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.parr...@calsolareng.com> peter.parr...@calsolareng.com Ph 323-258-8883, Mobile 323-839-6108, Fax 323-258-8885 _____ From: re-wrenches-boun...@lists.re-wrenches.org [mailto:re-wrenches-boun...@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 wmdors...@sbcglobal.net See Amory Lovins July 08 on Charlie Rose http://www.charlierose.com/guests/amory-lovins --- On Sat, 1/16/10, Dan Fink <dan...@hughes.net> wrote: From: Dan Fink <dan...@hughes.net> Subject: Re: [RE-wrenches] kW/MW v. kWh/MWh To: "RE-wrenches" <re-wrenches@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 > > > > ------------------------------------------------------------------------ > > _______________________________________________ > List sponsored by Home Power magazine > > List Address: RE-wrenches@lists.re-wrenches.org <http://us.mc819.mail.yahoo.com/mc/compose?to=re-wrenc...@lists.re-wrenches. org> > > Options & settings: > http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org > > List-Archive: http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org > > List rules & etiquette: > www.re-wrenches.org/etiquette.htm > > Check out participant bios: > www.members.re-wrenches.org > _______________________________________________ List sponsored by Home Power magazine List Address: RE-wrenches@lists.re-wrenches.org <http://us.mc819.mail.yahoo.com/mc/compose?to=re-wrenc...@lists.re-wrenches. org> Options & settings: http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org List-Archive: http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org List rules & etiquette: www.re-wrenches.org/etiquette.htm Check out participant bios: www.members.re-wrenches.org
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