Open and closed cycle ocean thermal energy conversion

The siting of an OTEC plant, either onshore or offshore, represents one of the key decision for any proposed project. The other key decision is the type of cycle to use. There are two principle options, an open cycle plant or a closed cycle plant. Hybrids of the two have also been proposed.

A closed cycle OTEC plant employs a thermodynamic fluid such as ammonia or a refrigerant like freon. This is contained in a completely closed system including the plant turbine. Hot surface seawater is used to evaporate the fluid and the vapour is then exploited to drive the turbine. The vapour from the turbine exhaust is condensed using the cold, deep ocean water, and returned to the beginning of the cycle where it can be reheated. A 50 kW closed cycle OTEC plant was built in Hawaii in 1979 and operated for a few months. A consortium of Japanese companies has also operated a 100 kW closed cycle OTEC plant in Nauru. Again this plant operated for only a few months to prove the concept. Neither was large enough to be commercially viable. Indeed, closed cycle OTEC is unlikely to be commercially viable in sizes of less than 40 MW.

In an open cycle OTEC system the seawater itself is used to provide the thermodynamic fluid. Warm seawater is expanded rapidly in a partially evacuated chamber where some of it 'flashes' to steam. This steam is then used to drive a steam turbine. From the exhaust of the turbine, the vapour is condensed using cold seawater. The vapour produced by flashing warm seawater is at a relatively low pressure so it requires a very large turbine to operate effectively. Practical limitations mean that the largest open cycle turbine that can be built today is around 2.5 MW, much smaller than for a closed cycle system.

One of the major advantages of the open cycle system is that the water condensed from the turbine exhaust is fresh, not salt water, and so the plant can also serve as a source of drinking water as well as electricity. A 210 kW open cycle OTEC pilot plant operated in Hawaii between 1993 and 1998.

In a hybrid OTEC plant warm seawater is flashed to produce steam and this steam is then employed as the heat source for a closed cycle system. This system is more complex that either of the other cycles but it marries the compact closed cycle system with the ability to produce drinking water.

Steam

Steam

Closed Cycle Otec System
Exhaust Fresh water water

Ammonia vapour

Ammonia vapour

Otec Plant Diagram

Figure 14.2 Block diagrams of (a) an open cycle OTEC plant and (b) a closed cycle OTEC plant

Figure 14.2 Block diagrams of (a) an open cycle OTEC plant and (b) a closed cycle OTEC plant

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Responses

  • haddas
    Is a turbine an open or closed system?
    7 years ago
  • vito
    Is a thermal plant an open or closed system?
    5 years ago
  • Johanna
    What is open cycke OTEC?
    2 years ago
  • rosa
    What is open cycle system in otec?
    2 years ago
  • robin
    What is open ocean thermal energy with diagram?
    2 years ago
  • IAN
    What is closed ocean thermak energy with diagram?
    2 years ago
  • Aziz Futsum
    What is closed type of otec?
    2 years ago
  • ODO MAGGOT
    Which type of turbine used in otec plant?
    2 years ago
  • Danilo
    What are types of otec?
    2 years ago
  • Sara
    What is meant by OTEC open CYCLE?
    2 years ago
  • GLEN
    What is mean by closed cycle and hybrid cycle?
    2 years ago
  • angel
    What is closed cycle OTEC?
    2 years ago
  • Primrose
    What is opened cycle otec?
    2 years ago
  • Hannu
    What is the difference between open cycle and closed cycle otec system?
    1 year ago
  • jonas
    What is open cycle system of ocean thermal energy conversion?
    1 year ago
  • quintilio
    What is open and closed cycle ocean thermal energy conversion?
    1 year ago
  • kati
    What is the difference in a closed or open ocean?
    11 months ago
  • danait simon
    What is differenc between open cycle and close cycle in ocean thermal energy conversion?
    11 months ago
  • arthur
    Is the Kernt cycle a open or closed system?
    28 days ago
  • leon hoffmann
    Which gas is generally used as a refrigerant in OTEC (Ocean Thermal Energy Conversion)?
    10 days ago

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