Human beings need energy to survive. But most of the time, we arent aware that we too generate a considerable amount of energy through our actions. Imagine the energy produced as we walk and step on things. Even the beating of our hearts produces energy. What if we could convert all this mechanical energy into electricity? Scientists are taking the first steps towards doing that, paving the way for small devices to power themselves.
At the University of Princeton, the US, assistant professor Michael McAlpine has developed ceramic nanoribbons embedded in silicon sheets that produce electricity when flexed even gently. In the near future, they could be placed in shoes to charge mobile phones, or embedded in cardiac pacemakers. Last week, the US Patent Office published a patent by Nokia for a self-charging phone using similar principles. And at the Georgia Institute of Technology, Professor Zhong Lin Wang is using zinc oxide and other materials to convert tiny flutters into electricity. We have crossed most of the technological hurdles for commercialisation, says Wang.
Small devices have penetrated our lives in a big way. They are charged usually by electricity or powered by small batteries. In many cases this presents problems. For example, using batteries inside our bodies is risky because they can leak. And if they drain out, replacing them is a problem. How do you periodically pierce the skull and change batteries of implants in the brain? Moreover, there are many situations where external devices need to be charged when electricity is not available. The ability to convert even small amounts of mechanical energy into electrical energy is useful in these circumstances.
Consider the work of McAlpine. Two months ago, he published research in the journal Nano letters. This was purely about the science behind a method to convert mechanical energy into electrical energy. McAlpine and his team used ribbons of lead zirconate titanate on rubber sheets. This material has one of the highest efficiencies known for converting mechanical energy into electrical energy, but it usually crystallises only at high temperatures. The Princeton scientists, with help from those from the California Institute of Technology, developed a scaleable process for making these materials.
Lead zirconate titanate belongs to a category of materials called piezoelectric. These materials produce a voltage and hence a current when flexed. And when a voltage is applied to them, they flex. These properties make them very useful, if we know how to exploit them. Some piezoelectric materials are biocompatible and hence suitable for use inside the body.
In the last two months, McAlpine has made a small device using these principles and applied for a patent. He has had enquiries from private companies, including a major shoe manufacturer. You can use this method to light up the shoe without batteries, says McAlpine. He has in mind more serious uses as well, including charging cell phones by walking. Putting these devices in malls anywhere where people walk to generate electricity is also not a far-fetched idea. If you walk briskly, each strike of the heel, in theory, can generate 67 watts. If you convert a small fraction of this power into electricity, it would be enough to power many devices.
The major advantage of McAlpines method is the efficiency it can convert 80 per cent of the mechanical energy into electricity. At Georgia Tech, Wang has prototype devices that can already do this job, but by using a material called zinc oxide. They are not as efficient as lead zirconate titanate, but Wang has now developed a new method using a material called gallium nitride. This is 10 times as efficient, and the work is going to be published soon. Wang has floated a company called Piezodyne to commercialise his technology.
Wang, who has recently been elected to the Chinese Academy of Sciences, is pursuing a dream: to power a network of sensors around the world. In the next few years, sensors are going to proliferate in our world for many reasons. They will provide inputs about the electricity grid, the weather, security, resources, our health and other vital data. Such data will be essential for the smooth running of our society.
Powering these sensors will be a major problem as their number could run into millions over the years. Many of them would be in places like mountaintops where electricity is not available. Moreover, they could be in places where it is not wise to use batteries. Unless we develop a method to power these sensors safely, reliably and in an environment-friendly way, we may not be able to deploy them widely.
People never used to worry about such things a few years ago, says Wang. Now energy is one of the most critical issues facing us.
Wangs dream is to power these sensors through piezoelectrics. And he feels this will soon come true, as there are no more major technology hurdles.