Category Archives: Physics

Whence Coal, Oil, and Natural Gas?

A primer on the origin, extraction, and emissions of everyone’s favorite fossil fuels

Three major forms of fossil energy together provide almost all the world’s energy: coal, oil (and the derived petroleum fuels), and natural gas. Moreover, humanity’s insatiable appetite for these fuels is the major driver of climate change. Therefore, it is essential to understand the fundamentals of these fuels: Where do they come from, what are their properties, and how do we extract them. Furthermore, it is generally understood that natural gas is the “cleanest” of the fossil fuels, while coal is the dirtiest. Again, we must ask why, and provide at least a basic answer. The purpose of this article is to answer (at least broadly) these questions, and to make the reader familiar with some key terms and concepts. In brief, the takeaways are:

  • All fossil fuels derive from organic matter buried at great depths and processed under pressure.
  • Oil and gas are produced by the same basic geologic processes, and are often present in the same wells/fields. Conventional oil/gas is formed when these hydrocarbons seep from low permeability source shales to collect in “traps,” where they can be pumped by conventional methods.
  • Unconventional oil/gas extraction via “fracking” involves drilling into and breaking up source shales directly.
  • Coal in the US is most commonly mined via surface mines, especially mountain top removal with valley fill, where whole mountains are blasted away to access coal seams.
  • Fossil fuels are mainly a mix of molecules with carbon (C) and hydrogen (H) bonds. Burning oxidizes these molecules to water and carbon dioxide, releasing heat along the way. The more C-H bonds, vs. C-C bonds, the more energy is released per atom of C, and thus the less CO₂ per heat energy released.
  • Natural gas, as CH₄, has the most C-H bonds of any fossil fuel (it is the most “reduced”), and burns the cleanest. Coal is the least reduced fossil fuel, and burns dirtiest.

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Why Solar Panels Can’t Get Much More Efficient (And That’s Okay!): Shockley-Queisser and the limits to converting sunlight into electricity

Commercially available solar panels now routinely convert 20% of the energy contained in sunlight into electricity, a truly remarkable feat of science and engineering, considering that it is theoretically impossible for silicon-based solar cells to be more than 32% efficient. This upper bound, known as the Shockley-Queisser Limit, was first calculated by the eponymous scientists (who actually gave 30% as their original limit) in the Journal of Applied Physics in 1961 [1] (see also updates by Rühle [2]).

Now, if we can answer why solar panels are thus limited, we can understand the essentials of photovoltaics (PV), which have their basis in the photoelectric effect, and p-n semiconductor junctions. While many have never heard of it, the photoelectric effect is of monumental importance, and when Albert Einstein received the 1921 Nobel Prize in physics, it was “for his services to Theoretical Physics, and especially for his discovery of the law of the photoelectric effect,” while p-n junctions lie at the foundations of modern electronics, including transistors and LEDs. Indeed, a solar cell is essentially an LED in reverse: Instead of an electric current generating light, light generates electric current!

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