Ocean Thermal Energy

A technology brief on ocean thermal energy converters (OTECs) that harvest energy from the temperature differential in the ocean

What is Ocean Thermal Energy?

Ocean Thermal Energy Conversion (OTEC) harnesses the temperature differential between warm ocean water at the surface and cold sub-surface at depth to operate a heat engine which can perform mechanical work, such as spinning a generator to produce electricity. OTEC operates in a similar way to your home air conditioning in reverse.

OTEC is best suited for tropical regions with access to deep waters. In such regions, the water at the ocean’s surface is relatively warm, upwards of 75°F to 80°F. As one descends in the water column, the temperature generally decreases. Below a thousand meters in depth the water is near-freezing, typically around while the 35°F to 40°F. This transition zone between the warm surface waters and cold deep waters is what’s referred to as a thermocline.

The water temperature in the deep doesn’t fluctuate too much spatially or temporally, but at the surface the seasonal difference (between summer and winter for example) can be 10°F or more. When sited correctly, these temperature variations can be minimized and OTEC can provide near constant renewable power.

While OTEC can run continuously in certain locations, it is not the most efficient method to harvest the ocean’s energy. The theoretical maximum efficiency of a heat engine is given to us by Carnot’s equation:

Efficiency = ( T_H-T_C ) / T_H

Where T_H is the hot (ocean surface) temperature and T_C is the cold (deep water) temperature in Kelvin. Since the temperature differential between the warm and cold ocean regions is only about 40°F, or 20°K, the theoretical maximum efficiency for OTEC is between 7 and 10 percent. Once we factor in the inefficiencies of other system elements like pumps, piping systems, turbines, etc. the real efficiency is closer to two or three percent.

To make these systems pencil-out with such low efficiencies, large volumes of water are needed. For example, with an efficiency of three percent, a one hundred megawatt OTEC plant would need to pump seawater at a rate of about 400 m3 per second. That’s about 40 dump-trucks-worth of water being supplied every second by massive pumps. The pipes and pumps needed for this, keeping in mind that the cold water needed for OTEC is 1000 meters deep or more, makes this a tricky engineering challenge.

Ocean Thermal Energy Design Archetypes

There are two types, a closed-loop and an open-loop system. Both have the same basic principle of operation, but differ in the type of ‘working fluid’ used and whether that fluid is open, or closed-off from the environment. These systems can be deployed on the coast or on floating offshore structures in some cases.

Closed-loop OTEC

closed-loop otec

In 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 Ocean Engineering 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.

Open-loop OTEC

open-loop otec

In an open-loop system, the working fluid is water vapor produced directly from seawater. It is considered “open” because the working fluid leaves the boundary of the system after performing work, that is the working fluid is returned to the ocean. 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.

The boiling point of water (the temperature at which the water changes from a liquid to a gas) decreases with altitude. Ask anyone who spends time making tea or coffee in the mountains. This is due to decreased atmospheric pressure at altitude relative to sea level. In a vacuum of 29 inches Hg, the boiling point of seawater is reduced to around 76^°F. When warm seawater is introduced into the evaporator section under vacuum, the seawater almost instantaneously boils and turns to a vapor.

The resulting steam is directed to a low-pressure 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 steam exchanges heat with cold seawater piped-up from the depths and is condensed into freshwater, the salts having been removed during the evaporation process. The condensed vapor is actually a high quality freshwater distillate at this point and is safe to use for any number of applications that need freshwater.

OTEC Blog Posts

A Decade of Ocean Energy Crowdfunding

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February 5, 2024

For ocean energy start-ups working on hard tech, raising funds can be more difficult than building the technology. Crowdfunding can help. Over the past decade more than 30,000 investors have contributed $69 million to ocean energy crowdfunding campaigns, and just two tidal companies account for half of the total funds. Read on to see ocean energy crowdfund trends of the past decade.