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Observing Ocean Temperatures For Thermal Energy Resources

James T. Potemra
University of Hawai’i at Manoa

Much attention in recent years has been placed on alternative energy, given our rapidly depleting reserves of fossil fuels. One potentially large energy resource is ocean thermal energy conversion (OTEC), and as OTEC facilities move from design to development, a need has arisen to determine where such plants would be most efficient. Ideally, these facilities would be placed close to shore-based electrical and port infrastructure in regions where the difference between warm surface waters and colder deep waters is found over a small vertical distance. The former reduces the amount of subsea cabling required and allows easier access for ships, while the latter improves efficiency by reducing the length of pipe needed to reach the cold water and power needed to pump it to the surface.

These factors provide the context for development of Ocean Thermal Extractable Energy Visualization (OTEEV), a GIS tool that incorporates in-situ and remote observations and numerical model output to determine optimal locations for OTEC facilities. The visualization tool, which is being developed by Lockheed Martin (Bethesda, Maryland) in cooperation with federal agencies and universities, incorporates in-situ and remote observations along with numerical model output to determine optimal locations for OTEC facilities.

In most designs, a 100-megawatt plant would require a temperature difference of about 20° C to be economically viable. In the ocean, sea surface temperatures (SSTs) range from near zero at the poles to 30° C in the western tropical Pacific. Deep-ocean temperatures, which are not exposed to atmospheric variability, are less variable in space and time. Mean temperatures at 1,000 meters, for example, range from near zero to about 5° C. As such, OTEC plants are only practical between the latitudes of 30° N and 30° S, where annual mean SST exceeds 20° C over depths less than 1,000 meters. The OTEEV project uses in-situ observations of ocean temperature to determine geographic locations where these conditions exist.

Large-scale estimates of ocean temperature, particularly at depth, are sparse, however, two recent efforts provide in-situ measurements of global ocean temperatures. One is the World Ocean Database complied by NOAA and the derivative World Ocean Atlas, which includes temperature on “standard” depth levels for the entire world oceans at 1° resolution and other parameters. Another global data set comes from the Argo program’s 3,000 drifting autonomous floats that measure temperature and salinity as a function of depth. These measurements are then telemetered to shore via satellite, and the data, which typically have a vertical resolution of a few meters, are released in near real time via two global data assembly centers. Using these data, along with numerical model output and reanalyses, the OTEEV team has identified regions where vertical ocean differences could provide the thermal criteria for an OTEC plant. Combining this information with other GIS capabilities, the OTEEV tool provides best estimates for OTEC plant sightings.

The oceanographic data show that poleward of about 30° latitude, the SSTs are not warm enough to establish a thermal gradient of 20° C, so most suitable OTEC sites would be within 30° of the equator. Conversely, the equatorial western Pacific warm pool has the warmest SSTs, typically reaching above 30° C. The thermocline, while deep, is sharp, and the underlying cold temperatures are relatively shallow. These regions in the western Pacific are where a temperature difference of 20° C can be realized within a few hundred meters. Another region of interest is the Hawaiian Islands, which have a 20° C thermal gradient very nearshore, about 500 to 600 meters in northern summer and 900 to 1,000 meters in northern winter. Other areas of relatively shallow thermal gradients can be seen off the coasts of Florida, and Central and South America.

These observations provide a valuable tool for a wide range of research and practical applications such as this. The OTEEV, once completed, will allow managers and decision makers with straightforward access to these data to apply them to OTEC plant planning.

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