[RE-wrenches] kW/MW v. kWh/MWh

Mon Jan 18 09:42:35 PST 2010

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:peter.parrish at calsolareng.com> peter.parrish at calsolareng.com  
Ph 323-258-8883, Mobile 323-839-6108, Fax 323-258-8885

  _____  

From: re-wrenches-bounces at lists.re-wrenches.org
[mailto:re-wrenches-bounces at lists.re-wrenches.org] 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
wmdorsett at sbcglobal.net

See Amory Lovins July 08 on Charlie Rose
http://www.charlierose.com/guests/amory-lovins

--- On Sat, 1/16/10, Dan Fink <danbob at hughes.net> wrote:

From: Dan Fink <danbob at hughes.net>
Subject: Re: [RE-wrenches] kW/MW v. kWh/MWh
To: "RE-wrenches" <re-wrenches at lists.re-wrenches.org>
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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