Need Energy? Look to the Sun
When world leaders assemble in Paris next month for the United Nations Climate Conference, they will have to consider a difficult issue: Where humans will get enough energy to meet their needs without destroying the planet.
From a purely scientific perspective, solar is the only solution.
Humanity's most ambitious goals -- such as pulling another 2 billion people out of extreme poverty, or getting a grip on global warming -- require an abundant source of clean, renewable energy. The question is how to choose among options such as solar, wind and biomass. To that end, a group of scientists from Germany's Max Planck Institute for Biogeochemistry decided to figure out which energy source has the most potential to satisfy humans' long-term needs.
Their result is striking: Solar can offer about 100 times as much clean energy as any other source.
The total solar energy hitting the Earth at the top of the atmosphere is about 175,000 terawatts, or about 10,000 times what humans currently use. Much of this gets absorbed in the atmosphere, where it fuels winds and storms and helps drive ocean flows. A bit less than half reaches the planet's surface in the form of radiation energy.
Humans collect the sun's energy in two ways. First, we employ solar technology to harvest the light radiation directly. Second, we get it indirectly, by burning organic matter (oil or coal) that the sun helped to grow, or by harnessing the wind and waves that the sun's light stirs up. Each method has its own physical limit -- the amount of sunlight that, according to the laws of thermodynamics, it can convert into usable energy.
The scientists from Max Planck find that the indirect method is by far the most wasteful. For wind energy, the best possible efficiency -- defined as the fraction of initial sunlight captured for human use -- is only about 0.5 percent. Making biofuels from plants operating through photosynthesis turns out to be only slightly better, with a maximum efficiency of 1.5 percent on land, mostly because plants manage to gather light energy only from a small fraction of the spectrum.
The direct approach is much better. The scientists estimate that energy can be harvested with 93 percent efficiency from direct sunlight, and 73 percent from diffuse, ambient light.
The combination of abundance and efficiency makes solar power far more promising than other energy sources. Using satellite data on global radiation patterns, the scientists estimate the maximum solar energy derivable over land at 16,300 terawatts, about 1,000 times our current energy usage. That's more than 100 times what can be had from either wind or biomass. Adding energy gathered over the oceans doesn’t change the picture much.
To be sure, photovoltaic technology remains far from ideal. The current average efficiency is only 20 percent, well below the theoretical maximum. Still, even with such devices, the available solar energy exceeds our current use by more than 250 times. Hence, it makes sense to keep expanding the use of solar technology, which has been growing at nearly 5 percent a year for 20 years. We can meet our energy needs with solar farms covering only a small fraction of the planet, particularly if one assumes that efficiency will improve.
The economics of various energy sources is another matter entirely. In dark or windy places, wind power may be the best choice. Elsewhere, local realities may make biofuel production sensible. Overall, though, there's plenty of energy to go around, and solar is by far the best way to get it.
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