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Ocean thermal energy conversion
Abstract
Ocean thermal energy conversion, or OTEC, is a way to generate electricity using the temperature difference of seawater at different depths. The method involves pumping cold water from the ocean depths (as deep as 1 km) to the surface and extracting energy from the flow of heat between the cold water and warm surfacewater.
OTEC utilizes the temperature difference that exists between deep and shallow waters within 20° of the equator in the tropics — to run a heat engine. Because the oceans are continually heated by the sun and cover nearly 70% of the Earth's surface, this temperature difference contains a vast amount of solar energy which could potentially be tapped for human use. If this extraction could be done profitably on a large scale, it could be a solution to some of the human population's energy problems. The total energy available is one or two orders of magnitude higher than other ocean energy options such as wave power, but the small size of the temperature difference makes energy extraction difficult and expensive. Hence, existing OTEC systems have an overall efficiency of only 1 to 3%. The concept of a heat engine is very common in engineering, and nearly all energy utilized by humans uses it in some form. A heat engine involves a device placed between a high temperature reservoir (such as a container) and a low temperature reservoir. As heat flows from one to the other, the engine extracts some of the heat in the form of work. This same general principle is used in steam turbines and internal combustion engines, while refrigerators reverse the natural flow of heat by "spending" energy. Rather than using heat energy from the burning of fuel, OTEC power draws on temperature differences caused by the sun's warming of the ocean surface.
INTRODUCTION
Ocean Thermal Energy conversion:-
Ocean Thermal Energy Conversion (OTEC) is a process which utilizes the heat energy stored in the tropical ocean. The world's oceans serve as a huge collector of heat energy. OTEC utilizes the difference in temperature between warm, surface seawater and cold, deep seawater to produce electricity. OTEC requires a temperature difference of about 36 deg F (20 deg C). This temperature difference exists between the surface and deep seawater year round throughout the tropical regions of the world.
In one, simple form of OTEC a fluid with a low boiling point (e.g. ammonia) is used and turned into vapor by heating it up with warm seawater. The pressure of the expanding vapor turns a turbine and produces electricity. Cold sea water is then used to reliquify the fluid. Other forms of OTEC also exist as explained in the sites listed below. One important bi-product of many of these techniques is fresh water.
This is also an indirect method of utilizing solar energy. A large amount of solar energy is collected and stored in tropical oceans. The surface of the water acts as the collector for solar heat, while the upper layer of the sea constitutes infinite heat storage reservoir. Thus the heat contained in the oceans, could be converted into electricity by utilizing the fact that the temperature difference between the warm surface waters of the tropical oceans and the colder waters in the depths is about 20 – 25o k. Utilization of this energy, with its associated temperature difference and its conversion into work, forms the basis of ocean thermal energy conversion (OTEC) systems.
The surface water which is t higher temperature could be used to heat some low boiling organic fluid, the vapours of which would run a heat engine. The exit vapours would be condensed by pumping cold water from the deeper regions. The amount of energy available for ocean thermal power generation is enormous, and is replenished continuously.
Several such plants are built in France after World War II (the largest of which has a capacity of 7.5 MW) with a 22oK temperature difference between surface and depths, such as exists in warmer ocean areas than the north sea, the carnot efficiency is around 7%. This is obviously very low.
OCEAN THERMAL ENERGY CONVERSION:OTEC: -
RANKINE CYCLE OTEC PLANT: -
The warm surface water is used for supplying the heat input in boiler, while the cold water brought up from the ocean depths is used for extracting the heat in the condenser. In India, Department of Non – conventional energy sources (DNES) has proposed to install a 1 MW OTEC plant in Lakshadweep Island at Kavaratti and Minicoy. Preliminary oceanographic studies the eastern side of Lakshadweep Island suggest the possibility of the establishment of shore based OTEC plant at the Island with a cold water pipe line running down the slope to a depth of 800-1000m. Both he Islands have large lagoons on the western side. The lagoons are very shallow with hardly any nutrient in the sea water. The proposed OTEC plant will bring up the water from 1000m depth which has high nutrient value. After providing the cooling effect in the condenser, a part of sea waster is proposed to be diverted to the lagoons for the development of aqua culture.
Rankine Cycle Description: -
1-2: Liquid water pumped to a higher pressure adiabatically: -
T1<T2, P1<P2
Work is added to run the pump Win= (-)
No heat is transferred Q = 0
2-3: Heat is added by boiling the water: -
T2<T3, P2=P3
No work is added W = 0
Heat is added QH= 0
3-4: High pressure steam drives the turbine adiabatically: -
T3>T4, P3>P4
Work is generated by the turbine Wout= (+)
No heat is transferred Q = 0
4-1: Steam is condensed to liquid water: -
T4=T1, P4=P1
No work is added W = 0
Heat is removed QL= (-)
Rankine Cycle PV diagram: -
•Water is the working fluid in the Rankine Cycle
•The water exists in two phases: liquid and steam
•The heat (QH) added to the boiler comes from burning coal, burning liquid fuels, or from a nuclear reactor
•The steam exiting the turbine is converted to a liquid in the condenser because it is more efficient to pump a liquid.
Background and History of OTEC Technology
In 1881, Jacques Arsene d'Arsonval, a French physicist, was the first to propose tapping the thermal energy of the ocean. Georges Claude, a student of d'Arsonval's, built an experimental open-cycle OTEC system at Matanzas Bay, Cuba, in 1930. The system produced 22 kilowatts (kW) of electricity by using a low-pressure turbine. In 1935, Claude constructed another open-cycle plant, this time aboard a 10,000-ton cargo vessel moored off the coast of Brazil. But both plants were destroyed by weather and waves, and Claude never achieved his goal of producing net power (the remainder after subtracting power needed to run the system) from an open-cycle OTEC system.
Then in 1956, French researchers designed a 3-megawatt (electric) (MWe) open-cycle plant for Abidjan on Africa's west coast. But the plant was never completed because of competition with inexpensive hydroelectric power. In 1974 the Natural Energy
Laboratory of Hawaii (NELHA, formerly NELH), at Keahole Point on the Kona coast of the island of Hawaii, was established. It has become the world's foremost laboratory and test facility for OTEC technologies.
In 1979, the first 50-kilowatt (electric) (kWe) closed-cycle OTEC demonstration plant went up at NELHA. Known as "Mini-OTEC," the plant was mounted on a converted U.S. Navy barge moored approximately 2 kilometers off Keahole Point. The plant used a cold-water pipe to produce 52 kWe of gross power and 15 kWe net power.
In 1980, the U.S. Department of Energy (DOE) built OTEC-1, a test site for closed- cycle OTEC heat exchangers installed on board a converted U.S. Navy tanker. Test results identified methods for designing commercial-scale heat exchangers and demonstrated that OTEC systems can operate from slowly moving ships with little effect on the marine environment. A new design for suspended cold-water pipes was validated at that test site. Also in 1980, two laws were enacted to promote the commercial development of OTEC technology: the Ocean Thermal Energy Conversion Act, Public Law (PL) 96-320, later modified by PL 98-623, and the Ocean Thermal Energy Conversion Research, Development, and Demonstration Act, PL 96-310. At Hawaii's Seacoast Test Facility, which was established as a joint project of the State of Hawaii and DOE, desalinated water was produced by using the open-cycle process. And a 1-meter-diameter col seawater/0.7-meter-diameter warm-seawater supply system was deployed at the Seacoast Test Facility to demonstrate how large polyethylene cold-water pipes can be used in an OTEC system.
In 1981, Japan demonstrated a shore-based, 100-kWe closed-cycle plant in the Republic of Nauru in the Pacific Ocean. This plant employed cold-water pipe laid on the sea bed to a depth of 580 meters. Freon was the working fluid, and a titanium shell-and-tube heat exchanger was used. The plant surpassed engineering expectations by producing 31.5 kWe of net power during continuous operating tests. Later, tests by the U.S. DOE determined that aluminum alloy can be used in place of more expensive titanium to make large heat exchangers for OTEC systems. And at- sea tests by DOE demonstrated that biofouling and corrosion of heat exchangers can be controlled. Biofouling does not appear to be a problem in cold seawater systems. In warm seawater systems, it can be controlled with a small amount of intermittent chlorination (70 parts per billion per hour per day). In 1984, scientists at a DOE national laboratory, the Solar Energy Research Institute (SERI, 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. Direct-contact condensers using advanced packings were also shown to be an efficient way to dispose of steam. Using freshwater, SERI staff developed and tested direct-contact condensers for open-cycle OTEC plants. British researchers, meanwhile, have designed and tested aluminum heat exchangers that could reduce heat exchanger costs to $1500 per installed kilowatt capacity. And the concept for a low-cost soft seawater pipe was developed and patented. Such a pipe could make size limitations unnecessary, as well as improve the economics of OTEC systems. In May 1993, an open-cycle OTEC plant at Keahole Point, Hawaii, produced 50,000 watts of electricity during a net power-producing experiment. This broke the record of 40,000 watts set by a Japanese system in 1982. Today, scientists are developing new, cost-effective, state-of-the-art turbines for open cycle OTEC systems.