OCEAN THERMAL ENERGY CONVERSION

OCEAN THERMAL ENERGY CONVERSION

INTRODUCTION :
Ocean thermal energy conversion (OTEC) is perhaps the most exciting world energy resource for the future-the near future. It promises vast amounts of energy (even ten times the current worldwide human utilization) that is cheap, naturally self-renewing, and ecologically friendly.
The oceans cover a little more than 70 percent of the Earth's surface. This makes them the world's largest solar energy collector and energy storage system. On an average day, 60 million square kilometers (23 million square miles) of tropical seas absorb an amount of solar radiation equal in heat content to about 250 billion barrels of oil. If less than one-tenth of one percent of this stored solar energy could be converted into electric power, it would supply more than 20 times the total amount of electricity consumed in the United States on any given day.
OTEC, or Ocean Thermal Energy Conversion, is an energy technology that converts solar radiation to electric power. OTEC systems use the ocean's natural thermal gradient—the fact that the ocean's layers of water have different temperatures—to drive a power-producing cycle. As long as the temperature between the warm surface water and the cold deep water differs by about 20°C (36°F), an OTEC system can produce a significant amount of power, with little impact on the surrounding environment. The oceans are thus a vast renewable resource, with the potential to help us produce billions of watts of electric power. This potential is estimated to be about 10 13 watts of baseload power generation, according to some experts.
The distinctive feature of OTEC energy systems is that the end products include not only energy in the form of electricity, but several other synergistic products.



HOW OTEC WORKS ?

Technologies for Ocean Thermal Energy Conversion are OPEN cycle, CLOSED cycle, HYBRID cycle. The working cycles may be chosen depending on the circumstances.


[ Fig. - Working of OTEC plant ]

Closed-Cycle
These systems use fluid with a low-boiling point, such as ammonia, to rotate a turbine to generate electricity. Warm surface seawater is pumped through a heat exchanger where the low-boiling-point fluid is vaporized. The expanding vapor turns the turbo-generator. Cold deep-seawater—pumped through a second heat exchanger—condenses the vapor back into a liquid, which is then recycled through the system.
In 1979, the Natural Energy Laboratory and several private-sector partners developed the mini OTEC experiment, which achieved the first successful at-sea production of net electrical power from closed-cycle OTEC. The mini OTEC vessel was moored 1.5 miles (2.4 km) 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 OTEC closed-cycle plant, the largest such plant ever put into operation.

Open-Cycle

These systems use the tropical oceans' warm surface water to make electricity. When warm seawater is placed in a low-pressure container, it boils. The expanding steam drives a low-pressure turbine attached to an electrical generator. The steam, which has left its salt behind in the low-pressure container, is almost pure fresh water. It is condensed back into a liquid by exposure to cold temperatures from deep-ocean water.
In 1984, the Solar Energy Research Institute (now the National Renewable Energy Laboratory) developed a vertical-spout evaporator to convert warm seawater into low-pressure steam for open-cycle plants. Energy conversion efficiencies as high as 97% were achieved. In May 1993, an open-cycle OTEC plant at Keahole Point, Hawaii, produced 50,000 watts of electricity during a net power-producing experiment.

Hybrid
These systems combine the features of both the closed-cycle and open-cycle systems. In a hybrid system, warm seawater enters a vacuum chamber where it is flash-evaporated into steam, similar to the open-cycle evaporation process. The steam vaporizes a low-boiling-point fluid (in a closed-cycle loop) that drives a turbine to produce electricity.


WHAT IS REQUIRED FOR OTEC ?

It is technologically easier to build and run an OTEC facility on shore rather than floating on the open ocean. This is because a shore-based facility is not as vulnerable to severe storms or high waves as a floating facility. In addition, siting a facility on shore allows local utilization of OTEC products and byproducts (power, fresh water, mariculture products, and chill-water). This saves on storage, distribution, and marketing expenses.
However, political (and particularly business and ecological constraints) may make it impossible to have initial OTEC sites near or on shore. It may be [ Fig. - Ideal Site for OTEC ]
necessary to have the first few OTECs demonstrating prolonged operational integrity and value afloat on the open tropical oceans. Nevertheless, shore-based OTECs are technologically easier to build and to run. It is therefore an important exercise to try to find a geographically, politically, and economically suitable coastal site for OTEC.
An OTEC facility can only be financed two ways: as a local utility or by venture capital. It should be possible to make a preliminary assessment of the feasibility of each of these financial paths. First, how extensive is the local utility infrastructure (power, water, transportation, etc.) and how is it funded for construction and maintenance? This information might provide some guidance as to whether the political structure of the area is amenable to OTEC development. Second, is there significant venture capital invested in the area? If there is not, it is probably because the effective political and economic forces are not conducive to it.

A careful analysis must be made of possible shore-based sites around the world. This involves several factors.
One must identify tropical sites where there is deep ocean water very near shore (within, say, ten kilometers or six miles). This is because the huge OTEC intake pipes are a major portion of the initial capital expense. It is important to make them as short as possible. This paper considers a maximum possible length to be ten kilometers (six miles). However, some authorities consider the practical maximum cold-water-pipe length to be much shorter than this, perhaps at most two or three kilometers (one or two miles).
One must then determine which of these sites have accommodating governments (for permitting, proprietary ownership, construction, labor, taxes, marketing products, etc.). George Lockwood points out (in a personal communication) that political risk insurance is available from the World Bank and the U.S. Overseas Investment Corporation at modest costs. He reports that there are very few countries where risks of expropriation or appropriation cannot be mitigated.
In addition, one must establish which of these sites have adequate local infrastructure to absorb an additional five or ten megawatts of power and several million gallons of fresh water per day. (Note that some authorities project practical OTEC facilities as small as one megawatt with a half-million gallons of fresh water produced per day.) In fact, one of the most critical factors affecting the feasibility of a shoreline OTEC site turns out to be availability of a willing buyer for the power. Furthermore, many power system engineers and financiers will not look much beyond diesel generation for power to remote locations since they consider diesel power to be proven and reliable. Calculation and projection of power and water needs can also be difficult. Socioculturally advanced communities use up to one megawatt of electricity and 400,000 liters (100,000 gallons) of water per day per 1,000 people. In less developed areas, people may use half as much water (though several times this amount may be needed in agricultural regions) and one-tenth as much electricity. Bearing in mind these complexities, as a rule of thumb, for OTEC development one is looking for a minimum population of at least 10,000 but, optimally, a population in excess of 50,000.


WHAT ARE THE BENEFITS OF OTEC ?


[Fig. – Figure showing Benefits of OTEC ]

General Benefits :
The energy resources for OTEC are vast.
The energy resources for OTEC are naturally self-renewing.
OTEC is non-polluting, in fact, it is ecologically positive since it enriches nutrient-poor surface water and tends to “sink” carbon. The nitrogen, phosphorus, silica, and other nutrients raised from the deep are combined via photosynthesis with atmospheric and ocean-dissolved carbon dioxide to produce increased biomass and reduce atmospheric carbon load.
OTEC is based on established turbine and refrigeration technologies.
OTEC readily produces, as side benefits, considerable quantities of fresh water, sea foods, and marine-life-based industrial products, as well as chill-water for air conditioning and cold-bed agriculture.

Economical Benefits :
We can measure the value of an ocean thermal energy conversion (OTEC) plant and continued OTEC development by both its economic and noneconomic benefits. OTEC's economic benefits include these:
Helps produce fuels such as hydrogen, ammonia, and methanol
Produces baseload electrical energy
Produces desalinated water for industrial, agricultural, and residential uses
Is a resource for on-shore and near-shore mariculture operations
Provides air-conditioning for buildings
Provides moderate-temperature refrigeration
Has significant potential to provide clean, cost-effective electricity for the future.

OTEC's noneconomic benefits, which help us achieve global environmental goals, include these:
Promotes competitiveness and international trade
Enhances energy independence and energy security
Promotes international sociopolitical stability
Has potential to mitigate greenhouse gas emissions resulting from burning fossil fuels.

In small island nations, the benefits of OTEC include self-sufficiency, minimal environmental impacts, and improved sanitation and nutrition, which result from the greater availability of desalinated water and mariculture products.


LIMITATIONS OF OTEC :

An OTEC facility requires a substantial initial capital outlay (in the range of $50 to $100 million for a “small” ten-megawatt plant).
OTEC has not been demonstrated at full scale over a prolonged period with integrated power, mariculture, fresh-water, and chill-water production.
OTEC is only feasible at relatively isolated sites (deep tropical oceans); from such sites, the power and marine products must be transported to market. (In general, the fresh water--and certainly the chill-water--cannot be transported more than a few miles economically.)
OTEC is ecologically controversial--at least untested--in large scale and over a long period.


THE FUTURE OF OTEC:

Operations such as the ones mentioned above continue to be studied and researched, as scientists must learn more about OTEC before it can be made for commercial use. The expenses of OTEC, including the transport of the energy or fresh water it produces, as well as the building and securing of OTEC sites, stop it from becoming a likely alternative in the near future. Still, through further research, it is very possible that mankind may be looking at a greener earth, whether it be decades, centuries, or millennia from now; one which is powered by the sun and the ocean.




CONCLUSION:


















REFRENCES:

World Health Council (http://www.worldenergy.com/)
OTEC News (http://www.otecnews.org/)
NREL Energy Conversion (http://www.nrel.gov/)
Energy source ocean thermal Pembina institute (http://www.pollutionprobe.org/)
Ocean Thermal Energy Conversion - wikipedia (http://www.wikipedia.com/)