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Unlocking Societal Innovation through "Ocean Thermal Energy Conversion (OTEC)" Sustainable Support Originating from the Sea (Part1)

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2024.10.09

In the "BLUE ACTION MOL", which challenges the creation of sustainable global environmental technologies and infrastructure, Mitsui O.S.K. Lines (MOL) declares: "If you look at the world from an ocean perspective, you can see a completely different future." One specific effort is advancing our participation in the operation of a demonstration plant and the development of related technologies for Ocean Thermal Energy Conversion (OTEC). OTEC has the potential to become a foundation for the economic independence of island nations, not only for power generation but also through the utilization of deep seawater used in power generation to create local industries and living infrastructures. This is a creation of a future world unique to MOL, expanding our field into social infrastructure enterprises originating from the sea. In this blog, we will introduce an overview of the project and our passion for the project, which includes an interview with the person in charge of OTEC project, in Part 1 and Part 2.

Please click here for Part2 →
Unlocking Societal Innovation through "Ocean Thermal Energy Conversion (OTEC)" Sustainable Support Originating from the Sea (Part2)

Global Spotlight on the "Kumejima Model" which Utilizes Ocean Energy

  1. Kumejima Island, situated approximately 100km west of Naha City in Okinawa Prefecture, Japan, is known for the beauty of "Hatenohama", an island made only of sandy beaches. However, Kumejima is also drawing global attention for the "Kumejima Model" initiative.


  2. The Kumejima Model involves "using OTEC facilities as the core to develop various industries and social infrastructures through the complex utilization of deep seawater used for power generation". MOL is participating in the operation of this OTEC, providing technical support and building a business model for commercialization and large-scale operation.


  3. The OTEC demonstration plant is located on the eastern side of the island in an area called Maja, with facilities nearby using deep seawater for cosmetics manufacturing, and aquaculture facilities for sea grapes, oysters, and Japanese tiger prawns. OTEC began operation in June 2012, installed by Okinawa Prefecture with the cooperation of Saga University, and has a maximum power generation capacity of 100kW.

OTEC demonstration plant on Kumejima IslandOTEC demonstration plant on Kumejima Island

According to Kumejima Town's Project Promotion Division, industries utilizing deep seawater have developed since the establishment of the Okinawa Prefecture Deep Sea Water Research Institute in 2000. Currently, the production value of deep seawater-related products amounts to about 2.5 billion yen annually. Moreover, Kumejima Town, with a population of about 7,600 people, has created over 140 new jobs in deep seawater-related businesses. The scale of this industry is almost the same as the town's agricultural production value and accounts for 70% of the total production value of the fishing industry. Furthermore, the total production value of Kumejima Town is 23.7 billion yen annually, meaning industries utilizing deep seawater represent about 10% of it, becoming a pillar of the regional industry. As a result of these excellent achievements, over 12,000 people from 76 countries around the world have already visited Kumejima's OTEC for inspection.

Various industries created by OTECVarious industries created by OTEC

Harnessing Ocean Temperature Differences for Power Generation and Multifaceted Utilization of Deep Seawater

OTEC is a system that generates electricity by exploiting the temperature difference between the surface and deep layers of the sea. The surface water heats up under the sun, reaching nearly 30°C (86°F) in areas like Hawaii. In contrast, at depths exceeding 200m (656ft), where sunlight hardly reaches, the water temperature drops to 4~5°C (39.2~41°F) at depths of 1,000m (3,280ft), with little regional variation.

The principle of OTEC is quite simple: 1) A low-boiling-point, easily vaporized medium such as ammonia is evaporated using warm surface seawater. 2) The generated steam turns a turbine to produce electricity. 3) The steam used to turn the turbine is then cooled back into a liquid using cold deep seawater. This process of steps 1 to 3 is repeated. This mechanism was proposed in 1881 by a French physicist, and has been intermittently developed since.

Power generation mechanism using OTECPower generation mechanism using OTEC

Unlike variable renewable energies such as solar and wind power, OTEC can provide a stable supply of electricity 24 hours a day, unaffected by weather conditions. OTEC requires a temperature difference between surface and deep seawater, with about 20°C being necessary for commercial deployment. Therefore, regions near the equator, where the average temperature difference between surface and deep seawater exceeds 20°C, are ideal locations, making it a promising energy source for island nations.

In the case of Kumejima, the island uses the temperature difference between deep seawater (6~7°C/42.8~44.6°F in the summer) drawn from the seabed 2.3km (1.4 miles) offshore at a depth of 612m (2007ft), and surface seawater (28~29°C/82.4~84.2°F in the summer) to generate a maximum of 100kW of power. The steam generated is cooled using 13,000 tons of deep seawater per day.

Map showing areas near the equator suitable for OTECMap showing areas near the equator suitable for OTEC

Moreover, deep seawater is not only used for cooling steam but also utilized for its purity, nutrient richness, and low temperature. It is used in cosmetic raw materials, aquaculture, and air conditioning systems. Such secondary uses of deep seawater are also significant benefits of OTEC.

One specific example is "sea grape cultivation". The demand for sea grapes surges during Okinawa's tourist season in summer, but cultivation is difficult due to high seawater temperatures. However, using cold deep seawater enables cultivation even in summer, allowing for stable supply throughout the year. Deep seawater is also used in the world's first "fully land-based oyster cultivation", with oysters grown exclusively in clean deep seawater being noted for being safe for raw consumption. Additionally, cooling soil with pipes carrying cold deep seawater enables the cultivation of leafy vegetables like spinach, which cannot typically be grown in tropical climates.

Model diagram of OTEC and related industriesModel diagram of OTEC and related industries

(Continued in part2)

 

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