Hi all,

 

Just really a theory of mine that i need clarifying:

 

So when we hear cars driving past us it is not the engine we hear, but in fact the friction caused by the tires and the road. When you think of the amount of high speed traffic passing through main motorways, surely we should be able to harness this "wasted" energy and put it to use in a heat engine of some description by running some kind of fluid under the road close enough to be heated. I am unsure of the amount of energy wasted but i am sure it is much more than negligible.

 

So who's with me ?

 

 

Under road heating is used in Iceland where they have geothermal energy to heat water.

Hi all,

 

Just really a theory of mine that i need clarifying:

 

So when we hear cars driving past us it is not the engine we hear, but in fact the friction caused by the tires and the road. When you think of the amount of high speed traffic passing through main motorways, surely we should be able to harness this "wasted" energy and put it to use in a heat engine of some description by running some kind of fluid under the road close enough to be heated. I am unsure of the amount of energy wasted but i am sure it is much more than negligible.

 

So who's with me ?

This is a science discussion board — let's do some science. How much energy are we talking about? Wheels are designed to minimize the rolling frictional losses, but having enough to maneuver and start and stop. Let's say a car gets 30 miles per gallon (12.75 km/L) A liter of gasoline has ~34 MJ/L. We'll assume 50% engine efficiency and 50% of that energy goes into overcoming rolling resistance. So 1 L is 9 MJ, deposited over 12.75 km, or around 2/3 kJ per meter. If you're going 60 km/hr that's 1 km/min or 16.67 m/s. That's 40 watts of thermal energy. Around half of which is going into the tire. Only a fraction of which could heat up a fluid under the pavement. That's not even worrying about how you would extract useful energy from the system.

 

Simply put: tires don't get all that hot when you're driving around.

 

Under road heating is used in Iceland where they have geothermal energy to heat water.

 

Which is the opposite of what the OP is discussing.

Which is the opposite of what the OP is discussing.

 

 

. When you think of the amount of high speed traffic passing through main motorways, surely we should be able to harness this "wasted" energy and put it to use in a heat engine of some description by running some kind of fluid under the road close enough to be heated.

 

People in Iceland use heated water (some kind of fluid). Realistically though the main reason for heating roads is to get rid of snow and ice. Snow is worst during the night when there is no sun to melt it and there is no cars driving on the road. The main problem with what the op suggested is that it isn't a constant supply of energy for example in the morning the roads will still be covered in snow because nobody has driven on them.

Ironically, it might be possible to "harvest" heat from roads because they are generally slightly warmer than their surroundings.

But that's because they are black and soak up heat from the sun.

Putting cars on them reduces this solar gain.

 

It's almost certainly not worth the investment.

When we hear cars driving past us it is not the engine we hear, but in fact the friction caused by the tires and the road.

 

No. Electric vehicles are much more silent. Petrol cars too when they roll on their momentum.

Noise isn't a good indicator of losses neither. An electric heater dissipates every kW fed in but rather quietly.

And at a good pace, aerodynamic losses exceed rolling losses for usual cars.