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Bridging the Carbon Gap: Fossil Fuel Use for the 21st Century
to electricity in fuel cells. As noted before, new technology has sharply reduced the parasitic power requirements of hydrogen compression. 16 Nevertheless, hydrogen from other sources, such as coal gasification, could conceivably become a fungible energy commodity similar to natural gas. The economic and operational feasibility of creating such a transmission, distribution, and storage system for hydrogen needs to be addressed, because of the obvious advantages of hydrogen as an energy carrier in tandem with electricity as a mainstay of a sustainable energy system.
Similar considerations apply to wind and solar-thermal power, so that the entire vision of using high-tech renewable energy sources as the replacement for fungible fossil fuels-oil, gas, and coal-depends on a solution to the intermittency and wide output swings problems. It also seems unlikely that it will be practical to rely on distributed photovoltaic systems to solve this problem, because of the large variations of solar insolation with latitude.
Small distributed systems with outputs as low as 0.5-1.0 peak kW may be practical with battery storage in much of the developing world, generally located in high insolation areas. Such small on-site sources of power could meet the most essential requirements of the more than two billion people still without electric service. Yet even if the installed cost of photovoltaic arrays is eventually reduced to $3/peak watt,17 with the added cost of batteries and power conditioning equipment, such small systems still may be out of reach of most of the people in the developing world.
The key challenge we face today is to evaluate the most promising technologies to bridge the Carbon Gap. That means achieving the most rapid and cost-effective transition from today's predominant dependence on fossil fuels and combustion processes for providing useful energy services (heating, cooling, lighting, refrigeration, shaft horsepower, passenger-miles, ton-miles, etc.) to high-tech renewable or essentially inexhaustible energy sources.
Nuclear breeder reactors would offer an ideal, emission-free, and essentially inexhaustible source of baseload power if generated in such inherently safe and proliferation-proof designs as the Integral Fast Reactor. 19 However, this would require enormous additional investments in research, development, and demonstration and faces much public opposition. And the economics of this power supply option are also highly uncertain.
Thus, the lead time for meeting most stationary energy requirements with renewable or essentially inexhaustible sources of power, and most transportation fuel needs with electrolytic hydrogen, at acceptable costs, may therefore substantially exceed the allowable time when anthropogenic carbon emissions must be sharply curtailed.
The simplest and most cost-effective approach to gain lead time for achieving sustainability of the United States and global energy systems would be to optimize the use of conventional hydrocarbon fuels by more aggressive development of their existing resource base, and further increases in their utilization efficiency. One key question is how natural gas can serve in this role.
Natural gas enjoys the virtue of a low carbon intensity. In terms of fuels, wood carries the highest ratio of atomic carbon to hydrogen, at 10:1. Coal offers a lower ratio of only 2:1, while oil carries a ratio of only