We walk here, we walk there, we walk everywhere. Maybe you’re headed to work or to lunch in a busy city. You’re expending energy, and the exercise is good for you. But what if, on top of that, we could recapture all that freely supplied energy and convert it to usable electricity?
This is a real thing. Systems have been installed in dozens of countries. Check out this video. And why stop there? You could put them in discotheques and harness that fancy footwork to power the strobe lights. Or build them into playground hopcotch grids. When you start thinking about it, the possibilities are endless.
But how does it work? And how much power can it generate? Obviously one person wouldn’t make much difference, but convert the teeming sidewalks of New York and you might really have something. Could we put this all over the world and stop using fossil fuels? Let’s find out!
Follow the Bouncing Ball
First we need a model of a walking human. No sweat, right? Walking is so easy a 1-year-old can do it. Well, actually, bipedal locomotion is horribly complicated from a physics perspective. Seriously, if you had to learn to walk from a physics model, you’d still be in a stroller. So let’s start with something simpler: a bouncing ball.
Believe it or not, this is a pretty good analogy. We can see immediately that there are three types of energy involved: kinetic energy, gravitational potential energy, and spring potential energy.
Kinetic energy has to do with the motion of an object—the faster it’s moving, the more kinetic energy it has. If you take a ball and drop it, it will accelerate downward, which means its kinetic energy is increasing. But where did that extra energy come from?
Answer: It’s stored in the gravitational field. This is gravitational potential energy. The amount depends on the strength of the field (g = 9.8 newtons per kilogram on Earth), the mass of the object, and how high above the ground it is. As a ball falls, the gravitational potential energy decreases and the kinetic energy increases.
Right there you can see something very powerful. We call it conservation of energy. This says that if we have a system with no energy inputs or outputs—what’s called a closed system—the energy can change form, but the total amount of energy remains constant.
Finally, we have spring potential energy. This is the energy stored in an elastic object when it’s compressed. When the ball hits the ground, it deforms and stops. If you had a high-speed camera you’d see it flatten out for a split second as the kinetic energy is converted to spring energy.
Then the ball rebounds to regain its shape. The spring potential energy is converted back to kinetic energy in the opposite direction and the ball trampolines upward. Here’s what it looks like:
Animation: Rhett Allain
If this is a truly closed system, the total energy would remain the same bounce after bounce, and you’d see the ball return to its original height. Of course, this isn’t very realistic. What happens instead is that some of the kinetic energy leaks away as heat or sound on impact. Then the spring energy is lower than the original kinetic energy, so each bounce gets lower and lower:
Animation: Rhett Allain
In the same way, each time your feet hit the ground, some energy leaks away. But wait! When humans walk, their height remains constant step after step. How is this possible? It’s because we make up the energy loss with internal energy from the food we eat. That’s your muscles working. If we include that source in the system, energy is conserved.
So, back to our original question: Is it possible to collect that “wasted” energy and actually use it? Yep, that’s what people-powered sidewalks do. Let’s see how it works.
Two Technologies
There are two basic methods you can use for this: a piezoelectric generator or an electromagnetic generator. Both of these technologies have been around for ages; it’s just the application to power-generating surfaces that’s new.
Piezoelectric devices are all around you. They’re in those wands you click to light your barbecue. They’re in elevator buttons, where their lack of moving parts makes them far more durable than old-style mechanical buttons. They’re what makes little kids’ sneakers light up at night.
It’s based on a special type of crystal, like quartz. When you press on the crystal, it changes the lattice structure of the atoms in a way that separates positive and negative charges. In other words, it creates voltage like a battery, and that causes electric current to flow.
Oh, it works in reverse too. If you apply voltage to a piezoelectric crystal it will expand. Apply a constantly changing voltage and the crystal will oscillate and produce sound. Now you have a tiny speaker. Because they’re so thin, they’re used in those greeting cards that play a song when you open them.
Here is a piezo speaker I took out of a toy and connected to an LED. When I hit it, there is enough energy to make the LED light up. Pretty cool, huh?
Courtesy of Rhett Allain
Now say you put a piezoelectric crystal in a floor panel. A walking human would push down on it, compressing the crystal and generating electricity. This is how Japan’s Soundpower Corp (now called Global Energy Harvest) created its Power-Generating Floor.
A British company, Pavegen, uses a floor with small flywheels underneath. When your foot pushes down on the floor, the flywheel spins, rotating a coil of wire in the presence of a magnetic field, which produces an electric current. This is an electromagnetic generator, and it’s how most of our everyday power is created—it’s just a question of what makes the generator spin. Usually it’s wind or water or (most commonly) steam from burning fossil fuels.
The Power of the People
So how much electrical energy could one person would generate? Let’s do a back-of-the-envelope calculation. (This is what physicists do when they want to get quick approximation—it lets us see if it’s even worthwhile to get a more precise number.)
First, a quick reminder on the difference between power and energy. In short, power is the rate of energy transfer. As shown in the following equation, it’s the change in energy (ΔE) per unit of time.
Courtesy of Rhett Allain
If ΔE is measured in joules and Δt is in seconds, then the power would be given in watts. To give you a feel for these units, lifting a textbook off the floor and putting it on a table takes about 10 joules of energy. The power used by a typical LED bulb is 10 to 20 watts.
Let’s say that our walking person is like a very terrible bouncing ball. During a single step, the person moves down and strikes the floor. In the process all of their kinetic energy is lost (for our estimation). Since their kinetic energy came from a change in gravitational potential energy, we can get the energy from one step just using the mass (m) and the height (h) that they move down. With that, our one-step energy would be:
Courtesy of Rhett Allain
I’ll use a human mass of 70 kilograms with a change in height of 2 centimeters. And remember, the gravitational field on Earth is g = 9.8 newtons per kilogram. But wait! Each step is just half the weight of a person, so we’ll multiply by 1/2. And suppose a fast-walking person takes two steps per second. That means we can use a time interval of 0.5 seconds.
Plugging in these values gives us an average power of 13.7 watts. Of course, that assumes the system is perfectly efficient. For a minimum estimate, let’s say the floor is just 10 percent efficient. That would put the average power output at 1.37 watts. I think if you had a value of anywhere from 1 to 10 watts, that could be legit.
So what if you had a people-powered floor in a busy airport, and you get 5 watts per person. That means 100 people would give you 500 watts of total power. That ought to be enough to light up the concourse. Of course, when the airport is deserted, like late at night, there’d be no light, so it might be a good idea to have some type of backup.
