Aquaculture

Aquaculture or mariculture is the cultivation of plants and animals of the sea in their own environment. Aquaculture represents a profitable industry in various locations in the world such as Hawaii and Florida, Hawaii having over one hundred of such marine farms.

Deep ocean water is rich in essential nutrients containing nitrate, carbon, and phosphate compounds. Since there is very little life at depths below 1000m the water is almost pathogen free and pure from competing species and disease, making it a perfect environment for cultivating marine life.

The integrated technology makes use of deep ocean water which is pumped to the surface where it is utilised in the condenser of an OTEC plant before being pumped in parallel through tanks where specific marine species are grown. Given that effective temperature control can be achieved by mixing with warm surface waters it is possible to grow a variety of marine species native to waters all around the world in far shorter periods of time than their growth cycle in the natural environment. Species such as salmon and red snappers, a variety of shellfish, micro algae and lobsters are examples of possible produce from this industry and it is likely that the demands and logistics of any particular site will dictate the most suitable product for cultivation.

Considerations are likely to include the local food requirements and preferences, local sustained tourist industry, locally established fishing industry, ease of exporting produce, dependence on imported products and nearby markets for exports. Further to this certain risk assessments may play a role given that if an operations failure should occur resulting in loss of stock then the time taken in cultivating certain species back to market size is entirely specific to the species. For example lobster can be grown in an aquaculture environment to 1/2kg in 3 years while in nature growth to the same weight takes 7 years due to hibernation periods in the winter season. If an operations failure occurred in a lobster or salmon farm then it would take at least three years to be back in business. In contrast certain algae’s can be re-cultivated for sale to pharmaceutical companies, as health food products or as fish/animal feed within three weeks.

Aquaculture alone is potentially enough t omake the technology of OTEC economically feasible in certain locations and in terms of local sustainability and the potential for significant economic gains through eliminating imports, creating jobs and developing new markets, this by-product of OTEC is of great significance for near future developments. In contrast, electrical power production, is viewed by many as a less significant by-product of OTEC developments.

Recent years have seen continued decline of the worlds fishing industry all over the globe as natural stocks become exhausted. In addition to providing sustainability deep ocean water technology may become more important in providing seafood which can alleviate pressures on natural reserves which may then be able to stabilise in numbers and eventually regenerate. TOP

Air Conditioning

Deep ocean water as a means to providing clean air conditioning is yet another valuable by-product of OTEC deep ocean water technology. Conventional air conditioning systems, which utilise internal pipes carrying water which are cooled by electrically powered chillers account for close to half the electricity costs of hotels in tropical island locations. Two thirds of this percentage is accounted for by cooling towers and chillers while the other third is in operating fans. Since fuels for electrical power generation to these regions is imported this energy intensive process is very costly to businesses and polluting to the environment. The diagram below illustrates the basic set up for utilisation of air conditioning as a sub-system of OTEC operations.

The cold sea water air conditioning acts to cool the building as shown (right). These buildings have the same internal AC fluids and flow rates as systems chilled by conventional chillers, however in this system the low temperatures in the buildings chilled water loop are achieved by passing the fresh water of the AC loop through a counter flow heat exchanger with the primary fluid being cold sea water. A titanium plate or other thermally conductive material transfers heat from the warmed AC loop of fresh water (after it has cooled the buildings) to the cold sea water. This process is possible since the sea water is several degrees colder than the temperature required for the chilled fresh water loop and large flow rates are required for the OTEC facility. After entering the heat exchanger the cold sea water can be further utilised in an aquaculture operation or immediately discharged to the ocean. As an integrated subsystem to any OTEC facility cold sea water air conditioning significantly increases the economic viability of development. Demand for AC varies throughout the day in which case during times of lower demand other subsystems can be optimised such as fresh water production at night time. It is also likely that if nutrient rich deep ocean water is to be diverted after the heat transfer process (use as air conditioning)for use in aquaculture farms that the choice of cultivation will be restricted given that the temperatures available after warming of the ocean water will be of a smaller range.

Predictions made by engineers presently pursuing this technology solely as an alternative to conventional air conditioning have suggested that savings made on electricity costs by replacing conventional AC systems with cold sea water systems can be extremely desirable financially. In reality this may be the case although the ease of access to deep ocean cold water still remains a dominant factor with respect to the length of cold water pipe which must be installed in any location. Island locations with steep declines to deep ocean (1000m) remain the focus although unlike OTEC technology, which is confined to 20° north or south, this technology can stretch its boundaries further a field. Locations which require shore side air conditioning at higher latitudes could especially benefit since at higher latitudes lesser depths may be required for access to cold ocean water.TOP

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The Integrated System

The big picture which is often misunderstood by people glancing at OTEC technology is an integration of all the by-products and subsystems discussed in previous pages. Electrical power production is seen by many experts as a less significant by-product when compared to cold sea water air conditioning, agricultural fertilization and aquaculture which at present offer greater social and economic benefits than power production. OTEC is capable of generating base load power and this can be used directly as domestic and industrial supply or can be converted into chemical feedstocks such as hydrogen or ammonia. It remains a fact however that as a source of electrical power OTEC will not be an economically feasible option in the near future. When all the subsystems and by-products are considered the picture is considerably different and a properly designed combination of any of aquaculture, agriculture, air conditioning, desalinated water and electrical power production could provide profitable small island sustainability. Of course like any development the location and in particular the successful engineering and installation of the massive cold water pipe will determine the outcome of any such project. The picture below demonstrates the possible offerings of OTEC to island locations as a network of subsystems.

All of the operations discussed are in reality technically proven and so the major issue that remains is to bring them together. Expertise exists in specific areas such as aquaculture and OTEC but trained personal to maintain and operate such a set up do not exist and the overlap of respective engineering skills is not established. New industry inevitably involves making mistakes. Lessons can be learnt from mistakes and history tells us that where there is a market eventually successful outcomes prevail. The present ideas and potential benefits associated with OTEC are untested on any reasonable pilot scale and until potential investors, which will likely involve subsidies from the US government, can be convinced and a successful operation can be delivered by those engineers involved it will sadly remain a good story. Like all new developments time for teething is required and presently it is hard to say how many decades more that might be.

Loss of the cold water pipe during connection of segments to each other and other surface interfaces of the system would be enough to render a project into liquidation. Wind power has taken twenty years of subsidies, research, the creation of artificial markets, and subsequent technological evolution to achieve play status in todays ever competitive market. Similarly renewable energies integrated into building design are facing huge problems since the infrastructure of knowledge and expertise in communications, design and installation do not exist. This again is something that will change with time backed by driven individuals and no doubt OTEC will be subject to even more extremes since so much of the potential market lies within borders of poor and often less stable countries.TOP

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Site Selection Considerations For Island Locations

Site selection

The first and foremost requirement for tapping this low-grade energy is that there should be at least a difference of 22^oK between warm and cold seawaters.  This requirement limits the potential sites to the tropical regions. This selection criterion for a specific location is very important in maximising economic benefits to an OTEC project. 

Why Islands?

Attention is focussed on those island communities that are not self reliant for their energy and other basic needs such as drinking water. Multi-purpose medium scale OTEC plants could certainly fit in the scenario which can make islands sustainable by the supply of electric power, drinking water and supporting agriculture, mari-culture and air conditioning as discussed above. Hence it is very important to identify a method for the selection of specific islands which can most benefit from OTEC and an integration of the sub systems discussed.

Important Site Selection Considerations for Island Locations

Thermal Gradient: As already mentioned, the thermal gradient of 22^oK preferably within a depth of less than 1000 m is the foremost requirement to allow the operation of an OTEC plant.

Shelf Distance: The distance of continental shelf from the coast should be as short as possible, makeing land based OTEC mosteconomical, as this can minimise the pipe lengths thus reducing the capital costs. Where distances are longer, the alternative is to build barge-mounted plants.

Climatic and Seismic Conditions: The selected location should be free from extreme climatic conditions such as storms, typhoons severe monsoons and seismic activities. Extreme conditions require plant to be designed with a higher factor of safety and this can increase the project costs.

Political Scenario: Stable Governments that support industrial and economical activities by providing subsidies, investment protection, additional infrastructure are vital for the establishment of an immature industry like OTEC. Political uncertainties often can derail project activities.

Infrastructure: Good infrastructure facilities such as ports, roads connecting them to project site, hotels, ancillary industries can help the developer to organise project activities efficiently and effectively.

Current Energy Scenario

Energy Mix: If the present electric generation is based on fossil fuels and imported fuels, OTEC could reduce the dependence on imports thus increasing energy security. It can also replace the use fossil fuels and thus reduce emissions of potential green house gases. If large scalehydro power is already available such as in the dominican republic OTEC is less likely to be competetive.

Transmision and Distribution: If the existing T&D network can absorb the additional generation, it can save additional capacity creation thereby vastly reducing project costs.

Source of Fresh Water: Where islands are dependent on seawater to produce fresh water, multi-purpose OTEC projects can integrate power generation and fresh water generation effectively thus eliminating additional costs for water production.

Aqua-culture and Mariculture: Islands having potential for these industries can vastly benefit from OTEC projects which can provide nutrient rich water for mari-culture and aqua culture.

POTENTIAL MARKETS- AN OVERVIEW

An economic analysis carried out by the OTEC researchers has indicated that, over the next 5 to 10 years, ocean thermal energy conversion (OTEC) plants may be competitive in four main markets.

The first market is the small island nations in the South Pacific and the island of Molokai in Hawaii. In these islands, the relatively high cost of diesel-generated electricity and desalinated water may make a small (1 MWe), land-based, open-cycle OTEC plant coupled with a second-stage desalinated water production system cost effective.

A second market can be found in American territories such as Guam and American Samoa, where land-based, open-cycle OTEC plants rated at 10 MWe with a second-stage water production system could be cost effective.

A third market is Hawaii, where a larger, land-based, closed-cycle OTEC plant could produce electricity with a second-stage desalinated water production system. OTEC could also become cost effective in this market, if the cost of diesel fuel increases sufficiently, for plants rated at 50 MWe or larger.

The fourth market is for floating, closed-cycle plants rated at 40 MWe or larger that house a factory or transmit electricity to shore via a submarine power cable. These plants could be built in Puerto Rico, the Gulf of Mexico, and the Pacific, Atlantic, and Indian Oceans. Military and security uses of large floating plantships with major life-support systems (power, desalinated water, cooling, and aquatic food) should be included in this last category.

Of all the markets studied for small OTEC installations (U.S. Gulf Coast and Caribbean regions, Africa and Asia, and the Pacific Islands), the Pacific Islands are expected to be the initial market for open-cycle OTEC plants. This prediction is based on the cost of oil-fired power, the demand for desalinated water, and the social benefits of this clean energy technology. U.S. OTEC technology is focused on U.S. Coastal areas, including the Gulf of Mexico, Florida, and islands such as Hawaii, Puerto Rico, and the Virgin Islands.