Can Ocean Thermal Energy Conversion Power the World?

Can Ocean Thermal Energy conversion provide a substantial amount of power for the world? The research is going on and engineers are confident. it’s good news. 

When I look at the ocean, the Sun, the only thing that comes into my mind is their massive energy. I always wonder why we burn fossil fuels and destroy the planet when we have these massive energy sources? 

Three-quarters of the earth’s surface is covered by oceans, a huge source of non -polluting and inexhaustible energy.  Did you know that we can use natural temperature differences in an ocean to produce clean electricity day and night and all year round?  

The process by which this is possible is called Ocean Thermal Energy Conversion (OTEC).OTEC uses the temperature difference between warmer and cooler layers of ocean water.

 The surface layer of an ocean will be warmer than the deeper layers since it is exposed to solar radiation. In Ocean Thermal Energy Conversion we consider the upper warmer layers of the ocean as a source and deeper cooler layers of the ocean as a sink. 

OTEC is a renewable energy system that not only focuses on electricity production but also provides freshwater as a by-product, this feature makes OTEC different from other renewable energy systems. 

For this energy conversion, the basic criteria are the appropriate temperature difference. Usually, for OTEC we need a temperature difference of at least 20 degrees Celsius to run the heat engine and produce useful work, in this case, electricity.

The system with which OTEC works is called Ocean Thermal Energy Conversion plant or simply called OTEC power systems. Well let us now discuss OTEC power systems in detail:

Turbine, Generator, water pump, condenser, and an evaporator

  

On the basis of the given diagram, it is clear to us that the area of higher temperature which is the warmer surface of the ocean is 26 degrees Celsius and the lower temperature of deeper layers of the ocean is 5 degrees Celsius. Thus temperature difference is maintained to be 20 degrees Celsius (approx). The evaporator and condenser should contain a working fluid of low boiling point, generally, ammonia is used. 

The process begins with the conversion of hot water into water vapor with the help of an evaporator. These vapors expand and spin the turbine which is coupled to a generator, which will in turn produce electricity. The extra vapors get transferred to the condenser, as a consequence the working fluid (ammonia) condenses back into a liquid and we can reuse it. This makes the OTEC power system a continuous electricity-generating cycle.

Since the conversion is ocean-based it is important for you to know about the classification of the system based on location.

This kind of power plant provides us easy access for construction and operation, thus helping in reducing costs. Since it is constructed on or near land, lengthy power cables, and extensive maintenance can be avoided. Land-based power plants are safe from storms as they are installed in sheltered areas. 

This type of OTEC also operates with mariculture which is an advantage. Additionally, all products such as electricity, desalinated, and cold water can be easily transferred to the water network.  But we must ensure that OTEC power plants water supply and discharge pipes are safe in protective trenches.

    

This type of power plant can be fixed to the sea bottom. As the name implies the plant is mounted on a shelf closer to the cold water resource. We must note that the expenditure involved in shelf based system is comparatively larger than the land-based system since the plant is mounted in deeper areas of open -ocean. 

The plant is mounted at about 100m depth so as to avoid turbulence. Some of the displeasing factors the system possesses are the requirement of long underwater cables and issues related to the transfer of electricity and freshwater produced.

As the name suggests it is a floating power plant that is located off-shore on special platforms. This power plant is generally preferred for systems with large power capacities. As we already mentioned the power plant is a floating type and hence difficult to stabilize. This difficulty results in power delivery issues. Platform cables are also subjected to repair and maintenance too often especially during heavy storms.

On the basis of the cycle, we can classify OTEC into the closed cycle, open cycle, and hybrid cycle

Let us now examine a closed cycle of OTEC. It includes a closed system. The closed cycle uses a working fluid of low boiling point normally ammonia. Under the appropriate temperature difference, ammonia (working-fluid) is subjected to evaporation inside the evaporator.

 The gas expands and rotates the turbine which is attached to a generator to produce electricity. In the closed type, there is another heat exchanger that condenses the vapors back into a liquid which is then recirculated through the system. Thus, the liquid gets recycled to start the process again.

If we turn to an Open cycle we can observe that an open cycle possesses an open system. Here, normal warm surface water of the ocean itself acts as the working fluid. 

When the fluid enters a low-pressure area it boils leaving behind freshwater which is free of contaminants. Now, the pure water thus obtained is condensed back into a liquid by exposure to the cold temperature of the deep-ocean. 

In an open cycle, the result is not only clean electricity but also pure desalinized water. The obtained water is convenient for both drinking and irrigation. Also, in the process, we do not require extra working fluid. So, we can surely say that an open system is preferred over closed systems.

After having explained two methods to produce electricity from ocean temperature difference, you may be wondering what is the need for a third -one? Well, the best features of closed and open cycles combine in the hybrid cycle. 

In this method, warm water passes through a vacuum chamber, evaporates into steam, and vaporizes ammonia (possess good transport properties, easily available, low-cost) which is present in the vaporizer. 

This eventually drives the turbine connected to a generator and supplies useful work in the form of electricity. The steam condenses to give fresh desalinated water. 

This hybrid cycle is expected to be in high demand in the market since it is the combination of the best characteristics. However, this works just theoretically and yet to be tested.

All these cycles and its results point to the importance of this energy conversion in a technology-centered world .OTEC has for many years been recognized as possible energy of the future. OTEC systems are, under normal conditions, environmentally friendly. 

We know that carbon dioxide is a greenhouse gas and can impact the global climate. However, compared to other fossil fuel power plants OTEC systems release lesser orders of magnitude. 

The freshwater produced by the OTEC plant is excellent for irrigation. Burying an array of cold water pipes in the ground, near the surface, create cool-weather growing conditions in tropical areas. The cold deep seawater can be used for air conditioning too. 

It has applications for open ocean mariculture. Artificial upwelling of deep water has been suggested as a method of creating new fisheries and marine biomass plantations. In this modern world, an environmentally friendly renewable energy conversion using temperature difference of ocean is a boon.