OceaN reNewable eNergy ’ S pOteNtial rOle iN SupplyiNg Future electrical eNergy NeedS

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" earth'S OceaN cONtaiNS large aMOuNtS combustion processes release great quantities of greenhouse gases, such as carbon dioxide, methane, and nitrous oxide, that are mounting in the atmosphere (Figure 1).The higher the greenhouse gas concentration becomes, the higher the global mean temperature will rise.
For this reason, it has become widely accepted that we must retard and stabilize the growth in CO 2 and other greenhouse gases, and there is much agreement that the best option for achieving stabilization is to deploy a portfolio of technologies, including ocean-derived energy, to achieve the target level.The world's primary energy demand is currently about 12,000 Mtoe (Mtoe = Million tonnes of oil equivalent), and approximately 85% of that is met by oil, coal, or natural gas.Primary energy is the intrinsic energy content of a naturally occurring fuel prior to being subjected to a conversion process.
The remaining 15% of the world energy demand is met by nuclear energy and renewable energy resources, which are currently dominated by biomass and hydropower.Figure 2 shows the projected world energy demand from 1980 to 2030 from a reference case scenario in the International Energy Agency's (IEA) World Energy Outlook for 2008 (Tanaka, 2008).
The reference scenario can be thought of as an extrapolation of today's energy usage trends.The slope of the overall demand curve shows an average growth rate of about 1.6% per year, a continuously growing demand for all of the carbon-based fuels, and a very small increase in renewable energy usage. 1 Does not include the fuel ethanol portion of motor gasoline-fuel ethanol is included in "Renewable Energy." 2 Excludes supplemental gaseous fuels.
3 Includes less than 0.1 quadrillion Btu of coal coke net imports. 4Conventional hydroelectric power, geothermal, solar/PV, wind, and biomass. 5Includes industrial combined-heat-and-power (CHP) and industrial electricity-only plants. 6Includes commercial combined-heat-and-power (CHP) and commercial electricity-only plants. 7Electricity-only and combined-heat-and-power (CHP) plants whose primary business is to sell electricity, or electricity and heat, to the public.Note: Sum of components may not equal 100 percent due to independent rounding.Sources: Energy Information Administration, Annual Energy Review 2008, Tables 1.3 1 Does not include the fuel ethanol portion of motor gasoline-fuel ethanol is included in "Renewable Energy." 2 Excludes supplemental gaseous fuels.
3 Includes less than 0.1 quadrillion Btu of coal coke net imports. 4Conventional hydroelectric power, geothermal, solar/PV, wind, and biomass. 5Includes industrial combined-heat-and-power (CHP) and industrial electricity-only plants. 6Includes commercial combined-heat-and-power (CHP) and commercial electricity-only plants. 7Electricity-only and combined-heat-and-power (CHP) plants whose primary business is to sell electricity, or electricity and heat, to the public.Note: Sum of components may not equal 100 percent due to independent rounding.• Put Americans on the path to generating 20% or more of our energy from renewable sources by 2020 Figure 5 shows that to limit the increase in global temperature to only 2°C at best, we must follow the lowest (green) trajectory for carbon emissions.Figure 6.estimated electric potential for uS ocean renewable energy resources.
1 resource includes: class 5 (approximately 7.5 m/s) wind or better; water depths between 0 m and 30 m; 60% resource area excluded; hi and aK not included; 0 to 50 nm from shore; 45% capacity factor.Source: Nrel. 2 resource includes: class 5 (approximately 7.5 m/s) wind or better; water depths between 30 m and 60 m; 60% resource area excluded; hi and aK not included; 0 to 50 nm from shore; 45% capacity factor.Source: Nrel. 3 resource includes: class 5 (approximately 7.5 m/s) wind or better; depths between 60 m and 900 m; 60% resource area excluded; hi and aK not included; 0 to 50 nm from shore; 45% capacity factor.Source: Nrel. 4 15% of incident wave energy; 20% conversion losses; aK and hi included; wave climate 10 kw/m or better Source: epri.
O b e r t t h r e S h e r a N d wa lt e r M u S i a l iNtrOductiON The world is facing enormous environmental issues as human consumption has begun to stress Earth's resources, and thus, our ability to sustain our existence in the way we are accustomed.In parallel with finding ways to mitigate the impact of climate change, we must address the important issue of depletion of conventional energy supplies.A diverse portfolio of energy sources must be developed that also achieves the needed atmospheric carbon reductions.Earth's ocean contains large amounts of untapped clean renewable energy resources that can play a significant role in our future energy portfolio.These resources are found in the waves, currents, tides, and ocean thermal gradients.Indeed, ocean energy sources could become the primary energy source for some resource-rich coastal communities.wOrld eNergy uSage Throughout recorded history, humans have energized their development of civilization, first with wood for heating, then with water and wind for grinding grain and pumping water.Then, nearly two centuries ago, we began using coal for heating and powering steam-driven machines for doing work and transporting people and goods.This technological step sparked a rapid increase in the rate of industrial development and a technological evolution.The increased use of fossil energy fueled our productivity and our ability to produce goods and services.During the last century, we have further evolved our technological skills and begun to use more versatile fuels like oil and natural gas that are easier to store and handle and that have less tangible residue for disposal after combustion.The unintended consequence of this increased use of the planet's carbon-based fuels is that these

FollowingFigure 5 .
Figure 5. (left panel) global cO 2 emissions for 1940 to 2000 and emissions ranges for categories of stabilization scenarios from 2000 to 2100.(right panel) The relationship between the stabilization target and global average temperature increase above preindustrial levels.Source: Figure SPM 11 in IPCC, 2007 energy from kinetic tidal and in-stream water currents 5 ; open ocean currents 6 ; and energy from ocean thermal gradients 7 .Other ocean energy sources such as salinity gradients at river mouths and marine biomass could also have some promise in the future, but the potential for these new resources has not yet been estimated.Although most engineers and scientists would consider these numbers to be upper bounds to the actual extractable potential, the ocean energy potential is still highly significant if only a fraction were extracted.In 2008, the American Wind Energy Association and the Department of Energy published an energy deployment vision for wind energy to supply 20% of US electricity by 2030 (USDOE, 2008).In this scenario, 54 GW of the 20% electric capacity is provided by offshore wind, and most of this capacity does not begin to tap the vast deepwater wind resources.In 2007, EPRI estimated that marine hydrokinetic technologies, including wave and water current turbine technologies, could potentially provide a similar contribution to the current electricity supply provided by conventional hydroelectric power plants (EPRI, 2007) 8 .The EPRI report estimated that 13 GW of new hydrokinetic technologies could be deployed by 2025.Of course, ocean energy industries are not likely to mature until after 2025, so these deployment estimates are likely to be low compared to the resource potentials that are estimated in Figure 6.Based on these approximations, ocean energy could ultimately provide at least 10% of the electric supply of the United States.In coastal areas, where these resources are plentiful, these indigenous sources may, on a regional basis, represent a much larger fraction of the local energy supply and may indeed be the best long-term energy option.