Ocean Thermal Energy Conversion
Covering over 70% of the earth's surface, water is one of the most amazing energy sources. From olden times, people had previously utilized the force of the waters in order to crush grain or help them to do their work more efficiently. Since then as a power source, water draws a lot of attention due to the efficient manner in which it gets to produce electricity. Ocean Thermal Energy Conversion (OTEC) is a method introduced in 1881 that employs naturally occurring temperature differences between warmer water and colder deep seawater (Thomas, 1993). Since its main source, which is water, is freely provided by nature. OTEC can be a feasible source of cost effective renewable energy in tropical coastal regions that have high shipping costs for fuels and goods.
Ocean thermal energy conversion generates electricity indirectly from solar energy by harnessing the temperature difference between the sun-warmed surface of tropical oceans and the colder deep waters (Masutani and Takahashi, 2001). To be effective a minimum temperature differences between the ocean surface layers is 20°C. These temperature gradients exist primarily in specific tropical regions near the equator (Takashi and Trenka, 1996). Originally proposed by French Engineer Jacques Arsene d'Arsonval in 1881, OTEC have developed for years along with advancements that have made in this technology. The three most common OTEC systems are open cycle, closed cycle and hybrid cycle, all requiring all requiring a working fluid, condenser and evaporator within the system. These three systems all employ the thermodynamics of a working heat exchanger and use the temperature differences naturally occurring in the ocean as the driving force.
D'Arsonval's original concept formed the first closed-cycle OTEC plant system. This cycle uses a working fluid with a low-boiling point, usually propane, or ammonia, in a closed flow path (Takashi and Trenka, 1996). The working fluid is evaporated to water vapour first; expand subsequently and do work before being condensed by cold seawater. This process was later adopted by Rankine cycle. In a closed-cycle OTEC plant system, heat transfer from warm surface seawater occurs in the evaporator, producing a saturated vapour from the working fluid which is now pressurized. This works due to the ideal gas law, that states that the temperature is directly proportional to the pressure; therefore, if the pressure increases in a system, the temperature does too. Then electricity is generated when this gas expands to lower-pressure through the turbine. Latent heat is transferred from the vapour to the cold seawater in the condenser and the resulting liquid is pressurized with a pump to repeat the cycle (Masutani and Takahashi, 2001).Figure 1 is a simplified schematic diagram of a closed-cycle OTEC system.
Rankine cycles are able to produce non-zero net power to the fact that less energy is required to increase the pressure of a liquid then is able to be recovered when the same fluid expands as a vapour (Wiser, 2000). It is for this reason that phase changes are essential when producing energy this way.
The advantages of using a closed-cycle system are that it can be designed to produce the same amount of power with less energy input compares to an open cycle system.
Figure 1 (Dr. Luis Vega, 2009)
B. Open Cycle
G. Claude, D’Arsonval’s former student, concerned about the cost and potential bio fouling of closed cycle heat exchangers. Therefore, he proposed a system called open cycle, which using steam generated directly from the warm seawater as the OTEC working fluid. It works by pumping the warm seawater into a low-pressure (vacuum) evaporator chamber where the water boils. The evaporator chamber vacuum is maintained through a series of valves and careful maintenance to avoid atmospheric...
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