Ocean Thermal Energy Conversion (OTEC) is recognized as a significant potential renewable energy resource for coastal communities, yet OTEC plants can only be constructed on sites that meet key logistical and technical requirements. Finding a suitable candidate site requires extensive study of the atmosphere, ocean and seabed environment surrounding the location where the plant will be installed. McCallister et al. (2010) studied the specific aspects of a candidate site for installation of an OTEC pilot plant off Pearl Harbor, O’ahu, Hawai’i. Using geophysical information and oceanographic data, the authors were able to conduct a preliminary survey of the installation area and identify potential risks and hazards at the installation site. The Pearl Harbor site was found to meet the key characteristics of a preliminary study, including a sufficient temperature differential (more than 20° C difference from deep water to cool water) and a favorable seafloor environment for anchoring the physical system. The site also presents low seismic and tsunami risk, moderate risk of mass sediment movement, and a significant long-term risk presented by tropical cyclones. Additionally, the authors explored the data collection and physical characteristics required to undertake installation of an OTEC system. –Meredith Reisfield
McCallister, M., Switzer, T., Arnold, F., Ericksen, T., 2010 .Geophysical and oceanographic site survey
OTEC works by utilizing the difference in temperature between deep water and surface water to drive turbines and generate electricity. A fluid with a low boiling point, typically ammonia, is circulated through a network of pipes. The fluid is vaporized by a heat exchanger warmed by surface water, and the resulting gas is used to spin a turbine and generate electricity. The gas is then cooled by a second heat exchanger and cooled by seawater pumped up from lower depths. It condenses into a liquid and the process is repeated. Pearl Harbor is the site of a ten-megawatt OTEC pilot plant that should be operational by 2015.
McCallister et al. obtained the geophysical data required to install an OTEC system through high-resolution sonar and video imaging. The O’ahu site was found to share common characteristics of favorable OTEC locations, including a steep slope, irregular bathymetry (depth of ocean floors), and uncertain sediment thickness. Sediment sampling revealed significant sediment thickness, suggesting that the O’ahu site could support the multiple anchoring locations required to secure the physical system in place. Additionally, an examination of the primary risks to facility found the highest potential risks to be from major caldera collapse slides, which would cause the anchors to destabilize. There is also the prospect of tropical cyclones damaging equipment.
McCallister et al. also discuss the need for extensive knowledge of physical and chemical seawater properties, recorded in situ over time to observe seasonal and local variations in temperature and current structure. Nutrient levels must be monitored at a variety of depths. The introduction of nutrient-rich deep waters into the nutrient-poor surface waters by the operation of the plant could stimulate plankton blooms and adversely affect the local ecological balance. Additional ecological challenges arise from the potential presence of endangered of threatened species, biological nursery areas, and other sensitive habitat. Since OTEC facilities must be located near islands or within several miles of coastal zones due to cabling constraints, near-shore circulation patterns play a crucial role in determining the thermal structure of the water column. Upwelling, bringing deep water closer to the surface, can benefit an OTEC facility by reducing the required length of a cold-water intake pipe. Downwelling, by contrast, can push colder waters out of range of the cold-water pipe. Thus, the construction of an OTEC plant requires a detailed model of the water column’s temperature structure across myriad conditions.