The infographic below shows how a closed cycle ocean thermal energy conversion (OTEC) power plant works.
Like all hydrothermal generation schemes, a heat source and a heat sink are required. In this case the heat source is warm water from the surface of the ocean and the heat sink is cold water from deep under the surface.
A closed cycle OTEC plant requires a working fluid which has an evaporating point between the temperature of the heat source and sink. This fluid runs in a closed loop between two heat exchange points, the evaporator and the condenser. When it enters the evaporator it is in liquid form, but as it passes through the evaporator the heat from the warm water causes the fluid to vaporize. When a fluid vaporizes (turns into gas) it expands. The force of the expansion is used to turn a turbine which generates electricity.
After the gas has passed through the turbine it enters the condenser. The condenser is cooler than the gas because it is in contact with the cold ocean water (the heat sink). As the condenser pulls heat from the gas, cooling it down, it condenses once more into a liquid. From there the liquid passes once more into the evaporator and the cycle repeats.
Each time the fluid passes through the evaporator heat energy transfers from the warm water into the fluid and the water cools accordingly. Each time the fluid passes through the condenser heat energy transfers from the fluid into the cold water and the cold water becomes warmer. If the water was never replaced, eventually the warm water and cold water would equalize at the same temperature. Without a difference in temperature no work could be performed and so power generation would stop.
That’s why this type of power plant requires constant replenishment of both warm and cold water. That is one of the challenges of this type of power plant – building and maintaining the infrastructure required to pump the water. While warm water may be easily available at the surface near the plant, it may be necessary to pipe the cold water from thousands of feet below the surface. Building and maintaining that much pipe is very costly.
Nonetheless, some very innovative groups continue to work through the challenges. If they can find a cost effective way to do this it would be revolutionary.
This type of power plant would be especially valuable for residents on tropical coastlines and islands. In these places, even though the water at the surface is very warm, at thousands of feet below the heat from the sun hardly penetrates, and the water is icy cold. This large temperature difference equates to potential for greater energy generation efficiency.