Hi Bill,
How would you propose dealing with this problem? The year is not a unit
in SI, but humans measure in years none-the-less. It is unavoidable
that there are numerous instances where the SI unit for time is
extremely inconvenient for a given application.
For example, imagine one is speaking in public or writing an article
about the health risks of obesity. Further, imagine one needs to
discuss the rate of weight-gain in adults. Speaking in terms of mass
gain in kg/s or g/Ms would bewilder the audience, even if they have a
good understanding and intuitive feel of kilograms and of seconds. On
the other hand, talking about adults gaining kg/year would be very
clear. And indeed, in this sort of context the uncertainty in
definition of the year (about a part in a thousand) would not be
significant.
One would have a similar problem with climate change. I intuitively
grasp somewhat the impact of someone saying that average temperatures
are rising 0.01° C/year. I immediately get a feel for how much warmer
it will be in 10 years or 100 years at this rate. But if we talk about
the temperature rising in pK/s, or even µK/days, then the impact of the
statement is completely lost, and I for one would be reaching for a
calculator.
Once again, I would be very interested to hear how you and the other
experts on this forum would deal with this issue.
J.
Bill Potts wrote:
There's a problem with y for year.
The unit y is neither an SI unit nor a unit approved for use with SI. The
largest unit that is approved is d, for day.
Bill Potts
Roseville, CA
http://metric1.org [SI Navigator]
-----Original Message-----
From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf
Of J. Ward
Sent: Thursday, August 23, 2007 19:32
To: U.S. Metric Association
Subject: [USMA:39273] Energy units
I've written two prominent experts in energy requesting that they use SI
units when talking about energy. One responded that he "entirely agrees,"
while the other says he's been going back and forth as to which units to
use. The biggest difficulty is how to define energy, since, for example,
the electricity produced by burning coal is the total thermal energy times
the efficiency of converting the heat into electricity. Anyway, here is
what I said in my letters.
-J
Dear ...,
I would like to suggest a way to help people understand energy issues more
clearly. It's very hard to understand energy issues when every source of
energy uses its own units of measurement. It would be much more clear to
use a single standard unit for power, preferably terawatts. For example, if
you express oil production in TW (instead of barrels per day), coal
production in TW (instead of tons / unit time), average solar production in
TW (instead of kWh / day or kWh / year), natural gas in TW (instead of
therms / unit time), etc. then it becomes very easy to compare all energy
sources on a more-equal footing, and also to relate them to consumption. As
an additional benefit, the watt is not only the world standard (SI) unit for
power, it is also a unit that most people are familiar with from direct
experience in their daily lives, and the "tera" prefix has become familiar
from computer data storage.
Choosing a single unit for energy would be equally helpful. The most
relevant time-scale for discussions of fossil fuels and climate change is
the year. Thus, if one were already consistently using TW for power, then
expressing energy in terawatt years would make comparisons between reserves,
production, and consumption extremely quick and simple. For example, 100
TWy of reserves being consumed at a rate of 4 TW implies a time-scale of 25
years.
On the other hand, exajoules would be the relevant SI unit for energy. The
UN World Energy Assessment uses exajoules to seamlessly compare different
energy resources, then uses EJ / year to relate this back to average power
to compare with consumption. Either way, I hope you will consider using TW
and TWy or EJ and EJ/y to help people understand energy issues more easily.
Sincerely....
