2008-03-09 19:29 | fche blog tech why anti-snow car lasers don't work
When I was about ten (?) years old, I had the brilliant idea that cars should carry laser beams in order to melt snow they’re about to drive on. My father, an engineer, said just that it couldn’t work, but didn’t explain that day why not. To dash the dreams of future youth with the same idea, let me spell out now why not.
Let’s pick some reasonable figures. Let’s work in SI, because, well, it’s a Sunday.
- two 12.5 cm wide front tires
- 1 cm deep snow, density equivalent to 1mm rain
- -1 degC snow temperature
- 4 m/s crawling speed
We need to compute how much dihydrogen oxide we want to liquify. Its total volume is: 4 m/s * (2 * 0.125 m) * 0.001 m = 0.001 m^3 = 1 L/s. And since water’s density is pretty close to 1 kg/L, let’s just say it’s 1 kg/s.
The energy required to heat up the -1 degC snow to its melting point of 0 is around 4.2 J/g. The additional energy needed to actually melt it is called the latent heat of fusion, which for water is about 333 J/g. The total energy is therefore around 337 J/g.
Now let’s calculate the amount of power needed to melt even this minimal snow cover. 1000 g/s * 337 J/g = 337000 J/s = 337 kW. A common power level for handheld laser pointers is on the order of 3 milliwatts, so you’d need 100000 of them. Before you try to find room for them in your car, consider that 337 kW is about 450 horsepower. Your car’s engine had better put out far more power than that, to power an unusually manly alternator, and of course to keep the car moving through the snow-cum-water.
So yeah, it is not going to work.