Understanding the OTEC cycles: Closed, open and hybrid

To convert the ocean thermal energy into power, OTEC systems operate using one of three main cycles: closed-cycle, open-cycle, and hybrid-cycle. Each has its own design, advantages, and technical considerations. Here is a closer look at how each one works. 

A) Closed-Cycle OTEC

This is the cycle used by Global OTEC in our OTEC Power Module®. In a closed-cycle system, a working fluid with a low boiling point, such as ammonia, is used to spin a turbine connected to a generator.

How it works: 

• Warm seawater from the ocean surface is used to heat the working fluid in a heat exchanger, causing it to vapourise.

• This high-pressure vapour spins a turbine, generating electricity.
• Cold seawater from the ocean depths is then pumped up to condense the vapour back into a liquid.
• The cycle repeats continuously in a closed loop.
  

B) Open-Cycle OTEC

The open-cycle system uses seawater itself as the working fluid, rather than a separate substance like ammonia. This requires operating turbines with low-pressure steam requires specialised designs.

How it works: 

• Warm surface seawater is placed in a low-pressure container, causing it to flash-evaporate into steam.

• The resulting low-pressure steam drives a turbine to generate electricity. 
• Cold deep seawater is then used to condense the steam back into liquid water.
  

C. Hybrid-Cycle OTEC

The hybrid-cycle combines elements of both the closed and open cycles, aiming to maximise both energy production and freshwater output.

How it works:

• Warm seawater is flash evaporated to create low-pressure steam (like in the open cycle). 

• This steam is used to heat a working fluid in a closed loop, which then drives the turbine. 
• Meanwhile, the steam is condensed using cold seawater, producing freshwater.