Mapping the Global Potential of Ocean Thermal Energy Conversion

A global transition to sustainable energy infrastructure will require long-lead-time research and development of myriad technologies such as ocean thermal energy conversion (OTEC). Although OTEC plants are capital-intensive installations, new technology is reducing the cost of investing in this technology. OTEC exploits the temperature difference across the thermocline to produce electricity. To explore the potential of the oceanic thermocline as a renewable energy resource on a global scale, researchers used a Geographic Information System (GIS) to map a detailed model of global climatology of oceanic stratification (Nagurny et al.2011). Nagurny and colleagues employed this information, coupled with statistics for an OTEC plant model, to estimate OTEC potential at locations worldwide, to understand the distribution of these resources, and to pinpoint locations with high potential for utilizing OTEC. This visual system enabled the researchers to estimate the potential renewable energy available in the ocean’s thermocline at any given location using a model of an OTEC installation. This model is an improvement over past thermocline studies; it yielded a worldwide distribution of power on a 1/12th degree grid across multiple time periods. It was obtained from a baseline single stage Rankine cycle design and can be simply adjusted to employ different plant designs. —Meredith Reisfield
Nagurny, J., Martel, L., Jansen, E., Plump, A., Gray-Hann, P., Heimiller, D., Rauchenstien, L.T., Hanson, H.P., 2011. Modeling global ocean thermal energy resources. Oceans, 2011, 1-7.

Nagurny and colleagues at Lockheed Martin and the National Renewable Energy Lab mapped global climatology model of ocean thermoclines based on open-source data from the Naval Research Laboratory’s (NRL’s) Hybrid Coordinate Ocean Model (HYCOM). The data for this model were gridded at 1/12th degree latitude and longitude using GIS, which improved spatial resolution over previous models. A closed cycle OTEC plant model developed by Lockheed Martin was applied to this GIS map. The nominal OTEC plant design produces 150 MW gross (100 MW net power) of electricity. The researchers chose this design because sufficient data about the plant was available, the size of the plant is feasible with existing technology, and this design produces enough power to have the potential to be economically reasonable in the near future. To calculate net power, gross power, and fixed and variable losses the researchers examined aspects accounting for major contributing and loss factors to OTEC power production. They focused on three groups of factors: gross power, cold water pipe pumping cost, all other pumping and transmission power costs. Gross power was calculated using established thermodynamic equations of a Rankine cycle. The model was formulated so that the gross power and variable loss factors were the variables that depended on oceanic conditions. Nagurny and colleagues added an algorithm that optimized the depth of the cold water source, relaxing the assumption that successfully installing an OTEC power plant would require 1-km-deep cold water. Previous global assessment of potential resources were limited by the assumption that sufficiently cold water lay 1000 m below the ocean’s surface and left out shallower regions with potential OTEC resources. The researchers chose the most shallow depth out of the bottom water, the water at a level where the temperature gradient of 3°C per km balances production and loss, and water at 1000 m (for consistency with previous studies).
The net power potential worldwide varied from 0 to 197 MWe. Large regions offer potential for net power production. Large areas of the Pacific, Atlantic, and Indian Oceans can supply 100 MWe or greater from the nominal design. Areas in the Philippines and off New Guinea where found to have the greatest potential for OTEC, with upwards of 190 MWe obtained from the 100MWe plant conditions. The higher resolution of this model and the inclusion of shallower cold water availability allowed the mapping of the Gulf Stream’s July thermocline off the U.S. east coast, the western Equatorial Atlantic, and the Central Pacific near and to the northeast of Hawaii. The study was limited by a lack of data concerning ocean currents at depth, which are needed to replace the cold water in OTEC for the power production to be sustainable, and climate change data affecting the solar heating of surface water. 

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