Eventually, there will come a day when people stop listening to “designers” with no background in physics who propose physics-based things. Until then, the word “designer” will continue to mean “dude with AutoCAD/Photoshop skills, a wild hair up his ass, and no connection to the actual world that real people live in.”
Now when you’re designing a lampshade that is made out of thousands of soda tabs for no obvious reason, that’s fine. Lampshades are not important. As long as it doesn’t catch fire or explode, it’s probably fine. It doesn’t have to “work” in any real sense because it doesn’t do anything.
But when your headline is “Magneter: Magnetic Highway Harvests Kinetic Energy From Cars To Generate Electricity,” you had better know what the fuck you’re talking about. And as usual, they don’t.
The short story is very simple: you cannot make a system that harvests kinetic energy from moving cars. It’s not a technological barrier, it’s not a materials thing or a budget thing or an infrastructure issue, it’s a physics problem. I’ll start from the top.
First, Inhabitat says that “Using highways to generate kinetic energy from cars is not a new idea,” which doesn’t make any sense. Most of these ideas are supposed to take kinetic energy from cars and turn it into electrical energy. This is an indicator of how delusional these people are. But now on to the idea itself.
Here’s an image with bad English telling you how it works.
So the idea is that the magnet will be in the road on a spring, then the car going by will bounce it, and that bouncing will generate electricity. He invokes Faraday’s law, and correctly at that, so you might be thinking that this is a smart idea. Now I’m going to explain why it’s stupid.
Let’s say a line of cars is going over this thing. There’s just one of them; we’re testing it or something. The cars all weigh 3700 pounds and are all going 75 mph. Let’s say they’re passing by every 110 feet, because that means they’ll be going by every second, and they go by for eight hours. That’s 28,800 cars.
Now let’s say we’d like this line of traffic to power one average house for 24 hours. An average house, according to the internet, uses 50 kilowatt-hours per day, so each car will have to provide .001736 kWh, or 6.25 kJ (kilojoules).
So how much kinetic energy do these cars have? If you do the math (or in this case, let Wolfram | Alpha do it for you), it turns out that they have 943.3 kJ apiece. If we subtract 6.25 kJ, they’re left with 937.05 kJ, which is a speed of 74.75 mph. So we’ve only stripped off a quarter of a mile per hour from each car, which doesn’t seem like much, but that has to be made up somehow.
Each car has to burn an extra 6.25 kJ worth of gasoline to get that speed back; gasoline they wouldn’t have otherwise had to burn. And the average internal combustion engine only turns about 25% of the chemical energy of gas into speed — the rest becomes heat — so we really need 25 kJ. Gasoline has 132 MJ (megajoules) of energy per gallon, so we’re only talking about 1/5280 of a gallon per car. But remember, we had almost 30,000 cars, so it’s actually about 5.45 gallons of gas.
So in order to power a house that required 50 kWh for the day, we had to burn 199.8 kWh worth of gasoline. What?
That’s the problem. In order to get energy out of a moving car, YOU HAVE TO TAKE IT AWAY FROM THE CAR. I don’t care if it’s magnets or pressure plates or whatever, that energy has to come from somewhere. That’s conservation of energy, which was first expressed mathematically by Liebniz OVER THREE HUNDRED YEARS AGO. And if you take kinetic energy away from the car, you slow it down, and it has to burn more gas to make up for it.
So what you’re doing is burning gasoline to make electricity. Like those gasoline power plants that you hear about in the news all the- wait a minute. Those don’t exist because gasoline is a horribly inefficient way of producing energy by burning shit. Like I said, gasoline engines only turn about 25%, maybe 30%, of their chemical energy into propulsion. Coal-fired power plants run at about 33%. Combined-cycle gas-fired plants run at about 50%. Nuclear plants run at 98%, though that’s not chemical energy.
My point is that the existing systems for turning deceased dinosaurs into electricity are all — all of them, across the board — better at it than the engine in your car, and most of them are cleaner too.
And that’s ignoring the friction of the system and all the inherent inefficiencies that would go with the infrastructure. All that math I did was assuming that every drop of energy coming off of the car would turn into lightbulb juice, which it can’t. That’s according to the laws of thermodynamics, which are over two hundred years old as well. So it’s worse than I described.
You see, there are hundreds of ideas out there (which I’ve written on before) to take energy from cars, but they all ignore the same glaring problem: those cars are using that energy to go places. They need it.
Leave them alone.
As a paleontologist, I have to comment that oil is not deceased dinosaurs… it's deceased phytoplankton. And coal is compressed deceased swamp plant material. Just for clarification.
Otherwise, well done, and well said. 🙂
Late to the game here, but nuclear plants make steam, and if you have a 98% efficient steam turbine I want in on your startup. To put it another way, all the processes you mention are quite efficient at turning fuel into heat (which must be where your 98% comes from), and kinetic energy into electricity – it’s that middle thermal-to-kinetic step that gets you. Your main point stands though.
Where cars need to slow down such as off ramps it could scavenge some energy that would otherwise be wasted on brake pads though.
So, the idea is, generate energy by making my gas mileage even WORSE than it already is. I hope there is some sort of revenue model involved where I get paid more than it costs me to generate that electricity.