[Greenbuilding] Energy Numbers

[Corwyn]

On 7/15/2012 11:10 AM, Reuben Deumling wrote:
> This comparison I didn't understand:
>
> Explosion of 1 kg of TNT 10e6 Joules
> Woman running for 1 hr  10e6 Joules
> Candy bar                       10e6 Joules

The conversion for TNT is 4.7 MJ/kg.
The conversion for Calories (food) is 4.2 kJ/Calorie.
A 2 oz Snickers bar has 271 Calories = 1.138 MJ = 20 MJ/kg.

>
> I'd like to know a little more about how these numbers are calculated.
> Presumably in the case of TNT it is energy released; in the case of the
> woman it is expended, but in the case of the candy bar I'm not sure -
> contained in the nuts and sucrose? Seems kind of gimmicky.

Calories are units of energy.  Not gimmicky at all, it is the standard 
way of referring to the energy content of food.


On 7/15/2012 4:51 PM, Joe Killian wrote:

 > Shows how efficient animal metabolism is.  We all know intuitively
 > that burning a candybar in any man-made vehicle (even one of 120 pounds)
 > would nowhere near move that machine the 7 miles or so the woman
 > would travel in the hour.

My Prius can carry 5 people, and gets 55 MPG.  Which equals 0.025 
Gallons per passenger, so for 7 miles is 2.9 MJ.  So, two and a half 
times the energy consumption at 7 times the speed.  Given that energy 
consumption is proportional to velocity to the fourth power, at seven 
MPH, I am sure the Prius would beat a human running (the Prius being one 
of a few cars which runs well at low speeds).

The most efficient gasoline 'cars' are getting somewhere around 3000 
MPG.  Which works out to 281 kJ for seven miles, or 67 Calories (1 cup 
of chopped onions).  Electric cars do even better.

So once again, what "we all know intuitively" turns out not to be true.


Thank You Kindly,

Corwyn

-- 
Topher Belknap
Green Fret Consulting
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