Rhett Allain is an Associate Professor of Physics at Southeastern Louisiana University. He
enjoys teaching and talking about physics. Sometimes he takes things apart and can't
put them back together.Here is an article from Wired.com that talks about a used cooking oil generator. It's a really neat idea in that the oil is right there anyway. Here is my problem - from the article:
A new garage-engineered generator burns the waste oil from restaurants' deep fryers to generate electricity and hot water. Put 80 gallons of grease into the Vegawatt each week, and its creators promise it will generate about 5 kilowatts of power.
If you put 80 gallons of grease into the thing, that has a finite amount of energy. 5 kilowatts tells you the rate the energy is created. This would be like saying "If the car drives 80 miles, that is the same as driving 5 mph".
The article should have said the energy you can get from 80 gallons of grease. A kilowatt-hour is a unit of energy. This was discussed in the comments for the article, but I just wanted to point that out. This is another example where people confuse something and how that something changes. Not too dissimilar from the confusion between acceleration and velocity.
Physical Science
Comments
It could actually makes sense, since they describe the input as grease/week. In other words, if you want a constant power output of 5 kilowatts, input grease at a constant rate of 80 gallons/week.
The problem is that burning 80 gallons of grease doesn't give you enough energy to output 5 kilowatts for a week, even assuming 100% efficiency.
5kwone week = 310^9J
80 gallons oil * 1g/cc * 9calories/gram = 1*10^7 J
Posted by: meichenl | January 8, 2009 6:55 PM
Meichnel - good point. Gallons per week WOULD be like power.
Posted by: Rhett | January 8, 2009 9:10 PM