Sunday, August 8, 2010

Degrees of Perfection, Part 4

Last of a 4 part series

We've been talking about exergy (or available energy and useful work) as part of this series. Last posting I reviewed the work of Professor Tim Gutowski of MIT on energy fundamentals in manufacturing. We'll continue along this line for this last in the series with an example from Professor Gutowski's work.

This series has generated some good comments and feedback. One pointed out a mistake in the previous posting (already corrected!) when I used the word "irreversibly" in place of reversibly - the correct word. This was in the quote from Gutowski's paper stating that exergy "represents the maximum amount of work that could be extracted from a system as it is reversibly brought equilibrium …" That is an important catch … sort of like using "nonpotable" for "potable." Thanks to that careful reader. More on some of the other comments below.

Now, on to an example.

Last time we spoke 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. We stated that we can replace (or augment) these arrows between boxes (or going into the box) with the systems mass, energy and entropy interactions. That means that each stage of a process can will have material flows or interactions as well as work and heat interactions. And there will be losses.

An earlier paper by Professor Gutowski used electrical energy in manufacturing from an energy perspective. The paper is titled "Electrical Energy Requirements for Manufacturing Processes and it was published in the Proceedings of the 13th CIRP Life Cycle Engineering Conference in 2006. You can find this publication on the web - it is number 23 under environmental publications.

In the last posting we talked about using exergy as a metric of performance. Gutowski tackles that in this paper.

Gutowski explains as a setup to the analysis that energy measures the potential of all materials to do work. "Fuels naturally have high values of exergy, but many other working materials, including pure metals, plastics and other organics, can have since we can then express these material and energy inputs and outputs in the same unit, usually joules (J).

He goes on. "Secondly, since the development of the concept of exergy is based upon the second law of thermodynamics, and not the first, it is not conserved. Hence this metric provides a measure of what is actually “used up” in the manufacturing process. As a result, a complex energy and material flow problem can be substantially simplified by using exergy analysis."

The process is broken up into two steps:
- 1) identify the system boundaries (that is the limits of the "box" we are analyzing, and
- 2) identify the exergy inputs and outputs.

Then, the difference between the inputs and the outputs is the exergy lost.

The paper explains that this "difference" can be used to "account for material transformations, including the conversion of raw working materials into products, wastes, and emissions, and the conversion of fuels (through combustion) into heat (to do work), wastes, and emissions." One can also extend the concept to incorporate all other energy sources, for example hydro, solar, electrochemical, and others.

Typically, we would consider the conversion of fuel (such as oil, coal or natural gas) to generate electricity which is then used in the manufacturing process for material conversion by, say, machining, grinding, welding, forming, forging, etc. As pointed out in an earlier posting on the variations of impacts depending on differing fuels for energy in different parts of the world, the exact fuel to energy relationship will vary.

The paper reminds us that to be fully consistent, we should take in to consideration the energy used to produce the materials we are "transforming" and, as this blog has argued, include the manufacture of the machinery to do the transforming as well.

The figure below, from Gutowski's paper, illustrates the energy and material inputs and outputs for a manufacturing process. This is a streamlined version of the input-output example discussed on the November 12th posting discussion whether or not lean is green and you can refer to that for additional background on "what's in the box." The example

followed in the paper deals with an automobile production machining line. As we've discussed in earlier postings, the machines used in these manufacturing lines have a number of elements and components that operate in parallel with the actual processing operation. For example, in the paper we are discussing, Gutowski mentions work handling, chip removal and treatment (removing oil) for recycling, tool changes, machine axis and spindle lubrication and temperature control, etc. in addition to actually machining the part. The figure representing this data for a typical automotive manufacturing machining line is below, from Gutowski, and shows the energy use breakdown as a function of vehicles produced.

So, as with our "tare heavy" and production "process heavy" discussion some postings ago) - this is an excellent example of tare heavy manufacturing. Here, a maximum of about 15% of the energy actually goes into machining the part. Granted, this is for a production line so there will be some expenditures of energy that might not be seen with a standalone machine tool. But, this is not very good.

One of the observations of the paper is that there is a variation with production rate. In fact, for standalone machine tools which may actually reach 60% or 70% energy usage for machining (and, thus, 40% or 30% for "all other") this maximum utilization varies with production rate as well. The takeaway is that, in production, there is a significant energy consumption for getting the machine ready for production and maintaining the machine (or line) readiness in the face of fluctuating production.

A more important observation from my perspective is that trying to estimate the energy consumption of manufacturing processes by looking only at the physical process (and the physics behind it - like metal cutting and the energy to form a chip, for example) will tell you almost nothing about the total energy consumption.

So what does this say about our "buy to fly ratio" analysis? To me, this is still a good way to characterize the efficiency of the process. In the example above (and under the assumptions detailed in the paper - the "academic fine print"!) we are utilizing at most 15% of the available energy coming into the process. That is, the transformation part of the manufacturing process is overwhelmed by the peripheral activities and requirements of the machine.

This is precisely what we were speaking about in our "low hanging fruit" discussion referenced above and what is motivating a lot of current development work by builders of and users of manufacturing machinery.

More on this to come.

Finally, one of the more prolific commenters to the blog talked about standardizing "by volume the process by which inputs, energy included, are transformed into outputs." A visual thinker! She goes on to say that with this approach "the perfect shape would be a cylinder, where all the outputs are useful, for nature, for humans or for both. The current processes are truncated cones with different "buy-to-fly" ratios symbolized by the ratio between the two bases. The cylinder's ratio is the perfect 1, no volume is lost."

The thought that came to mind when I read this was Rick Steves packing for a long trip on one of his adventures. He always seems to be wearing the same shirt and carries only a small backpack. How does he do that? If true, his "buy to fly" ratio must be close to cylindrical! That's perfection.

And, in the world of twitter - I learned of one called “50 Best Twitter Feeds To Stay On Top Of Green News”. The writer thought some of the blog readers might find it interesting. So, happy twittering!


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