Life of the Land Newsletter  March - April  1980

Alternative Energy: OTEC

By Jan Takeyama, LOL Staff Member

THE FIRST IN A SERIES OF ALTERNATE ENERGY ARTICLES

Ocean Thermal Energy Conversion (OTEC) is a system of obtaining electrical energy from the temperature differences existing in the ocean. OTEC uses warm water of about 75°-80°F (23.9° -25.7° C), which is present at the surface of the ocean, and cold water at 40°-45°F (4.4° - 7.2° C) which is present at depths of 2,000 feet.

In the "closed-cycle" system, the warm surface water is used to  vaporize ammonia (which has a boiling point of -27.4°F  (-33°C), through a heat exchanger. The ammonia vapor will in turn drive a turbine to power a conventional  electric generator. Cold bottom waters of the ocean are then used to cool and condense the ammonia, which is then recycled into the system.

Theoretical maximum efficiency rates from the OTEC system run at about 7%. However, due to thermal losses, energy losses in the turbine and generator, and the power required to pump the working fluid and large quantities of seawater for heating and cooling, actual net efficiency is calculated at about 2%. The OTEC fuel, however, is free and inexhaustible since it is a result of the sun's heat.

Correspondingly, an OTEC plant may be economically feasible if capital and operating costs are kept down especially as the price of fossil fuels increase. A low-efficiency system which utilizes free fuel can be as economical as a higher-efficiency system which utilizes expensive fuel.

Hawaii is the best place in the United States to build an OTEC site. The advantages our Hawaiian Islands possess -physical characteristics such as bathymetry and baththermal profiles, nutrient quality of the water, nearly constant (year-round) thermal differences of the Ocean (Approximately 36F) - have been documented in studies conducted at Ke-ahole Point on the Big Island, and make OTEC a dependable and competitive energy source.

A further benefit of OTEC may result in what is termed an "open-cycle" system. In this type of system, warm surface water is drawn into an evaporator where it boils because of the very low pressure. Seawater not turned into steam is discharged back into the ocean, while steam is sent to turn a turbine which turns a generator and produces electricity. The steam existing in the turbine travels through the condenser and is cooled by the deep ocean water. The condensate is a clean desalted, potable water supply -fresh water -which may be as beneficial as the electricity produced.

It is imperative that OTEC be considered as an important step to Hawaii's goal of energy and economic self-sufficiency. However, there are important environmental concerns such as: the effects of thermal stress which would be introduced into the immediate area; the effect of additional nutrients to coastal waters (creating the possibility of  biostimulation of marine life due to increased levels of food sources); and water quality standards as related to temperature and nutrient
changes. Another environmental concern is ammonia leakage into the ocean due to corrosion in the heat exchange system. One way to assure that this does not happen is to design the plant with a rotating maintenance schedule; in other words, to allow for a portion of the plant to be shut down for maintenance and
still maintain optimum capacity of the plant.

Observation of existing data and conclusions show that with further studies and research, OTEC can become environmentally acceptable as an energy alternative for Hawaii.

Recently, funding for OTEC has been threatened by President Carter's proposed budget cuts. In addition, the petroleum and car manufacturing industries of this country have set off on a program of such intent, equivalent, in their words, to NASA's space program in the '60's of putting a man on the moon, in researching and producing synthetic fuels. The amount of money being poured into this field is staggering, and the projected environmental effects are many.

It has been shown that three commercial OTEC plants could produce enough energy for all of Hawaii, yet this project is being threatened by further expenditure cuts. This country has always been guilty of short-sightedness in regards to our energy industry. Perhaps we will remain chained to our internal combustion engines and dependant upon rapidly deteriorating energy resources. Or perhaps we will move for long-term, cost-effective, natural energy sources. The choice is
ours. (Or is it?)

The deep ocean water that OTEC would pump up is rich in nutrients. This cool water can be used for agri/ aquaculture food production systems. Products such as prawn, oyster, opihi, seaweed, algae and catfish could be farmed. Preliminary studies of the possibility of installing an aquaculture system at the OTEC Seacoast Test Facility on Ke-ahole Point are beginning.