The Energy Challenge
The world now uses energy at a rate of approximately 4.1 × 1020 joules/yr, equivalent to a continuous power consumption of 13 trillion watts, or 13 terawatts (TW). Even with aggressive conservation and energy efficiency measures, an increase of the Earth’s population to 9 billion people, accompanied by rapid technology development and economic growth world-wide, is projected to produce more than double the demand for energy (to 30 TW) by 2050, and more than triple the demand (to 46 TW) by the end of the century. The reserves of fossil fuels that currently power society will fall short of this demand over the long term, and their continued use produces harmful side effects such as pollution that threatens human health and greenhouse gases associated with climate change. Alternative renewable fuels are at present far from competitive with fossil fuels in cost and production capacity. Without viable options for supplying double or triple today’s energy use, the world’s economic, technological, and political horizons will be severely limited.
Our primary source of clean, abundant energy is the sun. The sun deposits 120,000 TW of radiation on the surface of the Earth, far exceeding human needs even in the most aggressive energy demand scenarios. The sun is Earth’s natural power source, driving the circulation of global wind and ocean currents, the cycle of water evaporation and condensation that creates rivers and lakes, and the biological cycles of photosynthesis and life. Covering 0.16% of the land on Earth with 10% efficient solar conversion systems would provide 20 TW of power, nearly twice the world’s consumption rate of fossil energy and the equivalent 20,000 1-GWe nuclear fission plants. These comparisons illustrate the impressive magnitude of the solar resource, providing an energy stream far more potent than present-day human technology can achieve.
All routes for utilizing solar energy exploit the functional steps of capture, conversion, and storage. The sun’s energy arrives on Earth as radiation distributed across the color spectrum from infrared to ultraviolet. The energy of this radiation must be captured as excited electronhole pairs in a semiconductor, a dye, or a chromophore, or as heat in a thermal storage medium. Excited electrons and holes can be tapped off for immediate conversion to electrical power, or transferred to biological or chemical molecules for conversion to fuel. Natural photosynthesis produces fuel in the form of sugars and other carbohydrates derived from the reduction of CO2 in the atmosphere and used to power the growth of plants. The plants themselves become available as biomass for combustion as primary fuels or for conversion in reactors to secondary fuels like liquid ethanol or gaseous carbon monoxide, methane, and hydrogen. We are now learning to mimic the natural photosynthetic process in the laboratory using artificial molecular assemblies, where the excited electrons and holes can drive chemical reactions to produce fuels that link to our existing energy networks. Atmospheric CO2 can be reduced to ethanol or methane, or water can be split to create hydrogen. These fuels are the storage media for solar energy, bridging the natural day-night, winter-summer, and cloudy-sunny cycles of solar radiation.
In addition to electric and chemical conversion routes, solar radiation can be converted to heat energy. Solar concentrators focus sunlight collected over a large area to a line or spot where heat is collected in an absorber. Temperatures as high as 3,000°C can be generated to drive chemical reactions, or heat can be collected at lower temperatures and transferred to a thermal storage medium like water for distributed space heating or steam to drive an engine. Effective storage of solar energy as heat requires developing thermal storage media that accumulate heat efficiently during sunny periods and release heat slowly during dark or cloudy periods. Heat is one of the most versatile forms of energy, the common link in nearly all our energy networks. Solar thermal conversion can replace much of the heat now supplied by fossil fuel.
Although many routes use solar energy to produce electricity, fuel, and heat, none are currently competitive with fossil fuels for a combination of cost, reliability, and performance. Solar electricity from photovoltaics is too costly, by factors of 5–10, to compete with fossil-derived electricity, and is too costly by factors of 25–50 to compete with fossil fuel as a primary energy source. Solar fuels in the form of biomass produce electricity and heat at costs that are within range of fossil fuels, but their production capacity is limited. The low efficiency with which they convert sunlight to stored energy means large land areas are required. To produce the full 13 TW of power used by the planet, nearly all the arable land on Earth would need to be planted with switchgrass, the fastest-growing energy crop. Artificial photosynthetic systems are promising routes for converting solar energy to fuels, but they are still in the laboratory stage where the principles of their assembly and functionality are being explored. Solar thermal systems provide the lowest-cost solar electricity at the present time, but require large areas in the Sun Belt and breakthroughs in materials to become economically competitive with fossil energy as a primary energy source. While solar energy has enormous promise as a clean, abundant, economical energy source, it presents formidable basic research challenges in designing materials and in understanding the electronic and molecular basis of capture, conversion, and storage before its promise can be realized.
Global Energy Resources
Current global energy consumption is 4.1 × 1020 J annually, which is equivalent to an instantaneous yearly-averaged consumption rate of 13 × 1012 W [13 trillion watts, or 13 terawatts (TW)]. Projected population and economic growth will more than double this global energy consumption rate by the mid-21st century and more than triple the rate by 2100, even with aggressive conservation efforts. Hence, to contribute significantly to global primary energy supply, a prospective resource has to be capable of providing at least 1-10 TW of power for an extended period of time.
The threat of climate change imposes a second requirement on prospective energy resources:
They must produce energy without the emission of additional greenhouse gases. Stabilization of atmospheric CO2 levels at even twice their preanthropogenic value will require daunting amounts of carbon-neutral energy by mid-century. The needed levels are in excess of 10 TW, increasing after 2050 to support economic growth for an expanding population.
The three prominent options to meet this demand for carbon-neutral energy are fossil fuel use in conjunction with carbon sequestration, nuclear power, and solar power. The challenge for carbon sequestration is finding secure storage for the 25 billion metric tons of CO2 produced annually on Earth. At atmospheric pressure, this yearly global emission of CO2 would occupy 12,500 km3, equal to the volume of Lake Superior; it is 600 times the amount of CO2 injected every year into oil wells to spur production, 100 times the amount of natural gas the industry draws in and out of geologic storage in the United States each year to smooth seasonal demand, and 20,000 times the amount of CO2 stored annually in Norway’s Sleipner reservoir. Beyond finding storage volume, carbon sequestration also must prevent leakage. A 1% leak rate would nullify the sequestration effort in a century, far too short a time to have lasting impact on climate change. Although many scientists are optimistic, the success of carbon sequestration on the required scale for sufficiently long times has not yet been demonstrated.
Nuclear power is a second conceptually viable option. Producing 10 TW of nuclear power would require construction of a new one-gigawatt-electric (1-GWe) nuclear fission plant somewhere in the world every other day for the next 50 years. Once that level of deployment was reached, the terrestrial uranium resource base would be exhausted in 10 years. The required fuel would then have to be mined from seawater (requiring processing seawater at a rate equivalent to more than 1,000 Niagara Falls), or else breeder reactor technology would have to be developed and disseminated to countries wishing to meet their additional energy demand in this way.
The third option is to exploit renewable energy sources, of which solar energy is by far the most prominent. United Nations (U.N.) estimates indicate that the remaining global, practically exploitable hydroelectric resource is less than 0.5 TW. The cumulative energy in all the tides and ocean currents in the world amounts to less than 2 TW. The total geothermal energy at the surface of the Earth, integrated over all the land area of the continents, is 12 TW, of which only a small fraction could be practically extracted. The total amount of globally extractable wind power has been estimated by the IPCC and others to be 2-4 TWe. For comparison, the solar constant at the top of the atmosphere is 170,000 TW, of which, on average, 120,000 TW strikes the Earth (the remainder being scattered by the atmosphere and clouds). It is clear that solar energy can be exploited on the needed scale to meet global energy demand in a carbon-neutral fashion without significantly affecting the solar resource.
Solar energy is diffuse and intermittent, so effective storage and distribution are critical to matching supply with demand. The solar resource has been well established, and the mean yearly insolation values are well documented. At a typical latitude for the United States, a net 10% efficient solar energy “farm” covering 1.6% of the U.S. land area would meet the country’s entire domestic energy needs; indeed, just 0.16% of the land on Earth would supply 20 TW of power globally. For calibration purposes, the required U.S. land area is about 10 times the area of all single-family residential rooftops and is comparable with the land area covered by the nation’s federally numbered highways. The amount of energy produced by covering 0.16% of the Earth’s land area with 10% efficient solar cells is equal to that produced by 20,000 1-GWe nuclear fission plants. This many plants would need to be constructed to meet global demands for carbon-neutral energy in the second half of the 21st century if carbon sequestration were to prove technically nonviable and if solar energy were not developed.