Article

Ocean Thermal Energy: An Untapped Resource

Introduction and Definition of Ocean Thermal Energy
Theory Behind Ocean Thermal Energy
History of Ocean Thermal Energy
Technologies Used With Ocean Thermal Energy
Closed-Cycle Systems
Open-Cycle Systems
Hybrid Systems
Advantages of Ocean Thermal Energy
Issues with Ocean Thermal Energy
Sources

Introduction and Definition of Ocean Thermal Energy

Ocean thermal energy is derived from the solar energy that is absorbed by the oceans. Given that almost two-thirds of the planet’s surface is covered by the oceans, the majority of solar radiation hitting Earth is absorbed and stored here. In terms of heat energy, the amount of solar radiation absorbed by the oceans on an average day equates to about 250 billion barrels of oil. This means that if even a small amount of the heat energy stored in the oceans could be harnessed, it would have the potential to produce billions of watts of electrical power.

Theory Behind Ocean Thermal Energy

The sunlight that hits the ocean is stored as though the ocean was a giant solar panel. The ultraviolet radiation from sunlight excites the water molecules at the surface of the ocean, causing a rise in temperature. The ocean has a low ‘albedo’ (albedo is a measure of how well a surface reflects sunlight) of around 0.6. This means that the majority of solar radiation that hits the surface, around 94%, is absorbed. The warm water stays near the surface of the water as it is less dense than the cold water at depth.

As the ocean surface is heated by the sun, the depths of the ocean remain cold, thus creating a temperature gradient between the top of the ocean and the bottom. These differences in temperature can be used to drive turbines which can convert the thermal energy into electrical power.

Generally, the optimum situation for the conversion of ocean thermal energy to electrical power is when the temperature difference between surface and deep water is around 36^oF (20^oC). These conditions are usually found between the Tropic of Cancer and Tropic of Capricorn. Favourable conditions for ocean thermal energy conversion (OTEC) can be found off the coast of over 80 countries worldwide.

History of Ocean Thermal Energy

Though there has been relatively little investment and research into ocean thermal energy compared to other renewable energies, the idea of harnessing this energy source is not a new one. It was first proposed in 1881 by a French physicist, Jacques Arsene d’Arsonval, but was not implemented until 1930 when d’Arsonval’s pupil Georges Claude built the first energy conversion plant in Cuba.

Claude built a further conversion plant of the coast of Brazil in 1935; however this was destroyed by storms before it became economically viable.

Monetary expense has blighted the progression of ocean thermal progression. For example, construction of a large (3-megawatt) OTEC plant was planned off the coast of Ivory Coast, but was abandoned due to the lack of sufficient funds in the 1950’s.

In 1974, USA established the Natural Energy Laboratory of Hawaii Authority, which has become a leading centre for the development of ocean thermal energy conversion.

Technologies Used With Ocean Thermal Energy

There are several different types of Ocean Thermal Energy Conversion (OTEC) systems that have been implemented at various times. The main distinctions between the machines are briefly outlined below.

Closed-Cycle Systems

A liquid with a low boiling point, e.g. ammonia, is heated using the warm ocean surface waters. This produces steam, which then rotates a turbine to generate electricity. Cold water from the depths is then taken into the generator to cool the vapour which can then be recycled.

Open-Cycle Systems

In these systems, the seawater itself is used to drive the turbine, as it is taken into a pressurised container where it boils. Once again, the deep cold water is then used to turn the vapour back into liquid again.

Hybrid Systems

As the name suggests this combines elements of both systems. First, the warm surface water is vaporised, and this steam in turn vaporises a low boiling point liquid which turns the turbine.

Advantages of Ocean Thermal Energy

There are several great advantages to ocean thermal energy. Perhaps the most important of these is that it produces fresh water as a by-product. In an open-cycle system, when the surface water is vaporised, it precipitates out all of its salt, so once the vapour is condensed again it is drinkable. This could potentially solve many water shortage crises in communities across the planet.

The cold water pipes can also be beneficial to agriculture, as the temperature difference between warm plant leaves and cool roots produced by the cold pipe passing through the soil leads to temperate plants thriving in the subtropics.

Aquaculture is yet another important by-product. As nutrient-rich deep water is brought to the surface, it fertilises the ocean via artificial upwelling. This can lead to a thriving ecosystem around the conversion plant, and farmable fish can also be introduced into areas that they would not have previously survived in.

Air conditioning can also be produced from the system, as the cold water taken from depth can be directly input into an air conditioning unit.

Furthermore, it is a renewable energy, and one that never stops producing energy, unlike wind for example.

Issues with Ocean Thermal Energy

There are several logistical issues that have prevented ocean thermal energy from really taking off. As previously mentioned, the cost of a conversion plant is enormous and cash is often needed upfront. Currently there are few government initiatives to subsidise the technology.

Furthermore, the system has to be permanently located in very rough environments, and so must be robust enough to stand constant storms and waves.

The environmental impact of OTEC has not been investigated fully and special measures may be needed to make sure that a conversion plant does not harm local ecosystems. The effects of recycling the ocean water through the converter need to be more fully understood and steps also need to be taken so that debris and ocean species are not entering the converter. It has also been suggested that the electromagnetic field that is generated by the cables bringing electricity to the shore may impact the local ecosystem.