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Comment by james-rms | Hacker News Reader

This seems like a non-starter. A quick conservation of energy check:

Human beings put out around 75W of energy when they're not excercising. Most of that is body heat. They also take around 10,000 steps a day, which translates to around 85 mins walking per day. Even in the scenario where you could capture 10% of that 75 human watts for those 85 minutes, that's only enough energy to run a 15W LED bulb for 42 minutes.

Harvesting energy from cars is also a non-starter. Any energy you harvest there is added as a load on the engine of the cars, which are all powered by fossil fuels. It's a really inefficient way to turn a car engine into an electric generator.

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A 'classic' new technology that comes up every few years.

They often try and say how it can be used 'efficiently' but it's hard to see it ever working.

Could you really install it on roads at the cost of billions just so you can turn car petrol power to energy at peak times?

Were is it going to be useful in remote areas compared to just shoving up a solar panel?

Walked on every day vs a solar panel which is never trod on? Which is going to last longer?

> Harvesting energy from cars is also a non-starter. Any energy you harvest there is added as a load on the engine of the cars, which are all powered by fossil fuels. It's a really inefficient way to turn a car engine into an electric generator.

The vast majority of cars already have a small electrical generator installed. Plugging things into a cigarette socket to access that electricity causes 0 extra load on the engine.

> The vast majority of cars already have a small electrical generator installed. Plugging things into a cigarette socket to access that electricity causes 0 extra load on the engine.

No. Your use of energy from the generator through the cigarette socket manifests as a braking force in the generator. The more energy you use, the more it brakes. The car's engine has to work harder to overcome that braking.

What about regenerative braking?

My daughter missed a high school physics problem that seemed obvious to me until I understood her reasoning. The question was does playing the radio in a car reduce fuel efficiency, true or false? She didn't realize that the electricity to run the radio came from a rechargeable battery plus generator system. She had seen me replace a car battery and assumed that it was like an ordinary flashlight battery that provided electricity until used up.

The generator in a car doesn't run for free. It is powered by the engine and it's operation while charging is felt as a braking effect on the main engine. This additional drag is produced by a phenomena known as back electromagnetic force (back emf) that causes the generator to resist turning while generating current to power electrical devices in the car or recharge batteries.

Another way to realize that using electricity in a car isn't free is to consider why it wouldn't work to add a number of these generators and then use them to power electrical motors to propel the car. There is no free lunch.

On the other hand (and not really related to the points I'm making above), stopping a moving vehicle reduces the overall energy of the system because the kinetic energy is proportional to the square of the vehicle's velocity. Due to conservation of energy this energy has to go somewhere. Conventionally, this stopping energy is dissipated through heat (and to some extent breaking down a bit of the the brake pads). Some vehicles, like a Tesla, are able to capture a portion of this kinetic energy during braking, but its complicated, see [1].

[1] https://www.tesla.com/blog/magic-tesla-roadster-regenerative...

Exactly. Might as well say we're going to put a stationary bike connected to a generator in every home. Sure, it would work, but the energy expended by the human isn't free.

Reminds me of the crazy ideas to put plates on the roadway to generate power when cars drive over them. Robbing Peter to pay Paul....

>Reminds me of the crazy ideas to put plates on the roadway to generate power when cars drive over them. Robbing Peter to pay Paul....

That's easy, you just put them near stop signs. Fixed location, cars would be wasting the energy anyway, and you provide a benefit to them by taking it from them.

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