Published: Sunday, Jan 17, 2010, 2:10 IST By R Krishna | Place: Mumbai | Agency: DNA
Unreliability, inefficiency and high cost have been the bane of clean technologies such as solar energy and electric vehicles. That’s where nanotechnology comes in, because it holds the key to making these technologies commercially viable. George Crabtree, senior scientist at Argonne National Laboratory, USA who visited the Tata Institute of Fundamental Research in Mumbai this week, explains to DNA why nanotechnology is taking centrestage in the development of sustainable energy
Why have alternative sources of energy like solar and wind not become commercially viable? One simple reason — cost. Fossil electricity is cheaper to produce and it is cheaper to drive your car with gasoline than bio fuels. The energy industry will take the cheapest thing available. Of course, some costs, like the cost of the CO2 emissions, are ignored. Wind is more technologically ready than solar. In the windy areas of the US, electricity produced by wind is competitive to fossil fuels. I think the next big push will be wind rather than solar. Solar still has a cost issue. That cost has to do with materials. Plus, how well does the solar work? The best solar cells we get now are made of silicon and have about 20 per cent efficiency. In the best natural gas plants, the efficiency is about 60 per cent.
What do you mean by efficiency? Efficiency is electricity out versus energy in. If you take the energy that is there in natural gas and ask how much of that is turned into electricity, it can be 60 per cent. If you take the energy of the sunlight striking the solar cell, only 20 per cent comes out as electricity.
How will nanotechnology make a difference? Nanotechnology is everywhere. If you think about solar energy, you are taking a photon from the sun and turning it into an electron, which then goes into the grid.
So, a photon, whose size is about one micron, strikes the semiconductor. This photon liberates one electron and one hole. Then you have to conduct them to separate sites of the solar cell and connect them to the external circuit through wires. Those three things — absorption, charge separation and conduction — all have to be done by the solar cell. All this is a nanotechnology phenomenon.
The better we understand how a photon turns into an electron, the better we can control the process.
But how will this understanding make alternative energy commercially sustainable? For fossil fuels, the first thing we do is burn it. And we learned 150 years ago how to turn the heat thus generated into motion — like in an automobile engine or in a turbine. All that’s required are materials that can withstand the conditions of the conversion process.
But alternative energy is high tech, and doesn’t use heat. For example, in solar, you take a photon from the sun at room temperature to produce an electron. That is a much more complex material and a much more complex process. So we have to understand those materials and those processes.
The research we have to do is in order to make these technologies efficient, which will bring the cost down. If you have 10 photons and only one of them comes out as an electron, you need to get that number up to five instead of one. Then for the same cross-sectional area you get five times the electricity, and that makes solar five times cheaper. How far have we come in nano research? Nanotechnology is complex. You have to develop the tools which can see at the nano scale. It is only in the last 3-4 years that we have been able to get that with x-ray sources. The next step is to control what happens at the nano scale. You do that with things like lithography or self-assembly — you make materials with atoms positioned where you want them to achieve a certain functionality. Although it has been 10 years, we are only at the beginning of what nanotechnology can do.
Any examples of what has been achieved? Well, bio fuels are all nanotechnology. You may want to genetically alter plants so that instead of producing sugar they produce a fuel you can use like natural gas. The other way of using biology is to imitate it. Green plants somehow take photons from the sun at room temperature, crack water to produce hydrogen and they crack CO2 as well, and then they produce hydrocarbons. If we can understand that process, we can imitate it.
What problems is research trying to solve? The first is electrifying transportation. Instead of using gasoline why don’t we run the vehicle on electricity? In order to do that, we need better batteries. Now, the best battery we have is lithium ion and the energy it carries per unit weight as compared to gasoline is one-thirtieth or less. That tells you why we drive around with gasoline. You want to make better batteries and that again needs nanotechnology — you need to understand the process at the level of one charge.
Second is a solar cell. Silicon by itself is limited to 32 per cent efficiency. The problem is that silicon just absorbs one wavelength of the sunlight, and the rest is wasted. You want to have multi-junction silicon or other solar cells which take a greater fraction of the sun’s spectrum. So if you have a solar cell tuned to three different wavelengths, you can get three times the efficiency.
Recently there was a report on how green tech is causing deforestation in China where rare earth elements are mined. Will the need for new materials create new problems? Lithium too went through the same questions last year. You can ask the same question about platinum: There isn’t enough platinum to run every car on a fuel cell where it is an important catalyst. One way out is to plan in advance to develop those technologies which use materials that are abundantly available.
For solar cells, there’s more silicon in the world than we know what to do with. But some other solar cell technologies use cadmium telluride — I wonder whether there’s enough tellurium to go around. I saw a study recently where they looked at 20 different solar cell technologies, and asked what materials they needed. And if you are going to supply a big number like 30 per cent of the world’s energy, how much of those materials would you need, and is there enough? It turned out that for more than half of those technologies, there wasn’t enough material. So we shouldn’t be developing those technologies, at least not so as aggressively as the ones for which there is abundant material.