design used connected tube-shaped buoys to form a floating snake-like machine. As different sections are raised and lowered by the motion of the ocean, hydraulic pumps between the tube sections (in the joints) operate in a similar fashion as described above to drive onshore generators. In 2010, one of these machines was installed for testing off the coast of Orkney, Scotland. Unfortunately for ocean power enthusiasts, after a few years of poor funding, Pelamis went into bankruptcy in late 2014.
In terms of efficiency, the main drawback of wave power is its unpredictability. Ocean waves are caused by water being pushed by the wind, which is notorious for unforeseen fluctuations. As a result, the output of these types of power systems over a short period of time (daily, weekly) may be quite unknown. Another shortcoming is that because of their low profile, devices that operate near the surface may present a hazard to ships.
While wave power may be limited by its unpredictable nature, the rise and fall of tides is well known. The concept of harvesting energy from the tides has been around for centuries. In Europe, during the Middle Ages, dams were built to collect tidal flows, and the confined water was released to turn water wheels in grain mills. The same process is used in many of today’s tidal power plants, using submerged turbines to generate electricity. However, in order to produce sufficient amounts of energy, locations with significant differences in high and low tide levels must be chosen.
The La Rance tidal power plant opened in 1966 in northwestern France, and remained the largest plant of its kind until 2011, when it was surpassed by the Sihwa Lake Tidal Power Station in South Korea. The Sihwa Lake project has a capacity of 254 MW, over 25 times the capacity of the Wyandot Solar Farm, the largest solar installation in the state of Ohio. Learn more about the top five tidal power plants in the world in this article.
At an inlet near Belfast, UK, developers took advantage of a natural dam system. Almost 100 billion gallons of water flows through a narrow strait connecting Strangford Lough to the sea as a result of the tide. The large size of the inlet compared to the small width of the strait provides this natural collection system, where the high tidal water is contained in Strangford Lough and subsequently released into the ocean at low tide. Two 16-meter propellers have been connected to the floor of the channel which generates 1.2 megawatts of power from the flow of the water. Marine Current Turbines, the company responsible for building the system, is preparing to install five 2-megawatt machines off the coast of Wales by 2016.
Ocean Thermal Energy Conversion
This type of technology, known as OTEC, does not harvest the ocean’s mechanical energy. Instead, it uses the difference in temperature between the surface water and deep-sea water to produce energy. In order to achieve a useful level of efficiency in OTEC plant, a large difference in the temperature of the two water sources must be present. This temperature difference is only available within the tropics, near the equator.
Two primary approaches have been developed for OTEC exploitation:
- In a closed-cycle plant, warm surface seawater is pumped into a heat exchanger, where it is used to vaporize a fluid with a low boiling point (like ammonia). The vapor is used to power a generator, and the colder, deep-sea water condenses the ammonia vapor back into liquid form to be reused.
- An open-cycle plant pulls the warm surface water into a low-pressure container, where the water itself boils and is used to drive the generator. It is then allowed to cool when exposed to the deep-sea water.
After the water boils in an open-cycle plant, it leaves behind a great deal of salt and condenses into fresh water. As a result, an open-cycle power plant can double as a desalination plant. However, the deep-sea water used for cooling carries a great deal of nutrients. By mixing the nutrient-rich water with the condensed, purified water, the concentrations of these nutrients are reduced before pumping the water back offshore. Removing and diluting this deep-sea water could harm the surrounding ecosystem. The impacts of this effect are currently being studied in greater detail.
Hawaii has experimented with OTEC systems since the 1970’s, and multiple OTEC plants are currently operating throughout the Hawaiian islands. Lockheed Martin plans to install a 10 MW pilot plant off the southern coast of China, which would become the largest OTEC plant in the world. Lockheed Martin hopes the project will provide a learning experience and opportunity for greater study, and that more plants of this type, with capacities up to 100 MW, can be constructed in the near future.
Concerns and Reservations
The main drawback of almost all of these ocean power systems is the cost. The overwhelmingly positive asset of the mechanisms discussed above is that their operation is completely carbon-neutral, eliminating a great deal of emissions resulting from the burning of coal and natural gas. However, the ocean power industry is far behind the solar and wind industries in terms of its technological viability, as well as mass production.
After decades of research and development, commercial solar panels - while still needing to improve upon their efficiencies - can be installed for almost any home in the country, providing customers with renewable energy at a competitive cost.
Compared to solar technology, ocean energy projects are in their infancy. As a result, a great deal of money is spent to perform engineering calculations and develop prototypes, rather than install large-scale facilities. The hope for the future is that as more time and money are invested in ocean energy technologies, types of plants like the ones discussed can eventually be installed at a reasonable cost.
In addition to financial limitations, the environmental consequences of ocean power plants are also being investigated. While these plants operate completely emissions-free, placing a plant in the middle of the ocean could have an adverse effect on the surrounding ecosystem. Just as bird casualties are a concern for windmill developers, there are fears that ocean power systems containing a propeller (like the one near Belfast mentioned earlier) could strike migrating sea animals passing through the turbines. Researchers at the U.S. Department of Energy have performed various impact tests on fish and whale skins, as well as rubber model substitutes, to investigate the possibility of injury to these creatures. A Pacific Northwest National Laboratory study determined that a whale struck on the head by a turbine would escape with little more than a bruise.
Where We’re Going
In 2013, approximately 1.15 TWh (terrawatt-hours) of energy were generated from ocean systems around the world, with almost all of this production coming from Oceania and Europe. Almost 50% of all wave energy and tidal energy developers are located in the EU, and if current test projects remain on schedule, it is estimated that the EU’s combined capacity from tidal and wave energy could reach 66 MW by 2018.
While research has been performed in the U.S., there are limited installations currently. There is still a great deal to be done in assessing suitable sites, reducing costs, improving efficiency, and connecting offshore devices to the electrical grid. California estimates that with existing technology, there is enough ocean energy that can be harnessed along its 745-mile coast to account for 25% of the state’s energy needs.
The world’s oceans provide extraordinary potential as a source for producing clean energy. However, the task of constructing machines to efficiently and affordably extract this energy remains in the early stages of development. With greater investment and research, these technologies could make a great contribution toward our eventual goal of clean, affordable, and plentiful energy.