Tapping Ocean Thermal Energy - The History And The New Alternative Source Of Fuel

Oceans and seas constitute more than 70 percent of Earth's surface. On a given day, they absorb solar radiation comparable to approximately 250 billion barrels of oil. Thus they are a huge collector and store of solar energy. If even a small percentage of this energy could be harnessed, it would fulfill a significant part of the electricity needs of the world today.

OTEC, or Ocean Thermal Energy Conversion, converts this solar radiation to electric energy. Ocean's layers of water have different temperatures, forming a natural thermal gradient. The surface water is warm and the deep waters cold. If the difference between them is around 20°C, the OTEC technology can be used to run a heat engine used to produce electric energy. This makes the oceans a huge resources which can be used to produce billions of watts in electric energy.

A French engineer, Jacques D'Arsonval, envisioned OTEC in 1881. Currently NELHA or Natural Energy Laboratory of Hawaii Authority, founded in 1974, is the only place where OTEC is being practically pursued. NELHA is recognized worldwide as a unique and innovative distributed & renewable energy, ocean science and technology park.

While OTEC is a very promising alternative energy resource, the expenses involved in exploring this option are delaying the setup and sustained operation of an OTEC plant. In addition to high initial cost, the disadvantages include potential of damage to local ecosystem, and difficulty in finding suitable sites with deep waters nearby. However, it is a technology which needs to continue being explored and funded as it is a promising alternate energy source which could provide not only much needed power but also desalinated water and a number of mariculture products.

There can be four kinds of plants, based on location - shelf based (offshore platforms attached to seafloor), land based, floating platforms attached to seafloor, and on ships that move from one place to another.

There can be three types of plants, based on cycle -closed-cycle, open-cycle, and hybrid (various blendings of the earlier two options).

In Closed-Cycle process, warm ocean water is used for vaporizing a working fluid (a liquid with low boiling point, like propane or ammonia). The warm ocean water is pushed into a reaction chamber. As a result, the intermediate fluid boils and its vapor pushes the engine turbine, thus generating electricity.  Cold sea water is then used to cool the vapor.

Open-Cycle process is similar to closed-cycle process with the exception that an intermediate fluid is not used. With a vacuum, the warm surface water itself is changed into vapor. This low-pressure vapor, when released in a small area, can generate enough power that can run the turbine. After the vapor has generate the required power, cold ocean water is used to cool the vapor as well as to create desalinated water.

Hybrid-Cycle process. One other option is to use a combination of the two above processes into an open-cycle/closed-cycle hybrid, with the intention to capitalize on the benefits of both to produce electricity as well as desalinated water more efficiently.  In such a system, warm water would enter a vacuum to be flash-evaporated into vapor. 

The vapor would then be passed through an evaporator so as to vaporize the working. Then the fluid would drive a turbine which would generate electricity. The vapor would be condensed to produce desalinated water.