The news source for Ocean Thermal Energy Conversion (OTEC)

 

  by L. A. Vega, Ph.D., Hawaii, USA.

Previous: Technical Limitations

OTEC and the Environment

OTEC offers one of the most benign power production technologies, since the handling of hazardous substances is limited to the working fluid (e.g., ammonia), and no noxious by-products are generated. OTEC requires drawing seawater from the mixed layer and the deep ocean and returning it to the mixed layer, close to the thermocline, which could be accomplished with minimal environmental impact. The carbon dioxide out-gassing from the seawater used for the operation of an OC-OTEC plant is less than 1 percent of the approximately 700 grams per kWh amount released by fuel oil plants.  The value is even lower in the case of a CC-OTEC plant.

A sustained flow of cold, nutrient-rich, bacteria-free deep ocean water could cause sea surface temperature anomalies and biostimulation if resident times in the mixed layer and the euphotic zone respectively are long enough (i.e., upwelling). The euphotic zone is the upper layer of the ocean in which there is sufficient light for photosynthesis.  This has been taken to mean the 1 percent-light-penetration depth (e.g., 120 m in Hawaiian waters).  This is unduly conservative, because most biological activity requires radiation levels of at least 10 percent of the sea surface value.  Since light intensity decreases exponentially with depth, the critical 10 percent-light-penetration depth corresponds to, for example, 60 m in Hawaiian waters.  The analyses of specific OTEC designs indicate that mixed seawater returned at depths of 60 m results in a dilution coefficient of 4 (i.e., 1 part OTEC effluent is mixed with 3 parts of the ambient seawater) and equilibrium (neutral buoyancy) depths below the mixed layer throughout the year.  This water return depth also provides the vertical separation, from the warm water intake at about 20 m, required to avoid reingestion into the plant.  This value will vary as a function of ocean current conditions.  It follows that the marine food web should be minimally affected and that persistent sea surface temperature anomalies should not be induced.  These conclusions need to be confirmed with actual field measurements that could be performed with the pre-commercial plant described in Section 9. 

To have effective heat transfer it is necessary to protect the heat exchangers from biofouling. It has been determined that biofouling only occurs in OTEC heat exchangers exposed to surface seawater.  Therefore, it is only necessary to protect the CC-OTEC evaporators.  Chlorine (Cl₂ ) has been proposed along with several mechanical means.  Depending upon the type of evaporator, both chemical and mechanical means could be used. To protect marine life, the Environmental Protection Agency (EPA) in the USA allows a maximum Cl₂ discharge of 0.5 mg l^-1 and an average of 0.1 mg l^-1.   CC-OTEC plants need to use Cl₂ at levels of less than 10 percent of the EPA limits.  The power  plant components will release small quantities of working fluid during operations.  Marine discharges will depend on the working fluid, the biocides, the depth of intake and the discharge configuration chosen.

Other potentially significant concerns are related to the construction phase.  These are similar to those associated with the construction of any power plant,  shipbuilding and the construction of offshore platforms.  What is unique to OTEC is the movement of seawater streams with flow rates comparable to those of rivers and the effect of passing such streams through the OTEC components before returning them to the ocean.  The use of biocides and ammonia are similar to other human activities.  If occupational health and safety regulations like those in effect in the USA are followed, working fluid and biocide (most probably anhydrous ammonia and chlorine) emissions from a plant should be too low to detect outside the plant sites.  A major release of working fluid or biocide would be hazardous to plant workers, and potentially hazardous to the populace in surrounding areas, depending on their proximity.  Both ammonia and chlorine can damage the eyes, skin, and mucous membranes, and can inhibit respiration.  Should an accident occur with either system, the risks are similar to those for other industrial applications involving these chemicals.  Ammonia is used as a fertilizer and in ice skating rink refrigeration systems.  Chlorine is used in municipal water treatment plants and in steam power plants.  Chlorine can be generated in situ; therefore storage of large quantities of chlorine is not recommended.

Organisms impinged by an OTEC plant are caught on the screens protecting the intakes.  Impingement is fatal to the organism.  An entrained organism is drawn into and passes through the plant.  Entrained organisms may be exposed to biocides, and temperature and pressure shock.  Entrained organisms may also be exposed to working fluid and trace constituents (trace metals and oil or grease).  Intakes should be designed to limit the inlet flow velocity to minimize entrainment and impingement.  The inlets need to be tailored hydrodynamically so that withdrawal does not result in turbulence or recirculation zones in the immediate vicinity of the plant.  Many, if not all, organisms impinged or entrained by the intake waters may be damaged or killed.  Although experiments suggest that mortality rates for phytoplankton and zooplankton entrained by the warm-water intake may be less than 100 percent, in fact only a fraction of the phytoplankton crops from the surface may be killed by entrainment.  Prudence suggests that for the purpose of assessment, 100 percent capture and 100 percent mortality upon capture should be assumed unless further evidence exists to the contrary.  Metallic structural elements (e.g., heat exchangers, pump impellers, metallic piping) corroded or eroded by seawater will add trace elements to the effluent.  It is difficult to predict whether metals released from a plant will affect local biota.  Trace elements differ in their toxicity and resistance to corrosion.  Few studies have been conducted of tropical and subtropical species.  Furthermore, trace metals released by OTEC plants will be quickly diluted with great volumes of water passing through the plant.  However, the sheer size of an OTEC plant circulation system suggests that the aggregate of trace constituents released from the plant or redistributed from natural sources could have long-term significance for some organisms.

OTEC plant construction and operation may affect commercial and recreational fishing.  Fish will be attracted to the plant, potentially increasing fishing in the area.  Enhanced productivity due to redistribution of nutrients may improve fishing.  However, the losses of inshore fish eggs and larvae, as well as juvenile fish, due to impingement and entrainment and to the discharge of biocides may reduce fish populations.  The net effect of OTEC operation on aquatic life will depend on the balance achieved between these two effects.  Through adequate planning and coordination with the local community, recreational assets near an OTEC site may be enhanced. 

Other risks associated with the OTEC power system are the safety issues associated with steam electric power generation plants: electrical hazards, rotating machinery, use of compressed gases, heavy material-handling equipment, and shop and maintenance hazards.  Because the CC-OTEC power plant operates as a low-temperature, low pressure Rankine cycle, it poses less hazard to operating personnel and the local population than conventional fossil-fuel plants.  It is essential that all potentially significant concerns be examined and assessed for each site and design to assure that OTEC is an environmentally benign and safe alternative to conventional power generation.  The consensus among researchers is that the potentially detrimental effects of OTEC plants on the environment can be avoided or mitigated by proper design.

Next: Engineering Challenges

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