Talk:Exergy
From Patterns
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Apparent Contradiction in 'Efficient Vortex Motion'
I have had an ongoing discussion about vortex motion with a colleague who has a physics background. Water draining from a tub almost always forms a vortex, even at the equator, where the rotational direction is random. He argues that rotational energy is required to form and maintain the vortex, and that this energy must reduce the efficiency of the overall water flow. PAX Scientific argues the opposite, that vortex motion can increase flow efficiency, and has a number of products to back up the claims. Certainly, there are numerous examples in both the physical and biological worlds of vortex motion, but the underlying mechanism is not clear. It may be that "real world systems" behave in complex ways due to multiple interacting forces and processes. Straight-line flow may appear to be more efficient in isolation, but in a larger context, additional energy may be required to 'straighten' the flow, compared to the energy required to form the vortex.
Life as a Manifestation of the Second Law of Thermodynamics refers to an experiment using two bottles connected by a tube, such that water draining out of the top bottle passes through a orifice that restricts the flow. Normally, the top bottle drains in a straight flow in 360 seconds. However, imparting a slight rotation to the setup causes a vortex to form, draining the water in 11 seconds. Unfortunately, the setup is somewhat complicated by the need for air to flow from the lower to the upper bottle. This led to the following exchange which I also sent the January 12th thoughts to PAX Scientific for comment.
--Norbert 14:13, 12 January 2008 (EST) I may have stumbled on the answer, in a rather unlikely area. I have been working on a project to document and communicate 'patterns from nature', as a way of encouraging sustainable design. One path led to self-organising systems, non-equilibrium thermodynamics and energy quality (or exergy). I do not profess to understand this stuff in any depth. However, in a nutshell:
- in open systems, high quality energy flows set up energy gradients
- systems will attempt to reach energy equilibrium (2nd law of thermodynamics) by degrading the energy quality
- as energy flows reach critical values, coherent processes and structure emerge spontaneously, increasing energy dissipation (entropy is reduced locally, but increases globally)
- further increases in energy flows can result in more complex processes and structures,
- there is an upper limit to energy flows, at which point the system becomes chaotic
The classic example is the formation of Benard cells when a temperature gradient is imposed in a thin fluid film. Energy dissipation is much higher through Benard cells than conduction and increases in a non-linear fashion as the temperature gradient is increased. Temperature measurements across the film show that the temperature is constant outside the boundary layer near the plates that are imposing the temperature gradient. As the temperature is increased, the number of Benard cells increases, the boundary layers decrease in thickness, until turbulent flows form (maximises dissipation but is less interesting from a structural point of view).
The author (James Kay) implies that the same behaviour occurs when water in a bathtub rushes down a drain. At a critical water depth (energy gradient), a vortex will form because the vortex can dissipate the energy gradient more efficiently than simpler processes (friction in the drain pipe?). See page 12 of Life as a Manifestation of the Second Law of Thermodynamics for more details. In another article, Kay states that as the energy gradient increases, laminar flow results. He does not example how/why/whether laminar flow is even better at dissipating energy than vortex flows.
The key message is that many of the rules that we use are based on studies of systems at equilibrium, where the 1st law of thermodynamics and energy efficiency are most important. Kay and others believe that the 2nd law of thermodynamics is more important in explaining how non-equilibrium systems behave - this is the realm of energy quality/effectiveness/exergy. Kay argues persuasively that high quality energy dissipation creates order across a wide range of system types and scales: physical, chemical, biological and ecological. At this point, my brain goes really fuzzy. I can follow the argument but cannot fully see the implications.
--Response 09:28, 15 January 2008 (EST) I still don't buy it. :-) I think of an example of a piece of paper falling through air. The perfect way to fall is straight down. However, the paper edge almost is always tilted one way or another and it quickly turns horizontal and floats down from side to side. Once the paper turns, it hits a semi-stable mechanism that is hard to break out of. The falling (energy gradient) provides energy to swing back and forth and yet when the paper turns too much, it starts to rise up (the dampener). Eventually, it will stop rising up and start swinging back the other way, falls a bit more through the air as it does. There is an energy gradient and the system usually chooses the least efficient means because the least efficient means is a "stable mechanism". The most efficient means is an unstable mechanism.
With water flowing down a drain, I assume a funnel forms because the water is, in general (and despite a lot of other random movement), turning one way or another ever so slightly. As the water is forced down a small drain,
rotational momentum is conserved and yet the area is reduced... the "turning" speed is increased. It is the same principle as a figure skater who pulls their arms in to increase their turning speed.
Once the rapid turning is in place, the energy gradient provides energy to turn more and more water. In essence, the system sets up a stable mechanism where the water "falls" and this falling gives off energy to turn more
water. The wider body of water acts as a dampener that ensures the system doesn't spin out of control and the water doesn't end up spraying out of the bath and onto the walls of the bathroom. Energy to spin and a dampener
creates a stable mechanism that is the vortex. Falling straight down the drain, the most efficient mean... is unstable and therefore is not typically chosen.
Now if you look at the path that water takes in the vortex, it is much longer than a straight line. The water spins around and around several times before it reaches the drain. The shortest route is a straight line
and the fastest way to fall is to go straight down. Also, if you look at the actual amount of water shooting down the drain, the area of water flow is less (often there is a hole down the middle of the vortex). This
wouldn't be the case in a closed system but the principle is the same. Water must travel longer to reach the other side when in a vortex.
Seems very inefficient like the paper floating down instead of falling straight down. Boils down to a stable vs unstable mechanism for an energy gradient.
--Norbert 18:04, 15 January 2008 (EST) I will admit I have problems with these concepts. Part of the problem is that experts in non-equilibrium thermodynamics tend to focus on formulae and equations, but not the wider implications. People like James Kay are exploring the implications, but sometimes are not explaining the basics. Even the concept of efficiency is becoming fuzzy - somehow, I think rates are as important as quantities. I get hung up on 'moving energy from A to B', but suspect that the real objective is to dissipate the energy by whatever means are available. Kay also talks about degrading energy - destroying the ability of an energy influx to create gradients. One of the problems is that more organised energy dissipation happens whether we like it or not. I suspect Kay's Benard cell analysis of 'Conduction only' heat dissipation/entropy production/exergy degradation is a derived value. The 'Tornado Tube' example (Edmund Scientific still sells them) is interesting insofar as the gradient is insufficient for the vortex to form naturally, probably because of friction on the sides of the bottles. It would be worthwhile to do a full energy systems analysis to see what effect the rotational energy imparted on the bottles should have in reducing the drain time, which Kay claims is 360 seconds under straight-flow conditions, and 11 seconds under vortex conditions.
I am not sure how to analyse your falling paper example - I suspect this is a very complex situation. There is a potential energy gradient, but most of the examples I have seen involve a flow of time over an extended period of time. A similar example might involve a pair of low and high pressure areas, continuously maintained through some sort of energy influx. In addition, there needs to be some sort of resistance to the free energy flow, otherwise the energy gradient would disappear. Random motion of atoms and molecules is inherently inefficient. What we see instead is more directed or organised motion of air, namely wind. Off the top of my head, I am not aware of any mechanism that lines up air molecules and shoots them from the high to the low pressure area, like bullets out of a gun. I know the equilibrium end result of maximum entropy is more probable than any lower entropy solution, but that describes an end state, without specifying the mechanism for getting there. It would be interesting to repeat Kay's Benard cell analysis on air movement, to see if at low energy gradients random atomic/molecular motion is sufficient to explain the pressure dissipation, then scale that up to see if it is sufficient to explain the energy dissipation provided by wind. Hurricanes are usually explained as a more efficient way of moving vast amounts of energy between different parts of the earth's atmospheric system. These are more organised than wind, but also bring into play other forms of energy dissipation through evaporation and condensation.
Getting back to your piece of paper, an analogy might be a flag in wind. In this case, if the flag were to align itself perfectly with the wind, then there would be no pressure gradient across the flag. Effectively, the flag is not interacting with the wind and therefore has no ability to dissipate energy or degrade the larger pressure gradient. I get lost in the different gradients at this point - is the fluttering of the flag helping to dissipate energy in the wind by converting some of the wind energy to sound and heat? Or is it dissipating the pressure gradient across individual parts of the flag? Or both? Is the presence of the flag measurably reducing the pressure gradient? Or is the complex motion of the flag a reflection of eddies in the wind itself, much as you see in rivers as they start to speed up?
Efficient behavior may be unstable in the larger environment, which reduces the overall system effectiveness. Kay argues that there is more to it, that organised behaviour like convection cells can be shown to convert more of the energy gradient into work. He calls this exergy and relates it to the quality of energy. Some articles talk about efficiency as a measure of energy quantity, and relate it to the production of motion. Part of the problem is that energy efficiency/quantity is a good way to characterise systems at equilibrium. However, different rules apply to systems that are far from equilibrium - here energy effectiveness/quality seem to be more relevant. Whereas equilibrium systems have been well understood for hundreds of years, non-equilibrium systems are only now beginning to be studied in any depth.
I just remembered another example. Conduction of heat through a metal is relatively ineffective - even a thick copper bar will maintain a considerable temperature gradient. A heat pipe has far more capacity to transfer heat, through a combination of convection and evaporation/condensation. The heat pipe provides structure and a more organised process than conduction. I was going to say that I did not know of any naturally occurring analogy, when the hydrological cycle came to mind. The warm/moist earth surface evaporates water and leads to a convection current forming that raises the water molecules to higher altitudes where condensation occurs, leading to the downward cycle. I am sure this is much more efficient than non-convective heat conduction or equalisation of water molecule densities.
--Fil Salustri 14:56, 21 January 2008 (EST): Though I haven't had time to look into it, papers like "[Experiments on vortex funnel formation during drainage]" by M. Piva, M. Iglesias, P. Bissio and A. Calvo. The abstract reads: "We studied the vortex formation during drainage of water from a cylindrical vessel. The rate at which the vortex grows into a full funnel was found to depend on the initial velocity field characterized by Vθi, the maximum tangential velocity on the free surface and the nozzle location relative to the cylinder axis. We present a simple model to explain the observed results. This work provides important information especially for industrial process, for instance in steel continuous casting process."
I'm not saying that this negates the exergy arguments; I'm saying that the truth will emerge only if we engage appropriate experts - one of which I am certainly not. Indeed, it may be that the exergy-based explanation makes more sense in light of understanding the physics better.
--Norbert 16:28, 22 January 2008 (EST) Fil, interesting. Kay argues that the 2nd law is necessary but not sufficient for complex behaviour to occur. This article seems to address the other enabling conditions that need to be met. The abstract is not sufficiently exciting to get me to shell out U$30 for the full paper.
Implications of Energy Quality/Exergy
--John 12:33, 18 January 2008 (EST) Well Norbert,
It seems you have wandered into the always fascinating, often frustrating and sometimes fruitful intersection between thermodynamics and sustainability. You also seem to have encountered a problem I like to refer to as the dead worlds problem. The basic principles of physics, as useful and foundational as they are, do not preclude dead worlds. In fact, the body of evidence thus far gathered by astronomy suggests that they favor them. Therefore, an unguided search through the space of physical possibility more often than not leads to unsustainable ends. When taking an initial guess at a design for society that is both physically possible and sustainable, it is my contention that we need to start in the vicinity of a sustainable space, which led me back to the biosphere...
John
--Norbert 19:13, 20 January 2008 (EST) James Kay does point out that the imperative of the 2nd law of thermodynamics is necessary but not sufficient for the emergence of structure. Clearly, there are other conditions that must be met. It may be fruitful to look at what those enabling conditions might be. Is one measure of sustainability the degree to which these conditions are created, rather than being inhibited? Biodiversity may be an example.
I will admit that I hoped exergy would provide a more concrete 'grounding' for sustainability. It seems at best to encourage self-organized/hierarchical/open systems, but as John points out, does not demand that they be enduring. It begs the question: are there mechanisms that encourage sustainability, or is it a combination of geological time and the resilience of life that appears to bounce back even from massive extinction events? Do profligate spenders eventually run out of rope and simply disappear, leaving damage but nothing irreparable? A bit too philosophical for my liking...
Martha As I felt myself stepping toward the peatbog of the Exergy concept, I thought: there a two prongs here. One is what John has pointed out (and I acknowledged in my initial mention of non-equilibrium thermodynamics), i.e. that something else is essential for life to get going since there a lot of planets with great energy gradients to be dissipated but that don't have life. So, I too, found myself thinking, why not focus on some of the other essentials in the Conducive to Life pattern? So I should research Origins of Life more and RNA World, etc. And the other prong is...well, I'm interested in this exergy thing and combining it with other things I've read. It may not lead to anything fruitful for the Pattern Languages, but I've been putting one foot after the other into the peatbog... One reason I'm interested in it is that my knowledge of succession is contributing to my understanding. Also, the aspect of time and the sense that we humans are just a cog in some kind of machine (encompassing system) that will be selecting for best exergy. Acutally, that's depressing me.
--Norbert 11:23, 23 January 2008 (EST) Martha, I just received Into the Cool: Energy Flow, Thermodynamics, and Life and browsed the Preface. Schneider/Sagan argue that life is more than a genetic phenomenon and shares characteristics with other complex open systems driven by thermodynamics. It suggests a blurring of the line between inanimate and animate, just as our team early on decided that the really interesting patterns ought to span the nature and human worlds: "... we must now give up that remaining citadel, our faith that our intelligence and purpose are above the rest of nature." I find the notion exciting, not 'humans as cogs', but actors in a huge play or musicians in a vast orchestra. Schneider/Sagan even talk of "energy and wealth, energy and life, energy and exuberance."
At the risk of veering into philosophy, it implies that the problem is not in what we do, but how we do it. "In using up energy, in performing work and making themselves, they perform a natural function: the production of entropy mandated by thermodynamic's second law". In a sense, the 2nd law of thermodynamics suggests a scientific basis for the moral argument that we 'ought to fit in', and implies that understanding the 'rules of the game' will provide us the means to continue participating in the evolving history of the earth. The challenge is to turn the words into something practical that designers (of all stripes) can get their teeth into. How do we dramatically reduce our energy and other resource consumption, possibly along the lines of Factor 10 to make the transition to a renewables-based society possible and attractive?
John, the line "We are therefore ... far less concerned ... with simulations of biological reality that with biological reality itself." echoes your comments about the biosphere.
What Creates 'Wind'?
--Norbert 18:56, 20 January 2008 (EST) On the surface, wind sounds like a pretty simple phenomenon. You have low pressure areas and high pressure areas. Wind is nothing more than air molecules moving from a high pressure area to a low pressure area. In fact, there has to be some sort of a return loop or convection current. Kay argues that convection is an example of organized behavior and structure ([Exergy#Life_as_a_Manifestation_of_the_Second_Law_of_Thermodynamics_.281992.2F1994.29|Life 1992/1994, Benard cells).
From my foggy recollection of physics, pressure is related to the concentration and motion of air molecules. If a chamber with high pressure is connected to a chamber with low pressure, random movement of molecules will over a period of time (short to us) will equalize pressures, since statistically there are far more arrangements of are molecules even distributed between the two chambers than unequal distributions. The final state is in a sense more disordered or random than the starting state, where more air molecules are in one chamber. In other words, the pressure gradient has vanished and the entropy is increased.
Wind, on the other hand, is the coherent movement of many air molecules across considerable distances and often for long periods of time. The air molecules are not being shot from a gun or pulled by strings - I am not aware of any directed force from either the high or low pressure areas that affects air molecules at a distance. Kay argues that the probability of such coherent motion (at least in the Benard cell example) is extremely unlikely, and therefore an example of reduced entropy.
The key difference is an ongoing source of energy flow which maintains the pressure differential between the high and low pressure areas. Random motion of air molecules would tend to reduce the pressure gradient, but not very efficiently. Wind could be a 'first order' structure which speeds the degradation of the gradient. It should be possible to calculate the rate at which random motion can dissipate energy, and compare it to actual weather systems. Hurricanes and typhoons could be 'second order' structures that form when pressure gradients increase beyond what wind can dissipate. These structures are much more complex, enabling several energy dissipation pathways and releasing large amounts of energy over vast areas.
