<DrumBeat: August 13, 2006The Oil Drum

254 comments on Due Diligence: A reader's response to Khosla

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LJR on August 13, 2006 - 12:42pm Permalink | Subthread | Comments top
Excellent post!  One item that piqued my interest was your estimate of plug power utilization for the Prius.

745 watts per hour corresponds to 1 horsepower. I was quite surprised to read that the Prius can move an average mile on 260 watt hours.  That's 1/3 of a horsepower at 100% efficiency!  Furthermore, you say that that's at the plug, not at the wheels. Surely the charging, motor and drive train losses must be 30% or greater.

What am I missing here?  My 1/5 HP box fan motor won't move my car down the road at 50MPH.

Where did you get those numbers?

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Engineer-Poet on August 13, 2006 - 1:11pm Permalink | Subthread | Parent | Comments top
You're getting into unit analysis, one of my hobby-horses.

Watt-hours and watts are not the same thing.  Watts are power, watt-hours are energy.  You can burn a hundred watt-hours over an hour with a light bulb, or in 60 seconds with a 6 kilowatt heater.  100 W * 1 hr = 6000 W * 1/60  hr = 100 Wh.

I was quite surprised to read that the Prius can move an average mile on 260 watt hours.  That's 1/3 of a horsepower at 100% efficiency!  Furthermore, you say that that's at the plug, not at the wheels. Surely the charging, motor and drive train losses must be 30% or greater.

What am I missing here?

What you're missing is that the Prius+ is using that energy in around 2 minutes (at 30 MPH), so the 260 WH would be consumed at a rate of 7800 watts or a bit over 10 HP.

CalCars claims 262 WH/mile for the lead-acid conversion and 200 Wh/mile for Li-ion.

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LJR on August 13, 2006 - 3:04pm Permalink | Subthread | Parent | Comments top
I get it.  My box fan motor is using energy for an hour at a rate of 260 watts/hour whereas the car is using the same energy in 2 minutes for an overall rate of 7800 watts per hour.

Thanks.

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Engineer-Poet on August 13, 2006 - 3:52pm Permalink | Subthread | Parent | Comments top
Um, no.  "Watts per hour" is as meaningless as "horsepower per hour" (outside some esoteric things mostly suitable for physics education).  There's watts and watt-hours; if the sentence wouldn't make sense if you substituted "horsepower" for "watts", you're almost certainly mis-using the term.
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LJR on August 13, 2006 - 8:44pm Permalink | Subthread | Parent | Comments top
All right, already.  884 BTU per mile.  Watts suck.
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wimbi on August 16, 2006 - 7:56am Permalink | Subthread | Parent | Comments top
EP.  Many thanks for all your good work, not just this piece but all the great things you have on your ergosphere.  

I think a lot of people get self-confused when they get into the habit of abbreviating kilowatt-hrs to kilowatts, as in "My electric bill showed only 213 kilowatts last month". From there  to kilowatts per hour is a swift slide down the steep slippery slope.

BTW, I have finally got a thin stream of  vital fluids flowing again toward completion of my 1kW home power stirling engine and shall give a full report asap.  The military industrial vampire had sucked it dry for an entire year (goddamit).

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Engineer-Poet on August 16, 2006 - 9:38am Permalink | Subthread | Parent | Comments top
I'm looking forward to reading about it.
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Squalish on August 13, 2006 - 5:31pm Permalink | Subthread | Parent | Comments top
POWER:
Horsepower
Watts

ENERGY:
Joules
BTU
watt*hours

Power is energy divided by time.  Energy is power * time.

If charging a perfectly efficient battery for 10 hours takes 1 watt, then you have 10 watt hours in that battery.  That battery can't release all the energy at once after you're finished though, it maxes out at say, 5 watts.  So you can power a 5 watt lightbulb for two hours (until the batteries dead) on the amount of energy you charged it with.  

Alternately, you could power a 2.5 watt lightbulb for 4 hours, and so on and so forth.

We say that the battery has a capacity of 10 "watt hours," which is a distinct unit.

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DIYer on August 13, 2006 - 5:20pm Permalink | Subthread | Parent | Comments top
As TOD gets more subscribers, we'll probably see a smaller percentage who've had a tutorial in dimensional analysis. Watt-hours, electron-volts, Joules, BTUs, ... TOD is likely to toss around a few terms with which the average reader will be unfamiliar.

To make matters worse, I just looked up Joule in Wikipedia, and the entry starts with this misinformation: The joule (symbol: J) is the SI unit of energy, or work ... Gack, energy ≠ work!

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Kevembuangga on August 13, 2006 - 5:49pm Permalink | Subthread | Parent | Comments top
the entry starts with this misinformation: The joule (symbol: J) is the SI unit of energy, or work ... Gack, energy ≠ work!

Sorry, you should read closer or rehearse your courses from long ago.
The very first lines are absolutely correct :

The joule is a derived unit defined as the work done or energy required to exert a force of one newton for a distance of one metre, so the same quantity may be referred to as a newton metre or newton-metre with the symbol N·m.

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WebHubbleTelescope on August 13, 2006 - 5:51pm Permalink | Subthread | Parent | Comments top
same units, perhaps different meaning
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canbrit on August 13, 2006 - 6:41pm Permalink | Subthread | Parent | Comments top
Well, I certainly run out of energy when I do a lot of work!!

But seriously...

Units of Work = joules

Units of Energy = joules

Did I miss something during my Physics degree courses? Care to enlighten us?

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DIYer on August 13, 2006 - 8:39pm Permalink | Subthread | Parent | Comments top
Units are the same, but energy is generally defined as 'ability to do work'. However, I can burn X BTU's worth of energy without getting the equivalent in work. The difference is perhaps a subtle one.
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DIYer on August 13, 2006 - 8:44pm Permalink | Subthread | Parent | Comments top
Actually I was a little hasty with the post button, and would have withdrawn that post if I could. When looking at the Wiki article, I had a blonde moment and thought it said something like 'energy, or power'... Nonetheless, there is a distinction between work and energy.
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Engineer-Poet on August 13, 2006 - 6:50pm Permalink | Subthread | Parent | Comments top
The Wikipedia entry is correct, and now maybe you understand why the public has such a bad understanding of energy issues - it goes right down to the essential terms.
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Squalish on August 13, 2006 - 8:03pm Permalink | Subthread | Parent | Comments top
Just changed the line - energy is defined as the potential for a physical system to do work.
The usage on energy is "I use 7 million joules [of energy] a day to heat my home."

The usage on heat is "Theoretically, my car did about 7 million joules of work to get to the top of that mountain, not counting what it blew on heat"

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