OTEC – Oceans as electricity source

The oceans are constantly exposed to heat radiation from the sun, while the sea currents warm and cool the water at various depths and at various locations of the earth. Enormous amounts of electricity can be produced in the process of ocean thermal energy conversion (OTEC or OTE) which utilizes the water temperature difference in shallow and deep areas, without polluting the environment and almost without dangerous greenhouse gases. The advantage of OTEC, compared to other renewable energy sources where production depends on various factors, e.g. the weather, is uninterrupted production which is possible 24 hours a day, every day of the year. A great advantage of this process is ecological acceptability, in spite of some minor negative environmental impacts, and enormous amounts of available energy.

To set up the system, certain conditions need to be fulfilled: temperature difference has to be about 4.5C throughout the year; the water has to be of sufficient depth close to the shore, otherwise floating platforms have to be set up further away from the shore. In USA the locations suitable for OTEC can be found in tropical areas, such as Hawaii, and on a part of the Atlantic coast. Another problem is the construction of the structure. In India 1 MW OTEC plant started to be constructed in 2003 but the cold water connecting pipe at the depth of 800 m sank. The same thing happened the following year as well.

Regardless of the fact that the exploration of this energy source had began as early as in the 19th century (among others, Nikola Tesla introduced his concept in the magazine Everyday Science and Mechanics in 1931), most of the operating OTEC plants are still experimental. The largest is located near Japan, with the capacity of 120 kW, and the second largest (50 kW) close to the coast of Hawaii.

The basic principle of electricity generation is simple. Surface warm water is used to heat the liquid which has a low boiling temperature (e.g. propane). The steam created in this process drives the turbines of electricity generators. Cold water at the bottom of the ocean is used to cool down the steam, so that it becomes liquid again. This process could generate twice as much electricity than the other methods in which energy of the oceans is used, such as tide or waves. The greatest obstacle is the equipment which is at the moment very expensive due to its low thermal efficiency. The efficiency of the early OTEC systems was 1 – 3 %, but the efficiency of the most recent ones is already close to their theoretical maximum efficiency, i.e. 6 – 7 %.

In the recent years, the company that has contributed the most to the development of OTEC was Lockheed Martin which has been developing this technology since 1970. Only after substantial financial support from the American Ministry of Energy and American army in 2008 and 2009, the development has been drastically accelerated.

The process is presented in detail on this video.