P.I.: Prof. Frank Sansone and Dr. Kevin Weng, Department of Oceanography
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- Prof. Frank Sansone analyzing sediment samples onboard R.V. New Horizon
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- Dr. Kevin Weng diving at an offshore buoy located 10 nm WNW of Ka’ena Pt., where an acoustic listening device records the presence of tagged fishes. Photo credit: Jeff Muir (Pelagic Fisheries Research Program).
HINMREC has been working in coordination with federal regulatory agencies (FERC, BOEM, and NOAA) to define differences between ocean energy systems and already established regulated industrial activities. The conclusion is that: (i) Discharge of deep seawater below the photic zone is the OTEC differentiator; and, (ii) The effect of arrays/farms over large coastal regions (spacing and quantity) the WEC differentiator.
The OTEC environmental baseline database, for a potential site off Kahe Point in Oahu, has been documented and is available upon request ( Environmental Assessment of OTEC in Hawaii.) Key oceanographic parameters to be considered in assessing the OTEC environmental impact have been identified and can be grouped as follows:
Nutrients and Biological
- Nitrate
- Phosphate
- Silicate
- Chlorophyll a
CTD Data
- Temperature
- Salinity
- Dissolved Oxygen
Carbonate System
- Dissolved inorganic carbon
- pH
- Alkalinity
The primary indicators of impact during plant operations are: Chlorophyll a; CDT Data; and, pH. These should be monitored at the discharged plume Neutral-Buoyancy-Depth as well as the Far-Field and compared to baseline conditions.
Objectives: General assessment of the impacts of ocean energy installations on marine life; and, Effects of OTEC sea water removal and return on the food web.
Presently: The literature contains a significant body of information on animal responses to marine structures and anthropogenic disturbances. Chemical and biogeochemical impacts of OTEC operations have been studied.
Project: Analyze the full spectrum of possible impacts on marine animals by ocean energy installations.
Generalities: Wave Energy Devices Among the various possible impacts to be considered for each specific device are: electromagnetic effects on sharks, acoustic effects on whales, bird-strikes, entanglement, benthic effects of anchors, and aggregation of animals. Objects placed in the ocean often become powerful fish aggregators. The depths proposed for wave energy conversion (WEC) devices are less than 70m. In Hawai’i, these water depths are not major habitats for the commercial pelagic fishes such as tunas. There is a network of Fish Aggregation Devices (FADs) in Hawaiian waters than are installed for the purpose of aggregating tunas and other commercial species, and these FADS are in waters much deeper than 100m (http://www.hawaii.edu/HIMB/FADS/). The WECs under consideration may aggregate small schooling fishes such as opelu. The opelu is one of the main bait fishes used in fisheries for larger species such as tunas. Therefore, fishermen may be attracted to these shallow water facilities to catch bait.
Generalities: OTEC Operations Deep seawater used in OTEC operation contains elevated levels of dissolved inorganic nutrients, primarily phosphate, nitrate and silicate, which could be expected to promote blooms of photosynthetic organisms if and only if the seawater is discharged and contained within the upper ocean or in coastal waters. However, the density of the deep seawater is higher than that of surface waters, and thus deep seawater discharged above the thermocline would sink, mitigating this effect. Deep seawater also contains elevated levels of dissolved carbon dioxide, which would lead to the release of carbon dioxide to the atmosphere if and only if discharged water was allowed to come in contact with the atmosphere.
Two additional points are worth noting: (i) discharges of deep seawater within the photic zone of the ocean, but below the surface mixed layer, should result in photosynthetic production that would remove both the dissolved nutrients and the dissolved carbon dioxide at approximately the same stoichiometric ratio as they are elevated in deep seawater; thus, the only large-scale environmental impact would involve the fate of the resulting photosynthetically produced organic matter; and, (ii) the reduction in pressure of deep seawater as it is brought to the surface will lead to an increase in its pH, offering some relief to the acidification of seawater due to global increases in atmospheric carbon dioxide.
On this Earth Day, I find it remarkable that OTEC has survived to the point of consideration of building a large scale facility.
Points not addressed above: How does risk of environmental damage get weighted by the efficiency of the power plant, which I understand to be ridiculously low.
Is this really renewable energy? How?
How will deeper ocean aquatic life be affected?
For very large facilities, how will the local themal profile be affected?
1) Points not addressed above: How does risk of environmental damage get weighted by the efficiency of the power plant, which I understand to be ridiculously low.
Thank you for your inquiry but please avoid using such adjectives.
Yes, the thermodynamic efficiency (Carnot cycle) is relatively low meaning that one requires large amounts of seawater for the OTEC process. Analytical studies indicate that the environmental impact is small when compared to other energy technologies. However, because of the relatively large amounts of seawater required, we have concluded that it is judicious to deploy and operate, for at least a couple years, a pilot plant (sized at about 5 MW) to validate our environmental impact analysis and numerical models with relevant field data.
What drives our efforts can be explained as follows (taken from the references in our webpage). Given that oil reserves can satisfy world-wide demand for less than 50 years, it seems sensible to envision marine renewable energy resources as additional alternatives to our oil-based economy. Extending this assessment to include all fossil fuels as well as nuclear fuels, one reaches a similar conclusion. In theory, for example, the ocean thermal resource could be used to generate most of the energy required by humanity. We should consider the implementation of OTEC plantships providing electricity, via submarine power cables, to shore stations. This could be followed, in 20 to 30 years, with OTEC factories deployed along equatorial waters producing energy intensive products, like ammonia and hydrogen as the fuels that would support the post-fossil-fuels era.
What is pending, however, are realistic determinations of the costs and the potential global environmental impact of OTEC plants and this can only be accomplished by deploying and subsequently monitoring operations with first generation plants.
2) Is this really renewable energy? How?
As long as the sun shines the ocean thermal resource is renewable.
The question you meant to ask is: what is your estimate of worldwide power resources that could be extracted with OTEC plants without affecting general ocean circulation and thermal resource? Our answer is that, for example, the maximum steady-state OTEC electrical power is about 5 TW. This is about twice the amount projected for worldwide consumption by 2025.
3) How will deeper ocean aquatic life be affected?
In general, marine life requires the light intensity found in the ocean upper layers between the surface and about 150 m (referring to regions wherein OTEC resource is adequate). In the last 20 to 30 years, however, oceanographers have found new life forms occurring around the bottom of the oceans and in unexpected locations. The OTEC cold-water intake pipe would be located at least 200 m above the bottom and, therefore, should not impact bottom dwelling creatures.
4) For very large facilities, how will the local themal profile be affected?
(See webpage Section “OTEC Thermal Resource” under ongoing-projects-at-UH). The separation required between plants depends on site conditions (e.g., ocean currents, thermocline depth) and seawater pipes locations. A 100 MW plant, for example, would be housed in a 250 m x 60 m floating platform with warm water intake at 20 m ; cold water intake at 1000 m and return mixed water depth at 100 m such that a separation of less than 1 km would, in general, be adequate.