In order to reduce the United States’ dependence on fossil-fuel energy resources, alternate sources of renewable must be investigated in order to reduce the country’s emission of greenhouse gas emissions which contribute to global warming. At the recent COP21 Climate Summit in Paris, the United States committed to a reduction of greenhouse gas emissions by 26-28 percent below its 2005 level. This includes reductions in methane, carbon dioxide, perfluorocarbons, sulfur hexafluoride, nitrogen trifluoride and nitrous oxide, all of which contribute to global warming . In order to achieve these targets, the United States will need to employ alternative sources of renewable energy within the next ten years. One viable option is the use of Ocean Thermal Energy Conversion (OTEC).
The world’s oceans are the largest collectors of solar energy and OTEC technology is able to use this free and readily available source of energy to satisfy the global energy demand. Oceans cover over seventy percent of the Earth’s surface, and it is estimated that up to five tetrawatts (TW) of energy, about twice the world’s current energy usage, can be harvested without affecting oceanic temperature or the global environment . Unlike other renewable technologies, OTEC technology does not leave a carbon footprint, and thus has little if any, adverse impact on the environment. Dependent only upon sunshine and natural oceanic currents, this technology has the potential to make a significant contribution to a sustainable global energy mix .
OTEC is a renewable marine-based energy technology using the sun’s heat which is absorbed by the world’s oceans to generate electricity. Solar energy heating the oceans’ surface waters creates a gradient on temperature whereby surface waters are much warmer than the deeper ocean. By utilizing this natural temperature gradient, thermal energy can be used to produce electrical energy in a continuous cycle .
The technology is a heat engine system with a heat source and a heat sink . Warm ocean surface water (about 77F) –the heat source – is passed through a heat exchanger which is in contact with a closed system containing a low-boiling- point fluid such as ammonia, which is vaporized. This vapor then expands and drives a turbine to produce mechanical energy. This energy is converted to electrical energy by a generator connected to the turbine. Cooled by seawater at about 41F, which has been pumped from deeper in the ocean – the heat sink – the vapor is condensed back into a reusable fluid . The temperature differential between the ocean layers is an important factor in determining the efficiency of OTEC – the greater the differential, the more efficient the process. This technology is therefore most viable in regions close to the equator, where there is an ocean temperature differential of 36F or more, all year round .
Although the system can be located either onshore or offshore, it is more cost-effective to locate a facility offshore, where the cold water pipe hangs below an offshore platform, an integral element of the facility, moored to the ocean floor, which is used to house the necessary pumping and generating equipment. Electricity generated by the facility would be carried by power cables located on the floor of the ocean, to the onshore electricity grid. An onshore facility requires that the cold water intake pipe would need to be very long in order to reach the deep ocean cold water and would have to be entrenched on the sea floor and linked to the facility . Several types of OTEC systems are available but the most promising commercial facility is the offshore, closed-cycle system . Large volumes of continuously available warm and cold water are required by an OTEC establishment in order to be able to generate electricity. To generate 100 megawatts (MW), of electricity, a facility would need access to between 10-20 billion gallons of water daily .
OTEC technology has several unique and interesting features and by-products, one of which is the vertical cold-water intake pipe. To take advantage of the oceanic temperature differential which is critical to the success of the technology, the cold water pipe is a critical element of the system. The pipe must extend to a depth of 330 feet in order to draw cold water from the deeper oceanic layer and must have a diameter of about 33 feet in order to accommodate the large amounts of cold water which is required for generation of 100 MW of electricity .
Another interesting feature of the technology is its ability to generate useful byproducts such as fresh water, food and a method of cooling. A 100 MW facility would be able to produce some 450,000 m3 of fresh water per day, which compares favorably with the costs of standard desalination plants . Deep ocean water is by its nature, extremely nutrient rich, and the waste water from the OTEC cooling process can be used in containment ponds on land for mariculture production. Deep ocean cold water can also be used in cooling systems, air-conditioning systems and refrigeration systems, after which it can be discharged into the deep ocean . It can also be integrated with other commercial systems such as desalination plants and aquaculture facilities . Through the electrolysis of water, OTEC facilities can also produce hydrogen, and the process can also produce ammonia and biofuels. An offshore plant can also be employed as a ship refueling station .
In 1979, the United States first developed a Mini-OTEC facility on the Kona coast of Hawaii, and in 1980 built OTEC-1 to identify design parameters for large-scale heat exchangers need for the technology . At the same time the US Congress passed Public Law (PL) 96-320, The Ocean Thermal Energy Conversion Act, to encourage the commercialization and deployment of the technology, and formulated a national goal of generating 10,000 MW of energy from OTEC by 1999 . However, a subsequent global decline in energy generation costs made research and development of the technology no longer economically viable, and it was not until the 2000s, when concerns about the environmental harms of fossil fuel usage and energy security issues were raised, that interest was renewed in OTEC technology.
Theoretically, OTEC could be used to generate most of the world’s energy demand and while government interest in renewable energy resources has markedly increased in recent years, some challenges exist before the United States is able to forge ahead with implementation of this technology.
A regulatory framework with clear, consistent and predictable guidelines will need to be established in order that this technology may progress without compromise to human and environmental health and safety. The National Oceanic and Atmospheric Administration (NOAA) must be afforded adequate authority and technical expertise, together with adequate staffing and financial resources to successfully manage the implementation of the technology. Current licensing regulations are applied only on an individual basis by site, and are not specific or consistent enough to provide consistency and regulatory certain or to prevent adverse environmental effects and legal ramifications .
One of the main impediments to the implementation of this technology is the need to secure finance for the developmental technology . Prohibitive initial capital costs prevent developers from initiating projects, and for those projects already underway, the private funding to upscale the projects is not forthcoming due to lack of a proven results in the field . However, in 2009, the Obama administration increased federal support for research into renewable energy, sending a clear message to private and institutional investors that OTEC research will continue to be funded thus laying a foundation for the further development of this technology .
Hope for the United States’ lessening dependence on fossil fuels was greatly increased in August, 2015 when a 100 kilowatt (kW) onshore, closed-cycle OTEC plant was commissioned in Hawaii. While the plant’s output is still very small, it is the first of its kind in the world and the first to be connected to the US grid . American firm, Lockheed Martin Corporation, has recently been commissioned to design and built a 10 MW OTEC facility off the coast of the Peoples Republic of China. With lessons learned from this development, the corporation expects to apply this experience to the further development of OTEC plants worldwide .
Works Cited
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NOAA. Ocean Thermal Energy Conversion (OTEC) Technology. 11 July 2016. <https://coast.noaa.gov/czm/media/technicalfactsheet.pdf>.
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