First Generation 50 MW OTEC Plantship for the Production of Electricity and Desalinated Water
Authors: Vega Luis A. , Michaelis Dominic 
Published: July 2010

Preliminary designs for first generation Ocean Thermal Energy Conversion (OTEC) plants utilizing either closed cycle (CC) or open cycle (OC) concepts are presented. These are based on existing technology and current offshore industry practices. The CC-OTEC plant utilizes pressurized anhydrous ammonia as the working fluid to drive turbine-generators to produce electricity; and, the OC-OTEC plant makes use of low pressure steam generated in flash evaporators to drive steam turbine generators to produce electricity and surface condensers for the production of desalinated water.

Introduction

Given that oil reserves (˜ 1400 billion barrels) can satisfy world-wide demand (&gt; 30 billion barrels/year) for at most another 50 years, it seems sensible to envision marine renewable energy resources as additional alternatives to our oil-based economy. In theory, for example, the ocean thermal resource could be used to generate most of the energy required by humanity (Michaelis, 2002 and Nihous, 2007). We should consider the implementation of OTEC plantships providing electricity, via submarine power cables, to shore stations. This could be followed, in 20 to 30 years, with OTEC factories deployed along equatorial waters producing energy intensive products, like ammonia and hydrogen as the fuels that would support the post-petroleum era.

What is pending, however, is a realistic determination of the costs and the potential global environmental impact of OTEC plants and this can only be accomplished by deploying and subsequently monitoring operations with first generation plants (Vega, 2003).

In the 1990s, it was determined that to be cost competitive OTEC plants larger than about 50 MW were required in the USA market; and, that it was necessary to deploy demonstration plants as a prerequisite to commercialization (Vega, 1992). Unfortunately, development did not proceed beyond an experimental plant sized at about 0.25 MW (Vega, 1995).

A number of configurations ranging from offshore to land based concepts have been proposed. Large and small waterplane platforms have been considered. In general, the former (ship shape) is ideally suited for OTEC applications. Moored offshore configurations transmit electrical power to shore via a submarine power cable. The grazing configuration operates as a self-contained factory ship on which an energy-intensive product is produced (Nihous, 1993). The grazing plantship can cruise around tropical waters essentially decoupled from land.

Conceptual designs for 50 MW OTEC plants utilizing either closed cycle (CC) or open cycle (OC) technology are summarized herein. The CC-OTEC plant utilizes pressurized anhydrous ammonia as the working fluid to drive turbine-generators to produce electricity; and, the OC-OTEC plant makes use of low pressure steam generated in flash evaporators to drive steam turbine generators to produce electricity and surface condensers for the production of desalinated water.

The OTEC platform must interface with both the cold water pipe (CWP) and the deep ocean mooring system. The attachment between the vessel and CWP must provide freedom in at least pitch and roll. Deep Ocean mooring systems or dynamic positioning thrusters developed by the offshore industry can be used for position keeping.