OCEAN THERMAL ENERGY CONVERSION (OTEC) Meredith Baker

Definition of OTEC A method of making use of the thermal energy in the ocean by using the temperature difference between deep cold ocean water and warm tropical surface waters. The surface acts as the hot reservoir, and the deep water acts as the cold reservoir, and the transfer of heat allows for the extraction of mechanical energy.

How long has it been around? The idea of tapping into the ocean’s thermal energy was first introduced in 1881 by Jacques Arsene d’Arsonval, a French physicist. It was Jacques student, Georges Claude, who built the first OTEC plant in Cuba in 1930. The United States became involved in OTEC research in 1974 with the Natural Energy Laboratory of Hawaii Authority, becoming one of the world’s leading test facilities for OTEC technology.

Ocean energy research center in Hawaii, closed-cycle system

Where can it be done? OTEC is suitable only in tropical regions, because it relies on a temperature gradient between the surface and the very deep water in the ocean. A temperature difference of at least 20 degrees Celsius is needed for OTEC to be successful. OTEC systems can be constructed in three types of locations: onshore, mounted to the ocean floor, or floating. In the summertime especially, the temperature difference from the surface and the bottom of the ocean is very large.

At roughly 1000 m, the ocean temperature starts to become constant at about 4 degrees Celsius.

What is the efficiency? Maximum efficiency is limited by the Carnot efficiency. Carnot Efficiency of OTEC = 100 * ( 1 – Tc/Th ) If surface temperature were at 24 degrees Celsius (297 K)and water at 4 degrees Celsius (277 K) were used? = 6.73% efficiency If surface temperature were at 34 degrees Celsius (307 K) and water at 4 degrees Celsius (277 K) were used? = 9.77% efficiency These are relatively small compared to… Automobile engine ~ 17% Nuclear power plant ~ 40% Coal fired station ~ 40% These are low 354

3 Different Types of OTEC Systems System 1: Open Cycle Systems Warm water is pumped into a low-pressure container, where it boils. The expanding steam drives a low-pressure turbine attached to an electrical generator. The salt from the steam is left behind in the low-pressure container and the steam is condensed back into liquid by exposure to cold temperatures from deep-ocean water.

3 Different Types of OTEC Systems System 2: Closed-Cycle Systems Fluids with a low boiling point, such as ammonia, are used to rotate a turbine to generate electricity. Warm seawater is pumped through a heat exchanger, where the low boiling point fluid is vaporized. The expanding vapor then turns the turbo-generator. Cold seawater then condenses the vapor back into a liquid that is then recycled through the system.

3 Different Types of OTEC Systems System 3: Hybrid Systems Combines both the open and closed systems. Warm seawater enters a vacuum chamber, where it is quickly evaporated into steam. The steam vaporizes a low-boiling-point fluid in a closed-cycle loop that drives a turbine to produce electricity.

Advantages It is clean, green renewable energy. Doesn’t burn fossil fuels or release toxic air pollution. Can allow tropical island states to become self- sufficient. Easy to maintain. For land-based systems, the freshwater produced from the condensed working fluid can be used as a supply of quality drinking water.

Disadvantages and Challenges Relatively inefficient, looking at the Carnot efficiency of 7-9%. A large amount of water is needed to be pumped to produce this efficiency, and a significant amount of that energy produced has to be used to generate the OTEC system. OTEC systems have to be constructed at a large scale, making them costly. Pumping cold water from the deep ocean to the surface releases carbon dioxide, a greenhouse gas currently most responsible for global warming. However, it is only a fraction (~10%) as much as that produced by a fossil-fueled power plant.

Plans for the future? Makai, the Ocean Energy Research Center in Hawaii, has a 105 kW OTEC plant but wants to build a 100 MW system offshore. It would produce enough electricity for 100,000 Hawaiian homes. Creating this would drop the average price for electricity in Hawaii from 33.2 cents per kWh to 20 cents per kWh.

Thank you! 