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8 Other Renewable Sources of Energy B eyond fossil fuels, nuclear power, and biofuels are a variety of energy sources that will be part of the future portfolio, including solar power, wind power, hydropower, power from such sources as tides and waves, other forms of biopower, and hydrogen derived from these sources. Although most of these technologies were not much discussed at the summit, some received attention, and some more tentative sources of energy, such as enhanced geothermal energy, also were explored. SOLAR POWER AND WIND POWER Hydropower, which is a fairly mature energy source in the United States, was discussed very little at the summit. But several speakers touched on the prospects for greatly expanding the use of solar power and wind power. As Ray Orbach pointed out, more energy from sunlight strikes Earth in an hour than all of the energy consumed on the planet in a year. Yet less than a tenth of 1 percent of our primary energy is derived directly from sunlight. “We have barely scratched the surface on solar energy, and the amount of energy available is so enormous that it is to our advantage to pay particular attention,” Orbach said. There are several ways of generating electricity from sunlight. One is to concentrate solar energy and use it to heat a liquid that can drive a turbine- generator set. This is the technology being pursued at a full-scale pilot plant that the Department of Energy is helping to fund in Nevada. Other techniques involve converting solar energy into electricity or into fuels. The challenge, 59

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60 THE NATIONAL ACADEMIES SUMMIT ON AMERICA’S ENERGY FUTURE said Orbach, is to reduce the costs and increase the efficiency of the conver- sion process, which often requires understanding and controlling phenomena at the nanoscale level. One drawback to the increased use of solar power and wind power is that they tend to be intermittent sources of energy, in that they can be generated only when the sun is shining or the wind is blowing. Taking full advantage of these technologies will require the development of both energy storage tech- nologies and “smart grids” that can control the flow of electricity from energy producers to energy users. Several energy storage technologies are being developed. Although cur- rent batteries are not suited for large-scale storage, advanced batteries and electrochemical capacitors offer potential for the future. An older technology that remains useful is to pump water to a higher level and subsequently use the stored potential energy to generate electricity. Another such technology is compressed air storage, in which air is pumped into underground caverns or aboveground storage areas and then drives electricity-producing turbines when it is released. “There’s some very good new technology on compressed air energy storage that can use existing gas turbine designs,” said Steven Specker. “There’s a plant in Alabama that has been operating for a number of years with compressed air energy storage. It works very well. [And] as wind power grows rapidly in certain parts of the United States and the world, we need a storage approach.” Today, intermittent sources of electricity, such as wind or solar power, can make electricity grids unstable. To manage large amounts of renewable power generation, smart grids using advanced power electronics and electri- cal storage devices are needed to manage the transmission and distribution of electricity from where it is produced to where it is being used. Specifically, these grids must be able to “communicate” between utilities and electricity meters, enabling such advances as provision of power to the grid by plug-in hybrid automobiles and powering off of home appliances during peak load periods. ENHANCED GEOTHERMAL ENERGY Several speakers at the summit discussed sustainable sources of energy that are farther in the future than the expanded use of solar power and wind power. One is energy from engineered geothermal systems. As Dan Reicher pointed out, if a hole is drilled into the ground, it eventually will encounter rocks that are heated by Earth’s interior. If several such holes are drilled near each other and the rock between them is fractured, water can be injected into one well and returned from the others much hotter, and steam from that water could turn a This depends on local grid characteristics, but in general grids are able to handle between 10 and 20 percent of intermittent renewables without requiring storage.

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OTHER RENEWABLE SOURCES OF ENERGY 61 turbine to generate power (Figure 8.1). “It is a vast and ubiquitous base load resource, unlike solar and wind, which are obviously an intermittent resource,” said Reicher. “If we can figure out how to exploit it, it could be developed in the large megawatt range.” There are no scientific showstoppers to exploiting this resource, said Reicher, but there are “great engineering challenges.” No such system has been developed anywhere in the world; existing geothermal energy sources rely on heated water and steam that is near the surface of the earth. Furthermore, exploiting this resource requires that holes be drilled as deep as 10 kilometers into the earth and that rocks be fractured at great depths. Can such deep wells be drilled? “The answer,” said Reicher, “is that the oil industry does drill to those depths. And the oil industry does fracture rock at those depths. Ten kilometers is not an insignificant piece of work, but it is a distance the oil industry knows how to get to.” Injection pump Makeup water Power Plant Sediments and/or Volcanics Injection Well Low-Permeability Crystalline Production Well Basement Rocks 10,000-30,000 ft. Depth 3-10 km Depth FIGURE 8.1 Enhanced geothermal systems could extract energy from Earth’s interior. SOURCE: INL (2006). 8-1 new

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62 THE NATIONAL ACADEMIES SUMMIT ON AMERICA’S ENERGY FUTURE Furthermore, an array of technological advances could cut costs, and the industry is working on these advances. For example, seismic technology can provide a good snapshot of fractured rock at depth to enable large areas underground to be connected, creating what is essentially a heat exchanger. Advanced drilling, control, and high-temperature technologies are all being investigated. Today the Energy Department spends only $20 million per year to pursue this option. “This effort should be significantly expanded,” said Reicher. “The Australians, who lead the world in this technology, are spending hundreds of millions of dollars. There are 30 companies in Australia working at this today, and we’re playing catch up.” ADVANCED ENERGY R&D Orbach also discussed ways of generating energy that will require what he called “transformational discoveries” in basic research. Over the past 5 years the Energy Department has conducted a series of workshops on basic research needs for a secure energy future, examining such topics as superconductivity, the hydrogen economy, solid-state lighting, advanced nuclear reactor designs, energy storage, and materials science. Using scientific and engineering research, Orbach asked, “What can we do to break out of the straightjacket in which we find ourselves?” For example, he mentioned studies of photosynthesis as a possible way to take advantage of techniques that living things have evolved to meet their energy needs. “It really comes down to how nature works,” he said. “Plants are almost 100 percent efficient at room temperature. Is there any way for us to mimic what nature does so well?” The technologies of this century will be rooted in the ability to direct and control matter at the molecular, atomic, and quantum levels, according to Orbach. Research challenges include the synthesis of new forms of matter with tailored properties, predictions of the properties of novel materials, and the fabrication of manmade nanoscale objects with capabilities that rival those of living things. Incremental changes will not be sufficient, Orbach said. Transfor- mational discoveries and disruptive technologies will be essential.