This is one of my pet peeves too...
A few snippets and observations from my frequently given electrical
terms presentation:
If I asked you "How far is it to Vancouver?" and you answered "45
miles per hour," everyone listening would know that someone was
confused. But we tolerate and even perpetuate this kind of confusion
daily with the terms "watt" and "watt-hour", which are directly
comparable to miles per hour and miles (NOT the reverse). The terms
themselves are very confusing, since a watt (which might sound like a
quantity/unit) is shorthand for "joule per second," the rate of
energy generation, transfer, or use, while a watt-hour is a unit of
energy (even though it might remind some people of "miles per hour",
with "hour" in it).
I'm amazed when some long-time RE folks say things like, "Oh, it
doesn't matter..." when they say "kilowatt" when they mean
"kilowatt-hour". This sort of ignorance and brush-off just makes our
job of energy education harder.
I've begun to teach watts and watt-hours FIRST in my presentations,
instead of the more common order of volts, amps, amp-hours, watts,
and watt-hours. Watts and watt-hours are the most important measures
and the most important distinction, and if I lose students after
that, it's better than losing them before understanding these crucial
terms.
For the general public, I completely downplay amps and amp-hours,
which are really an internal measurement perhaps analogous to
something like cylinder displacement in an engine, in a world where
people want to know fuel economy and speed. If I ran the circus, we'd
rate batteries in watt-hours, have watts and watt-hours on every
consumer RE system gauge, and leave amps and amp-hours to the geeks
who need them (for wire sizing, for example).
And going down that road a bit, one of the most helpful moments of
understanding for me was when years ago my guru Bill Beaty said that
"two things flow in electrical circuits -- charges and energy". Amps
and amp-hours describe charge flow and quantity, while watts and
watt-hours describe energy flow and quantity.
Of course, as others have noted, we can talk about generating or
storage sources in terms of their power (kW) or energy (kWh)
capacity, but there is such confusion about the terms, it's better to
be redundantly clear about what we mean, and make sure we're accurate.
Encouraging electrical terminology literacy, one wrench and newbie at
a time, ;-)
Ian
At 9:42 AM -0800 1/18/10, Peter Parrish wrote:
Just a pedagogical note. Since I have been teaching PV Installation
now fairly continuously since July of this year. The kW/kWh problem
is pretty pervasive: I even slip once and a while. Here is the
problem as I see it.
Most basic units like mass, distance, energy are simply defined:
kilogram, meter and joule. The rates of change are derived:
kilometers per hour, meter per second, joule per second (watt).
The basic units related to energy (at least within the electrical
utility and related fields) are defined first with the rate of
change: watts (or joule per second). Then the other quantities are
derived from the watt: watt-hour (energy), watts/meter-squared
(insolation) and the like.
Well that's just a fact, and in my classes I drive that point home
early and often - so as to minimize confusion down the road. To
enliven things a bit, I also add a little history.
In the 18th and 19th centuries, there was a lot of work done trying
to understand the nature of and interrelationship between mechanical
work, heat and then later electricity. The early work focused on the
rate at which work could be done, and concepts like horsepower and
later watts were used and compared. An exception to this trend is
the unit of heat the British Thermal Unit (BTU) and the rate BTU/hr.
Another interesting fact: since the beginning of time (wo)man has
invented various units of measure to assist them in carrying out
business. The rule of thumb: invent a unit of measure that can be
used for the puposes at hand, that is convenient (gives you the
level of accuracy you need) and doesn't use zeros (invented in India
in 800 AD) or the decimal point (an even later invention).
Some examples:
(1) A cord of wood (128 cu-ft) is rougly the amount that can fit
in the back of a wagon (or large pick up truck). In New England, you
can sustainably harvest about 1 cord per acre (if I remember
correctly). Whether you cut your own wood or order it from someone
else, you would probably think for example in terms of 3 or 4 cords
of wood (not 3.5 cords).
(2) A bushel is a convenient measure of fruit and vegetables,
about a cubic foot. If you needed bake a few of pies and have some
left over for eating, you might buy a bushel of apples.
(3) A peck is one fourth the volume, maybe more convenient for
nuts and berries.
(4) A hand (4 inches) is used to measure how tall a horse is. An
average horse is about 16 hands; a pony 14 hands or less and a big
draft horse could be 19-20 hands.
(5) When talking to a customer about their usage per day, the
unit of energy kWh is similarly convenient. A small family might use
10 kWh/day and a larger family might use 35 kWh/day. In either case
1 kWh/day one way or another won't make much difference in designing
a system that will eliminate their bill. I must admit that with
larger commercial accounts we would be looking at numbers like 150
kWh/day and we start using zeros to reflect the fact that we aren't
particularly interested in measuring energy usage to a precison of
less than 1%. In any event, the kWh is much more convenient than the
Joule: there are 3,600,000 Joules in 1 kWh. Of course we could
"shorten" the notation to MJ and now we would have a unit of energy
measure that fits the criteria: typical usage and the level of
precision doesn't rquire zeros or decimal points. So the small
household might use 36 MJ/day and the large household 126 MJ/day.
Perhaps one day, we could switch from kWh to MJ!
Happy MLK Jr. Day to everyone! NOW what do I do to keep busy?
- Peter
Peter T. Parrish, Ph.D., President
California Solar Engineering, Inc.
820 Cynthia Ave., Los Angeles, CA 90065
CA Lic. 854779, NABCEP Cert. 031806-26
<mailto:[email protected]>[email protected]
Ph 323-258-8883, Mobile 323-839-6108, Fax
323-258-8885
From: [email protected]
[mailto:[email protected]] On Behalf Of
William Dorsett
Sent: Sunday, January 17, 2010 4:38 PM
To: RE-wrenches; Dan Fink
Subject: Re: [RE-wrenches] kW/MW v. kWh/MWh
But you know they keep talking about the potential contribution that
plug-in hybrids might have for storing energy for utility support.
Most of our systems have hugely larger capacity than current Prius'.
And I suspect that many RE system owners will want some backup,
especially if they are compensated for installing it.
Bill Dorsett
SunwrightS
1715 Leavenworth
Manhattan, KS 66502
Home/Office 785/539=1956
Cell 785/564-2583
[email protected]
See Amory Lovins July 08 on Charlie Rose
http://www.charlierose.com/guests/amory-lovins
--- On Sat, 1/16/10, Dan Fink <[email protected]> wrote:
From: Dan Fink <[email protected]>
Subject: Re: [RE-wrenches] kW/MW v. kWh/MWh
To: "RE-wrenches" <[email protected]>
Date: Saturday, January 16, 2010, 8:51 PM
Hello Marco.
No d'oh!
Your assessment is correct; you can't store kilowatts of power, you
*can* store kilowatt hours of energy.
Author Lyn Corum might have slipped up a couple times in there and
dropped an 'h' from the kw, but maybe not -- because when storing
energy on a utility scale, the engineers are greatly concerned with
how fast the energy can come in, and how fast they can send it out.
So when they refer to a '50 kw flywheel storage unit,' that often
means the max power coming in or out at any given instant.
In our (comparatively) puny end of the energy storage business,
batteries, we rarely have to deal with the issue. Assuming our
battery bank is big enough to keep the charge or discharge rate
below C10, or even C1, we just stack more inverters.
DAN FINK
Marco Mangelsdorf wrote:
I'm reading a piece on energy storage in the latest issue of
Distributed Energy mag.
>
The writer, Lyn Corum, repeatedly refers to the capacity of
storage mediums (thermal, compressed air, flywheel, battery) in kW
and MW terms.
Heck, even my friends at Beacon Power refer to their flywheel
systems in MW terms.
Am I missing something here? Shouldn't all references to energy
storage be in kWh and MWh terms?
Or am I experiencing a d'oh! moment?
Marco
ProVision Solar
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