Energy conversion is not an efficient process. Only a small part of the energy that is produced is used productively, the rest is wasted as heat. For example, when we burn fuel in a car engine, only about 30% of the total energy content is used to drive the vehicle. The rest is released as heat, which is wasted. It is possible to tap this waste heat and use it for other meaningful purposes. It requires further conversion of heat energy into something more convenient, like electric energy, which can be used for various purposes.
This conversion too is inefficient, and typically not more than 15% to 20% of the waste heat can be utilised. However, as the demand for energy continues to grow, along with a greater emphasis on increasing efficiency, this additional source of energy is not insignificant, if produced economically.
Scientists have been trying to tap this waste heat, and convert it into electrical energy, by utilising a well-known physical phenomenon called thermo-electric effect. It has been known for long that if two ends of a electrically-conducting material, like a metal, are maintained at different temperatures, a current flows from the hot to the cold end.
This phenomenon makes it possible, for example, to generate a current in a metallic attachment that is connected to the exhaust pipe of an automobile engine. The end connected to the exhaust pipe will be hotter than the one that is not, thereby inducing a current, which can be tapped by connecting the other end to a battery or some other device.
Although it may appear to be simple, there is a huge obstacle. The problem is that most materials which conduct electricity are also good conductors of heat. Such a material is useless for thermo-electric effect because both the ends gets heated up and there is no temperature difference to exploit.
Traditionally, thermo-electric effect has been demonstrated by using two different metals joined together, and by mechanically maintaining different temperatures at the ends. For the kind of solution that is being explored for tapping waste heat, mechanical maintenance of different temperatures at the ends is not possible. The quest, therefore, has been to find a material that is a good conductor of electricity but a bad conductor of heat. That is, it should have free electrons to move the current but heating up one of its ends does not result in heating the entire thing up.
It is here that Dr Kanishka Biswas and his PhD students at the Jawaharlal Nehru Centre for Advanced Scientific Research, Bengaluru, have managed to make a big breakthrough. Biswas has synthesised a new material, Silver Copper Telluride (AgCuTe), a compound of three elements, Silver, Copper and Tellurium, that has this desired property — conduction of electricity but not of heat. Biswas says this property is due to the unique structure of the new compound. While electrons are free to move around ensuring electrical conductivity, the lattices of Tellurium are unusually rigid. The lattices of Silver also vibrates very slowly, thereby, inhibiting the conduction of heat. It is thus a good candidate for exploiting thermo-electric effect. The team’s work has just been published in the chemistry journal, Angewandte Chemie.
A few years ago, Biswas, while working in the US, had come up with an even better candidate, Lead Telluride (PbTe). This material had a calculated efficiency of about 18% in converting waste heat into electricity. However, since Lead adversely affects the environment, the material could not adopted. Silver Copper Telluride, on the other hand, has a calculated efficiency of 14% but is environmentally benign and has much greater chances of adoption.
Biswas says the new material can be extremely useful in big industries where the amount of waste heat generated through various processes is huge, and the potential to convert it into electricity is large. Of course, the engineering works to make the attachments of this material is still to be done.
Silver Copper Telluride is a good conductor of electricity and not of heat. The new compound can now be used for thermo-electric effect.
Dr Kanishka Biswas and team Jawaharlal Nehru Centre for Advanced Scientific Research, Bengaluru