The news source for Ocean Thermal Energy Conversion (OTEC)

 

  by L. A. Vega, Ph.D., Hawaii, USA.

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Background

It is estimated that, in an annual basis, the amount solar energy absorbed by the oceans is equivalent to at least 4000 times the amount presently consumed by humans.  For an OTEC efficiency of 3 percent, in converting ocean thermal energy to electricity, we would need less than 1 percent of this renewable energy to satisfy all of our desires for energy.  However, even assuming that the removal of such relatively small amount of ocean solar energy does not pose an adverse environmental impact we must first identify and develop the means to transform it to a useful form and to transport it to the user.

The first documented reference to the use of ocean temperature differences to produce electricity is found in Jules Verne's "Twenty Thousand Leagues Under the Sea" published in 1870.  Eleven years after Jules Verne, D'Arsonval proposed to use the relatively warm (24 °C to 30 °C) surface water of the tropical oceans to vaporize pressurized ammonia through a heat exchanger (i.e., evaporator) and use the resulting vapor to drive a turbine-generator. The cold ocean water transported (upwelled) to the surface from 800 m to 1000 m depths, with temperatures ranging from 8 °C to 4 °C, would condense the ammonia vapor through another heat exchanger (i.e., condenser).  His concept is grounded in the thermodynamic  Rankine cycle used to study steam (vapor) power plants.  Because the ammonia circulates in a closed loop, this concept has been named closed-cycle OTEC (CC-OTEC).  D'Arsonval's concept was demonstrated in 1979, when a small plant mounted on a barge off Hawaii (Mini-OTEC) produced 50 kW of gross power, with a net output of 18 kW.  Subsequently, a 100 kW gross power, land-based plant was operated in the island nation of Nauru by a consortium of Japanese companies.  These plants were operated for a few months to demonstrate the concept.  They were too small to be scaled to commercial size systems.

Forty years after D'Arsonval, Georges Claude, another French inventor, proposed to use the ocean water as the working fluid. In Claude's cycle the surface water is flash-evaporated in a vacuum chamber. The resulting low-pressure steam is used to drive a turbine-generator and the relatively colder deep seawater is used to condense the steam after it has passed through the turbine. This cycle can, therefore, be configured to produce desalinated water as well as electricity. Claude's cycle is also referred to as open-cycle OTEC (OC-OTEC) because the working fluid flows once through the system.  He demonstrated this cycle in 1930 in Cuba with a small land-based plant making use of a direct contact condenser (DCC). Therefore, desalinated water was not a by-product.  The plant failed to achieve net power production because of a poor site selection (e.g., thermal resource) and a mismatch of the power and seawater systems.  However, the plant did operate for several weeks. This was followed by the design of a 2.2 MW floating plant for the production of up to 2000 tons of ice (this was prior to the wide availability of household refrigerators) for the city of Rio de Janeiro.  Claude housed his power plant in a ship (i.e., plantship), about 100 km offshore. Unfortunately, he failed in his numerous attempts to install the vertical long pipe required to transport the deep ocean water to the ship (the cold water pipe, CWP) and had to abandon his enterprise in 1935.  His failure can be attributed to the absence of the offshore industry, and ocean engineering expertise presently available. His biggest technological challenge was the at-sea installation of a CWP.  This situation is markedly different now that there is a proven record in the installation of several pipes during experimental operations.

The next step towards answering questions related to operation of OTEC plants was the installation of a small OC-OTEC land-based experimental facility in Hawaii. This plant was designed and operated by a team led by the author. The turbine-generator was designed for an output is 210 kW for 26 °C warm surface water and a deep water temperature 6 °C.  A small fraction (10 percent) of the steam produced was diverted to a surface condenser for the production of desalinated water. The experimental plant was successfully operated for six years.  The highest production rates achieved were 255 kWe (gross) with a corresponding net power of 10³ kW and 0.4 l s^-1 of desalinated water. These are world records for OTEC.

A two-stage OTEC hybrid cycle, wherein electricity is produced in a first-stage (closed cycle) followed by water production in a second-stage, has been proposed by the author and his coworkers to maximize the use of the thermal resource available to produce water and electricity.  In the second-stage, the temperature difference available in the seawater effluents from an OTEC plant (e.g., 12 °C) is used to produce desalinated water through a system consisting of a flash evaporator and a surface condenser (basically, an open cycle without a turbine-generator).  In the case of an open cycle plant, the addition of a second-stage results in doubling water production.

The use of the cold deep water as the chiller fluid in air conditioning (AC) systems has also been proposed.  It has been determined that these systems would have tremendous economic potential as well as providing significant energy conservation independent of OTEC.  For example, to produce 5800 tons (roughly equivalent to 5800 rooms) of air conditioning only 1 m ³ s^-1 of 7 °C deep ocean water is required.  The pumping power required is 360 kW  as compared to 5000 kW for a conventional AC system.  The investment payback period is estimated at 3 to 4 years.

A number of possible configurations for OTEC plants have been proposed.  These configurations range from floating plants to land-based plants, including shelf-mounted towers and other offshore structures.  The primary candidate for commercial size plants appears to be the floating plant, positioned close to land, transmitting power to shore via a submarine power cable.

Next: Technical Limitations

© 1999. L. A. Vega. All rights reserved.
Published here with the kind permission of the author.