Climate Solutions: Chapter 4

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Published: June 16, 2010, 6:53 pm
Updated: May 7, 2012, 11:52 am
Author: Leo Wiegman
Author: David Blockstein
Topic Editor: Peter Saundry
Topics:

Ocean Thermal Energy Conversion

As seawater warms up, it expands, increasing the volume of the global ocean. [28] —Gerald Meehl and his IPCC colleagues, 2007

Global warming raises the potential of unlocking large amounts of fresh water now frozen in the vast Greenland ice sheet and in Arctic Ocean sea ice. Warming air temperatures could also increase evaporation in low latitudes and transport freshwater vapor toward high latitudes, where it falls as rain or snow into the oceans. Could these factors tip the freshwater balance in the North Atlantic in the future? [27]

—Jerry McManus and Delia Oppo, Woods Hole Oceanographic Institution, 2006

As we have learned, the ocean is a vast reservoir, not only of water, but also of heat. The thermal layers are not uniform. Surface water warmed by the sun tends to contain more heat than layers 100 meters below the surface or deeper. The bigger the temperature difference between the warmer top water and colder bottom water, the more potential exists to convert that difference into other kinds of energy, such as electricity. Ocean Thermal Energy Conversion (OTEC) is an energy technology that converts solar radiation to electric power. OTEC systems use the ocean's natural thermal gradient—the difference in temperatures of the ocean's layers of water—to drive a power-producing cycle. As long as the temperature between the warm surface water and the cold deep water differs by about 20°C (36°F), an OTEC system can produce a significant amount of power, with little impact on the surrounding environment. As the OTEC Web site notes, “The oceans are thus a vast renewable resource, with the potential to help us produce billions of watts of electric power.” [31] According to some experts, this potential may be as large as 10,000 billion watts of continuous baseload power generation.

Essentially, the technology involves pumping cold deep ocean water to the surface, exchanging the thermal energy between the two reservoirs in a heat engine, and returning the water to the mixed layer between the warm top and cold deep layers. Experimental OTEC stations have been in operation since the late 1990s. The by-products of the heat exchange include clean freshwater (which rivals in quality that of modern desalination plants) and cold "waste" water, which could be used for marine aquaculture or even for growing plants on land, as the Seawater Greenhouse project shows.*

  • www.seawatergreenhouse.com

Online figures

Figure 4.1 Sea level rise Global average sea levels have been rising for about 130 years. The rate of the rise has accelerated in the past decades. Sea level rise can be attributed to both thermal expansion, when ocean water becomes warmer, and the melting of formerly frozen water from the polar regions and mountain glaciers. Sea level data for the most recent period, shown here in the black curve, is based on satellite altimetry. Source: [2]
Figure 4.2 Arctic steam above a polynya Polynyas form where currents keep thick surface ice from forming. Sunlight that reflects off open water has a solar heating effect on that water that is nine times stronger than sunlight that reflects off snow-covered ice that is floating on water. [30] Therefore, the seawater exposed in a polynya heats up more rapidly, creating steam in the cold Arctic air above. Source: [18]
Figure 4.3 Follow the waterCold, relatively fresh water from the Pacific Ocean enters the Arctic Ocean through the Bering Strait. It is swept into the Beaufort Gyre and exits into the North Atlantic Ocean through three gateways around Greenland. Warmer, denser waters from the Atlantic penetrate the Arctic Ocean beneath colder water layers, which lie atop the warmer waters and act as a barrier, preventing them from melting sea ice. A global system of currents, often called the ocean conveyor, carries warm surface waters from the tropics northward. At high latitudes, the waters cool, releasing heat to the atmosphere and moderating wintertime climate in the North Atlantic region. The colder (and denser) waters sink and flow southward in the deep ocean to keep the conveyor moving. Source: [7], illustration by Jack Cook, WHOI
Figure 4.6 The Atlantic Ocean as a sea level systemThis schematic of the observed changes in the ocean state includes ocean temperature, ocean salinity, sea level, sea ice, and biogeochemical cycles. The legend identifies the direction of the changes in these variables. NADW represents the North Atlantic Deep Water that sinks as the Gulf Stream cools. Notice that the Southern Ocean has no cold deep water of this kind, because Antarctica sits on a landmass, unlike the Arctic, which is water covered with ice. Source: [2]

Figure 4.10 The carbon budget: 1956–2000The fate of the emitted carbon dioxide, including the increase in atmospheric carbon dioxide plus the sinks of carbon dioxide on land and in the ocean: Flux is a measure of how much flow passes through a surface in a given time. Carbon flux is given in petagrams of carbon per year (PgCy?1 in the left axis), and carbon dioxide flux is given in petagrams of carbon dioxide per year (PgCO2y?1 in the right axis). A petagram is 10 to the 15th grams, or 10,000 billion kilograms (a lot!). Source: [5]
Figure 4.11 Fraction of carbon emission absorbed by ocean, land, and atmosphere: 1960–2007Fraction of the total carbon dioxide emissions (from fossil fuel use + from land use change) that remains in the atmosphere (A), the land biosphere (B), and the ocean (C). The ocean’s share has been in steady decline from roughly 30% of total emissions in 1960 to about 25% of a much larger volume today. Source: [5]

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Online resources

Action items

Action 18: Coastal Management and Climate Change

Action 19: Forest Management and Climate Change

Action 25: Ocean Fertilization for Carbon Sequestration

Instructor resources

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This is a chapter from Climate Solutions Consensus.
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Citation

Wiegman, L., & Blockstein, D. (2012). Climate Solutions: Chapter 4. Retrieved from http://editors.eol.org/eoearth/wiki/Climate_Solutions:_Chapter_4
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