The World's Oceans: Earth's Largest Heat Sink
Heat pumps are being used in wells and ponds, so what about using them in the oceans?
This is a good news/bad news story.
The good news is that the oceans are a vast renewable resource, with the potential to help us produce billions of watts of electric power. On an average day, tropical seas absorb solar radiation equivalent in heat content to about 250 billion barrels of oil. Less than one-tenth of one percent of this stored solar energy could supply more than 20 times the total amount of electricity consumed in the United States on any given day.[i]
The bad news is that we haven't managed to do it yet in a cost-effective manner.
And the good news, again, is that we are working on it, with a technology called OTEC, or "ocean thermal energy conversion."
Most of the United States's OTEC experiments in recent years have taken place in the Natural Energy Laboratory of Hawaii Authority (NELHA), which is recognized as the world's foremost laboratory for OTEC research. The facility has been funded by the State of Hawaii with significant USDOE and private sector participation.[ii]
What is OTEC?[iii]
OTEC converts solar radiation to electric power. OTEC systems use the ocean's natural thermal gradient the different temperatures of different layers of ocean water to drive a power-producing cycle. As long as the temperature between the warm surface water and the cold deep water differs by about 36 degrees F., an OTEC system can produce a significant amount of power.
The economics of energy production have delayed the financing of a permanent, continuously operating OTEC plant. However, OTEC is very promising for tropical island communities that rely heavily on imported fuel. OTEC plants in these markets could provide islanders with much-needed power, as well as desalinated water and a variety of mariculture products.
The Basic Process[iv]
There are three types of OTEC processes: closed-cycle, open-cycle, and hybrid-cycle.
In the closed-cycle system, heat transferred from the warm surface seawater causes a working fluid (such as ammonia, which boils at a temperature of about 78 degrees F. at atmospheric pressure), to turn to vapor.
The expanding vapor drives a turbine attached to a generator which produces electricity. Cold seawater passing through a condenser containing the vaporized working fluid turns the vapor back into a liquid which is then recycled through the system.
Open-cycle OTEC uses the warm surface water itself as the working fluid. The water vaporizes in a near vacuum at surface water temperatures. The expanding vapor drives a low-pressure turbine attached to a generator which produces electricity.
The vapor, which has lost its salt and is almost pure fresh water, is condensed back into a liquid by exposure to cold temperatures from deep ocean water. If the condenser keeps the vapor from direct contact with seawater, the condensed water can be used for drinking water, irrigation or aquaculture.
A "direct contact" condenser produces more electricity, but the vapor is mixed with cold seawater and the discharge water is salty. That mixture is returned to the ocean. The process is repeated with a continuous supply of warm surface seawater.
Hybrid systems use parts of both open- and closed-cycle systems to optimize production of electricity and fresh water.
Advantages of OTEC[v]
Disadvantages and Challenges[vi]
- OTEC uses clean, abundant, renewable, natural resources.
- Suitably designed OTEC plants will produce little or no carbon dioxide or other polluting chemicals which contribute to acid rain or global warming. Extensive research indicates little or no adverse environmental effects from discharging the used OTEC water back into the ocean at prescribed depths.
- OTEC systems can produce fresh water as well as electricity. This is a significant advantage in island areas where fresh water is limited.
- There is enough solar energy received and stored in the warm tropical ocean surface layer to provide most, if not all, of present human energy needs.
- The use of OTEC as a source of electricity will help reduce our dependence on imported fossil fuels.
- The cold seawater from the OTEC process has many additional uses, including air-conditioning buildings, assisting agriculture, and growing fish, shellfish, kelp and other sea plants which thrive in the cold, nutrient-rich, pathogen-free water.
- OTEC-produced electricity at present would cost more than electricity generated from fossil fuels at their current costs. The electricity cost could be reduced significantly if the plant operated without major overhaul for 30 years or more, but there are no data on possible plant life cycles.
- OTEC plants must be located where a difference of about 40° Fahrenheit (F) occurs year 'round. Ocean depths must be available fairly close to shore-based facilities for economic operation. Floating plant ships could provide more flexibility.
- Although extensive and successful testing of OTEC has occurred in experiments on component parts or small scale plants, a pilot or demonstration plant of commercial size needs to be built to further document economic feasibility.
- Construction of OTEC plants and laying of pipes in coastal waters may cause localized damage to reefs and near-shore marine ecosystems.
- Some additional development of key components is essential to the success of future OTEC plants (e.g., less-costly large diameter, deep seawater pipelines; low-pressure turbines and condensers for open-cycle systems; etc.).
- The U.S. Department of Energy's Energy Efficiency and Renewable Energy Network, a good starting place for links to other sites on renewable energy.
- The National Database of State Incentives for Renewable Energy (DSIRE), a constantly-updated database of information from the 50 states on financial and regulatory incentives for all renewable energy systems.
- OTEC, history, facts, and diagrams of open- and and closed-cycle OTEC systems.
- Sea Solar Power, a vendor of OTEC technology, with projects in several locations; diagrams, discussions of "land-based or sea-based?", and a comprehensive set of links.
- U.S. Department of Energy (DOE) and Electric Power Research Institute (EPRI) Renewable Energy Technology Characterizations.
- best estimates of USDOE and EPRI regarding technical and economic status and future performance and cost of renewable energy technologies through the year 2030.
- Ocean Thermal Energy Conversion http://www.nrel.gov/otec/what.html
- Ocean Thermal Energy Conversion (OTEC) Fact Sheet.
- See reference 1.
- See reference 2.