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OTEC Principle:
The Ocean is the largest solar collector in the world and due to its mass, there is little temperature difference between day and night.
The surface layer, the Euphotic zone, receives all the solar energy and extends from the ocean surface to a depth of 200 meters. It is the warmest layer, and depending on geographical location, can reach temperatures of over 30°C. (36°C has been recorded in the Persian Gulf)
The deep layer, the Disphotic zone, occurs at depths from 200m to 1,000m and is sometimes referred to as the twilight zone since the sunlight is very faint. Due to the lack of solar energy, the water temperature decreases rapidly with increasing depth. This phenomenon is referred to as the thermocline.
The temperature differences below depths of 1000m are small and therefore not considered for OTEC. Graph 1 illustrates this point.
OTEC Principle: "Rankine Cycle" illustrates the basic cycle
With its high latent heat, ammonia is the working fluid of choice. At specific pressures, ammonia evaporates at a temperature lower than the warm seawater, and condenses back to a liquid at a temperature higher than the cold seawater.
At the time, the Rankine cycle was a technological breakthrough, and is still used in many power generating systems. For OTEC conditions, however, the practical efficiency is approximately 3%. In order to improve this efficiency, Dr. Uehara invented a new cycle to reduce the actual losses at each point and this cycle approaches the Carnot cycle efficiency. This new cycle is called the "Uehara Cycle", figure 2.
In addition to the new cycle, each component’s function and design has been thoroughly tested and optimized. The overall cycle efficiency depends greatly on reducing the evaporator and condenser losses. Dr. Uehara is an expert with over 30 years research experience in this area, and is published in the, "Handbook of Phase Change: Boiling and Condensation" by Kandlikar.
With the above improvements, efficiency above 5% is possible. Although this may seem low, it is easily justifiable since all the energy used in this cycle is 100% renewable. With no fuel costs, OTEC is the leading base load renewable energy option as it can operate 24 hours/day.
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