Short note on ocean thermal energy conversion (OTEC)

Ocean thermal energy can be harnessed to produce electricity using a phenomenon termed as Ocean Thermal Energy Conversion (OTEC). This blog post is broken up into several pieces, so you can easily find the one you’re looking for.

Ocean Thermal Energy

History of Ocean Thermal Energy Conversion

Ocean Thermal Energy Conversion (OTEC)

Types of Ocean Thermal energy Conversion

Challenges in Ocean Thermal Energy

Advantages of Ocean Thermal energy

Disadvantages of Ocean Thermal energy

Oceans can be the source of two main types of renewable energy  thermal energy (produced from the sun’s heat) and mechanical energy (produced from wave action and tides).

Ocean Thermal Energy

Ocean thermal energy refers to the heat energy stored in the ocean waters which they receive from the sun during the day. Ocean thermal energy can be harnessed to produce electricity using a phenomenon termed as Ocean Thermal Energy Conversion (OTEC).

History of Ocean Thermal Energy Conversion

In the year 1881, French physicist Jacques-Arsène d’Arsonval was the first person to suggest the notion. Since then, there have been quiet times in the pace of progress; however, in response to the current rise in interest in renewable energy, there has been a pick-up in the pace of development in nations such as the United States of America, France, China, India and Japan.

Ocean Thermal Energy Conversion (OTEC)

In the equatorial and tropical seas, there exists a substantial temperature differential between the surface waters and the deeper waters, often to an extent of 20 degrees Celsius. This temperature gradient can be utilized to produce electricity. The warm surface waters are made to pass through a heat exchanger in which the heat stored in ocean waters is transferred to a fluid with a low boiling point. This heat vaporizes the fluid which can be used to run turbines (similar to steam run turbines) and generate electricity.

Unlike other renewables, OTEC is a non-intermittent source of electricity i.e., electricity can be produced day and night, all-round the year. Hence, OTEC has the potential to be a source of base-load. OTEC is particularly suited for small island nations and island communities which are largely dependent on imported fossil fuels for their local power needs. Naval bases and other strategic, protected assets located in the tropical ocean waters can also rely on OTEC for a sustainable power supply. OTEC power plants can be set up offshore or on the coast.

Types of Ocean Thermal energy Conversion

Ocean Thermal Energy Conversion (OTEC) systems may be broken down into three distinct types: closed-cycle, open-cycle, and hybrid.

  1. Closed-cycle: Closed-cycle systems (figure:1) make advantage of the warm water that is found at the surface of the ocean in order to vaporize a working fluid that has a low boiling point, like ammonia. The rotating turbine is driven by the expanding vapors.
  2. Open-cycle: In open-cycle systems (figure:2), the saltwater is really brought to a boil by working at low pressures, and the boiled water is then sent through a turbine as part of a Rankine thermodynamic cycle.
  3. Hybrid-cycle: Closed-cycle and open-cycle architecture are both components of hybrid systems (figure:3). After that, the ammonia or sea water is cooled down and made liquid by the cooler water that has been pulled up from deep.
Closed-cycle Ocean Thermal Energy Conversion (OTEC) systems make advantage of the warm water that is found at the surface of the ocean in order to vaporize a working fluid that has a low boiling point, like ammonia. The rotating turbine is driven by the expanding vapors.

Figure:1 Closed-cycle Ocean Thermal Energy Conversion (OTEC) system

In open-cycle systems Ocean Thermal Energy Conversion (OTEC) the saltwater is really brought to a boil by working at low pressures, and the boiled water is then sent through a turbine as part of a Rankine thermodynamic cycle.

Figure:2 Open-Cycle Ocean Thermal Energy Conversion (OTEC)

Closed-cycle and open-cycle architecture are both components of Hybrid-cycle Ocean Thermal Energy Conversion (OTEC). After that, the ammonia or sea water is cooled down and made liquid by the cooler water that has been pulled up from deep.

Figure 3: Hybrid-cycle Ocean Thermal Energy Conversion (OTEC)

Challenges in Ocean Thermal Energy

  1. OTEC can be highly capital intensive, as against conventional sources such as coal, diesel-based power.
  2. OTEC technology is still in its nascent stages and has not been deployed on a large scale anywhere in the world.
  3. Ocean Thermal Energy Conversion becomes financially viable only when power is produced on a large scale.

Advantages of Ocean Thermal energy

  1. Ocean Thermal Energy Conversion is a renewable source of energy that is clean and friendly to the environment.
  2. Additionally, unlike solar plants, which are unable to produce energy at night, and wind turbines, which can only generate power when there is a breeze, OTEC can generate power whenever it is needed. These are two of the most significant advantages of OTEC.
  3. The open-cycle system may function as a desalination plant and provide populations in its immediate vicinity with freshwater. This is one of the system’s many advantages.
  4. Once the cold water pipes are established to pump the cold water up from the deep, another advantage is that the cold water may then be utilized for other purposes, such as air conditioning and refrigeration.

Disadvantages of Ocean Thermal energy

  1. OTEC’s drawbacks include the fact that it is prohibitively expensive from an economic standpoint, that the energy generated offshore must be transported to land via underwater electrical lines, that a significant amount of energy is expended to bring the frigid deep seawater to the surface, that the installation of the pipes and plant may disturb marine life, and, perhaps most importantly, that it can only be used in a limited number of locations.
  2. OTEC requires a 20-degree Celsius temperature gradient between warm surface water and chilly deep water in order to function. One can only find such a large temperature differential in tropical water, which eliminates a large number of potential sites for an OTEC facility.

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