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Capturing Ocean Heat

Ocean thermal energy conversion offers a timely renewable alternative.

Fortnightly Magazine - October 2009

fuels ( e.g., hydrogen, ammonia, and biofuels); foods from mariculture and greenhouses (including cold-weather crops utilizing chilled-soil agriculture to provide the right growing conditions); resource extraction from the brine ( i.e., lithium, molybdenum and uranium may be profitably extracted from seawater considering the flow rates needed to operate the OTEC plant); and, if close enough to shore, provide moderate-temperature refrigeration and air-conditioning for buildings or for on-board facilities. In addition, utilizing the “Energy Island” concept 14 developed by Dominic Michaelis, the OTEC plant could incorporate other renewable energy-gathering technologies ( e.g., wind, photovoltaics, concentrating solar, wave, current, and reverse-pumped energy storage (PES) to increase the overall energy production capabilities. Finally, the OTEC plant could be co-located with other industrial facilities ( e.g., computer server farms, cargo transhipment facilities, shipping refueling facilities), in order to provide additional revenue streams.

OTEC Advantages

While there are challenges to bringing OTEC to commercial viability, building this energy infrastructure would offer many advantages.

First, they provide clean, renewable, and independent baseload energy production. Unlike other sources of renewable energy that vary depending on weather and time of day, OTEC power plants can produce electricity 24 hours a day, 365 days a year, 15 providing customers with enough power and water to make them independent of costly fuel imports. OTEC has a virtually non-existent carbon footprint, which leads to little if any adverse environmental impacts, particularly when compared with other energy sources. Since OTEC isn’t exothermic (like fossil-fueled and nuclear power plants), and since the cold or mixed water will be discharged at depth, it doesn’t contribute directly to global warming.

Second, it can produce fresh water for various purposes. Both open-cycle and hybrid plants directly can produce potable water as well as electricity (at a rate of about 700,000 gallons/MW) that is suitable for human consumption, as well as agriculture and livestock needs, which can be significant for areas that have little rainfall or increasing fresh water needs. 16, 17

OTEC plants can produce fuels in addition to heat and electricity. OTEC plants can produce hydrogen (through electrolysis of water), ammonia or biofuels ( e.g., growing algae), which could be transported virtually anywhere. Alternately, an OTEC plant can be used as a deep-water refueling station for ships.

OTEC facilities can serve mariculture and agriculture production. The large quantities of cold ocean waters (around 4 degrees C) pumped from 1,000 meters deep are nutrient-rich and relatively pathogen-free, which provides an excellent medium for growing phytoplankton (microalgae), which is the feedstock for the production of a variety of commercially valuable fish and shellfish, 18 as well as growing other algae that can be turned into biofuels. Further, the cold waters can support greenhouses growing cold-weather fruits and vegetables if suitably mixed for the ideal growth temperature either ashore or afloat. 19

Additionally, these plants can provide air-conditioning and refrigeration capacity. The deep-ocean cold water can be used as a cooling medium in air-conditioning systems. For example, only 1 cubic meter per second (1 m 3/s) of water at a temperature of 7 degrees