The news source for Ocean Thermal Energy Conversion (OTEC)

 

  by L. A. Vega, Ph.D., Hawaii, USA.

Previous: Background

Technical Limitations

The performance of OTEC power generating cycles is assessed with the same elementary concepts of thermodynamics used for conventional steam power plants. The major difference arises from the large quantities of warm and cold seawater required for heat transfer processes, resulting in the consumption of  20  to 30 percent of the power generated by the turbine-generator in the operation of pumps.  The power required to pump seawater is determined accounting for the pipe-fluid frictional losses and in the case of the cold seawater for the density head, i.e., gravitational energy due to the differences in density between the heavier (colder) water inside the pipe and the surrounding water column.  The seawater temperature rise, due to frictional losses, is negligible for the designs presented herein. 
 
The ideal energy conversion for 26 °C and 4 °C warm and cold seawaters is 8 percent.  An actual OTEC plant will transfer heat irreversibly and produce entropy at various points in the cycle yielding an energy conversion of 3 to 4 percent.  These values are small compared to efficiencies obtained for conventional power plants; however, OTEC uses a resource that is constantly renewed by the sun.  Considering practical sizes for the cold water pipe OTEC is presently limited to sizes of no more than about 100 MW.  In the case of the open-cycle, due to the low-pressure steam, the turbine is presently limited to sizes of no more than 2.5 MW.  The thermal performance of CC-OTEC and OC-OTEC is comparable.  Floating vessels approaching the dimensions of supertankers, housing factories operated with OTEC-generated electricity, or transmitting the electricity to shore via submarine power cables have been conceptualized.  Large diameter pipes suspended from these plantships extending to depths of 1000 m are required to transport the deep ocean water to the heat exchangers onboard.  The design and operation of these cold water pipes are major issues that have been resolved by researchers and engineers in the USA. 

It has been determined that approximately 4 m³ s^-1 of warm seawater and 2 m³ s^-1 of cold seawater (ratio of 2:1), with a nominal temperature difference of 20 °C, are required per MW of exportable or net electricity (net = gross - inhouse usage).  To keep the water pumping losses at about 20 to 30 percent  of the gross power, an average speed of less than 2 m s-1  is considered for the seawater flowing through the pipes transporting the seawater resource to the OTEC power block.  Therefore, a 100 MW plant would use 400 m ³ s^-1 of 26 °C water flowing through a 16 m inside diameter pipe extending to a depth of 20 m; and 200 m³ s^-1 of 4 °C water flowing through an 11 m diameter pipe extending to depths of 1000 m.  Using similar arguments, a 20 m diameter pipe is required for the mixed water return.  To minimize the environmental impact due to the return of the processed water to the ocean (mostly changes in temperature), a discharge depth of 60 m is sufficient for most sites considered feasible, resulting in a pipe extending to depths of 60 m. 

The amount of total world power that could be provided by OTEC must be balanced with the impact to the marine environment that might be caused by the relatively massive amounts of seawater required to operate OTEC plants.  The discharge water from a 100 MW plant would be equivalent to the nominal flow of the Colorado River into the Pacific Ocean (1/10 the Danube, or 1/30 the Mississippi, or 1/5 the Nile into the Atlantic).  The discharge flow from 60,000 MW (0.6 percent of present world consumption) of OTEC plants would be equivalent to the combined discharge from all rivers flowing into the Atlantic and Pacific Oceans (361,000 m³ s^-1).  Although river runoff composition is considerably different from the OTEC discharge, providing a significant amount of power to the world with OTEC might have an impact on the environment below the oceanic mixed layer and, therefore, could have long-term significance in the marine environment.  However, numerous countries throughout the world could use OTEC as a component of their energy equation with relatively minimal environmental impact.  Tropical and subtropical island sites could be made independent of conventional fuels for the production of electricity and desalinated water by using plants of appropriate size.  The larger question of OTEC as a significant provider of power for the world cannot be assessed, beyond the experimental plant stage, until some operational and environmental impact data is made available through the construction and operation of the pre-commercial plant mentioned above.

Next: OTEC and the Environment

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