What is Ocean Thermal Energy Conversion?
Ocean thermal energy conversion (OTEC) is a process that uses the temperature difference between warmer surface water and colder deep ocean water to generate electricity. OTEC plants pump large quantities of warm surface seawater into a flash evaporation chamber. The warm water flashes into steam, spinning a turbine that generates electricity. The steam is then condensed into water using cold seawater pumped from the deep ocean. The condensation of steam produces desalinated freshwater as a byproduct.
OTEC technology offers a constant flow of baseload electricity 24 hours a day. It does not rely on sunlight as solar power does. The oceans provide an enormous renewable energy resource, with the potential to generate gigawatts of electrical power. The technology also has environmental benefits, since it does not emit greenhouse gases or require fossil fuels.
A Brief History of OTEC
The basic principle of OTEC has been understood since the late 1800s. One of the first documented descriptions of using ocean temperature differences to generate electricity was by French engineer Jacques Arsene d'Arsonval in 1881. D'Arsonval conceptualized using the alternation of warm surface water and cold deep water to run an engine.
The modern era of OTEC research began in the 1930s with studies by French physicist George Claude. He built the first OTEC plant in Matanzas Bay, Cuba in 1930. The experimental open-cycle plant produced 22 kW of net electrical power using a low-pressure turbine. Claude's OTEC plant demonstrated that electricity could be produced from the oceans. However, his research was stymied by the technology limitations and high costs of the time.
Interest in OTEC was revived in the 1970s, sparked by the global oil crisis. The U.S. government established the Natural Energy Laboratory of Hawaii Authority (NELHA) in 1974 to research renewable ocean resources including OTEC. NELHA built a small experimental OTEC plant off the coast of Hawaii in 1979, which successfully generated net power of about 18 kW.
How OTEC Plants Work
There are three main types of OTEC plant designs:
Closed-Cycle OTEC
Closed-cycle OTEC plants use warm surface water to vaporize a working fluid with a low boiling point, such as ammonia, which drives a turbine. Cold seawater condenses the vapor back into a liquid to be recycled through the system.
Open-Cycle OTEC
Open-cycle OTEC is the simplest design. Warm seawater itself boils in a low-pressure container. The steam vapor expands through a turbine. Cold seawater then condenses the steam into desalinated freshwater.
Hybrid Cycle OTEC
Hybrid cycle OTEC combines closed-cycle and open-cycle systems. Warm seawater first flashes evaporate in a vacuum chamber. This steam vaporizes a low-boiling point fluid like ammonia in a closed loop to power the turbine. Exhaust steam is then condensed into desalinated water.
All OTEC designs require a constant flow of warm surface seawater and cold deep seawater. Existing concepts use large diameter intake pipes to pump seawater between the ocean's surface and depths of around 1000 meters. The cold water intake is the most technically challenging component.
The Potential of OTEC
The ocean thermal resource far exceeds humanity's energy demands. Just 0.1% of the heat content in the tropical oceans could meet all current electricity needs. OTEC is also a reliable baseload resource, unaffected by weather or seasons.
OTEC plants have a theoretical maximum efficiency of 7-8%, limited by thermodynamic laws governing heat engines. In the real world, efficiencies of 1-3% are more realistic with current technology. Still, OTEC has an enormous sustainable energy generation potential equalling tens of gigawatts.
Island nations and developing tropical countries with access to deep cold water, like Indonesia and the Philippines, are ideal locations for OTEC plants. Clustering multiple OTEC plants together improves cost efficiency. The plants have additional benefits like freshwater production and aquaculture. OTEC technology has the potential to provide sustainable renewable baseload power, drinking water, and food production.
Challenges Facing OTEC Development
Despite its promise, OTEC has struggled to become commercially viable. There are substantial technological and economic obstacles facing OTEC:
-
Large capital costs - Building the underwater intake/outflow pipes and seawater pumping systems requires major upfront investments.
-
Technical complexity - OTEC plants have many custom components that must withstand corrosion from seawater over decades. Maintaining and protecting equipment on the ocean floor is difficult.
-
Low efficiencies - The maximum Carnot efficiency is low compared to mechanical engines. Real-world net efficiencies are lower still after electrical conversion losses.
-
Remote locations - Most potential sites are far from electricity grids, increasing transmission costs.
-
Environmental concerns - Large seawater flows may affect ocean nutrients and marine life habitats. Avoiding impacts requires careful siting and design.
These challenges have limited OTEC adoption despite its tantalizing potential. Only a handful of small prototype OTEC plants have ever been built. For OTEC to live up to its promise, these engineering and economic obstacles must be overcome through sustained research and development support.
Recent Progress in OTEC Technology
While commercial-scale OTEC has proven elusive so far, promising advancements keep happening:
-
In 2015, Makai Ocean Engineering completed a 105 kW closed-cycle OTEC plant connected to Hawaii's power grid, the largest built to date.
-
Lockheed Martin is developing an offshore 10 MW OTEC pilot system called Ocean Thermal Energy Conversion Cold Water Pipe. Their project focuses on novel materials and designs for long cold water intake pipes.
-
Researchers at UC Berkeley patented a low-cost hybrid foam OTEC concept using floating concrete and plastic foam. The design may enable smaller 100 kW modular OTEC barges.
-
OTEC projects have been built for niche applications like air conditioning and aquaculture. For example, a resort hotel in Hawaiʻi uses a 120 kW OTEC mini-plant for air conditioning and water desalination.
-
Japan and India have emerged as leaders in OTEC development, with ambitious plans to build floating 100 MW OTEC platforms.
While full commercialization remains over the horizon, these examples showcase that OTEC technology continues advancing. With sustained long-term support, OTEC may yet fulfill its promise as a major energy source.
The Future of OTEC
OTEC offers tantalizing potential as an emissions-free, renewable baseload power source located directly where electricity demand is greatest on tropical coasts and islands. The thermal resource alone could meet a substantial share of global energy demand far into the future.
Realizing the full promise of OTEC will require surmounting remaining engineering challenges and bringing down costs through research and commercial demonstration programs. OTEC technology readiness levels must progress to build confidence for the major investments needed to scale up to utility-size systems.
With rising global energy needs and the urgency of reducing greenhouse gas emissions, innovative renewable technologies like OTEC warrant greater attention. Alone, OTEC cannot solve our energy challenges. But it could make a meaningful contribution as part of a diverse mix of sustainable energy solutions. OTEC provides a technological foundation to build upon in humanity's transition to a carbon-free energy future.