Renewable Energy and the Terawatt Challenge

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Technological advancement and a growing world economy during the past few decades have led to major improvements in the living conditions of people in the developed world. However, these improvements have come at a steep environmental price. Air quality concerns and global climate impact constitute two major problems with our reliance on fossil energy sources. Global warming as a result of the accumulation of greenhouse gases such as CO2 is not a new concept. More than a century ago, Arrhenius put forth the idea that CO2 from fossil fuel combustion could cause the earth to warm as the infrared opacity of its atmosphere continued to rise.1 The links between fossil fuel burning, climate change, and environmental impacts are becoming better understood.2 Atmospheric CO2 has increased from ~275 ppm to ~370 ppm (Figure 1); unchecked, it will pass 550 ppm this century. Climate models indicate that 550 ppm CO2 accumulation, if sustained, could eventually produce global warming comparable in magnitude but opposite in sign to the global cooling of the last Ice Age.3 The consequences of this lurking time bomb could be unpredictably catastrophic and disastrous as recent hurricanes and tsunamis indicate.

Every year, a larger percentage of the 6.5 billion global population seeks to improve their standard of living by burning ever-increasing quantities of carbon-rich fossil fuels. Based on United Nations forecasts, another 2.5 billion people are expected by 2050 with the preponderance of them residing in poor countries.4 Coupled with this growing population's desire to improve their quality of life are the developed countries already high and rising per capita energy use which promises to add to the environmental pressure.

Oil, coal, and natural gas have powered cars, trucks, power plants, and factories, causing a relatively recent and dramatic buildup of greenhouse gases in the atmos-

1960 1970 1980 1990 2000

Fig. 1. Atmospheric carbon dioxide record from Mauna Loa. Data courtesy of C. D. Keeling and T. P. Whorf.

1960 1970 1980 1990 2000

Fig. 1. Atmospheric carbon dioxide record from Mauna Loa. Data courtesy of C. D. Keeling and T. P. Whorf.

phere, most notably CO2. The anthropogenic buildup of heat-trapping gases is intensifying the earth's natural greenhouse effect, causing average global temperatures to rise at an increasing rate. We appear to be entering into a period of abrupt swings in climate partially due to buildup of human-released CO2 in the atmosphere. Most alarming is not the fact that the climate is changing but rather the rate at which the buildup of CO2 is occurring.

Ice core samples from Vostok, Antarctic, look back over 400,000 years before present at atmospheric CO2 levels by examining the composition of air bubbles trapped in the polar ice buried over 3623 m (11,886 ft) deep.5 These data show that the range of CO2 concentrations over this time period have been relatively stable, cycling between about 180 and 300 parts per million by volume (ppmv). According to the World Meteorological Organization the CO2 concentration in 2005 reached an unprecedented 379.1 ppmv.6

This environmental imperative requires us to quickly come to terms with the actual costs, including environmental externalities, of all of our energy use. Only then will the economic reality of energy consumption be realized and renewable sources expand through true market forces. That is not to say that fossil fuels like oil, natural gas, and coal do not have a future in helping to meet this growing demand. However, it should go without saying that all new sources of CO2 should be captured and stored (i.e., sequestered). Although integrating the systems required to safely and economically storing CO2 deep underground have not been realized. More than ever, CO2 released into the atmosphere by coal-fired power plants must be addressed to effectively deal with global climate change. In addition to greenhouse gas emissions, destructive extraction and processing of the fuel, fine particulates of 2.5 micrometers (|im) released from coal-fired power plants are responsible for the deaths of roughly 30,000 Americans every year.7

Even notwithstanding this climate change and global warming concern are issues with the supply side of a fossil-derived energy economy. Gasoline and natural gas supplies will be under increasing stress as the economies of heavily-populated developing countries (such as India and China) heat up and become more energy intensive. It is pertinent to note that this supply problem is exacerbated because the United States alone consumes a disproportionately higher fraction (more than the next five highest energy-consuming nations, Ref. 8) of the available fossil fuel supply. There are no signs that the insatiable energy appetite of the U. S. and other advanced parts of the world are beginning to wane. While there is considerable debate about when global oil and natural gas production is likely to peak,9 there is no debate that fossil fuels constitute a non-renewable, finite resource. We are already seeing a trend in some parts of the world (e.g., Alberta, Canada) of a switch to "dirtier" fossil fuels, namely, coal, heavy oil or tar sand as petroleum substitutes. This switch would mean an increase in CO2 emissions (note that the carbon content of these sources is higher than gasoline or natural gas), a greater temperature rise than is now being forecast, and even more devastating effects on the earth's biosphere than have already been envisioned.10

Currently, renewable energy only constitutes a very small fraction of the total energy mix in the U. S. and in other parts of the world (Figure 2). For example, in 2000, only about 6.6 quads (one quad is about 1018 J) of the primary energy in the U. S. came from renewables out of a total of 98.5 quads.11 Of this small fraction supplied by renewable energy, about 3.3 quads were from biomass, 2.8 from hydroelectric generation, 0.32 from geothermal sources, 0.07 from solar thermal energy and 0.05 quads from wind turbines.8 This profile would have to switch to an energy mix that resembles the right-side panel in Figure 2 if the CO2 emissions are to be capped at environmentally safe levels. This is what the late Professor Rick Smalley, winner of the Nobel Prize in Chemistry, referred to as the Terawatt Challenge. Recent analyses12 have posited that researching, developing, and commercializing carbon-free primary power to the required level of 10-30 TW (one terawatt = 1012 W) by 2050 will require efforts of the urgency and scale of the Manhattan Project and the Apollo Space Program.

This book examines the salient aspects of a hydrogen economy, particularly within the context of a renewable, sustainable energy system.

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