In the previous column dealing with this subject, we presented the Diesel Electric Power Plant. In this column, we tackle what could be said as a not commonly known type of energy source for generating electricity, the Ocean Thermal Energy Conversion (Otec). Today, the only two countries that have a continuous operation of Otec are Japan and the United States of America. In Japan, its Otec facility is located in Kume Island, Okinawa Prefecture. In the USA, its Otec facility is located in Kailua-Kona, Hawaii.
Otec is a type of energy source that utilizes the temperature difference between the surface water and the deep water of the ocean. This type needs to be constructed in a place where there is available significant temperature difference between the surface and deep seawaters.
The main component parts of the Otec system are a) vaporizer, b) turbine and generator, c) condenser and d) pump. It uses a low-boiling-point kind of fluid. Ammonia is the very likely candidate for this purpose; it is used as a working fluid or medium for driving the turbine. Ammonia has a low boiling point of about -330C at atmospheric pressure (i.e., 14.7 psi (pounds per square inch) at sea level). It is for the closed-cycle type of Otec.
It could be seen then in this system that the vaporizer is connected to the turbine and the turbine is connected to the generator. The turbine is connected to the condenser. The condenser is connected to the pump. The pump is connected to the vaporizer.
There are three types of the Otec system: 1) closed-cycle, 2) open-cycle and 3) hybrid-cycle. The closed-cycle is a closed-loop type. It uses ammonia as the medium for driving the turbine. Basically, the working principle of the closed-cycle Otec system is as follows. Let’s begin with the pump. The pump circulates (and recirculates) the ammonia at cold temperature. It pushes the ammonia to the vaporizer. The vaporizer, with warm or hot temperature at the surface ocean water, vaporizes the ammonia. The warm temperature at the surface of ocean water, say, at 200C, easily vaporizes the ammonia fluid. The ammonia expands in gaseous form. The ammonia’s expansion creates a high pressure in a pressure tank. Then this high pressure vaporized ammonia is sent to the turbine, causing the turbine to rotate. As the turbine rotates, the generator connected to it also rotates. Thus, the generator produces electricity. Now, from the turbine, the ammonia gas goes to the condenser at the deep ocean water. At the condenser, with cold temperature typically ranging from 50C–100C, the ammonia is being cooled down back to its liquid form. Then, the pump recirculates the liquid ammonia to the vaporizer. And the cycle repeats.
The working principle of the open-cycle Otec system has the same procedure. But this time, it does not use ammonia to vaporize. Instead, it directly boils warm sea water itself. The pump pushes warm sea water directly to the vaporizer. This produces steam which is housed in a container or tank. The steam then is sent to drive the turbine, causing it to rotate. The generator, rotating together with the turbine, thus produces electricity. Then the sea water in gaseous form can either be released to the atmosphere directly or it can be desalinated to get fresh water. In this case, there is no need for a condenser, since no fluid needs to be cooled down back to its liquid form.
The working principle of the hybrid-cycle Otec system is as follows. It combines the closed-cycle and the open-cycle Otec systems. Here, the open-cycle type is used to “flash-evaporate” the warm surface water to create steam. But the steam is not directly used to drive the turbine. It is rather used to boil the working fluid or medium, such as ammonia, which is of low-boiling-point. In other words, the steam is used as the agent to vaporize the ammonia, which in turn is used to drive the turbine, in the closed-cycle type of Otec.
It is said that the Otec is a continuous renewable energy source in the oceans of tropical countries. It is a clean energy, with zero-emission. No carbon emissions as in coal-powered electric plants. However, one disadvantage that can be seen in this type of energy source is that it cannot work in cold oceans in the polar regions of the earth. It can only work in tropical oceans where there is available continuous warm surface water and cold deep water.
Nevertheless, it is a clean energy source that may be utilized in the Philippines, since it is a tropical country.