Same issue. When you are standing on the tile, you are pushing down on the tile with your weight (let's use the moment when you are on one foot, having just barely transferred the weight off the other foot and getting ready to lift it to start the step). In order to start your center of mass moving upward, you have to exert a force greater than your weight on the tile.

Correct, but the peak force is for a very small percentage of the time of the total cycle.

Also keep in mind that the entire body is not lifted in walking, more the torso which is 60% of the total bodyweight. Then add in the damping effect of viscous muscles and rubber shoes both efficient at removing a lot of the energy before the ground, and elasticity in both so that much of that vertical movement is coming from bounce and not delivered to the ground.

You can't use a simple math formula to quantify such a complex process. I like my solution better; we know a typical man will produce maybe 200W max with his legs at peak power ie heart pounding and sweat pouring. A casual walk has to be significantly less total power and delivered through a very inefficient damped system to the ground. Brownout said 3.4Wsec per step delivered to the ground, I said 1Wsec per step and even that was probably generous. And they still have to convert that to electricity.

Are you saying that walking is more efficient than biking? ...

Nope, I used the generalisation of a normal man at 200W MAX produced with his legs, just to get a base idea of how much power the man is making with his legs.

 

I stand by 3.4J. The idea is the walker is lifting himself out of a shallow depression, not walking on a perfectly straight walkway. The simple calculation get us very close in this case.

 

Correct, but the peak force is for a very small percentage of the time of the total cycle.

Yep. The peak can be a pretty high number (10x or so), but what counts (and which I used in my first response) is the average, which will still be greater than the person's weight in order to cause the net acceleration.

Also keep in mind that the entire body is not lifted in walking, more the torso which is 60% of the total bodyweight.

Agreed, but that is why I have been using the change in height of the center of mass, in which case you use the entire weight. If part of the body doesn't rise, then the rise in the center of mass will be less than the rise of the portions of the body that do rise.

Then add in the damping effect of viscous muscles and rubber shoes both efficient at removing a lot of the energy before the ground, and elasticity in both so that much of that vertical movement is coming from bounce and not delivered to the ground.

Agreed. My point is that the claimed numbers are absurd even if you allow ALL of that energy to be delivered to the ground. Hell, I even allowed all of the energy associated with the total Calories that are burned during walking to be delivered to the ground and the numbers claimed are still absurd!

You can't use a simple math formula to quantify such a complex process.

But you can bound it and if the claimed numbers are way on the wrong side of the boundary, you have an answer.

I like my solution better; we know a typical man will produce maybe 200W max with his legs at peak power ie heart pounding and sweat pouring.

The problem is that it doesn't produce a relevant bound. This is a number for the useful output, not the total energy expenditure including waste energy. The manufacturer is claiming to recover waste energy, so you need to bound the waste energy.

Consider an engine that produces 10kW of useful output. Someone comes along and says that they have a technology that can capture 20kW of waste energy from it. Is this impossible? You can't make that claim. What if the engine consumes 210kW of energy in order to produce that 10kW of useful output? The engine is wasting 200kW of energy and the person is only claiming to capture 10% of that waste. So the fact that they're claiming that their device is able to capture twice the energy output of the engine does not make it impossible.

In this case, to refute your line of reasoning, all the manufacturer has to say is that the human body is so terribly inefficient that a huge fraction of energy is wasted in ground/foot interface and they are just capturing that energy. It may be an outright lie, but your argument doesn't refute it.

 

I stand by 3.4J. The idea is the walker is lifting himself out of a shallow depression, not walking on a perfectly straight walkway. The simple calculation get us very close in this case.

Close to what? You calculated the amount by which the walker's gravitational potential energy increased. Upon what basis do you make the claim that the energy expended and delivered to the sidewalk to make that happen is not significantly more than that?

The degree to which you are establishing a bound at all, you are establishing one such that the manufacturer's claim places them on the side of the bound they are claiming to be on and, as such, cannot address whether or not their claims places them so far away from that bound as to be unbelievable.

 

Close to what?

Close to the energy that can be recovered.

You calculated the amount by which the walker's gravitational potential energy increased. Upon what basis do you make the claim that the energy expended and delivered to the sidewalk to make that happen is not significantly more than that?

Upon the basis of conservation of energy.

The degree to which you are establishing a bound at all, you are establishing one such that the manufacturer's claim places them on the side of the bound they are claiming to be on and, as such, cannot address whether or not their claims places them so far away from that bound as to be unbelievable.

I don't even know what that means.

 

Upon the basis of conservation of energy.

There are two opportunities for this tile to recover waste energy. One as the person steps down and one as the person pushes off. All you have shown is a bound on the MINIMUM amount of energy that the person converts into kinetic/potential energy of their body as they push off. They could have (and in practice do) put more. You don't bound how much more, so the maker of the tile can claim that it is a lot more and you haven't shown that it can't be. It is true that the energy recovered as they step down can't exceed the amount that actually made it into the kinetic/gravitational energy of their body from their body (on average), but you leave completely unbounded how much additional energy was wasted in getting that energy into their body and the maker can simply claim that it is a lot and that they are harvesting that energy.

Imagine I have a fire and I put turbine fan above it that captures some of the energy and turns a motor that outputs 1kW. Now someone claims to have a device that they surround the fire box with and that can produce 2kW of power from the waste heat of the fire. Does the fact that the fire was previously only resulting in a useful output of 1kW mean that it is not possible to recover 2kW of additional power from the waste heat?

 

Actually, what I've shown in the MAXIMUM amount of energy that can be harvested from a person stepping on the tile. You can either harvest the potential energy, or convert it to potential energy for harvesting on the step up, but you can't do both. And I don't believe their harvesting anthing else, according to the write up. It wouldn't be practical anyway.

 

When you step up, your muscles exert a downward force on the tile that exceeds your weight in order to accelerate your body upward by some amount. Some of the energy involved, A, is dissipated in the tile, some of it, B, goes into your kinetic and potential energy, and some of it, C, is expended as heat. The total energy expended by your body is D = A+B+C. Some fraction of A can be harvested by the tile on the step up. On the step down, you have to dissipate B, somehow, in order to return to your starting state. Thus, the total energy available for harvesting is A+B. The actual energy harvested is aA+bB where 'a' and 'b' are the harvest efficiencies of the step up and step down, respectively. Thus, but putting a lower bound on B, you are establishing a MINIMUM amount of energy that is available for harvesting. They can simply claim that A is large and that 'a' and 'b' are close to one. The actual energy harvested is also dD, where once again 'd' is an efficiency between 0 and 1. By establishing an upper limit for D, then the it places an upper limit on the amount of energy that can be harvested because they can only claim a 'd' no large than 1.

 

I don't understand everything you two are talking about, but from my layman's perspective it seems you're looking at the energy used to walk in a straight line on level ground, and then trying to figure out how much of that can be captured. That's the wrong way to look at it in my opinion. The presence of the tiles changes the terrain. It's no longer level ground. You should be looking at it more from the perspecive of walking up stairs. Walking on tiles that sink down when you step on them, you're going to be perpetually stepping up out of a hole. So it's totally plausible to me that, by changing the sidewalk into a stairstpepper machine, they would be able to harvest more energy into their streetlight system than what's typically expended by a person walking on level ground. It would be much more of a workout than walking on a normal sidewalk. Like walking on mattresses, or sand, or up a staircase with 5mm steps.

 

@WBahn,
I understand what you're saying. But the dissipated energy, A, would be a tiny fraction of the potential energy, and not practical to try to harvest. Anyway, although the write-up was vague, I don't think they are even trying to harvest the dissipated energy, impractical as it is.

 

There's no basis upon which to claim that A is even smaller than B, let alone only a tiny fraction. In many instances it is not. A typical engine wastes considerably more energy than it converts to useful work. And whether it's practical, or even possible, to harvest it is largely beside the point because the snake oil salesman can simply make the claim. But if you show that the energy isn't available to begin with, then they no longer have a hook to hang their claims on.

 

The presence of the tiles changes the terrain. It's no longer level ground. ...or up a staircase with 5mm steps.

I think gym mats typically compress more than this and most people wouldn't notice much difference. Also, the 'gel' insoles that people wear may approach this as well (and would be yet another source of energy getting dissipated and made unavailable to the tiles, as I think someone else mentioned much earlier).

But the effect certainly does increase the energy expenditure and if my bounds don't account for it, then the snake oil salesman has a new hook upon which to hang his claims. We can consider it to be a staircase, but it is easier to think of it as a slope that rises 5mm per 2ft, or about 43ft/mi, or slightly less than a 1% grade. On a treadmill, measurements have shown that, as a rule of thumb, energy expenditure increases by 12% per 1% increase in the incline. Those are even better numbers to use because on an inclined treadmill your center of mass is not actually rising against gravity progressively with each step and this is the same for walking on the tiles. So we can increase the bounds I gave by twice that, say 25%, and take away the hook.

 

But we're not talking about typical engines, and there is pleanty basis to make the claim that the amount of waste energy is too small to try to harvest. We could calculate it, but I would consider that a waste of time, since I already know the outcome.

 

What's the basis? The increase in gravitational potential energy by walking up a 1% incline for a mile is about 3 Calories (for a 180lb person), yet that person will expend an additional 12 Calories doing it. The snake oil salesman will claim that they are harvesting a significant portion of that extra 9 Calories. It's not enough to say that you believe they won't be able to do it, you have to show that they can't. Or, you can come at it from the other direction and say, "So what? It doesn't matter. I'll assume you can harvest 100% of it. You still won't have the energy available to do what you claim."

 

I think gym mats typically compress more than this and most people wouldn't notice much difference. Also, the 'gel' insoles that people wear may approach this as well (and would be yet another source of energy getting dissipated and made unavailable to the tiles, as I think someone else mentioned much earlier).
.

True, but these things act as springs, probably returning the majority of the force that you apply to them, back into your foot. This thing takes the energy that would have been returned to your foot (if it were a gym mat) and uses it to light streetlights. I think walking in dry sand best illustrates the point I am trying to make. The sand really robs you of energy; it takes the energy from your foot, and turns it into little piles of potential energy surrounding your footprint.

 

What's the basis? The increase in gravitational potential energy by walking up a 1% incline for a mile is about 3 Calories (for a 180lb person), yet that person will expend an additional 12 Calories doing it. The snake oil salesman will claim that they are harvesting a significant portion of that extra 9 Calories. It's not enough to say that you believe they won't be able to do it, you have to show that they can't. Or, you can come at it from the other direction and say, "So what? It doesn't matter. I'll assume you can harvest 100% of it. You still won't have the energy available to do what you claim."

My baises is imperical data, logic and intuition. If you think they can be harvested, then show us how. Presently there are no good methods, and noting in the write up shows they have something to do it. If you have information the rest of us don't, then share it. Otherwise, this discussion is just getting silly, and pointless.

 

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In this case, to refute your line of reasoning, all the manufacturer has to say is that the human body is so terribly inefficient that a huge fraction of energy is wasted in ground/foot interface and they are just capturing that energy. It may be an outright lie, but your argument doesn't refute it.

You made some good points and I don't disagree with any of them, except maybe for "refuting";

Their claim that their device; "A single step provides sufficient power to run an LED street lamp for 30 seconds"... Considering a streetlamp must be something like 20 watts, that's 20*30 or 600W seconds and a "single footstep" of 0.5 second would mean the person is producing a continuous average 1200W! That is thoroughly ridiculous (and re my earlier post) a person producing 200W is at their peak exertion on an efficient bike with heart pounding etc.

As far as to recover energy from "walking" the available energy is miniscule. You can't calc based on vertical movement of a device and applied mass as that is not "walking" it becomes an additional load applied to the normal walking, so (as Strantor has said) this is not recovering energy from "walking" but would be like walking on sand, or up a flight of stairs, ie a high additional load on top of walking.

Casual human walking is highly efficient and has pendulum and sprung properties. As an analogy you could make a model with a 1 pound weight on top of a spring, on a concrete floor. You can keep the weight bouncing 1" up and down with a tiny amount of power. The energy that can be extracted from that efficient process is NOT found by a calculation of 1 pound mass and 1" vertical distance.

 

The article says the device harvests energy from people stepping on it, and doesn't say anything about 'casual' walking. So, in fact, you can use the vertical movement and mass to calculate the energy harvested. Casual walking, however that is defined, is irrelevant.

 

I have a strong feeling that the bike would be more efficient.

True generally. Except when going up steep hills. Then the bike loses all of it's mechanical advantage.

In my younger days doing sports, we used to sprint hills as an intense workout. Usually we would do 200 m sprints up a steep hill and then jog back down. After 10 cycles, we were pretty much wiped out. Very often bicyclists would be going up the hill at the same time. We would always beat them up to the top while sprinting, even if they were using all of their strength.