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Dario D.
01-31-2010, 08:03 AM
Does anyone here know if you can harvest enough energy from pressure-plates to be worth anything?

I'm trying to think if there could be some *meaningful* way to collect energy from the weight of people walking on a surface, or cars driving over a road, by using pressure-sensitive plates that move very slightly. (or some other form of generator that responds to pressure)

What I heard was that you can collect energy from "gravitational potential energy", but the object has to be able to move... (in other words, you can collect energy from a boulder that you set on a scale, but only as long as it moves downward)

Any ideas?

I'm trying to figure out if any of these would ever make sense in the future:

- Pressure-plates on the sides of tall buildings, that are compressed by the wind
- Sensors under carpets that produce energy when stepped on
- Pressure-plates under floors/roads that respond to "gravitational potential energy"

I *THINK* the only question is whether or not these things could produce enough energy to be meaningful, but I don't yet know exactly how energy-harvesting works.

RainyDayNinja
01-31-2010, 08:32 AM
You should read up on piezoelectrics (http://en.wikipedia.org/wiki/Piezoelectricity). I believe there is talk about putting them in highways to harvest electricity from the cars driving on them. Also, using piezoelectric materials in shoes could provide enough power to run all of your personal gadgets (iPod, phone, etc.). As far as I know, this is all in the theoretical stage, though.

benbradley
01-31-2010, 08:54 AM
Does anyone here know if you can harvest enough energy from pressure-plates to be worth anything?

I'm trying to think if there could be some *meaningful* way to collect energy from the weight of people walking on a surface, or cars driving over a road, by using pressure-sensitive plates that move very slightly. (or some other form of generator that responds to pressure)

What I heard was that you can collect energy from "gravitational potential energy", but the object has to be able to move... (in other words, you can collect energy from a boulder that you set on a scale, but only as long as it moves downward)
Yes. This provides energy much like a weight on a string powers a mechanical (as in grandfather) clock. Whether it's practical, well, it depends.
Any ideas?

I'm trying to figure out if any of these would ever make sense in the future:

- Pressure-plates on the sides of buildings that are compressed by the wind

This would generate very little energy. Some when the wind speed increased, and some when it decreased. You would get a lot more energy by putting a windmill on top of the building.

- Sensors under carpets that produce energy when stepped on

Yes, this is possible, but it would not be much energy. If this is in a typical building with electric power, it's easier to run a power cord to whatever you want to power.

- Pressure-plates under floors/roads that respond to "gravitational potential energy"

This would work, but you would be effectively stealing energy from the people or vehicles that go buy. Each plate would sink slightly as something rolled or stepped on it, providing movement to power some generator (a rack-and-pinion type thing or whatever), but the person/vehicle would be going slightly uphill all the time (stepping up from one plate to the next, which would then sink down), taking more energy to travel a distance than if the road stayed flat. This isn't really "free" energy in any sense.


I *THINK* the only question is whether or not these things could produce enough energy to be meaningful, but I don't yet know exactly how energy-harvesting works.
A plate or weight dropping to generate energy is "simple" physics. Something dropping through a gravity field converts potential energy into kinetic energy. And there are other energy sources that can be harvested.

Also, it depends on what you mean by meaningful. Many electronic devices thesedays work on very small amounts of energy. A digital watch is one such device that comes to mind - most run for several years on one small battery.

There's a sign with a flashing yellow light on a highway in a rural area "in the middle of nowhere." It's got a solar panel on top of it and a controller box below. The solar panel provides enough power to run the light in the daytime and also to charge a battery in the box that powers the thing at night. No doubt it's a somewhat expensive and complicated contraption, but it's cheaper than adding utility poles to run a power line from perhaps a mile away to power the thing. That's one example of "energy harvesting."

Here's a similar example:
http://www.intelligent-systems.info/bridge.htm

These examples provide very low amounts of energy (the thing on the bridge could perhaps turn on a flashlight bulb for a second with the amount of energy it collects for an hour), but are cost-effective solutions - powering these devices in any other way would be more expensive.

If this is an idea for an alternative method to generate energy for buildings and cars, it won't work. You can't get substantial power out of energy harvesting, unless you consider energy harvesting to include large solar panels, large windmills and traditional hydroelectric plants.

Dario D.
01-31-2010, 09:33 AM
Oh man, I love your avatar. :eek:
Sorry, I'll reply when I can get past this...

Mac H.
01-31-2010, 09:56 AM
The BIGGEST problem with what you are describing is simply understanding the scale involved.

Imagine your piece of equipment extracted the amount of energy from people walking.
How much energy could you extract?

Well - someone riding a bike for 30 minutes can burn about 4kJ - let's double that for a safety margin giving us about 8 kJ. So let's use that as a guide - imagine your expensive equipment could extract that amount of energy from the carpet every single day.

That's pretty good - after all, it's free energy, right ?

Well, around here energy costs (retail) are the equivalent of about 15.17 cents USD ... per kilowatt hour.

So to figure out how much that energy you've extracted is worth, type this into google:
[ (8 kJ)*15.17 (cents/kWh) in cents ]

The answer ? 0.0337 cents. After a year you've saved about 12.3 cents.

That's how much money you'll be saving by extracting energy from the carpet !

So if you can run wires to the equipment it will never be competitive. Never.
(And putting piezoelectric materials in your shoes could never provide enough power to run all of your iPod, phone, etc! )

---

However, the good news is that even though can only extract tiny, tiny,tiny amounts of energy this is still useful when you don't have wiring.

For example, there are projects putting piezoes 'cat eye' reflector bumps by the side of the road. This extracts enough energy from cars running over them to power the tiniest of LEDs in them .. so they can still glow by the side of the road in darkness.

(Whoever manages the highway could just 'recharge' them by running the maintenance vehicle over the bumps deliberately.)

This works because in total darkness a tiny LED can still be very visible - even though it takes the tiniest amount of current. We aren't talking high brightness here ... just little pinpricks of light flicking on once every few seconds for a brief moment to mark the edge of the road.

The main competition to energy harvesting for non-wired devices is the fact that battery technology is getting pretty good now. If I have a piece of equipment that I want to run the for next 10 years without wiring, I could put an energy harvester in it with unknown reliability .. or I could just put a precharged battery in it. (There are some pretty fancy batteries with very low self-discharge)

Mac

RJK
01-31-2010, 08:17 PM
How about installing piezo-electric crystals in truck wheels? A 70 ton, 18-wheeler would generate nearly 8,000 pounds of pressure on each wheel. Make the truck a hybrid, and the wheels could charge the batteries as they turn.

Tsu Dho Nimh
01-31-2010, 09:10 PM
The problem is that the energy harvested from the pressure plates comes by sucking it from the cars ... the extra flex required to make the piezo-electric stuff work lowers the efficiency of the vehicles.

For pedestrians it's less of an issue, because people like a bit of cushion on a walking surface.

Kathie Freeman
01-31-2010, 10:21 PM
Theoretically it could work, but in fact energy-generating shoes already exist. Check out http://www.technovelgy.com/ct/Science-Fiction-News.asp?NewsNum=1932 This technology uses water sloshing around to generate enough wattage to power an Ipod or cell phone.

Kitty Pryde
01-31-2010, 10:28 PM
They are using this in some train stations and places with loads of pedestrians: http://www.telegraph.co.uk/earth/energy/3721841/Japan-harnesses-energy-from-footsteps.html

"An average person, weighing 60 kg, will generate only 0.1 watt in the single second required to take two steps across the tile," said Yoshiaki Takuya, a planner with Soundpower Corp. "But when they are covering a large area of floor space and thousands of people are stepping or jumping on them, then we can generate significant amounts of power."

Mac H.
02-01-2010, 02:46 AM
How about installing piezo-electric crystals in truck wheels? A 70 ton, 18-wheeler would generate nearly 8,000 pounds of pressure on each wheel. Make the truck a hybrid, and the wheels could charge the batteries as they turn.That's equivalent to connecting a generator to an electric motor, and using it to generate the electricity to run the motor !

Theoretically it could work, but in fact energy-generating shoes already exist. Check out http://www.technovelgy.com/ct/Science-Fiction-News.asp?NewsNum=1932 This technology uses water sloshing around to generate enough wattage to power an Ipod or cell phone.The article claims it generates 1.2 Watts.

But the maths doesn't add up - unless it feels like you are walking around with bricks on your feet !

Mac

Mac H.
02-01-2010, 10:32 AM
They are using this in some train stations and places with loads of pedestrians: http://www.telegraph.co.uk/earth/energy/3721841/Japan-harnesses-energy-from-footsteps.html

"An average person, weighing 60 kg, will generate only 0.1 watt in the single second required to take two steps across the tile," said Yoshiaki Takuya, a planner with Soundpower Corp. "But when they are covering a large area of floor space and thousands of people are stepping or jumping on them, then we can generate significant amounts of power."

And let's see how much electricity they are generating with 2000 people over an 8 hour day:

google: [ (2000 * 0.1 watts) * 15.17 (cents/kWh) * 8 hours ]

Yep - 24 cents of electricity every day. After a year you've generated $89 worth of electricity.

Those kind of things are just media stunts - not reality.

Mac

benbradley
02-01-2010, 10:53 AM
And let's see how much electricity they are generating with 2000 people over an 8 hour day:

google: [ (2000 * 0.1 watts) * 15.17 (cents/kWh) * 8 hours ]

Yep - 24 cents of electricity every day. After a year you've generated $89 worth of electricity.

Those kind of things are just media stunts - not reality.

Mac
That's the point I've tried to make, none of these energy harvesting techniques can compete with either the low cost or the large amount of power available if you can easily connect to the electric power grid or to a generator running off a motor as in a vehicle. Energy harvesting only exists in practical application for places where only a little electric power is needed, and no power is available from any traditional source.

Kitty Pryde
02-01-2010, 08:49 PM
And let's see how much electricity they are generating with 2000 people over an 8 hour day:

google: [ (2000 * 0.1 watts) * 15.17 (cents/kWh) * 8 hours ]

Yep - 24 cents of electricity every day. After a year you've generated $89 worth of electricity.

Those kind of things are just media stunts - not reality.

Mac

That's with 2000 people a day, but this train station gets 400,000 people a day! So that's more like $1800 a year. But what if ALL the main tiles people walk on in the station are replaced with piezoelectric tiles? 100 tiles in that station would generate $180,000 a year. Still not a ton, but it's something.

thothguard51
02-01-2010, 09:55 PM
That's with 2000 people a day, but this train station gets 400,000 people a day! So that's more like $1800 a year. But what if ALL the main tiles people walk on in the station are replaced with piezoelectric tiles? 100 tiles in that station would generate $180,000 a year. Still not a ton, but it's something.

The cost has to equal or be better than traditional sources, otherwise it is not worth the effort. Maintenance and replacement parts for hybrid systems are generally more expensive than traditional sources. A lot of local government agencies are not going to invest in a system that has higher cost or upkeep because they do not have the budgets. Private companies using such systems will do so on a limited basis, until such time as the cost savings exceed traditional methods.

veinglory
02-01-2010, 09:59 PM
The cost has to equal or be better than traditional sources, otherwise it is not worth the effort.

Unless it has added value for "green" reasons, like publicity, the lack of pollution tariffs, willingness of sponsors to pay for upkeep etc.

BillPatt
02-02-2010, 01:10 AM
Unless it has added value for "green" reasons, like publicity, the lack of pollution tariffs, willingness of sponsors to pay for upkeep etc.

Except for the publicity value, your other two options represent a cost shifting from the operator of the facility to other parties. The total costs remain the same, therefore the economics of the scheme remain unfavorable over the long run.

veinglory
02-02-2010, 01:17 AM
If you can shift the costs off your books they are essentially gone so long as that remains the case. You can hardly say, for example, that Recaptcha is paying to have book digitised. They have found a way to get people to do it for them for free without feeling ripped off. Clean energy has similar potential to exploit people without them objecting. Not just the foot traffic making the energy but potentially the group that pays for the upkeep in return for advertising space. The good PR can, itself, be monotised in the way a smoke stack can't be.

Sarpedon
02-02-2010, 01:41 AM
Just wait to see how cost effective it is when someone trips on the damn thing and sues them.

abctriplets
02-02-2010, 01:48 AM
I first read this as harvesting energy from the pressure between tectonic plates....

Mac H.
02-02-2010, 03:51 PM
That's with 2000 people a day, but this train station gets 400,000 people a day! So that's more like $1800 a year.I'm afraid not.

That was assuming that that 2000 people all walk the tiles non-stop for 8 hours continually.

And the reason I used '2000' people was that the article claimed that it was significant with 'thousands' of passengers.

Let's assume that there are 400,000 people a day, each of them spending 5 minutes walking within the station every morning, and 5 minutes walking in the evening. (The time spent standing waiting for the train doesn't generate anything.)

google: [ 400000* (0.1 watts) * 15.17 (cents/kWh) * (10 minutes / day) in dollars per day (http://www.google.com/search?q=400000+*+(0.1+watts)+*+15.17+(cents/kWh)+*+(10+minutes+/+day)+in+dollars+per+day) ]

Yep - about a dollar of electricity every day. Even worse ! (Since they aren't walking around continually)

After a year you've generated $368 worth of electricity.

Perhaps I was too conservative. Let's imagine that instead of taking 5 minutes every morning that they have 400,000 passengers spending an entire hour walking around the station in the morning non-stop, and another hour at night. Every single day of the year.

google: [ 400000 * (0.1 watts) * 15.17 (cents/kWh) * (2 hours/day) * 1 year (http://www.google.com/search?q=400000+*+(0.1+watts)+*+15.17+(cents%2FkWh )+*+(2+hours%2Fday)+*+1+year) ]

Even with the most optimistic projections: $4,400 per year.

So even if you've convinced 400,000 people to devote 2 hours of their life every frigging day of the year to generate electricity for you ... you can only get $4,400 per year !!! That won't even cover the energy cost to install the damn things.

But what if you made the tiles compulsory .. so every surface in the entire city is covered ?

Let's imagine that the government also made it illegal to NOT walk. People are permitted 8 hours sleep, but every other moment of their existence they need to be walking on the pressure plates.

Each person would generate ...

google: [(0.1 watts) * 15.17 (cents/kWh) * ((1 day - 8 hours)/day) * 1 year] (http://www.google.com/search?hl=en&q=(0.1+watts)+*+15.17+(cents/kWh)+*+((1+day+-+8+hours)/day)+*+1+year)

... about 9 cents of electricity every year.

So even making the tiles as widely available as possible isn't going to help.

The maths simply doesn't support it.

Even if the efficiency of the tiles increased by a hundred fold (which would probably violate the laws of physics) ... you would still only have $9 per person being generated by the slaves. Slave labour isn't a good way to generate electricity.

Mac

GeorgeK
02-02-2010, 05:11 PM
I don't recall the link, but I read about it being used, I think in Israel as a test section of road and they found that if it's placed on a busy section of highway it can power the street lights

abctriplets
02-02-2010, 07:18 PM
I don't recall the link, but I read about it being used, I think in Israel as a test section of road and they found that if it's placed on a busy section of highway it can power the street lights

Street lamps bright enough to read under during the daytime!

Mac H.
02-03-2010, 09:11 AM
I don't recall the link, but I read about it being used, I think in Israel as a test section of road and they found that if it's placed on a busy section of highway it can power the street lightsIt is probably this system: http://www.inhabitat.com/2008/12/16/energy-generating-roads-by-innowattech/

It's by an Israeli company.

This is more like it. Basically the strategy is simple ... drive up the fuel bills of all the cars and extract some of the energy as electricity.

It's a lousy deal for the people on the road, and a bad one for the environment (since cars aren't the most environmentally suitable way to generate electricity) but a good deal for the people using the electricity.

This is the way to steal energy ... not the dubious one of going by people walking !

Mac
(PS: They claim it harvests only waste energy. I'm not convinced - especially when we consider the change in fuel efficiency a change in tire pressure makes. I'd expect intuitively that this would be similar.

It also seems that the road surface would normally get extremely hot if a 10 metre stretch with a car every 10 seconds is really generating 240 watts normally.

A quick crunch of the numbers from the February 2009 trials don't add up.

Then they say that they calculated that they could generate 1kWh of energy .. if they had a 17 ton truck passing every four seconds at 72km/h.

But that confuses energy and power. They should say '1kW' not '1kWh'.
They claim also that they simply used 'an electric lamp' to indicate the amount of energy harvested by the system. The whole point of a trial is to take proper measurements ... not use an electric lamp and then calculate how much energy COULD be generated !

--

However, it seems like a technology that would be very useful for road-side applications like Billboards, etc.

But notice how none of their trials verified their claims that it doesn't affect fuel consumption !)

GeorgeK
02-07-2010, 06:13 PM
However, it seems like a technology that would be very useful for road-side applications like Billboards, etc.

But notice how none of their trials verified their claims that it doesn't affect fuel consumption !)


The way you explain it seems to make sense. (I'm certainly no physicist) However, if you are right then I'd be very irate with such a system to power lights for billboards. That's up there with how in the late 90's some product and pharmaceutical businesses were faxing unrequested advertisements to doctors' offices so they would use our paper and ink. A roar of outrage to politicians soon made it illegal here, but in the meantime some of us were known to fax back a response of black construction paper taped into a loop.

Mac H.
02-09-2011, 04:44 AM
However, the good news is that even though can only extract tiny, tiny,tiny amounts of energy this is still useful when you don't have wiring.I just came across a nice article which led me to resurrect this old thread.

Energy Harvesting without Batteries (http://bit.ly/eAIFtv)

This method generates enough to power a transmitter which uses about half a milliwatt.

It's a nice implementation.

Mac

blacbird
02-09-2011, 05:01 AM
How about installing piezo-electric crystals in truck wheels? A 70 ton, 18-wheeler would generate nearly 8,000 pounds of pressure on each wheel. Make the truck a hybrid, and the wheels could charge the batteries as they turn.

Not much different in concept from what happens in any gasoline-powered car, when the battery is charged as the car runs.

You can't, however, get more power back than you put into any system. In fact, you can't ever get as much as you put in. Damn that Isaac Newton, anyhow!

Kenn
02-09-2011, 04:49 PM
Well - someone riding a bike for 30 minutes burns about 8 kJ.
This can't be right. I think you are confusing food calories (kCal) and calories.

Let's assume that there are 400,000 people a day, each of them spending 5 minutes walking within the station every morning, and 5 minutes walking in the evening. (The time spent standing waiting for the train doesn't generate anything.)

google: [ 400000* (0.1 watts) * 15.17 (cents/kWh) * (10 minutes) ]

Yep - a total of 9 cents of electricity every day.
Then Google is wrong by just over a factor of 10.

Sorry!

Mac H.
02-10-2011, 06:55 AM
Oops. You are right Kenn. That's the problem with me tweaking posts after I originally write them.

I'd changed the numbers and not updated the text. I made the calculations clickable to ensure that they are right now.

It's a bit sad that you are the first person to notice that I was out by a factor of ten. Just as well I was posting on a message board rather than doing calculations for the shuttle's O-rings...

The '30 minutes of cycling on a bike is 8 kJ' is near enough for a back-of-the-envelope calculation like this. It's high - but not drastically so given my other guesses.

For example, using a treadmill or exercise bike for a workout can burn 300 food calories (ie: kCal) in 10 minutes. So that's about 4kJ if you could keep it going for half an hour.

So I probably over-estimated by about double .. which I'd like to think is a safety margin of 100% in my calculations rather than a mistake.

Yep - that's how I'll explain it.

Thanks !

Mac

Kenn
02-10-2011, 04:28 PM
Still something not right, Mac H.

1 Cal = 4.2 J

So, 900 kCal (in 30 minutes) = 3780 kJ

It's the confusion between food calories and proper ones (CGS units) causing the problem.

Sorry again!

Kenn (PS I think you are right that these things are just a gimmick though)