“Be kind, for everyone you meet is fighting a hard battle” - Often attributed to Plato but likely from Ian McLaren (pseudonym of Reverend John Watson)

Monday, June 17, 2013

Wasted energy

Nope, I'm not talking about the 75% of the energy in the gasoline in your car going out the exhaust pipe as waste heat, or the cooled/heated air escaping your house because it's not as "tight" as it could be. I'm not talking about anything to do with the inefficiency of the use of primary energy. I'm (against my usual nature) using a more colloquial meaning of "energy" here. I'm talking about foolish expenditures of "mental energy" on schemes that either will produce no useful energetic results or such a trivial amount of energy that it's a waste of time to bother.

I've previously written of the foolishness of humans as generators via walking and revolving doors, the silliness of one of the Discovery Channel's "Project Earth" programs (they were all fairly silly but I focused on a specific one). I've got two more examples.

The first is something akin to the "human powered generator," that is, it would probably work from a technical point of view but be useless in any practical way. I was pointed to it by a post in Tom Sanwson's Swans on Tea blog entitled "This Claim Won't Fly." The post referred to an article on CNN's site about Airbus working with UNESCO to challenge engineering students in a "Fly Your Ideas" competition.

Of the ideas described in the article, one sounds far out (shape shifting engines to reduce noise footprint), one marginally practical (powering jets with a supercooled mixture of biomethane and LNG), and one ... let's just say silly. That last one is to use seats upholstered in a thermoelectric fabric to use passengers' body heat to generate electricity. University Putra Malaysia team leader Tan Kai Jun envisions generating "100 nanowatts of voltage." Like Tom Swanson, I'll ignore the fact that watts is not a unit of voltage, but rather a unit of power.
"It's a small amount, but imagine this collected from 550 seats throughout 10 hours of flight. A plane has a lifespan of a few hundred flights -- over time that's a big reduction," Mr. Jun Tan says."
Tom ran some numbers, but I'll do the same. The "age" of an airliner is measured in pressurization cycles, and a "typical" airliner may have 51,000 flight hours and 75,000 pressurization cycles in its useful life. Let's consider an airliner with 400 seats (say, a B777) and 51,000 hours. We'll assume the plane flies with an average load capacity of 95%. So we have 400*51000*.95 = 19,380,000 seat hours. Using the the Google Chrome extension "Cloudy Calculator" and multiplying hours times 100 nanowatts (the calculator does all conversions) we find that we've generated 1.9 watt hours over the life of the aircraft. I pay about $0.12 (12 cents) per kilowatt hour at my house, so this almost two watt hours is worth a smidgen (one of my favorite units - slightly smaller than a skosh) over a fiftieth of a penny.

Looked at another way, Jet A fuel (used in airliners) has an energy density of 35.3 megajoules per liter. 1.9 watt hours is about 7,000 joules, or 0.007 megajoules. So, over the lifetime of the airplane, the passengers would generate electrical energy equal to the chemical potential energy in 0.2 milliliters of jet fuel. Of course, a heat engine such as a turbofan might operate at, say, 45% efficiency so we'd actually need to burn a bit under half a milliliter to generate those 1.9 watt hours. I suspect that this development will not revolutionize flying.

The next post will cover a different type of silliness - one that's either a fraud or pie in the sky.

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