Ocean Thermal Energy Conversion (OTEC) is the process of converting the ocean waters temperature difference at different depths into usable energy. From the solar heated upper depths of the ocean to the cooler depths below 2,000 feet a 40 degree temperature difference must be maintained to generate electricity. This is a relatively new and unheard of source of energy. Many people are not aware of the many benefits of OTEC and the impact it will have in the future as the use of fossil fuel conversion declines.
Heightened environmental concerns in today’s world makes OTEC an attractive alternative for energy conversion. The ocean offers a natural and renewable resource which will make it possible for converting energy this way to last for a very long, long time. OTEC is a very cost effective method of using the oceans waters for energy conversion but its use and benefits to society have been obscure to the general public. In the coming years OTEC will probably be at the forefront of new energy resources and a good understanding of it know will prove invaluable to power engineers of the future.
How does Thermal Energy Conversion Work? OTEC is a form of solar power in that it uses the sun’s energy to heat the upper depths of the oceans waters. Since the sun’s energy cannot penetrate very deep depths (approximately 2000ft. ) the temperature below these levels is much colder then those near the surface. Most of the solar energy is absorbed in the top layer (35 to 100 m) of water. This upper layer of warm water is very uniformed in temperature because wind and waves cause the water to circulate and mix. The temperature in this layer can reach close to 82 F (28 C ) from regions close the equator.
Beyond this layer the temperature drops noticeably with the depth until 800 m to1000 m. At this depth the temperature is 40 F (4. 4 C ). From here the temperature changes very gradually. The colder water at these great depths is the result of melting ice in the polar regions. The colder water has a higher density than the warmer water which causes two important events. First the higher density water will sink and displace the lighter, warm water. Secondly, the rotation of the earth forces the heavier cold water to move toward the equator.
Because of this, the ocean provides a large reservoir of cold and warm water that can differ in temperature of 40 to 45 F ( 22 to 25 C). This can be seen in the following diagram. There are three basic processes utilized in Ocean Thermal Energy Conversion: Closed-cycle, Open-cycle, and Hybrid-cycle. To give you an ideal of the process, a picture of the open-water cycle is shown below. The closed-cycle process uses a working fluid with a low-boiling point such as ammonia, propane, or Freon. The warm surface water will be used in a heat exchange which will then transfer its heat to the working fluid.
When the working fluid vaporizes it propels a turbine to turn a generator which will produce electricity. After the working fluid has been vaporized it is passed through a condenser where cold water from the deeper depths of the ocean is used to convert the vaporized fluid back to a liquid state. The working fluid will now be ready to start the process over again. A picture of the internal workings can be found below. In an open-cycle process the seawater is used as the working fluid unlike the closed-cycle which used a low-boiling point liquid for its working fluid.
A near vacuum is used on the surface water which causes it to vaporize. Like the working fluids in the above paragraph the vaporized sea water is used to turn a low pressure turbine which is connected to a generator, thus producing electrical energy. The vaporized surface water is then condensed using the cooler water from deeper in the ocean, then introduced back into the ocean. When the warm surface water is vaporized the salts are left behind thus creating almost pure fresh water. The hybrid-cycle process is a combination of closed and open cycle processes.
In the initial stages the hybrid-cycle acts as an open-cycle process because it uses water as its working fluid. This water is vaporized and turns a turbine to produce electrical energy, but then acts as a closed-cycle system during the condensation process since the water is not put back into ocean. The condensed water is instead used as a fresh water resource since the water will have lost its salt contents in the vaporization process. The History of OTEC Jacques Arsene d’Arsonval was the first to proposition the idea of using the oceans temperature difference for energy in 1881.
In 1930 an open-cycle system was built and produced 22,000 Watts of power in Cuba. Then in 1935, aboard a ship, an open-cycle plant was again used to produce energy from the oceans thermal differences. Both of these plants were destroyed as a consequence of hazardous weather conditions. A laboratory test site was built in Hawaii in 1974 and has become the leading test plant for OTEC operations today. The Natural Energy Laboratory of Hawaii also made the first closed-cycle test plant ( Mini -OTEC ) in 1979. This plant produced 52kW of power and had a net power gain of 15kW.
The U. S. Department of Energy built OTEC-1 aboard a Naval ship in1980. OTEC-1 was a closed-cycle system that was used to study OTEC effects on marine environments. Tests showed that there was little effect on the marine environment. In the same year two laws were passed giving OTEC the go ahead with further development for commercial and private use. An experimental open-cycle plant produced 50kW of net power at Keyhole Point, Hawaii. This broke the previous mark for net electric power from an OTEC system by 10kW. The gross power from this plant exceeded 200kW!
Further testing and research is continually being done and rapid improvements are being made to increase the net power output. OTEC and the Environment One of the main concerns with OTEC is its environmental impact on the marine area that surrounds an OTEC plant. Since the energy conversion plants are built in or near the ocean there is great concern about harmful effects of an OTEC plant breakdown which could cause harm to the environment. After extensive research it looks as though the positive environmental impacts outweigh the negative.
Following is some examples of what exactly OTEC’s impact will have on the environment and you can decide for yourself if it is a safe alternative to current energy sources. One of the main benefits to the environment that OTEC brings to the table is that it is a renewable resource. This means that the resources originally used to produce energy can be used again and again ( unless of course the sun decides to burn out). Present means of energy conversion, such as the burning of fossil fuels, will only last as long as the supply in the earth allows us to use them, which some fear may run out very quickly.
The burning of these fossil fuels emits harmful carbon dioxide into the air which is a main contributor to global warming. OTEC depends on solar energy to produce the layer of warm water on the surface. In an average day the heat absorbed by the surface water in only one square mile is equivalent to the burning of 7000 barrels of oil. Since OTEC is a renewable energy resource and produces far less carbon dioxide as fossil fuel conversion the effects OTEC will have on global warming will be greatly reduced.
Another environmental benefit of OTEC is made possible from the cold water extracted from the deeper depths of the ocean. This cold water can be used in a variety of ways to support aquaculture environments such as sea animals and plants. This could prove beneficial because endangered marine life that is supported in the deeper cooler water could be transplanted to land based agencies to help monitor and restore the species. When seawater is used as the working fluid, the salts are removed from the water, creating fresh water. This is an easy way of producing fresh water for lands that have very little fresh water resources.
Separate distillation plants to create fresh water would no longer be needed and would cut out the pollution produced by these plants. The following figure illustrates these beneficial points. Of course there are some environmental drawbacks to OTEC. One concern is that the pipes laid in the ocean to extract the cold water may disturb reefs and the present ecosystem existing there. This would only be for a small localized area though and would not damage as much of an area as say an oil spill would. In a closed cycle system chemicals such as ammonia, propane, and Freon are used as the working fluid.
There is a fear that these chemicals could leak into the ocean environment which could cause damage to marine life. The likelihood of this happening though is minimal and a very large leak of these working fluids would be needed to cause significant damage to the environment. Leakage fears from present day energy conversion plants are greater since the harm they can cause would be much greater (nuclear meltdown, oil spills, toxic fumes, hazardous waste, etc.. ). As you can see from the above analysis the benefits that OTEC brings to the environment are much greater then the benefits produced by present day systems.
The environmental drawbacks are limited compared to fossil fuel burning and nuclear reactors and since OTEC is still pretty much in the developmental stages extensive research now and in the future will help reduce these environmental concerns even more. The Economics of OTEC An important concern about any new item being introduced is its economic viability. Questions about it s production costs, market costs, benefits to consumers and others are concerns that we all have. People have the same questions about OTEC and it’s impact on the economy if it were to become a main energy conversion source.
The following discussion will help answer your questions and give helpful insight into the economic impact of OTEC now and in the future. The impact OTEC can have on the world’s economy will be astounding, especially to those people who live in the remote areas where OTEC plants would be built. Since the plants will be built in tropical areas and island lands, the people who live their are very dependent on imported oil for their energy resources. This is often quite expensive and being able to use OTEC for their energy resources would prove to be quite cost effective.
As described earlier the ocean absorbs quite a large amount of heat from the sun. It is so astounding that only about one tenth of this amount needs to be converted to usable energy resources to have an impact on the economy. This low amount means that costs will not have to be as high then if it were necessary to convert 50% of this solar energy for people to use. Another impact OTEC can have on the economy is the fact that it will create many new jobs. Not only positions for engineers, researchers, biologists, and management but for low skilled labor as well.
The requirements for low skilled laborers to help make a plant work would help reduce unemployment and help build the economies of many poor nations where OTEC plants would be built. With the influx of money created from the plant, many new opportunities for small businesses arise because of the need for stores, restaurants, and the like to meet the needs of those working at the plant. The cold and freshwater produced from OTEC creates many uses for commercial and private businesses. Aquaculture companies can use the cold water to market marine products such as wildlife and marine plants.
Other useful purposes include air-conditioning, irrigation, and fresh drinking water, as illustrated in the previously seen picture, re-shown here. Technological breakthroughs in recent years have contributed to declining costs of producing OTEC systems. The General Electric Company of Great Britain has made giant strides in reducing the costs of evaporators and condensers by researching new designs for them. Other breakthroughs that are leading to lower costs include an aluminum heat exchanger, and a new kind of pipe that would be laid in the sea which would make size limitations unnecessary.
The pipe already has been developed and patented. One thing that is still holding back full scale OTEC operations is the fact that it is still very expensive for people to use as an energy source. The price per kilowatt hour is much more expensive then its fossil fuel counterparts. Some estimates right now say that the price could range from $200-$1500 per kilowatt hour. Obviously for OTEC to be part of the future the cost must decline. The Uses of OTEC in Today’s World OTEC plants must be built in the worlds tropical climate zones which lies between latitudes 23 deg N and 23 deg S, which can be seen from the diagram.
Part of the U. S. and Australia lie in this zone and much of the funding for further research and development comes from these two countries, since most of the other lands contained in this zone are not as industrialized. The plants must be built between these latitudes because that is where the necessary temperature difference for OTEC operation takes place. There are many different types of plants that can be built in these locations and include: land based, near shore, continental shelf mounted, and floating facilities.
The amount of energy produced from OTEC today is not enough to replace existing energy resources but continuing advancements are producing more and more energy. The highest output produced so far was an open-cycle plant in Honolulu which produced 255 kilowatts gross. A closed-cycle plant in Hawaii is being tested which produces 50 kilowatts of power. This closed-cycle plant is making use of the aluminum heat exchanger and is providing data for economic feasibility of the aquaculture and expansion of the plant. OTEC is still not in use today due to the cost of implementation.
The system is still in heavy research to bring these costs down and also to make improvements on the system. Although the full scale commercial size systems are not practical at this time, small scale experimental plant systems are being for further research. OTEC is expected to make an impact on the future. The Future of OTEC OTEC is a very exciting new way of energy conversion. People are skeptical about new and innovative ideas because of their unfamiliarity and lack ofknowledge about the idea. This holds true with OTEC. Many people are unaware of what OTEC is and the benefits that OTEC holds for the future of our Earth.
Much more development and research must be made to make OTEC a large scale operation and to bring the cost to consumers down. This is only going to happen if people are made aware of OTEC and its many possibilities. OTEC has many environmental benefits and can produce the world s necessary energy needs for a very long time (it is after all a renewable energy source! ). We are quite certain that in the upcoming years OTEC will become a much more publicized topic and greater research will come about to produce a fully sized commercial plant.
