Kobe Steel Ltd has developed a high heat transfer titanium sheet called Heet for use in plate heat exchangers, one of the important applications of commercially pure titanium sheet. Kobe Steel has supplied this material for use in the heat exchangers of an ocean thermal energy conversion (Otec) power system in a demonstration project in Kumejima, Okinawa prefecture, Japan.

The high heat transfer titanium sheet enhances the heat transfer performance by more than 20 per cent. Moreover, the new sheet is environmentally friendly and conserves resources. It is the first time that Heet has been applied in an Otec power system. This Heet application is hoped to make large advances in lowering the power generation cost to verify the practical use of Otec power generation, which is attracting attention as a renewable source.

THE PROCESS
Otec power generation utilises the temperature difference between deep seawater and surface water. Warm surface seawater is pumped through a heat exchanger where a low-boiling-point fluid, such as ammonia, is vapourised. The resulting steam drives a turbine to generate electricity. Cold deep seawater goes through a second heat exchanger to condense the vapour back to liquid. Otec is drawing attention as a renewable source of energy that efficiently uses the heat energy in the ocean.

Electricity generated in the demonstration project is supplied to a power company, a world first. The Otec power system successfully started generating electricity on March 30 and commenced verification tests in April. Stability tests of the heat cycle, validation of controllability, and performance tests of the heat exchangers are also being conducted. Aiming to commercialise the Otec power system in the future, the project is examining ways to reduce power generation costs and is looking to set up an Otec power system in the Okinawa area.

From melting to final products, Kobe Steel is a comprehensive manufacturer of titanium products. Kobe Steel supplies titanium alloys to the aerospace industry, as well as commercially pure titanium for the energy, chemical and construction fields.

BUMPS BOOST
Kobe Steel’s high heat transfer titanium sheet, made of commercially pure titanium, has minute convex-shaped bumps to increase heat conductivity. When using this high heat transfer titanium sheet in heat exchangers for Otec power systems, the boiling heat transfer increases by more than 20 per cent without changing the design of the heat exchanger. By increasing the heat transfer performance, it is possible to generate electricity from smaller temperature differences. This broadens the areas where electricity can be generated.

In addition, reducing the amount of seawater that’s drawn up, decreases electricity consumption within the Otec plant, leading to an actual increase in power-generation capacity. The compact size of the heat exchanger lowers equipment costs, which contribute to lowering power generation costs. In this demonstration project, the economic benefits of using high heat transfer titanium sheet in the power generation business will be specifically studied, as will the commercial application of the Otec power system.

DEDICATED EFFORTS
In 2000, Kobe Steel began research on titanium sheets for heat exchangers in Otec power systems. It collaborated with Saga University’s Institute of Ocean Energy to increase the advancement of titanium materials. Kobe Steel is actively working to expand sales of the high heat transfer titanium sheet to domestic and overseas users in industrial sectors that use heat exchangers including the energy, plant engineering and shipbuilding fields. At the same time, by contributing to the effective utilisation of renewable energy, Kobe Steel aims to help build a sustainable society.