Ocean Thermal Energy Conversion: Open, Closed, and Hybrid Systems

Open Cycle OTEC

Warm seawater is the working fluid.

The warm seawater is "flash"-evaporated in a vacuum chamber to produce steam at an absolute pressure of about 2.4 kilopascals.

The steam expands through a low-pressure turbine that is coupled to a generator to produce electricity.

The steam exiting the turbine is condensed by cold seawater pumped from the ocean's depths through a cold-water pipe.

If a surface condenser is used in the system, the condensed steam remains separated from the cold seawater, resulting in a supply of desalinated water.

Closed Cycle OTEC

• Warm seawater vaporizes a working fluid, such as ammonia, flowing through a heat exchanger (evaporator).
• The vapour expands at moderate pressures and turns a turbine coupled to a generator that produces electricity.
• The vapour is then condensed in a condenser using cold seawater pumped from the ocean's depths through a cold-water pipe.
• The condensed working fluid is pumped back to the evaporator to repeat the cycle.
• The working fluid remains in a closed system and circulates continuously.

Hybrid OTEC

A hybrid cycle combines the features of both the closed-cycle and open-cycle systems.

Warm seawater enters a vacuum chamber where it is flash-evaporated into steam, which is similar to the open-cycle evaporation process.

The steam vaporizes the working fluid of a closed-cycle loop on the other side of an ammonia vaporizer.

The vaporized fluid then drives a turbine that produces electricity.

The steam condenses within the heat exchanger and provides desalinated water.

Factors for Site Selection

• Thermal gradient in the ocean
• Topography of the ocean floor
• Seismic activity
• Availability of personnel to operate the plant
• Infrastructure: airports, harbors, etc.
• Local electricity and desalinated water demand
• Political and ecological constraints
• Cost and availability of shoreline sites

Advantages

• Supplies steady power without fluctuations and is independent of unpredictable weather.
• Availability hardly varies from season to season.
• At a suitable site, the resource is essentially limited only by the size of the system.

Disadvantages

• Low efficiency.
• Large size.
• High installation cost.
• Ocean depths should be available fairly close to shore-based facilities to avoid transmission/distribution losses.
• The plant must withstand ocean conditions like storms.