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Co-Products: A Multiple Systems Approach
Fresh Water | Air Conditioning | Aquaculture | Agriculture
One of the principal innovations and benefits of the OCEES’ systems approach to OTEC power generation is the synergistic integration of multiple revenue generating co-products that significantly improve the economic viability of such a system under current amenable economic conditions (relatively low interest rates and high fuel costs). Potential co-products for tropical island communities include clean power, pure fresh water, cold water air conditioning, aquaculture, agriculture, ice and hydrogen. All of these co-products are viable and can be integrated utilizing existing, proven technologies. The most immediate economic and social benefits for tropical island communities employing an OCEES OTEC system can be realized through the integration of these co-products according to local societal requirements and resource limitations.
An integrated systems approach has never been implemented in evaluating the economic feasibility of OTEC technology. Only the economic merits of the power producing components have been addressed when analyzing and comparing to more conventional energy systems. Previous analyses have completely ignored the additional economic, social and environmental benefits an integrated systems approach incorporating ancillary technologies can provide. Recent advances in the power-cycle and cold-water pipe (CWP) materials and deployment technologies have greatly reduced initial capital costs and risks commonly associated with and currently hindering the effective commercialization of OTEC.
Most of the progress in OTEC systems in the U.S. has occurred in Hawaii with the participation of governmental, university and private entities. The result has been a significant improvement in OTEC technology and a multi-product systems approach to take full advantage of the current favorable economic climate. Multi-product OTEC systems can now be based on demonstrated technology and can be shown to be economically competitive with fossil fuel based systems in many oceanic island communities.
The co-production of large quantities of fresh water is one of the main advantages of the OTEC process. Up to 0.7 to 0.8 MGD (Million Gallons per Day) of fresh water can be produced per MW (Megawatt) of installed gross electric power capacity. The fresh water results from the evaporated warm seawater used as the working fluid in the Open-Cycle OTEC power production process or as an additional parallel system component in a Kalina Cycle® power system configuration. The evaporation occurs because the warm seawater is exposed to a partial vacuum which turns 0.5% of it into low temperature – low pressure steam. In an Open-Cycle OTEC power configuration, the generated steam is passed through a turbine (which powers a generator to produce electricity) and then is condensed into liquid fresh water by transferring heat to the cold seawater through a heat exchanger. The fresh water is then pumped out for storage and distribution.
The efficiency of the Open-Cycle OTEC process (as well as the efficiency of any seawater distillation process) is increased by the removal of non-condensable gases from the seawater. Research done at the University of Hawaii by Drs. Hans Krock (OCEES), Stephen Oney (OCEES), and Manfred Zapka resulted in two patents related to the enhanced exchange of non-polar gases into and out of seawater. These patented processes allow the design of a pre-deaeration and a gas reinjection system which make the OC-OTEC system (or the separate fresh water production system in conjunction with a Kalina Cycle® system) inherently more efficient, less costly, and environmentally benign.
The production of fresh water from the Open-Cycle OTEC process was demonstrated at a functioning plant at the Natural Energy Laboratory of Hawaii (NELHA) on the island of Hawaii. The fresh water produced was of high quality with approximately 80 mg/l of TDS (Total Dissolved Solids). The TDS of water from a full production integrated Kalina Cycle® OTEC plant or an OC-OTEC facility is expected to be even lower because the "steam" used for fresh water production in the NELHA plant was diverted before the mist screen allowing for more salt carry-over via water droplets than would be experienced in current OCEES fresh water production plant designs. As with any desalination product water (which is essentially distilled water), it will necessarily require minor adjustments in pH and dissolved oxygen in order to improve the taste and control corrosivity.
When fresh water is produced in a parallel process to the Kalina Cycle® power system, the method used is essentially the same as that used for the Open Cycle except that no turbine is used. This means that the thermodynamic control parameters are set at slightly different values and the configuration of the vacuum chamber is optimized for fresh water production. The desired quantity of fresh water and the temperature difference dictate the size of the facility independent of the power cycle.A third method to produce fresh water in the OTEC systems approach is to exploit the natural humidity inherent in the tropical regions of the earth by passing the cold seawater through a heat exchanger which is in contact with this humid air, effectively condensing fresh water on the surface of the irrigation tube. This method is primarily utilized in the Cold Water Agriculture application discussed in the corresponding section of this website as developed by Dr. John Craven of the Common Heritage Corporation. This method not only provides a natural irrigation technique further utilizing the cold seawater resource inherent in an OTEC system but also works well at conditioning temporal plants for suitable cultivation in an "unnatural" environment like the tropics.
Practical and Economical
Nearly all tropical island nations have a relatively uniform need for air conditioning. Traditional electrical air conditioning systems (HVAC systems) are energy intensive and represent 35% to 45% of energy use in typical office buildings and hotels in tropical island communities.
As a general rule of thumb, the amount of cold seawater required to generate one megawatt (1 MW) of electrical energy from and OTEC plant will provide the equivalent of 10 MW of air conditioning or process cooling. This implies that in certain instances where the air conditioning load density is concentrated in shoreline areas in close proximity to an OTEC facility, the plant will not only provide base load power to the local community, but also reduce load requirements by nearly 40% by supplying cold water air conditioning! Likewise, in areas too developed which restricts the construction of an OTEC plant directly adjacent to a load center with reasonable access to deep cold water, SeaWater Air Conditioning (SWAC) can provide a cost effective, environmentally attractive energy technology on its own.
Additional Benefits
The effluents from an integrated OTEC system or SWAC facility can be further utilized in appropriate locations to improve water quality in local harbors or marinas by providing necessary flow rates to "flush" the harbor or similar relatively stagnant water resource. This could significantly improve water quality in the harbor or marina over current practice. Also, since the effluent streams still possess significant "cold" or heat absorption capabilities, utilization as process cooling in conjunction with conventional power plants or industrial facilities is also an option. Providing cooling waters at significantly reduced temperatures from ambient sources currently employed would significantly improve plant efficiencies above current practices.
Air conditioning and cooling with deep, cold seawater for some coastal communities situated in the tropical island regions of the world is economically and technically viable today. SWAC in conjunction with an integrated OTEC system, or stand alone, is most attractive to communities with three or more of the following features (1) the community is sufficiently close to a source of cold seawater, (2) the overall air conditioning demand is large — more than 1000 tons, (3) the local cost of energy is high, (4) the air conditioning utilization is high (air conditioning required year-round, as is inherent in tropical ocean communities), (5) the onshore distribution is not extensive (easy access to the air conditioning load centers). Under these circumstances, energy savings can be greater than 80% and air conditioning costs significantly reduced.
Sustainable Food
The seawater which is pumped ashore for an integrated land-based OTEC system has many potential secondary uses which have proven to be economically beneficial and can be incorporated into the plant design to offset and reduce the effective costs associated with OTEC energy. Aquaculture of both plants and animals is a particularly attractive by-product potentially included in an integrated OTEC system design.
OTEC Resource Advantage
The cold, deep seawater required for OTEC has three principal advantages for aquacultural systems:
Mature Industry
Mature and developing commercial aquaculture applications utilizing cold, deep seawater and surface water are currently being implemented at the Natural Energy Laboratory of Hawaii Authority (NELHA) in Hawaii to grow micro algae, macro algae, crustaceans such as shrimp and lobster, mollusks such as oysters and abalone, and finfish such as tilapia, flounder and salmon.
The largest and most successful of the aquaculture applications at NELHA are growing micro algae for health food, pharmaceuticals, and fish and animal feed products. Micro algae are commercially attractive because they grow quickly in the strong tropical sun. Cyanotech, Inc. grows Spirulina, a health food supplement. The tremendous advantage of micro algae over other aquaculture organisms is their rapid growth cycle. If the Spirulina cultures experienced catastrophic failure (which has never happened to date at NELHA), they can be back in production in 2 to 3 weeks. The corresponding time period for salmon, lobster, abalone, etc. is 3 to 6 years.
Kona Cold Lobster, Ltd., another NELHA tenant, grows Maine Lobster (Homarus americanus) in Hawaii by using the cold water to optimize the lobster’s aquatic environment. They keep the water at the optimum 20oC year round, so that the animals grow to 500 gm size in about 3 years. In nature they hibernate in the winter, so the animals require seven years to reach this size!
Other marketable aquaculture products have also been, and are currently being developed for commercial applications appropriate for integration into an OTEC system for a tropical island community. Other available products include, but are not limited to, oysters (both as food stock and pearl culturing), abalone, clams, shrimp (as food source and brood stock), giant clams (Tridacna), Dungeness Crab, salmon, tilapia and tropical reef fish for aquariums (see accompanying table).
Natural Energy Laboratory of Hawaii Authority
Tenants: September 1998
(Courtesy: Natural Energy Laboratory of Hawaii Authority)
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Aquaculture Business |
Products
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| Kona Cold Lobster | Maine Lobster, Dungeness Crab |
| Aquasearch | Micro algae, astaxanthin |
| Royal Hawaiian Sea Farms | Ogo (gracillaria), opihi, tilapia, namako |
| Uwajima Fisheries | Hirame, shrimp, tilapia |
| Kona Bay Oyster & Shrimp Co., Inc. | Shrimp, oysters, algae |
| Cyanotech Corporation | Spirulina, Dunalellia (beta carotene) |
| Indo Pacific Sea Farms | Giant Clams (tridacnida) |
| Cultured Technologies, Inc. | Pearls |
| Taylor United, Inc. | Oyster, clam hatchery |
| High Health Aquaculture, Inc. | Shrimp (disease-free) |
| Black Pearls, Inc. | Local oyster re-seeding, black pearls |
| Hawaiian Bred Tropicals, Inc. | Tropical reef fish (for aquariums) |
| Hauser Chemical, Inc. | Beta Carotene extraction from micro algae |
OTEC aquaculture not only holds the benefit of self-sustaining food resources for tropical island communities and its tourism related industry, but also holds the potential of creating a readily available, commercially attractive, exportable industry to further the diversity of the tropical island economy as well as providing necessary job opportunities to local island residents.
Innovative Techniques
Temperate fruit and vegetable crops have been grown successfully in the Natural Energy Laboratory of Hawaii Authority’s (NELHA) sub-tropical climate by utilizing the cold seawater to chill the soil. The combination of high solar insulation and cold roots has enhanced production and quality of many crops, including strawberries, grapes, asparagus, pears, gourds, alstroemeria flower, and many others.
In the ColdAg process (developed at NELHA by the Common Heritage Corporation), designed for utilization in an integrated OTEC system, the deep ocean water (following usage in the OTEC power facility) flows through enclosed pipes embedded in the soil at the root zone. Temperatures as low as 10oC are produced. This results in temperatures at the soil surface of approximately 25-30oC. This is below the dew point in most tropical regions and moisture condenses at the surface of the soil. As the water continues to cool, it migrates through the soil until it reaches the coldest spot in the root zone. As the water migrates through the soil, it accumulates minerals and nutrients. The heat from the sun (as high as 37oC in tropical regions) warms the flower and fruit and the heat diffuses down the stem to the root. The resulting thermal gradient carries nutrients up the stem at a rate proportional to the temperature difference between root (10oC) and fruit (37oC). Nature rarely produces a temperature difference greater than 9oC. Thus, the ColdAg process transports nutrients at least three times as fast as nature! The cold water flow necessary to provide this effect is very small and costs associated with this form of agriculture in conjunction with a working OTEC system in which the cold water is readily available as a secondary contributor are far outweighed by the potential benefits. Likewise, associated costs are small compared to other costs (labor) generally attributed to the agriculture process.
Enhanced Production
As a result of this fundamental process, the Common Heritage Corporation has been able to successfully produce more than one hundred crops from nearly every known climate. Dormancy can be induced by turning off the deep ocean water flow periodically and as a result four or more seasonal cycles can be achieved in one year. Fruit trees can produce as many as four crops on a single tree in one year. This is also true for grapes. Sunflowers can be "tricked" into producing more than a dozen flowers per stalk. The fruits are large and have a high sugar content (sweetness). It is estimated that 100 acres of agriculture can be had from the secondary use of water that has been employed to generate one megawatt (1 MWe) of electrical power. The system employs standard irrigation pipes and standard farming techniques and can be maintained by agricultural workers.
Enormous Potential
As you can see, agriculture utilizing the ColdAg process in conjunction with an integrated OTEC system can provide new and exciting industries to tropical island communities. Agricultural cash crops previously limited to commercial applications in the temporal zones of the globe can now be successfully, and profitably, grown in the tropical regions. Thus, an integrated OTEC system not only provides tropical island communities a reliable source of renewable, environmentally friendly energy and fresh, potable water, but also provides them with a renewable and reliable food sources to support the island’s inhabitants and tourism industry as well. Likewise, the potential exists to help diversify the local economy beyond conventional crops towards higher value exportable fruits and vegetables. This diversification can help establish a much needed extensibility to the current economic base and create more jobs for the local tropical island communities embracing OTEC technologies.