September 28, 2015


Need Energy? Look to the Sun (Mark Buchanan, 9/28/15, Bloomberg View)

[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.

Posted by at September 28, 2015 9:11 PM