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<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'>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.</span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'> </span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'>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).</span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'> </span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'>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.</span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'> </span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'>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.</span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'> </span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'>In the 18<sup>th</sup> and 19<sup>th</sup>
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.</span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'> </span></font></p>
<div>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'>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).</span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'> </span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'>Some examples:</span></font></p>
<p class=MsoNormal style='margin-left:.5in;text-indent:-.25in'><font size=2
color=navy face=Arial><span style='font-size:10.0pt;font-family:Arial;
color:navy'>(1)<font size=1 face="Times New Roman"><span style='font:7.0pt "Times New Roman"'>
</span></font></span></font><font size=2 color=navy face=Arial><span
style='font-size:10.0pt;font-family:Arial;color:navy'>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).</span></font></p>
<p class=MsoNormal style='margin-left:.5in;text-indent:-.25in'><font size=2
color=navy face=Arial><span style='font-size:10.0pt;font-family:Arial;
color:navy'>(2)<font size=1 face="Times New Roman"><span style='font:7.0pt "Times New Roman"'>
</span></font></span></font><font size=2 color=navy face=Arial><span
style='font-size:10.0pt;font-family:Arial;color:navy'>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. </span></font></p>
<p class=MsoNormal style='margin-left:.5in;text-indent:-.25in'><font size=2
color=navy face=Arial><span style='font-size:10.0pt;font-family:Arial;
color:navy'>(3)<font size=1 face="Times New Roman"><span style='font:7.0pt "Times New Roman"'>
</span></font></span></font><font size=2 color=navy face=Arial><span
style='font-size:10.0pt;font-family:Arial;color:navy'>A peck is one fourth the
volume, maybe more convenient for nuts and berries.</span></font></p>
<p class=MsoNormal style='margin-left:.5in;text-indent:-.25in'><font size=2
color=navy face=Arial><span style='font-size:10.0pt;font-family:Arial;
color:navy'>(4)<font size=1 face="Times New Roman"><span style='font:7.0pt "Times New Roman"'>
</span></font></span></font><font size=2 color=navy face=Arial><span
style='font-size:10.0pt;font-family:Arial;color:navy'>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. </span></font></p>
<p class=MsoNormal style='margin-left:.5in;text-indent:-.25in'><font size=2
color=navy face=Arial><span style='font-size:10.0pt;font-family:Arial;
color:navy'>(5)<font size=1 face="Times New Roman"><span style='font:7.0pt "Times New Roman"'>
</span></font></span></font><font size=2 color=navy face=Arial><span
style='font-size:10.0pt;font-family:Arial;color:navy'>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!</span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'> </span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'>Happy MLK Jr. Day to everyone! NOW what do
I do to keep busy?</span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'> </span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'>- Peter</span></font></p>
<p><font size=2 color=navy face=Arial><span style='font-size:10.0pt'>Peter T.
Parrish, Ph.D., President<br>
California Solar Engineering, Inc.<br>
820 Cynthia Ave., Los Angeles, CA 90065<br>
CA Lic. 854779, NABCEP Cert. 031806-26<br>
<a href="mailto:peter.parrish@calsolareng.com"><font size=1><span
style='font-size:9.0pt'>peter.parrish@calsolareng.com</span></font></a> <br>
Ph 323-258-8883, Mobile 323-839-6108, Fax
323-258-8885
</span></font></p>
<p><font size=2 color=navy face=Arial><span style='font-size:10.0pt'> </span></font></p>
</div>
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<hr size=2 width="100%" align=center tabindex=-1>
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<p class=MsoNormal><b><font size=2 face=Tahoma><span style='font-size:10.0pt;
font-family:Tahoma;font-weight:bold'>From:</span></font></b><font size=2
face=Tahoma><span style='font-size:10.0pt;font-family:Tahoma'>
re-wrenches-bounces@lists.re-wrenches.org
[mailto:re-wrenches-bounces@lists.re-wrenches.org] <b><span style='font-weight:
bold'>On Behalf Of </span></b>William Dorsett<br>
<b><span style='font-weight:bold'>Sent:</span></b> Sunday, January 17, 2010
4:38 PM<br>
<b><span style='font-weight:bold'>To:</span></b> RE-wrenches; Dan Fink<br>
<b><span style='font-weight:bold'>Subject:</span></b> Re: [RE-wrenches] kW/MW
v. kWh/MWh</span></font></p>
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<p class=MsoNormal><font size=3 face="Times New Roman"><span
style='font-size:12.0pt'>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.<br>
<br>
Bill Dorsett <br>
SunwrightS<br>
1715 Leavenworth<br>
Manhattan, KS 66502<br>
Home/Office 785/539=1956<br>
Cell 785/564-2583<br>
wmdorsett@sbcglobal.net<br>
<br>
See Amory Lovins July 08 on Charlie Rose<br>
http://www.charlierose.com/guests/amory-lovins<br>
<br>
--- On <b><span style='font-weight:bold'>Sat, 1/16/10, Dan Fink <i><span
style='font-style:italic'><danbob@hughes.net></span></i></span></b>
wrote:</span></font></p>
<p class=MsoNormal style='margin-bottom:12.0pt'><font size=3
face="Times New Roman"><span style='font-size:12.0pt'><br>
From: Dan Fink <danbob@hughes.net><br>
Subject: Re: [RE-wrenches] kW/MW v. kWh/MWh<br>
To: "RE-wrenches" <re-wrenches@lists.re-wrenches.org><br>
Date: Saturday, January 16, 2010, 8:51 PM</span></font></p>
<div>
<p class=MsoNormal style='margin-bottom:12.0pt'><font size=3
face="Times New Roman"><span style='font-size:12.0pt'>Hello Marco.<br>
No d'oh!<br>
Your assessment is correct; you can't store kilowatts of power, you *can*
store kilowatt hours of energy.<br>
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.<br>
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.<br>
DAN FINK<br>
<br>
<br>
<br>
Marco Mangelsdorf wrote:<br>
> I’m reading a piece on energy storage in the latest issue of
Distributed Energy mag.<br>
> <br>
> <br>
> The writer, Lyn Corum, repeatedly refers to the capacity of storage
mediums (thermal, compressed air, flywheel, battery) in kW and MW terms.<br>
> <br>
> <br>
> Heck, even my friends at Beacon Power refer to their flywheel systems in
MW terms.<br>
> <br>
> <br>
> Am I missing something here? Shouldn’t all references to
energy storage be in kWh and MWh terms?<br>
> <br>
> <br>
> Or am I experiencing a d’oh! moment?<br>
> <br>
> <br>
> Marco<br>
> <br>
> ProVision Solar<br>
> <br>
> <br>
> <br>
> ------------------------------------------------------------------------<br>
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