Dossier Océan et énergie - Énergie Thermique des Mers

Sommaire IOA News Letters

AN ECONOMIC DEEP OCEAN WATER APPLICATION DEVELOPED FROM OTEC TECHNOLOGY

Dr. Joseph Van Ryzin
President, Makai Ocean Engineering, and Member, IOA

--Seawater air conditoning, a concept that is technically and economically feasible today; environmentally safe; and renewable.

Deep, cold seawater has long been recognized as a valuable energy resource.  During the past 20 years, various experimentation and research in ocean thermal energy conversion and cold water mariculture has sought to solve the technical problems that prevent their economic development.   From this effort, the use of deep ocean seawater for air conditioning has emerged as the first technically and economically viable option and one which is environmentally benign.

The concept of seawater air conditioning systems (SWACS) is not a new one. the major difficulties in its utilization were the lack of proven pipeline concepts and heat exchangers were expensive with major uncertainties relative to corrosion and fouling.

During the last decade, research in the development of  OTEC has addressed these unknows.  Makai Ocean Engineering has successfully designed, engineered, and deployed several deep ocean pipelines.  The largest of these pipelines is  1m in diameter and was deployed to 700m.  A 1400mm diameter pipeline to deployed to 950m is currently being designed.

Development of low cost aluminum heat exchangers, compatible with deep, ocean seawater has also occurred, and tests at the Natural Energy Laboratory in Hawaii by ALCAN indicate minimal and controllable corrosion and fouling problems.

Another important factor is that the seawater cooling system can be conveniently incorporated within the conventional chilled fresh water circulation systems working in most large air conditioning plants.  The interior of the buildings remains unchanged, fresh water still circulates as with conventional A/C.

Air conditioning with deep, cold seawater also has a significant advantage over an OTEC power plant.  A seawater air conditioning system of comparable megawatt size can be built at a small fraction of the size and cost of an OTEC facility.  For example, a 5 MW OTEC plant would require approximately a 3m diameter pipeline for both cold and warm water intakes, heat exchangers and a power plant.  A seawater air conditioning system that replaces 5 MW of electrical power only requires 10% of the OTEC cold water flow through a 1m cold water pipeline and has no warm pipe or power plant system.  A pipeline of this size has already been installed and operated for several years in Hawaii.   Therefore, while developments for OTEC have brought it just to the edge of being economically feasible, the seawater air conditioning system is clearly already economically viable.

For large building and hotels in tropical and subtropical climates, air conditoning represents the major energy demand.  As a rule-of-thumb, a typical hotel room requires approximately 1 ton of air conditioning with an energy requirement of 0.9 kw. A conventional system utilizes about 900 kw/1000 tons but a similar sized A/C system using seawater requires only pumping power in the order of  40-80 kw/1000 tons, representing a 90% electrical saving over the chiller power requirement.

This high reduction in energy consumption makes the seawater A/C system quite profitable for certain locations.  The primary factor impacting economic success are:

	Distance offshore to cold water in the 4-9¢XC range
	Size of the air conditioning load
	Percent utilization of the air conditioning system
	Local cost of electricity.

Each potential site should be analyzed separately.  As an example, Makai has conducted a detailed analysis for a SWACS on Curacao in the Netherlands Antilles.  We studied the feasibility for three sites on the island, with air conditioning loads ranging from 540 to 2100 tons (540 to 2100 hotel rooms) the length of the seawater intake pipelines ranges between 5,150' and 11,750' depending on the site and the seawater intake temperature.

Capital costs, including pipeline, heat exchangers, and chilled water distribution system, was on the order of $2-5 million.  The payback period for these systems range between five and six years for the most feasible sites.

Makai Ocean Engineering has also completed some preliminary analyses for Guam in the Tumon Bay area where there is a high density of hotel rooms.  This preliminary analysis indicates that 10,000 hotel rooms could be air conditioned with cold seawater and that the capital payback period for installing such a system would be approximately five to six years.

The technology required to install and operate an economic seawater air conditioning system is available today.