8.9 Ocean Thermal Energy Conversion

Introduction

Ocean Thermal Energy Conversion, often shortened to OTEC, is a technology that uses the natural temperature difference between warm surface seawater and cold deep seawater to generate useful energy. It targets tropical and subtropical oceans where surface waters are warm throughout the year, while deep waters remain cold. This chapter focuses on how OTEC works, the main technological approaches, and its potential uses and challenges, without repeating broader hydropower or marine energy topics from other chapters.

The Physical Principle Behind OTEC

OTEC relies on the fact that sunlight heats the top layer of the ocean, typically the upper 100 to 200 meters, to temperatures around 24 to 30 °C in tropical regions. Below about 700 to 1000 meters, water is much colder, often around 4 to 5 °C, because it has not been in recent contact with the atmosphere or direct solar heating.

This vertical temperature gradient represents a form of stored solar energy. OTEC systems treat the warm surface as a low temperature heat source and the deep cold water as a heat sink. The concept is similar to a conventional thermal power plant, but with much lower temperatures.

The maximum theoretical efficiency of any heat engine that operates between a hot source at temperature $T_{hot}$ and a cold sink at temperature $T_{cold}$ is given by the Carnot efficiency:

For a typical OTEC situation with $T_{hot} \approx 298 \text{ K}$ (25 °C) and $T_{cold} \approx 278 \text{ K}$ (5 °C), the Carnot efficiency is about 6.7 percent. Practical OTEC efficiencies are lower, often only a few percent. This low efficiency is a central characteristic of OTEC and shapes its technology choices and design.

Main OTEC Cycles

OTEC systems translate the small temperature difference into usable power through thermodynamic cycles. Three main cycle types are commonly discussed: closed-cycle, open-cycle, and hybrid systems.

Closed-Cycle OTEC

In closed-cycle OTEC, a working fluid with a low boiling point, such as ammonia or certain refrigerants, circulates in a closed loop. Warm surface seawater does not boil directly. Instead, it transfers heat to the working fluid through a heat exchanger.

The basic steps are:

1. Warm surface seawater passes through an evaporator heat exchanger. It transfers heat to the working fluid, which boils at a low temperature and becomes high volume vapor.
2. The vapor expands through a turbine connected to a generator, producing electricity.
3. After the turbine, the vapor enters a condenser cooled by cold deep seawater. It condenses back into a liquid.
4. A pump returns the liquid working fluid to the evaporator, and the cycle repeats.

Closed-cycle systems have the advantage of separating seawater from the turbines and generators, which reduces corrosion and scaling. However, they need large and efficient heat exchangers to make up for the small driving temperature difference, and they must handle potentially hazardous working fluids safely.

Open-Cycle OTEC

In open-cycle OTEC, warm seawater itself is the working fluid. No separate low boiling point fluid is used. Instead, the system operates at low pressure so that seawater boils at a temperature close to that of the warm surface layer.

The process can be described as follows:

1. Warm surface seawater enters a low pressure chamber, often under vacuum. The pressure is low enough that a portion of the water flashes into steam at near-ambient temperature.
2. The resulting steam passes through a turbine to generate electricity.
3. After the turbine, the steam is condensed back into liquid using cold deep seawater in a condenser.

Since salt does not evaporate, the condensed water is essentially desalinated freshwater. This is a distinctive feature of open-cycle OTEC: it can produce both electricity and freshwater in a single process.

Open-cycle systems avoid the use of synthetic working fluids but must manage very low pressures and large volumes of low pressure steam. Turbines and vacuum systems need to be designed to handle this.

Hybrid OTEC Systems

Hybrid OTEC systems combine elements of closed and open cycles. A common arrangement uses an open-cycle stage to produce low pressure steam from seawater and then uses that steam to drive a heat exchanger that boils a secondary working fluid in a closed-cycle loop.

In this configuration, the open-cycle portion can generate desalinated water, while the closed cycle is optimized for electricity production. Hybrid systems aim to capture the benefits of both approaches, but they add complexity and require careful integration of multiple subsystems.

Key Components and Infrastructure

Although there are different OTEC cycle types, many physical components are shared. These include large intake pipes, heat exchangers, turbines, condensers, pumps, and structural platforms.

Warm and Cold Water Intake Systems

OTEC requires a continuous flow of warm surface seawater and cold deep seawater. Warm water intake systems are relatively simple since they operate near the surface. The main challenge is to avoid entraining marine organisms and to prevent fouling of pipes and heat exchangers.

Cold water intake is much more demanding. A long and large diameter pipe must reach down to depths of 700 to 1000 meters or more to access sufficiently cold water. This cold water pipe must withstand external pressure, wave action, and currents, while maintaining structural integrity and alignment with the platform or plant.

These pipes are often made of high density polyethylene or composite materials to balance strength, flexibility, and weight. Their design is one of the most critical and costly elements of an OTEC plant.

Heat Exchangers and Turbines

Heat exchangers are at the heart of OTEC performance. Because the temperature difference between warm and cold water is small, heat exchangers must be large, efficient, and have minimal thermal resistance. Common materials include titanium and corrosion-resistant alloys, especially where seawater is in direct contact with metals.

Turbines in closed-cycle systems resemble those used in low pressure steam or organic Rankine cycle plants, but they are optimized for the specific working fluid and relatively small temperature difference. In open-cycle systems, turbines must handle very low pressure and large specific volume steam, which influences their size and blade design.

Platforms and Plant Configuration

OTEC can be implemented on land, nearshore structures, or floating offshore platforms.

Land-based and nearshore plants attach to the coast. The cold water pipe runs from deep offshore to the shore-based plant. These arrangements can simplify maintenance and allow easier integration of electricity and freshwater into local infrastructure, but suitable coastal sites with access to deep water close to shore are limited.

Floating platforms are anchored offshore in deep water. The entire plant, including the cold water pipe, is located at sea. Electricity can be brought to shore through underwater cables. This configuration expands the potential OTEC locations but increases engineering demands related to stability, anchoring, and maritime conditions.

Power Output and Scale

Individual OTEC plants are envisioned at different scales, from small experimental units to large commercial plants.

Small demonstration plants may produce on the order of tens of kilowatts to a few megawatts. These are often used to test components, validate designs, and support small island communities.

Commercial scale concepts usually aim for tens to hundreds of megawatts. Achieving such scales requires very large water intakes and heat exchangers, and careful optimization of all energy flows. Because of the low thermodynamic efficiency, a large mass flow of water is needed to produce significant power.

For example, an OTEC plant generating several tens of megawatts may need to move hundreds of cubic meters of seawater per second through its system. Pumping this water consumes part of the generated power, so the design must ensure that net power output remains positive and economically meaningful.

Co-Products and Integrated Uses

One of the distinctive features of OTEC, especially open and hybrid systems, is the potential to provide useful co-products alongside electricity.

Desalinated freshwater is a central co-product in open-cycle and hybrid OTEC systems. Since the process naturally distills seawater, it can be tailored to provide water for drinking, agriculture, or industry. In water-scarce tropical islands, this combined electricity and water production can be very attractive.

Cold deep seawater has other uses beyond condensation. After passing through the condenser, it still has a relatively low temperature that can be used for air conditioning and refrigeration. This concept is known as seawater air conditioning. It can cool buildings with much lower electricity use than conventional air conditioning systems.

Nutrient rich deep water may also support aquaculture and agriculture. In some concepts, deep seawater is used to support fish farming, algae cultivation, or even to cool greenhouses in hot climates. By integrating these uses, OTEC complexes can become multi-purpose resource hubs rather than stand-alone power plants.

Environmental and Ecological Considerations

OTEC operates continuously and does not emit greenhouse gases during operation. However, it interacts directly with the ocean environment, and this raises several environmental questions.

The intake of large volumes of water can affect plankton and small organisms. Design of intake structures, flow velocities, and screening systems influences how many organisms are entrained or impinged. Careful design can reduce these effects, but they cannot be ignored.

Discharge of used water must also be considered. OTEC discharges typically involve mixing cold deep water and warm surface water and then returning the mixture to the ocean. If discharged at inappropriate depths, this could disturb local temperature profiles or nutrient distributions. Poorly designed discharges might encourage excessive algal growth, or they might affect local marine habitats.

The long term effect of large scale OTEC deployment on ocean thermal structure and circulation is still a subject of research. Since OTEC essentially extracts energy from the vertical temperature gradient, widespread deployment could in principle have subtle effects on regional ocean dynamics, though likely smaller than many natural variations for the scales currently envisioned.

Material choices, such as metals and plastics for pipes and heat exchangers, must be managed to avoid corrosion products or microplastics entering the marine environment. OTEC plants must comply with marine environmental protections and be monitored for impacts.

Technical and Economic Challenges

Despite its attractive concept and the huge theoretical resource in tropical oceans, OTEC has not yet reached widespread commercial deployment. Several technical and economic challenges explain this situation.

The low temperature difference means low efficiency and high sensitivity to any additional losses. Every pressure drop, thermal resistance, or extra pumping step erodes net power output. This pushes designs toward large, high quality components that are expensive to build and maintain.

Cold water pipes to great depths are technically demanding. They must survive storms, currents, and fatigue over long periods. Their size and complexity represent a large initial investment, and failures would be very costly.

Heat exchangers must be resistant to corrosion, fouling, and scaling. In warm tropical waters, biological growth such as algae and barnacles can quickly reduce performance. Regular cleaning or antifouling strategies are required, adding to operating costs.

Economically, OTEC competes with other renewable technologies that have matured more rapidly, such as solar and wind. In many locations, solar photovoltaics and wind power now have much lower levelized costs of electricity. To be competitive, OTEC must either reduce its costs or target niche applications where its specific strengths, such as constant baseload power and co-production of freshwater and cooling, have high value.

Financing large offshore infrastructure is also demanding, especially in small island states where the technology might be most beneficial. Limited experience, perceived risk, and lack of track record can make investors cautious.

Resource Potential and Suitable Locations

OTEC is essentially limited to regions where the temperature difference between warm surface water and deep cold water is large enough all year. Typically, a minimum difference of about 20 °C between surface and deep water at around 1000 meters depth is considered necessary for practical OTEC operation.

This condition is met primarily in tropical and some subtropical oceans. Many potential sites lie around small island developing states, equatorial coastal regions, and archipelagos where deep water is relatively close to shore. Examples include parts of the Pacific and Indian Oceans and some Caribbean locations.

The global theoretical resource is large, since oceans cover most of the planet and solar heating maintains warm surface layers. However, only a fraction of this resource is practically accessible when technological, economic, and environmental constraints are taken into account.

OTEC is particularly attractive for locations that currently rely on imported fossil fuels for electricity and water desalination. In such cases, the combined provision of continuous renewable electricity, freshwater, and cooling can support local resilience and reduce dependence on supply chains that are vulnerable to price shocks and disruptions.

Future Prospects and Research Directions

Research and development in OTEC continues in several areas. Engineers and scientists are working on more robust and cost effective cold water pipes, improved heat exchanger materials and designs, and better integration of OTEC with local energy and water systems.

There is interest in modular OTEC units that can be scaled up gradually, which could reduce financial risk and allow learning by doing. At the same time, studies are examining long term environmental impacts and ways to minimize or even use the thermal and nutrient effects of OTEC discharges in beneficial ways.

Digital tools, such as advanced modeling and monitoring, can help refine plant operations, improve reliability, and optimize the combined use of electricity, water, and cooling services.

If technological hurdles and economic barriers can be reduced, OTEC could become an important part of the energy mix for some tropical regions. Its ability to provide continuous baseload power, along with co-benefits such as desalinated water and cooling, makes it a distinctive member of the family of marine energy technologies.