Thursday, July 22, 2010

Degrees of Perfection, Part 3

Part 3 of a series

Let's talk about exergy (or available energy and useful work).

With, again, apologies that Wikipedia is not a scholarly resource, the definition of exergy from Wikipedia goes like this: "the exergy of a system is the maximum useful work possible during a process." So, a measure of energy is a measure of our ability to achieve the most with what we have - sort of a thermodynamic "buy to fly" ratio!

The paper by Gutowski I referenced at the start of this series on July 2  gives an excellent discussion of the fundamentals of applying this to manufacturing. Gutowski explains that exergy "represents the maximum amount of work that could be extracted from a system as it is reversibly brought to equilibrium with a well-defined environmental reference state." This is usually comprised of physical energy (the portion of the system that can be removed from the system while bringing it's state to a "dead state" at a reference temperature and pressure, and chemical energy. Chemical energy refers to additional available energy potential by bringing the chemical potentials of a compound to equilibrium with its surroundings. Gutowski explains where these reference state data come from. And there are a lot of equations.

Ultimately, you can derive an expression that represents the work rate of a system derived from the explicit terms representing the physical and chemical exergy of the system.

To illustrate exergy flows, an excellent graphical image of the global energy flow, accumulation and destruction starting with sources of energy (solar primarily) to the eventual natural and anthropogenic destruction (that is, due to human activities, as opposed to that occurring in the biophysical environments without human influence) is presented by the Stanford Global Climate and Energy Project. The site also shows the global carbon flow and accumulation. Fascinating stuff.

Now comes applying this to manufacturing systems.

We defined some time ago the characteristics of a "typical" manufacturing system represented by a series of "boxes and arrows" connected serially and representing the individual processes and the connecting material transport between processes. (See the posting of November 12, 2009 for a refresher).

We can replace (or augment) these arrows between boxes (or going into the box as in the process box discussion in the posting referenced above) with the systems mass, energy and entropy interactions. Recall that entropy is a measure of "disorder" in a system and it increases over time. A typical example of entropy increasing is ice melting. This from the work of the person most credited with putting forth the idea of entropy, Rudolf Clausius in 1862. There is a change from solid, molecularly ordered ice, to "disordered" water as the water increases in temperature over time. Temperature is usually a conjugate variable of entropy in thermodynamics.

So, each stage of a process can have material flows or interactions as well as work and heat interactions. And, with each step and its associated interactions, there will be losses. These are the materials wasted (and accounting for the buy-to-fly ratio) as well as energy losses. Gutowski's paper goes into this analysis in great detail.

First we need to identify all of these "losses" so we can determine the system performance. Then, we can look at how the losses can be avoided, reduced, or "recovered" to improve the performance of the system.

That is, we then have another "metric" for manufacturing system design, operation and optimization.

More to come on this next time. But, I have some small items of (potential) interest to conclude with this time.

I don't "tweet" and don't follow those who do … but if I did … I would have been madly tweeting away the 13th of July from San Francisco. I was invited to a very splashy event hosted by General Electric touting the successes of their "ecomagination" initiative and announcing a new $200 million "Power Grid Challenge" to spur innovation and entrepreneurship in the electrical grid. The show included the GE Chairman and CEO Jeff Immelt, assorted venture capitalist who are helping with the program (like Emerald Technology Ventures, Foundation Capital, Kleiner Perkins Caufield & Byer, and RockPort Capital), Dr. Arun Majumdar, head of ARPA-E (DOE's advanced research agency for energy technology), the President of PG&E, our local utility, among others. One of two panels was chaired by the editor in chaired of Wired magazine and they have a short writeup on the funding part.

You can also check up on this at a GE website which gives the details and a link to the "challenge" website. The site includes a "tracker" listing the latest statistics on ideas submitted, comments and votes on ideas. They even have an app for an iPhone so you can track this on the road.

The comments of the panelists, including Mr. Immelt, were very interesting. Much was said about the potential for "low hanging fruit" - for example, the use of monitoring technology so the consumer can see their energy use (sometimes called "smart meters) is claimed to drive an immediate 10% reduction in consumption. If you see how much you are using, you use less of it! This relates to energy dashboards for manufacturing we've discussed.

Immelt's comments about the business aspects of conservation and sustainability were exceptionally noteworthy. There was a lot of discussion about the inevitability of the jump to eco-consciousness and clean energy. No one can tell when it will happen but it will. The Wired article referenced above quotes Immelt as saying, with respect to companies like GE that want to stay ahead of the curve in terms of investing to maintain competitiveness and profitability,  "…it’s going to change in like, 15 minutes one day.”  “I guarantee that’s going to happen.” He followed on commenting that since no one can predict when this will happen - you have to plan for this in your business strategy.

Wow! I felt like he was speaking to me (or maybe that he'd read the blog!)

To top it all off, during a Q&A session the inevitable question came up about all this potential regulation and conservation (specially pricing to encourage reducing consumption) and the impact it will have on business. Immelt stated "you can have a complete industrial base, and it can grow, while reducing green house gas" emission. This has been GE's experience based on information presented as part of their Ecomagination initative. Granted, this is not your small or medium enterprise but a Fortune 100 company (actually a Fortune 6 company!) But that really makes the case for getting on with it!

Finally, I was interviewed on a very interesting radio program the other day. The program, hosted by Colonel Mason, is called "The Promise of Tomorrow" and deals with the business of emerging science and nanotechnology. We spoke about green manufacturing for quite some time. You can listen to the broadcast at his website - it is program #114 broadcast on July 19th (see archives). He also mentions our upcoming book titled Green Manufacturing: Fundamentals and Applications  from Springer due out late this fall. More to come on this of course. It is already listed on Amazon if you want to "pre-order" a copy!


  1. I applaud companies who are trying to go green. It's nice to see and hear people getting interested in green practices and actually actually wanting to learn. I read a report that said schools have even seen an increased interest in their courses. I believe the University of San Francisco's Green Supply Chain Management certification program said they received the most interest. I hope more and more people and companies will see the value in going green not only to save money but to help the environment.

  2. ...that exergy "represents the maximum amount of work that could be extracted from a system as it is irreversibly brought equilibrium with a well-defined environmental reference state."...

    Prof. Dornfeld, I would like to point-out the typo "irreversibly", I think it should be "reversibly". It's fundamental definition so I think it's a significant typo.

    Looking forward for your next discussion on exergy issue, since I'm currently working on this topic :)