When it comes to potential new renewable energy sources, Ocean Thermal Energy Conversion (OTEC) has a lot going for it. Greenhouse gas emissions and their climate change effects, ongoing ecosystem and biodiversity loss and degradation, and already high fossil fuel prices — with the likelihood of only going higher over time — have come together in alignment to spur serious, government-backed efforts at finding a cost-effective technology capable of generating clean, renewable electricity derived from the natural temperature gradient that exists deep throughout the world’s oceans.
Besides the technological and economic hurdles, the ecological impacts of OTEC systems remain uncertain. Described in a 2011 environmental report on OTEC as “an unprecedented environmental modification [that] must be rigorously evaluated,” the daily flow of a 5 MW pilot OTEC plant has been estimated at more than 2 million cubic meters of water.
A focal point in this regard is the biological and ecological effects of pumping and discharging massive volumes of nutrient-rich deep ocean water up to near surface depths. One cause for concern is the potential for this to result in phytoplankton blooms. Another is the potential entrapment and mortality of organisms in OTEC system intake pipes. A simulation and analysis performed by O’ahu’s Makai Ocean Engineering recently published by the Department of Energy (DOE) indicates that perturbations resulting from operation of a 100MW OTEC plant in the waters off O’ahu would not have significant impacts on phytoplankton reproduction.
The Making of Makai’s Bio-physical OTEC Model
Makai Ocean Engineering has been involved in OTEC research and development since way back in the late 1970s. In late September this year, the DOE published a technical report describing the modeling Makai has done in simulating the “biochemical effects of the nutrient-enhanced seawater plumes that are discharged by one or several 100 megawatt (MW) OTEC plants.”
In order to simulate the biochemical response for three classes of phytoplankton, Makai’s biological and physical model entailed setting up grid cells with three-hour time steps for the waters surrounding O’ahu, in conformance with the Environmental Fluid Dynamics Code (EFDC) approved by the EPA.
Makai calibrated the dynamic biological phytoplankton model using data collected for the Hawaii Ocean Time Series (HOTS) project and then had it peer reviewed. The physical oceanography model component made use of “boundary conditions from a surrounding Hawai’i Regional Ocean Model (HROM) operated by the University of Hawai’i and the National Atmospheric and Oceanic Administration (NOAA).”
Makai ran its model for a “100 MW OTEC plant consisting of four separate ducts discharging a total combined flow rate of 420 cubic meters/second of warm water and 320 m3/s of cold water in a mixed discharge” at a depth of 70 meters. At this depth, the HOTS system observations indicate concentrations of phytoplankton at a density of 10-15 mg of carbon per cubic meter, according to the technical report.
After first running simulations without the OTEC system in order to calibrate its model with the HOTS system, Makai ran the simulation with the addition of the model for the 100MW OTEC plant.
The researchers, among other things, found that, “because this terminal near-field plume is well below the 1% light limited depths (~120m), no immediate biological utilization of the nutrients occurs.
They explain that, “As the nitrate is advected and dispersed downstream, a fraction of the deep ocean nutrients (< 0.5 umol/kg perturbation) mix upward where they are utilized by the ambient phytoplankton population.” They found that this occurs around 25 kilometers downstream from the plant at 110–70 meters depth.
For pico-phytoplankton, the modeling results indicated that “this nutrient perturbation causes a phytoplankton perturbation of approximately 1 mgC/m3 (~10% of average ambient concentrations) that covers an area 10×5 km in size at the 70 to 90m depth. Thus, the perturbations are well within the natural variability of the system, generally corresponding to a 10 to 15% increase above the average pico-phytoplankton biomass.”
Furthermore, this perturbation “exhibits a meandering horizontal plume trajectory and spatial extent, but remains similar in magnitude (generally 1-2 mgC/m3).”
Diatom perturbations become more noticeable after three weeks of running the simulation, “when the nearshore diatom population trends towards a greater concentration of 1 to 3 mgC/m3. They note that this increase is a fraction of ambient, background concentrations, “with perturbations remaining within fluctuations of the existing system.”
Makai’s researchers conclude by explaining that, “The perturbations were quantified by post-processing each time-step of model simulations without OTEC plants, with identical simulations that included OTEC plumes. Without this post processing, the 10-25% perturbations were obscured by the larger dynamic variations naturally caused by ocean circulation.”
I've been reporting and writing on a wide range of topics at the nexus of economics, technology, ecology/environment and society for some five years now. Whether in Asia-Pacific, Europe, the Americas, Africa or the Middle East, issues related to these broad topical areas pose tremendous opportunities, as well as challenges, and define the quality of our lives, as well as our relationship to the natural environment.