Entropy and the Meaning of “Renewable”

Fighting Entropy takes Energy (photo: Printemps, flickr)

Fighting Entropy takes Energy (photo: Printemps, flickr)

Can we give hard currency to the terms ‘renewable’ and ‘sustainable’?

An excellent book by Guy Deutscher, a physicist from Tel Aviv University, is trying to do just that, from first principles. What follows is in no way a summary of the book “The Entropy Crisis”, but the ideas in this post are, to say the least, inspired by the book, so full credit to Deutscher here.

But while Deutscher focuses on the role of entropy versus energy, I will focus on the role of life itself in this delicate equation. There have been many attempts to answer the question “what is life?”  Instead of that, we will start with the converse: What happens in the absence of life?

(1) In the absence of life, things tend in a direction where they attain lower states of energy, and in doing so they release entropy.

That’s just a physicist’s way to balance everything when we know energy must be conserved. If one thing tends to a lower state of energy, it has to release some energy into the environment. If that energy is ‘lost’, i.e. it doesn’t do anything useful or interesting, we call it entropy. Examples might help: If protons and electrons combine to form an atom, they release one (or more) photon(s), to keep the energy balance. The photon flies out into space at the speed of light. The bound atom-state is of a lower energy than the previous state in which the particles were separate and ‘free’, and the emitted photon carries the energy difference away with it into the environment. We’d have to find another photon and shoot it straight at the atom to undo the combination and split the atom again into the constituent particles. Another example, when galaxies and stars form, the loss of (gravitational) potential energy is compensated by a production of kinetic energy (things start to spin faster, collide,and cause heat). This is great, because it helps getting stars’ nuclear reactions started, so we get useful things like the sun, but ultimately all that heat gets radiated out into space from where it will never be recovered. Without energy input from the outside, eventually we will have a stable closed system where nothing interesting will happen. It is an irreversible processes that bring the system into a lower state of energy, releasing the energy difference in the process, and thereby increase the entropy in the total. Eventually nothing interesting will happen, when all the ‘fuel’ (the potential, chemical, and nuclear energy in the galaxy) is spent.

For the last decades of the 19th century, every physicist and engineer was able to use the concept of entropy without ‘knowing’ precisely what it is, so I think we can get away with doing the same here:

(2) We can simply say entropy is a book-keeping entry for reversibility. If entropy increases in a process, it takes outside energy to bring the entropy back down again.

If entropy increases (which, in a system left to itself, it always will), things happen that will not undo themselves. This is more useful and correct than the usual ‘explanation’ that entropy somehow represents ‘disorder’.

Burning fuel is one such entropy-increasing process: Atoms or molecules recombine to other molecules which put the constituent particles into a lower-energy state (a more strongly bound state), and the excess energy gets released into the environment. You can’t reverse this process without putting that energy back ‘in the box’ where it came from, and if your reverse process isn’t 100% efficient, you’ll actually have to put more energy back than you ‘won’ when you burned things.

So how can we ever ‘clean up’ after burning something? If it’s a law of nature that the only way of doing this is to put in energy from the outside, we also have to recognize that the only meaningful amount of energy that comes from outside our planet is from the sun. No process that derives the energy from an Earthly source can reduce the net entropy on the planet. And since solar energy is limited, there is a limit on how much ‘burning’ we can do before things become irreversible down here. Worse, only a tiny fraction of the energy from the sun can be ‘used’ and ‘converted’ to reduce the ‘planetary’ entropy. Plants and photosynthesis are not the most efficient entropy-hoovers, using solar energy for a process that ultimately ‘un-burns’ stuff, but they are by far the most prevalent. We could conceivable construct more efficient ‘un-burners’ with the help of solar panels, except no-one would seriously suggest this could be done on a scale to compete with the overall entropy-reduction rate of the Earth’s plant life.

All the other life-forms in the food-chain, those who do not use sun-light directly as a source of energy are net entropy producers, i.e. they ingest ‘fuel’ to harvest some useful chemical free energy, and produce useless energy (entropy) in the process, in the form of CO2 from breathing, and bodily waste. Because none of these organism is even remotely close to being 100% efficient in a thermodynamic sense, energy is tied up in the entropy production that cannot be undone unless plants (or other things that use sun-light directly or indirectly) get to work specifically on entropy reduction. By the way, most of the ‘useful’ energy that organisms harvest from ‘fuel’ is needed just to combat the entropy production that comes with being highly organised organisms, the entropy of natural decay processes. Literally, the calories you need to just stop your body from disintegrating outnumber by a wide margin the calories you need to move around and do more interesting things than just staying alive.

So what about wind power. In what way is it ‘renewable’? It is not because wind and weather are ultimately powered by the sun (although they are), but because in ‘farming’ wind one is tapping into a process that is happening anyway. The energy released by the wind will produce the same amount of entropy (irrecoverable energy) if you first convert it to electricity and then use it and cause heat-loss later, or if you just leave it alone. So it is ‘renewable’ because it doesn’t shift the entropy production rate of wind from what it would have been otherwise. In that sense ‘wind farming’ is a very appropriate term: We are only tapping into an entropy-producing process that would happen anyway. Geo-thermal energy falls into that same category of what we might call ‘second-order’ renewable. It doesn’t lower the entropy in the biosphere, but it also doesn’t disturb it negatively. Even wood-burning is renewable in that sense (as attested by no lesser source than the Carbon Trust), because the wood would decay and release its CO2 into the atmosphere anyway eventually.

Because hardly any living being harvests wind, geo-thermal, or other renewable resources other than the sun, the food (and therefore energy) chain of living things has a more than critical dependence on the first component, the plants: They are the only ones that restore order and keep the cycle going after everyone else in the food chain has caused a ‘mess’, burning fuel. They pump the necessary outside energy into the food/energy chain to produce fuel for the food chain; They ‘un-burn’ stuff that the other living things can burn again. But that ‘fuel’ is stuff that would rot anyway, so in a second-order sense tapping into the food-chain is as ‘renewable’ as wind-farming is: We are all just harvesting a decay process that would take place anyway, drawing energy from it and only releasing entropy which would have been released without us too. Then the plants get to work on the ‘waste products’ (CO2 and bodily waste) again, to make it a cycle we can live off.

If that’s not enough to convince you of the super-critical nature of plants and microbial or sub-microbial life, consider the fact that for some three billion years (i.e. most of the life of the planet) the atmosphere (and therefore land) was inhospitable to life, and it was the under-sea life during those three billion yeas which created the conditions for life on land, by detoxifying the atmosphere and filling it with some oxygen. What we call the biosphere today was created by life itself, and wouldn’t be there without it.

So what has changed to make things potentially unsustainable? About 2000 yeas ago, humans started farming wind. This reduced the need for human and animal ‘power’, but it didn’t actually impact net entropy ‘production’: Both wind farmed energy and muscle energy are tapping into entropy-producing processes that would have happened anyway. Not having the humans or animals do so much work was progress, but it wasn’t fundamentally more or less sustainable. In the industrial revolution, humans started to burn stuff like coal, which would not simply have disintegrated on its own.  This then increased entropy at a faster rate than would have happened without humans, and plants had to work harder to make up for it. Then we started burning oil, and we are doing that at a faster and faster rate. According to the latest IPCC report, plants must have kept up with all this well into the 1970s: The impact of human activity on climate in their new models is not measurably large until the 1970s. Before then, the biosphere somehow seems to have kept up and hoovered up again. The planet’s 1 degree increase in average temperatures actually happened over the last 40 years or so. That’s an alarming rate.

Looking at it from the viewpoint of entropy, the true scale of irreversibility is apparent on a systems level: If petrol engines have 20% efficiency, it will take 5 times as much energy to undo the environmental (entropy) damage than the amount of useful energy they produced in the first place. And we have to adjust that figure for the efficiency of the entropy-reducing process too. Plants need five times the ‘solar power’ to break up that CO2, and plants have really low efficiency in doing that. In practice, the amount of solar energy they need to absorb is 200-500x larger than the useful energy we got out of burning the carbon-based fuel. Planting a tree is no way to offset your emissions. Calculating net entropy production in a process chain allows us to make meaningful relative comparisons of (un)sustainability.

Again, looking at it from the viewpoint of net entropy production, some bio-fuels will rank worse than petrol, but I will not attempt a simplistic analysis here. Too many factors go into it, for example when biological fertilizers that would otherwise not be produced can be formed as off-shoot products, thereby increasing CO2 absorption capacity of the biosphere. But where bio-fuel production competes for land-space with food-production, we could have a major problem for entropy balance (never mind food prices) if the food production also competes with spaces like forests that would absorb much more CO2. Biofuel’s net CO2 balance is complex to analyse to say the least.

What entropy balancing calculations tell us is that what really has changed since the industrial revolution is that mankind has already unwittingly been in a process of geo-engineering by modifying the entropy of the biosphere. We have run a lot of processes that are irreversible without outside energy input. When we ‘consume’ non-renewable energy, we create entropy which will take more energy than we won just to clean up again.

This puts a real dampener on geo-engineering solutions to global warming. Any such concept must be given a ‘moral’ code that it must not increase the entropy of the biosphere. In plain English, it has to be reversible. The reason is simple: Imagine we could burn some stuff (perhaps sulfur) that would somehow reduce the greenhouse effect. If you consider this a magic bullet, you have to offset the burning of fossil fuels with the burning of yet another thing. It’s combating one irreversible thing by adding yet another. So if this process hits the end of the road somewhere for any unforeseen but undesirable reason, you can’t go back! It would take almost inconceivable arrogance on the part of our species to assume that we are so smart that it’s ok to engineer our biosphere in an irreversible way.

The problem with making people think in this way is this: In our everyday lives we never have to worry about (sh)it: Because our households, our workplaces, and our roads aren’t closed systems, we don’t notice the entropy we put into the world when we flush the toilet, burn fuel, leave the printer on, or dispose of our rubbish. We think of it as ‘keeping the house clean’, not as making the world dirty. Our thinking would change dramatically if our household became a closed system tomorrow, with nothing being allowed in our out. Yet our biosphere is exactly that.

One response to “Entropy and the Meaning of “Renewable”

  1. I think the discussion about sustainability is an especially tricky one because you also have to consider the speed of things changing not just the change itself.
    Let me explain: The second law states that entropy in a closed system stays the same (which never happens in an interesting system) or increases. So even if mankind did not start the industrial revolution, entropy would increase anyway. And if you look at your Earth on a long enough timescale, say several billion years, life will dwindle for sure because we run out of solar energy. (Or actually die as a result of an outburst of too much energy from the aging sun, but that’s a different story.) No matter what, Earth will get an forbidding place sooner or later.
    The problem with using non-renewable resources is the “sooner-option”, the speedup of the intrinsic decay. The problem is not the increase of entropy itself, as Karl explained above, the trouble comes from the increase of the increase.
    Whatever we do, we will at least slightly “pollute” our environment with additional entropy. Wether this will be compensated by nature or shorten the “usefull lifetime” of our planet is one of the most intractable questions of our time.


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