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Capturing Ocean Heat

Ocean thermal energy conversion offers a timely renewable alternative.

Fortnightly Magazine - October 2009

funding made available in the economic stimulus and recovery law passed in early 2009, sustaining these amounts is important to send signals to private and institutional researchers that their pursuits of OTEC research will continue being funded. Despite the need for expanded R&D funding, the existing authorizing laws, grant programs and university-based research centers will help lay the institutional foundation for not only developing this technology, but also for growing the knowledge and workforce capacity necessary for long-term domestic development of OTEC.

With the above caveats, the time appears ripe for revisiting the development and deployment of commercial OTEC plants.

OTEC Plant Types

There are three types of OTEC plants: closed-cycle, open-cycle, and hybrid-cycle. The main differences between the first two is that the closed cycle uses the warm surface waters to heat a low-boiling point fluid, such as ammonia, that is used to drive the turbine-generator, while the open cycle utilizes a vacuum to flash sea water to steam, which then is used to turn the turbine-generator. The hybrid cycle uses features of both the closed- and open-cycles.

In d’Arsonval’s closed-cycle OTEC system ( see Figure 3 ), the warm surface water is sent through a heat exchanger (evaporator) where the low-boiling point working fluid is vaporized. This vapor is used to turn the turbine-generator, generating electricity. The vapor is then sent to a condenser where the cold sea water from the depths removes the remaining heat, which condenses it back to liquid. A pump sends the fluid back to the heat exchanger, completing the closed loop, and ensuring that the working fluid that remains continuously is circulated in a closed system, achieving relatively high efficiencies at a smaller scale when compared to the open-cycle system. This is essentially the same technology as is used in standard refrigeration systems, and the technology is well-understood and fairly mature, allowing for a straightforward scale-up to commercial sizes.

For the open-cycle system pioneered by Claude ( see Figure 4 ), the warm surface sea water is pumped into a low-pressure (vacuum) flash evaporator, causing it to boil into desalinated water vapor. This low-quality steam drives a low-pressure turbine-generator, and then is condensed into potable water in the condensor. This system allows for not only the generation of electricity, but also fresh water. In 1984, the Solar Energy Research Institute (now the National Renewable Energy Laboratory, NREL) developed an evaporator for open-cycle plants that had conversion efficiencies as high as 97 percent.

The hybrid OTEC system combines the features of both the closed- and open-cycle systems ( see Figure 5 ). Similar to the open-cycle process, warm sea water is flash-evaporated into steam in a vacuum chamber; however, this steam then is used to vaporize a low-boiling-point fluid, like the closed-cycle system, which then drives a turbine to produce electricity. The major advantage to the hybrid system is that is considered a more efficient producer of both electricity and side products like desalinated water.

Siting

OTEC facilities can be built on: 1) land or near the shore; 2) deep-water platforms moored to the continental shelf