Re: Bicycles, trucks, trains

[Leigh Palmer <palmer@SFU.CA>]

At 8:20 AM -0700 9/16/99, Laurent Hodges wrote:

> I remember also their insistence that driving 65 mph was really more
> energy-efficient (as well as time-efficient) than driving 55 mph.  The
> Department of Energy (or maybe ERDA in those days) did a test with a group
> of identical trucks driving an identical route all around Virginia, but
> going at different maximum speeds.  The slowest (30-mph) truck was the most
> energy-efficient, of course, blowing the truck drivers' argument away.

I teach my thermodynamics students that "efficiency" when used in the
technical sense means exactly what Prof. Hodges means here. Somehow
the term "figure of merit" doesn't seem to be known to them, so I
introduce it using, of course, refrigerators. My definition:

                        the value of what you get that you want
   figure of merit = ---------------------------------------------
                      the cost of getting it that you have to pay

For example, one can't create energy, but a heat pump produces heat
at a specified temperature (which you want, and which has a value you
must calculate) by extracting heat for which you don't have to pay,
using a much smaller amount of energy for which you do have to pay.
Thus the figure of merit of a heat pump is the "coefficient of
performance", really just a special name for something which is much
more broadly meaningful, and which subsumes efficiency when that is
really the correct figure of merit to consider.

One can compare refrigerators with refrigerators using such a figure,
and trains with trucks, and trucks at one speed with trucks at
another speed. In doing this last comparison I (and the truck
drivers) would argue that efficiency is not the proper figure of
merit to use. The cost of the driver's time and other factors must
appear in the denominator of any figure of merit. It is also the
case that different interests will calculate the numerator and
denominator differently. Greenpeace, for example, will place a large
value on the coefficient used to multiply the number of column
inches they achieve in major newspapers in their denominator*.

"Figure of merit" is my physicist's way of labeling what economists
call "cost benefit analysis". It is also that of the engineers. I
would like to encourage all teachers to include some mention of this
topic and its broad applicability at appropriate points in their
courses.

It is an evident extension of the silliness that gave us the "energy
crisis" of the seventies that even considering the operation of
transport trucks at 30 mph was undertaken. I imagine the silly
bureaucrat who commissioned that test thought he was very clever in
doing so. The outcome should have been evident before the test was
done; he could have asked a physicist! His test was a waste of time,
money and energy. Clearly he could have used a course in physics,
suitably augmented as I've suggested. Physicists spend some time
considering what questions to ask before they do experiments.

Leigh

*You may guess that I don't much like Greenpeace. I know where it
came from.