Comment Comment by one of the inventors (Score 5, Interesting) 128
Hello Slashdot community. My name is Max Donelan and I am one of the inventors of this energy harvesting technology. I thought I might try and clear up some of the misconceptions that people have about what we did. Here goes.
When you walk, your muscles are constantly taking some of your mechanical energy away from your body and dissipating it as heat. Other muscles (or even the same muscles at a later time) are acting to put mechanical energy back in to the system. This is a little like stop-and-go driving. Perhaps more accurately, it is like driving with one foot on the gas and one foot on the brake. While walking this way may not sound like a good idea, it is what we do. We can take advantage of the fact that walking is inherently uneconomical to generate electricity economically. The idea is to use a generator to help the muscles in taking away the mechanical energy. But instead of dissipating it all as heat like muscles, the generator also produces some electricity.
Here is a thought experiment that may make it a little clearer. If you stand up from your chair, your muscles that run down the front of your leg act to extend the knee. They increase your mechanical energy because by the time you are standing you have more gravitational potential energy. When you sit back down, the same muscles are active but now their job is to take the energy away from your body and dissipate it as heat (your kinetic energy is the same whether you are standing or sitting but your potential energy is less when you are sitting). Unlike traditional car brakes, your muscles require substantial "gas" (i.e. food) to decrease the energy of the system. And muscles are totally different than an electric motor - if you run an electric motor in reverse it takes mechanical energy and produces electric energy (i.e. a generator) but when you run muscles in reverse, they don't take mechanical energy and produce chocolate bars (i.e. food or chemical energy).
OK, back to the thought experiment. If we were to couple a generator to your knee motion, it would always resist the motion. So, it would make it harder to stand up and easier to sit down. It would produce electricity in both directions. What if we had some way to engage and disengage the generator and we disengaged it when you are going from a sit to a stand and engaged it when you are going from a stand to a sit. While this would only produce electricity for half the time, it would actually make the whole task easier. You can get electricity and lower the effort required to do the task! Of course this requires you to already have the need to do the task and that is why it makes more sense to do it during walking.
For the commenters that think it is too heavy, they are right. We are a year in to the next version and you can check it out on http://www.bionic-power.com/ The graphic on the splash page will give you an idea of what it will look like. It will be less than 1 kg.
With regards to other energy harvesting technologies, I think they are all pretty cool. My favorite is the self-winding watch. The drawback is that it gets only about 5 micro watts. The shoes are all very cool and will likely serve a real need but they also get much smaller amounts of power. If you are already carrying a heavy load, the backpack is fantastic.
I am enjoying reading your comments so keep them coming!
When you walk, your muscles are constantly taking some of your mechanical energy away from your body and dissipating it as heat. Other muscles (or even the same muscles at a later time) are acting to put mechanical energy back in to the system. This is a little like stop-and-go driving. Perhaps more accurately, it is like driving with one foot on the gas and one foot on the brake. While walking this way may not sound like a good idea, it is what we do. We can take advantage of the fact that walking is inherently uneconomical to generate electricity economically. The idea is to use a generator to help the muscles in taking away the mechanical energy. But instead of dissipating it all as heat like muscles, the generator also produces some electricity.
Here is a thought experiment that may make it a little clearer. If you stand up from your chair, your muscles that run down the front of your leg act to extend the knee. They increase your mechanical energy because by the time you are standing you have more gravitational potential energy. When you sit back down, the same muscles are active but now their job is to take the energy away from your body and dissipate it as heat (your kinetic energy is the same whether you are standing or sitting but your potential energy is less when you are sitting). Unlike traditional car brakes, your muscles require substantial "gas" (i.e. food) to decrease the energy of the system. And muscles are totally different than an electric motor - if you run an electric motor in reverse it takes mechanical energy and produces electric energy (i.e. a generator) but when you run muscles in reverse, they don't take mechanical energy and produce chocolate bars (i.e. food or chemical energy).
OK, back to the thought experiment. If we were to couple a generator to your knee motion, it would always resist the motion. So, it would make it harder to stand up and easier to sit down. It would produce electricity in both directions. What if we had some way to engage and disengage the generator and we disengaged it when you are going from a sit to a stand and engaged it when you are going from a stand to a sit. While this would only produce electricity for half the time, it would actually make the whole task easier. You can get electricity and lower the effort required to do the task! Of course this requires you to already have the need to do the task and that is why it makes more sense to do it during walking.
For the commenters that think it is too heavy, they are right. We are a year in to the next version and you can check it out on http://www.bionic-power.com/ The graphic on the splash page will give you an idea of what it will look like. It will be less than 1 kg.
With regards to other energy harvesting technologies, I think they are all pretty cool. My favorite is the self-winding watch. The drawback is that it gets only about 5 micro watts. The shoes are all very cool and will likely serve a real need but they also get much smaller amounts of power. If you are already carrying a heavy load, the backpack is fantastic.
I am enjoying reading your comments so keep them coming!
