Study on the Cost Effective Ocean Thermal Energy Convertion Power Plant
Authors: Srinivasan Nagan , Sridhar Meena , Agrawal Madhusuden 
Published: July 2010

Ocean Thermal Energy Conversion (OTEC) is a promising renewable energy resource from the ocean near equator that had been neglected for decades due to its enormous capital cost. The technologies that are developed for oil and gas industry for deep water applications could be adopted into OTEC power plant and the capital cost could be reduced by fifty percent than the estimation using conventional OTEC technology. This paper addresses a new OTEC technology for the OTEC power plant. Design and workability of the individual components of the new OTEC engine is shown. The global architecture of the power plant is illustrated. The engineering analyses studied show that the new OTEC power plant is feasible to construct and the cost analysis studied shows that the new OTEC power plant is viable for applications.

Introduction

Clean renewable energy is a necessity of today that more and more people are aware of. Countries all over the world support this effort, considering that there is no carbon dioxide emission and no environmental impact. Ocean Thermal Energy Conversion (OTEC), as one of the renewable energy source, is very attractive because no fossil fuel needed and is available with unlimited large quantity. USA is now spending several billion dollars for various renewable energy source projects. The operating cost of most of the renewable energy is very low but the capital cost is enormous. Because of the environmental nature, OTEC is very stable and reliable energy resource for tropical islands like Hawaii. The main challenge is to make the OTEC electric power generation cost-effective and comparable to the nuclear energy source.

OTEC

OTEC is a method for generating electricity utilizing the source of stored solar energy available in the ocean near the equator, where the surface water temperature is about 25 degree C or more due to heat from sunlight and at 3000 ft depth the water temperature is about 4 degree C. A thermodynamic heat transfer method is used to convert the thermal energy to run the gas turbine. Ammonia gas is used as working fluid for the OTEC engine. Heat transfer is the major engineering task to be involved on the success of any efficient OTEC power plant. Conventional OTEC does the heat transfer effort on the top of the platform deck. The amount of water needed to cool the working fluid, ammonia, is enormous. There is a cold water pipe in the conventional OTEC that is used to bring the cold water from 3000 ft down to the free surface. Similarly the gas heating process, where the surface warm water is used in the evaporator, faces the similar problem. Enormous amount of heavy equipment for handling large volumes of water on the surface of floating vessel has increased the vessel size and consequently the capital cost was very high in the conventional OTEC. This technology is addressed in Reference 1.

New OTEC

This paper addresses an OTEC engine that uses new technologies like sub-sea condenser, sub-sea pump, submerged evaporator and independent floating-pipe buoy platform to transport working fluid from turbine outlet to the subsea condenser. Oil and gas field has offered innovative and cost effective solutions for deepwater applications. The technologies developed for deepwater are well proven and are working in water depth over 6,000 ft. OTEC water depth is only 1,500 ft, which is much smaller than the depth capabilities of today oil and gas field operations. The new OTEC design uses subsea solutions for the heat conduction problem. The condenser is located subsea area where the enormous cold water is available without the need of storage. The condenser is directly exposed to the underwater environment with 4 degree C. The condenser is designed for 1,500 psi water depth pressure for its structural strength integrity. Subsea condenser is tested for flow simulation and heat conduction problem for workability by using modern ANSYS Computational Flow Dynamic (CFD) engineering software. The equipments and the piping are tested for 1,500 psi water pressure to work at 3,000 ft depth.