The second law of thermodynamics

OK, please don’t switch off right away. Despite the nerdy title, this page will be written especially for the 99% of people who are not engineers. No equations or engineering jargon will be used, I promise.

The reason for dedicating a special page to the second law of thermodynamics is simply because it has profound implications on our energy future and, as stated on another page, the world is totally and utterly dependent on cheap and abundant energy. The average developed world citizen consumes the energy equivalent of 140 slaves working 24/7 (almost 600 manual laborers working 40 hour weeks). If all of us suddenly lose the cheap work done by these 600 diligent laborers (aka fossil fuels), our society will instantly crumble to dust. Our energy future should therefore be at the very top of the list of priorities of each and every world citizen.

The law

So, let’s get straight to it then. The implications of the second law of thermodynamics as it applies to our energy future is this: energy will always try to move from more concentrated forms to less concentrated forms and can only be made to do some useful work during this transition. The more well-known first law of thermodynamics states that energy cannot be created or destroyed, but this law has very little to do with the energy future of our civilization. We can have all the energy in the world, but if this energy is not in a sufficiently concentrated form, it is physically impossible to make it do any meaningful work for us.

This is why fossil fuels are so incredibly fantastic; they contain their energy in a highly concentrated form and, by burning these fuels (freeing this concentrated energy to disperse as heat throughout the biosphere), we can easily make them do a lot of meaningful work for us. However, as already discussed in the aforementioned page, we are now running out of easily accessible fossil fuels and therefore need to expend a lot more effort and a lot more of the fossil fuels we already have to extract further fossil fuels. Naturally, when the energy return on investment (EROI shown below) of extracting fossil fuels becomes too low, the entire exercise becomes futile. And then of course there is also this other little issue called climate change…

The fact is that we are constantly moving closer to the point where new difficult-to-access fossil fuels (especially oil) become too expensive to extract and many people think that climate change might already make our futures very uncomfortable even if we manage to magically cut all CO2 emissions right now. It is therefore pretty obvious that we seriously need to start planning for the decarbonization of our economy. As the global energy consumption figure below shows, however, we sure have a very long way to go in this regard.

Global energy mix

Most people intuitively feel that this transition must be led by renewable energy (the purple line at the bottom of the graph). However, as this article will illustrate, the second law of thermodynamics poses significant challenges to this ideal. The declining red line (nuclear energy) is a much more thermodynamically favoured option (actually much more so than fossil fuels), but faces a great deal of public resistance.

Renewable energy and the second law of thermodynamics

Renewable energy from solar, wind and bio-matter certainly has the fundamental potential to carry our civilization into an age of sustainable prosperity (illustrated by the figure below), but the vast majority of people severely underestimate the challenges that these technologies will face from (among several others) the second law of thermodynamics.


The primary aim of this page is to inform readers about this fundamental limitation and grant some much needed perspective. Optimism about renewables is nice, but misguided beliefs that the market will deliver renewable energy solutions and allow us to continue our exponential economic expansion after the age of fossil fuels unfortunately belong in the same league as Santa Clause and the Tooth Fairy. It is vitally important that the world wakes up to this fundamental fact ASAP.

Ultimately, all of our energy (except for nuclear and geothermal) comes from the sun. You might have heard that the earth receives enough sunlight in one hour to satisfy human energy needs for an entire year, but, thanks to the second law of thermodynamics, this massive amount of energy is totally useless to us unless we manage to massively concentrate it from the diffuse form in which it hits the surface of the earth. And unfortunately for us, energy does not like to be concentrated. Just like many other things in nature, it naturally diffuses away from regions of high concentration and towards regions of low concentration. Our task of concentrating the sunlight hitting our planet into useful forms like electricity or biofuels will therefore be very, very challenging. Let’s take a closer look at the challenges related to the two options we have for solar energy concentration: technology and photosynthesis.

Solar energy concentration through technology

Direct solar power is the best example of the technology option and will form the backbone of this little section. However, it should be acknowledged that wind, hydro and wave power are all caused by the earth’s great weather engine which is powered almost exclusively by solar energy. Conclusions regarding solar power are therefore just as applicable to wind, hydro and wave power.

In general, solar energy can be converted to electricity either directly through the photovoltaic effect (solar panels) or through thermal concentrators using mirrors to concentrate solar energy to a central point in order to drive a steam turbine (solar thermal power plants). In principle, these technologies are very simple, but the second law of thermodynamics quickly shows its ugly face when you consider the scale on which these technologies have to be rolled out in order to have any meaningful effect.

On average, the surface of the earth receives about 180 W/m2 over a 24 hour period, but, because energy does not like to be concentrated, we can only get access to small amounts of this solar energy influx. More specifically; solar panels made from common materials which are sufficiently abundant for mass production typically manage to convert about 10% of the incoming solar radiation to useful electricity and solar thermal is even worse. When considered relative to the solar radiation falling on the total plant area, a large solar thermal power plant produces electricity at an efficiency of less than 3%.

Solar panels therefore can generate about 18 W/m2 of electricity on average while solar thermal is restricted to about 5 W/m2. Actually solar PV farms (which require significant spacing between the panels) also have a power density of about 5 W/m2This implies that you would need about 20 m2 of solar farm to power a single traditional 100 W light bulb 24/7 (if you also have the energy storage capacity to smooth out the intermittent nature of solar power of course).

For some more perspective, consider that many modern fossil fuel power plants produce electricity at a rate of 1000 MW or more. Achieving the same result through solar plants will require roughly 20000 hectares (40000 football pitches) of land covered with solar towers and steam turbines surrounded by millions of mirrors (each with its own automated sun-tracking motor and regular cleaning mechanism) to achieve the same result. 

If we look at rooftop solar, we need about 15 relatively large 200 Wp panels to satisfy the ravenous electricity demand of the average American. Getting the roughly 1 billion rich people on Earth electricity independent will therefore require about 15 billion solar panels. (Note that “electricity independence” is a bad term because it implies that the home is no longer connected to the electric grid. This is impossible at present because it would require very expensive battery storage technology and substantial lifestyle changes from the occupants of the home.)

For perspective, we are currently installing about 150 million 200 Wp solar panels per year (most of which is at utility scale). Even if we eventually manage to install rooftop solar at this rate, we will require an entire century to get the homes of the richest billion people “electricity independent”. And yes, residential electricity demand is only about 30% of the total. And electricity consumption is only about 40% of total energy consumption (on a primary energy basis)…

As a final little bit of perspective, it must be acknowledged that it is highly unlikely that the limping global economy will be able to keep on subsidizing the projected exponential growth in solar power generation. Even China, which is striving for a solar PV monopoly through government subsidies which are so large that the US thinks they are illegal, is now slowing down. If subsidies go away, solar growth will vanish with it simply because solar electricity is projected to be at least twice as expensive as increasingly hard-to-get-at fossil fuels by the end of the decade.

And yes, then it must also be recognized that today’s fossil fuel prices are already too high for achieving sustained economic growth. For example, a previous post discussed how the total credit market debt in the USA has grown faster than GDP for the last several decades, culminating in 2008 when debt grew 2.5 times faster than GDP. Naturally, if your monthly debt repayments grow 2.5 times faster than your monthly income over an extended period of time, you will not last for very long. Indeed, the fossil fuels we need for sustained growth is not the $70/barrel shale oil everyone is so excited about nowadays, but rather the $3/barrel conventional oil still being extracted in the Middle East. These super-cheap conventional fossil fuels are what built our civilization and, in the absence of such cheap energy, the only remaining path to temporary growth is cheap credit (which has predictably transformed virtually the entire developed world into one big debt crisis).

Renewable energy prices IRENA

For some real-world examples of the economic challenges facing renewables one can look to Germany, which now faces growing public resistance to the electricity price increases caused by renewables, and Denmark, where large-scale wind generation is helping to provide the most expensive electricity in the developed world (about 33% more expensive than their closest rival, Belgium, according to Wikipedia). Unlike Germany, however, Denmark still has wide public support for its ambitious renewable energy program (possibly mainly because they make so much money from selling wind energy technology to the rest of the world). I sincerely hope that this public support stays strong within an environment of escalating prices and dwindling economic growth.

Solar energy concentration through photosynthesis

Along with water, photosynthesis is the primary reason for life on earth. Without photosynthesis, living organisms would have no way in which to absorb the energy required for daily life. In short, photosynthesis is the wonderful process by which plants can capture solar energy and use it to convert the carbon in atmospheric CO2 to high-energy carbon fuels that can be consumed as food by other living organisms or burned to release energy for human use. In common language, this option is known as biofuels.

Unfortunately, the prospects of employing photosynthesis as an energy concentration mechanism are even worse than doing this through technology. While solar PV can capture about 10% of solar radiation in a form that can do useful work for humans, plants capture only about 1%. In addition, the plants need to be processed to a form that can be used as liquid fuels or for power generation, consuming large quantities of additional energy and labor in the process. And yes, then there is of course the fact that the most common type of biofuel nowadays – corn-based ethanol – competes directly with the world’s dwindling food supplies.

But hey, what about algae? Yes, it is true; a lot of recent hype has been going on about algae, primarily because of its ability to capture solar energy with an efficiency possibly even as high as 5% and the fact that it does not compete with human food stocks. However, producing algae-based biofuels on a large scale in a manner where it actually produces substantially more energy than it consumes, is cost-competitive with fossil fuels and does all this without having serious environmental impacts will be incredibly difficult. Take a look at this page to get an idea about the challenges facing algae based biofuels.

With these challenges in mind, it really seems highly unlikely that 30 years of operating about 3 square meters of algae farm turns out to be more economical both in terms of money and energy than building a 1 square meter solar panel and simply letting it sit in the sun for 30 years. Yep, like so many other “miracle solutions” to our impending energy crisis, I’m afraid that algae-based biofuels will probably also run into the unbreakable brick wall called reality.


In summary, I think that further economic hardship over the coming years will get society to slowly realize just how amazing fossil fuels really are. Necessity will force the common man to get a lot better acquainted with the second law of thermodynamics and finally understand the blatantly obvious: real-time concentration of diffuse solar energy (be it through solar, wind or biofuels) will never ever be able to compete on a level playing field with burning five years of ancient sunlight every single minute in the form of fossil fuels. The laws of physics simply make this a total mismatch. And yes, it is only when we accept this fundamental truth that we can really start building a sustainable energy future.

But before we can start building our sustainable energy future, the next page will provide a further dose of objective reality by discussing three additional challenges faced by renewable energy (as if the second law of thermodynamics was not enough already).

12 thoughts on “The second law of thermodynamics”

  1. Excellent summary of the issues, Schalk. Did you post this after being inspired by my Entropy – an unauthorised biography (6 September 2012)?

    Does all this mean that Tom Blees’ nuclear – Prescription for the Planet – was right all along? James Hansen was certainly convinced by him but, if he is right, how do we make nuclear power generation low-tech and cheap-enough to prevent the creation of a new form of technological colonialism/dependency in poorer countries?

    1. The inspiration for this page came from reading John Michael Greer’s interesting book “The Wealth of Nature”. I just read your post now. It was certainly interesting although I doubt that we will see economically viable nuclear fusion technology in our lifetime.

      To me, the medium term energy future of our society lies between nuclear combined with electric transportation and fossil fuels with CCS combined with hydrogen (from natural gas with CCS) transportation. Currently I think CCS is the cheaper, more practical, more politically acceptable and safer option. That is why I’m working on it. Only time will tell if this is a wise investment of time though…

      1. I too think nuclear fusion is improbable. Fast Breeder Reactors, however, should probably not have been abandoned in the late 80s/early 90s. Although I wish it were not being used as an excuse for not maximising renewables (George Osborne in the UK) and/or unwarranted optimism (Barrack Obama in the US), I do now agree that we need to make CCS work…

      2. Glad to hear that you now also think CCS has an important role to play in the medium term. It certainly is not the ideal case, but I think it is probably the best we can do under the circumstances. Now all we need are those carbon taxes…

      3. I thought I made clear that I had reluctantly conceded our need of CCS some time ago (but reserve the right to keep calling it – along with the notion of “clean coal” – fossil fuel lobby propaganda).

  2. Hi Schalk. Please see Gillian’s response to my post. This is what I had hoped for – but completely failed to get – when I posted links to your blog on that of others. You have said that you do not want to argue with emotive people who are rude to (or about) you; and no-one can blame you for that. However, to her great credit, Gillian is neither emotive nor rude, so I hope you will feel able to respond

  3. Great its a very helpful blog. You explain the basic concept of second law of thermodynamics. There are many important points that you should explain breifly. I just read your post . It was certainly interesting. Nice one.

  4. Another way of looking at this is. The REAL economy is about energy. Always has been and always will be. The paper economy is nothing more than a quasi representation of that economy. Life is ultimately a competition for scarce energy resources. All known accessible energy sources to man are finite and rapidly running out.

  5. Nuclear or any other sources of energy you care to mention do not solve the problem. Fossil fuel energy is extremely cheap, very potent and very portable. Energy conveyed by electricity is not. Not to mention the entire global food supply is dependent on fertilizers made from finite fossil fuels. In a nutshell there are simply too many people (7 Billion +) for any of the usual “solutions” to work. Which leaves one obvious one…

  6. Thank you for writing this awesome article. I’m
    a long time reader but I’ve never been compelled
    to leave a comment. I subscribed to your blog and shared this on my Facebook.
    Thanks again for a great post!

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