Jim,
You missed a factor of 1000 somewhere. 1 kWh is 1000 W for 3600 s, hence
3.6 MJ.
In a drivethru application, this might hit 2 kW instantaneous power, but
the window will have a significant transaction time severely limiting the
average power.
He, of course, may have slightly different numbers, but my estimates are
an average vehicle with mass 2.5 t, driver accelerates to 2 m/s in a
stop-and-go line and needs to stop at the window, where he will have a 60
s transaction to receive his food, pay, receive change
Kinetic energy, (½mv²) is
0.5* 2500 kg * (2 m/s)² = 5000 J
If the car stops in 2 s, 2500 W would be generated during that period.
However, with a 60 s transaction at the window, the average power is 5000
J/60 s = 80 W more or less. Even this (useless) level of power assumes
100% efficiency, so real world results will be lower.
Assuming a line of cars awaiting their turn at the window, perhaps one
device for each waiting position in line could improve this somewhat. I
don't see it making a lot of power. Especially if anyone is stopped in
the wrong place and everybody has to use their real brakes.
--- On *Fri, 9/4/09, James R. Frysinger /<j...@metricmethods.com>/*
wrote:
From: James R. Frysinger <j...@metricmethods.com>
Subject: [USMA:45733] [Fwd: Energy and power]
To: "U.S. Metric Association" <usma@colostate.edu>
Date: Friday, September 4, 2009, 2:44 PM
I recently posted this email to Rick Leventhal at FoxNews.com.
Jim
Dear Mr. Leventhal,
I have just finished reading your online article
N.J. Burger King Testing Energy-Producing Speed Bump
http://www.foxnews.com/story/0,2933,546512,00.html
posted on FoxNews.com.
In this article you have made an error that detracts significantly
from
your report. You apparently confused the two distinctly different
quantities energy and power. Power is the rate at which energy is
produced, used, or transferred. Think "power equals energy divided by
time". Conversely, "energy equals power times time".
In your article you state, "That force turns gears inside, generating
2000 watts of electricity instantaneously, according to the engineers
who designed it." The implication is that some amount of electrical
energy is produced in a short period of time. But energy is measured
in
joules (J), not in watts (W). In the electrical utility industry,
they
often use kilowatt hours to measure energy; a kilowatt hour is equal
to
3600 joules, or 3.6 kilojoules (kJ).
The watt (W) is used to measure power. It is defined to be 1 J/s.
Let's
assume that object 1 transfers 1000 joules (1000 J) of energy over
the
time span of 1 second (1 s) to object 2 and this generates
electricity
with 100 % efficiency. The power of this generation event would then
be
1000 J divided by 1 s or 1000 W. If instead the transfer of energy
and
energy production took 0.5 seconds, the power level would be 2000 W.
Or
if the transfer and generation took 2 seconds, the power level would
be
500 W. All of these would of course be the average power levels
during
the time span of the interaction; between interactions the power
level
would be zero.
The way you should have worded your sentence would be of the form,
"That
force turns gears inside, generating an average of 2000 watts of
electrical power during the time span of the energy transfer,
according
to the engineers who designed it."
The website for New Energy Technology states:
"All vehicles in motion possess kinetic energy. The amount of
kinetic
energy a vehicle possesses is based upon the vehicle’s speed and
weight.
The faster the vehicle is moving and the more it weighs, the more
kinetic energy it possesses."
It would have been more informative if you had given us a typical
value
for interaction time and the power produced during that span of time
OR
a typical value for the amount of kinetic energy delivered and the
amount of electrical energy produced for some typical car and speed
circumstance.
I encourage you to study the difference between energy and power, and
the units used to report them, before writing anything else that uses
them in the discussion.
regards,
/s/
-- James R. Frysinger
632 Stony Point Mountain Road
Doyle, TN 38559-3030
(C) 931.212.0267
(H) 931.657.3107
(F) 931.657.3108
-- James R. Frysinger
632 Stony Point Mountain Road
Doyle, TN 38559-3030
(C) 931.212.0267
(H) 931.657.3107
(F) 931.657.3108
