Physical Insights

An independent scientist’s observations on society, technology, energy, science and the environment. “Modern science has been a voyage into the unknown, with a lesson in humility waiting at every stop. Many passengers would rather have stayed home.” – Carl Sagan

Thermodynamics, stars, uranium, life and everything: Part I

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We hear a lot about this phrase “renewable energy” these days. But what exactly is “renewable energy”?

Why are certain energy systems considered “renewable”, whilst others are not? What makes, say, solar power “renewable” energy, but nuclear power not, supposedly, “renewable energy”? These questions bear thinking about.

Now, uranium is technically a finite mineral resource, just like the bauxite used to construct wind turbines is a finite resource and the silica used to construct silicon photovoltaic devices is a finite mineral resource from the Earth.

Five billion or so years from now, the hydrogen within the Sun’s interior will be exhausted, and it will begin to use that in its less dense upper layers. It will expand to eighty times its current diameter, about 7.5 billion years from now, to become a red giant, cooled and dulled as a result of its vastly increased surface area. As the Sun expands, it will swallow up the planet Mercury. However, Earth and Venus can be expected to survive, since the Sun will lose about 28 percent of its mass, and its lower gravity will send them into higher orbits. The Earth will be left scorched, its land surface reduced to the consistency of hot clay by a flux of solar heat a thousand times more powerful than that today, and our atmosphere will be stripped away into space by a now-ferocious solar wind. Not one living cell on this planet will remain alive.

Eventually, the helium produced in hydrogen fusion in the Sun’s outer regions will fall back into the core, increasing the density until it reaches the levels needed to fuse helium into carbon. A “helium flash” will then occur; the Sun will shrink abruptly to slightly larger than its original radius, as its energy source has fallen back to its core. Due to the increase in the reaction rates, due to the increased temperature and pressure at the stellar core, and the smaller amount of helium compared to hydrogen, the complete helium-burning stage will last only 100 million years. Eventually it will have to again resort to its reserves in its outer layers, and will again attain a red giant form. This phase lasts a further 100 million years, after which, over the course of a further 100,000 years, the Sun’s outer layers will fall away, ejecting a vast stream of matter into space and forming a planetary nebula.

Eventually, all that will remain of the Sun is a white dwarf, a hot, dim and extraordinarily dense object; half its original mass but only the size of the Earth. Were it viewed from Earth’s surface, it would be a point of light the size of Venus with 100 times its current apparent luminosity. Eventually, after trillions of years, it will fade and die, finally ceasing to shine altogether.

Why is geothermal energy produced by the \mathrm{\alpha -decay} of uranium in the ground considered as “renewable” when that produced by fissioning those same atoms in a reactor is not?

The answer, of course, is that that’s not the point. The point is that “renewable”, as we hear the term used in society today, doesn’t have any rigorous physical meaning. Loosely, the popular definition of “renewable” means “not fossil fuels and not nuclear energy”, and fossil fuels do not meet the above definition when used at today’s consumption rates (if oil use were cut by a factor of 100,000, it would also be renewable). More correctly, “renewable” energy has come to refer to anything that the Green lobby hasn’t chosen to oppose – anything except fossil fuels or nuclear reactor-derived energy. (Nuclear geothermal energy seems to be OK, though.)

When something does meet the definition that the environmentalist lobby doesn’t like, they amend the mysterious unwritten non-scientific definition to exclude it.

For example, whale oil could produced by farming whales, constituting “renewable biofuel”, in exactly the same way that sugarcane-derived ethanol is in principle a “renewable” fuel. I can’t see the capital-G Green lobby being too keen about the idea, though.

Now, it seems reasonable to argue that, for example, wind energy or solar energy are not in fact consuming any significant finite resources at all during their ongoing operation, the raw materials such as aluminium, silicon or concrete used in the construction of their infrastructure not withstanding.

Opponents of nuclear energy often seem keen to point out that nuclear fuels are what they often describe as “finite, nonrenewable” resources. However, there’s no such thing as a source of energy that we can use without consuming any finite resource, because the energy that we can extract from any isolated system is in itself a finite resource. When the free energy of the universe is expended, the “heat death” of the entire universe is the result. This is the end of all that is, all that was, and all that ever will be, and this is going to happen.

There’s no such thing as “renewable energy”.

The free energy of any isolated system, for any reasonably literal, sensible definition of the word renewable, is not “renewable”.

“Renewable energy” does not exist. That’s the second law of thermodynamics.

Written by Luke Weston

October 12, 2008 at 12:58 pm

One Response

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  1. I think it’s merely a matter of scale. Of course, no source of energy will last for ever; that much is obvious, but those commonly referred to as “renewable” are those that, if we do decide to tap into them, we’ll have no significant effect on the system from which they derive. If we decide to erect wind turbines, we won’t significantly change the amount of wind on the Earth. If we build tidal turbines, we won’t significantly reduce the rotation of the Earth.

    However, If we burn billions of barrels of oil per year or mine uranium, we will have a significant effect because these resources are far more limited. Moreover, oil is not continually “replenished” by natural processes (at least at a useful rate). As long as the sun exists, we can derive energy from it, whereas we cannot get more uranium until another nearby star undergoes a supernova, and so forth.

    I think this kind of argument is mainly semantic (or pedantic) and doesn’t really have any scientific basis. Of course, no source of energy is renewable; resources are always depleted in one way or another. On a Human scale, though, that really isn’t relevant.


    October 12, 2008 at 2:20 pm

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