Ocean Thermal Energy Conversion

OTEC Overview

  • Energy Source: Temperature difference between surface and deep ocean water
  • Energy Type Converted: Thermal
  • U.S. Theoretical Energy Potential: 576 TWh/year
  • Power Density: Unknown
  • Device Types: Open-loop, closed-loop, and hybrid

 

 

Source: Florida Solar Energy Center

Source: Florida Solar Energy Center

Ocean Thermal Energy Conversion (OTEC) is pretty much a big heat exchanger connected to a heat engine. There are two types, a closed-loop and an open-loop system. Both systems rely on the temperature difference between warm surface waters and cold sub-surface waters, ideally 36oF or more. When sited correctly, these temperature differences can be near constant making OTEC a viable candidate to provide renewable base load power.

In a closed-loop system, the fluid that produces work (working fluid) is a refrigerant and the amount in the system remains fixed. While the amount of refrigerant remains relatively constant, the physical state of the refrigerant changes as it moves throughout the system.

In an open-loop system, the working fluid is water vapor produced directly from seawater. It is considered an “open” system because the working fluid leaves the boundary of the system after performing work, i.e. returns to the ocean. There is also a hybrid system, but you can think of those of a combination between an open and closed-loop system.


Closed-Loop

A closed-loop OTEC system needs three different fluids. The first is warm ocean water, typically around 75oF. The second is sub-surface cold seawater, generally around 41oF. And lastly, OTEC needs a working fluid with a low boiling point, typically a refrigerant. Ammonia and R-134a are common fluids for this purpose, since they have boiling points of -28oF and -15oF respectively (at atmospheric pressure).

otec-diagramIn a closed-loop system, liquid refrigerant at slight pressure and the warm surface water are both
introduced into a heat exchanger (evaporator), but they aren’t allowed to mix. Heat is transferred from the seawater to the refrigerant, causing it to warm and undergo a phase change into a vapor.

The refrigerant vapor is then directed to a turbine where heat energy is converted into mechanical energy. The turbine is used to drive a generator, which in turn produces electricity. The refrigerant having served its purpose is now directed to a second heat exchanger, the condenser.

Cold seawater and vapor refrigerant both enter the condenser, but again they don’t mix. The cold seawater cools the refrigerant, causing it to change back to a liquid. From here the liquid refrigerant is pumped back to the evaporator where the process begins anew.

Makai and Lockheed Martin have a 100 kW prototype of this technology currently in testing out in Hawaii, with plans to scale it up to 100 MW

 

Source: Makai Ocean Engineering

Source: Makai Ocean Engineering


Open-Loop

Open-cycle systems don’t need refrigerant, just warm and cold seawater. These systems use the warm seawater as the working fluid by flash-evaporating it into a vapor.

otecIn a vacuum of 29 inches Hg, the boiling point of seawater is reduced to 76oF. When warm seawater is introduced into the evaporator section under vacuum it boils and turns to a vapor, leaving its salt behind. The ultra-salty brine is rejected back out to the ocean.

The resulting steam is directed to a turbine where it performs useful work and drives an electrical generator. The steam then flows to the condenser section where it is cooled by the cold sub-surface seawater. The condensed vapor is actually a high quality freshwater distillate at this point and is safe to use for any number of freshwater applications.

 

References: Florida Solar Energy Center, Masutani and Takahashi at UH-Manoa, DoE, Makai Ocean Engineering