Introduction

Presently OTEC technology although technically feasible is not economically viable and in recent years investors and researchers in Ocean Thermal Energy Conversion have moved the focus from plant ships which utilise the closed cycle technology to generate electricity, to hybrid type cycles in tropical island environments which can benefit from specific by-products.

Cold Water Pipe

The fabrication and installation of a cold water pipe which pumps cold deep ocean water at 1000m up to the surface can constitute up to half the cost of any OTEC facility and so minimising the length and thus cost of this pipe will have great financial implications for any OTEC facility. Mainland sites are surrounded by a continental shelf which can stretch tens of kilometres out to sea and encompasses shallow waters of insufficient depths to capture the low temperature waters required for operation (4oC at 1000m depth). In contrast islands have shelves, which drop off steeply to deep ocean water within a few kilometres or less. This means firstly that the cost of the pipe is lowest in such sites and due to the comparatively short length of pipe required many island locations are suitable for onshore plants. Maintenance, operaterational, and capital costs will in general be a great deal less for onshore technology than for offshore technology due to obvious difficulties associated with engineering and operations in the marine environment. Furthermore, considering additional integrated benefits from the bi-products of OTEC in island sites, onshore operations greatly simplify the systems integration by eliminating the need for expensive underwater cables and simplifying successful distribution of bi-products. TOP

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Fresh Water Production

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 tomake 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 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 20o 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

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

Site Selection Considerations For Island Locations

It is important to identify the factors involved in the selection of an island for OTEC and to establish an adequate criteria to select an optimum site for the plant.
The main factors to consider are:
The tropical climate thinking about the average monthly temperature and the average monthly surface water temperature both through the year and it is important too to know if there are severe storms like typhoons, tornadoes, monsoons, and others.
The location and the distance off shore to 1000 meters ocean depth are important data. It is necessary to know factors like shore line construction and how densely is the shore line developed in order to know how expensive and available it is.
The political climate to support an infrastructural development. Important data are the large hotel construction in the area, the local tourist infrastructure, the local infrastructure for transport of products as fish, industrial products and water and transport of people all by highways, railways, ports and airports.
The population is an important factor. Necessary to know the area population, the number of inhabitants and the population of tourists through the year to study supplies and demands.
The electrical capacity and energy use data as electricity generation, power generation development plants, uses of diesel, hydro, nuclear, wind and others, the local capacity of the local power grid and the energy uses in general have to be considered to study the possibility for OTEC.
Finally, to study the market and needs it is important to consider points as the present sources of fresh water, the local market for fresh water (in volume and cost), the local markets for fish and marine life products (in price, varieties and volumes) and the industry, the local agriculture, aquaculture and the needs of air-conditioning, water and products for people.

Figure X: Bi-products for OTEC

It is important to consider the last points in a qualitative and in a quantitative way, by this reason it is necessary to quantify the next factors to think about a real case study.

When the possibility of OTEC system is considered, it is necessary to carry out a previous study to quantify different important factors to know if it is feasible or not the application of OTEC in the island selected.

LOCATION IN AN ISLAND

GENERAL FACTORS TO QUANTIFY

General Data: as location, area (km2), and population.
Climate Data: as ambient temperature over the year, sea surface water temperature and deep sea water temperature, climate effects, hazards and in general average annual data.
Resources: as water, products, by products. Important to know the demand needed to supply completely the population. The transportation in the island
Energy Data:
The total energy capacity (MW)
The electricity production (kWh)
The electricity consumption (kWh)
% of fossil fuels (important to know and to try to reduce the %)
Economics and industry: it is important to consider internal policies, the environmental currents issues in the country and for example other economic data as the oil products imported, the imports and the exports to do an study of the market and the relation with other islands and countries.

SITE POINT
The specific point site must be studied before to install the OTEC system. Important factors are:
Geography
Meteorology (Climate effects and hazards)
Seismic activity
Physical oceanography
Ocean Temperature, water temperature (20C)
Wave environment
Littoral transport
Properties of sea water
Nutrients, Oxygen, salinity, pH
Depth (1000 m).
Shelf size.
Natural resources
Environmental current issues (like for example the supply of potable water)
Marine life
Land Properties
Population (size and type)
Economy: economics and developing island
Electricity
Total energy capacity (MW)
Oil products imported
Economy and industries
Electricity production
Electricity consumption
% of fossil fuels
Recreational activities
Naval operations

LOCATION AND ECONOMICS OF OTEC
An economic analysis indicates that, over the next 5 to 10 years, ocean thermal energy conversion (OTEC) plants may be competitive in four main markets. Defining this main markets it is possible to see the potential site for OTEC considered.
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 would 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 should quickly become cost effective in this market, when the cost of diesel fuel doubles, 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 the three worldwide 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.