Often on this page, OGJ editors have discussed promising new technologies-such as windmills, harnessing ocean waves, and various types of biofuels-that might someday replace oil and natural gas as power sources. One not-so-new technology that hasn’t been discussed, but that is just as intriguing, is ocean thermal energy conversion (OTEC).
The OTEC technology uses differences in seawater temperatures at various depths to generate electricity, desalinate water, and even condition air. The Earth’s ocean acts as an enormous collector of solar energy.
According to the National Renewable Energy Laboratory’s OTEC web site, “On an average day, 60 million sq km of tropical seas absorb an amount of solar radiation equal in heat content to about 250 billion bbl of oil.” And although the economics of OTEC do not compete currently with conventional power generation technologies, the vast potential of this amount of energy is very compelling.
It makes sense that the hotter the surface water temperature, the greater the potential of the OTEC technology. The technology is therefore most promising for tropical island communities that have to depend on diesel-generated electricity. The fact that the technology produces desalinated water is an added benefit. And the chilled water can be used for mariculture ponds in which cold-water ocean species, such as lobster and salmon, can be raised.
OTEC history
As previously mentioned, this technology is not a new idea. In fact, it was first proposed in 1881 by a French physicist named Jacques Arsene d’Arsonval.
One of his students, Georges Claude, was the first to build an OTEC plant in 1930 in Matanzas Bay, Cuba. Using a low-pressure turbine, the plant produced 22 kw of electricity.
Claude constructed another plant off the coast of Brazil in 1935. Both of his plants, however, were destroyed by the sea. Neither plant ever produced net positive electricity.
In 1974, with oil prices rising after the Arab oil embargo, the US government got involved with OTEC research. It established the National Energy Laboratory of Hawaii Authority (NELHA) at Keahole Point on the Kona coast. In 1979, the facility started up a 50-kw OTEC demonstration plant. The plant produced 52 kw of gross power and 15 kw of net power.
In 1981, Japan produced 31.5 kw of net power during a demonstration of a shore-based plant. Then in 1993, NELHA produced 50 kw of net electricity in an open-cycle OTEC plant. Finally, in 1999, NELHA tested a 250-kw pilot OTEC plant.
OTEC technology
The OTEC technology has three different designs-closed cycle, open cycle, and hybrid-and it can be located on land, on a sea shelf, aboard a floating vessel, or theoretically in a submerged plant.
The closed-cycle system pumps cold water from depths of up to 1 km into a heat exchanger that condenses a vapor, such as ammonia or propylene. The heat-transfer fluid is then pumped through a second heat exchanger, where it is vaporized with the warmer surface seawater. The expanding vapor turns a turbine, which generates electricity.
In an open-cycle system, warm seawater is boiled in a low-pressure container. The steam expands in a low-pressure turbine to generate electricity. The steam, now nearly pure water, is condensed with the cold deep-sea water.
The hybrid system combines features of the closed and open-cycle systems. Steam is still generated at low pressure, but in this system it is used to vaporize the fluid, which is in a closed loop.
Future work
A company called Sea Solar Power Inc. in Pennsylvania is currently developing plants based on OTEC technology. The company has two models in development: a 10-Mw, land-based plant and a 100-Mw floating plantship.
The company reports that the land-based plant is specifically designed for small tropical islands and that the plantship is suited for continental applications. The company has recognized that conventional heat exchangers and turbomachinery are inefficient for OTEC plants and is designing those units to be most efficient for their specific application.
Sea Solar Power says the land-based plant should cost $45-50 million, for which the company already has financing. The first plantship should cost about $250 million.
Indeed, it will be interesting to see if OTEC can take advantage of further technological advancements to someday compete with conventional oil-based electricity generation.
