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Saturday, October 23, 2021

Bengaluru-based scientists develop material to tap waste heat generated by appliances and reuse it

The heat from the laptop, for example, can be used to charge a mobile phone. Or, that from the phone can be used to charge a small watch.

Written by Amitabh Sinha | Pune |
Updated: February 20, 2021 5:12:16 pm
A representative image demonstrating thermo-electric effect. (Photo credit: Kanishka Biswas)

Bengaluru-based scientists have developed a new material that can help in tapping waste heat produced by all kinds of domestic and industrial appliances, and use it to accomplish other useful work. The heat from the laptop, for example, can be used to charge a mobile phone. Or, that from the phone can be used to charge a small watch.

Industrial processes and power plants produce ample amount of waste heat that can be utilised to do significant amounts of work. The key is to find an energy conversion process that can convert heat into the more convenient electrical energy which can then be deployed to drive other machines or processes. Kanishka Biswas and his team, at the Bengaluru-based Jawaharlal Nehru Centre for Advanced Scientific Research, now claim to have found a new material, Silver Antimony Telluride, that can facilitate this energy conversion. Their latest finding, which improves upon their own earlier work in this field, has been reported in the Science journal.

Energy conversion, in general, is not a very efficient process. Only a small fraction of energy that is used is actually productive. The rest of it becomes waste, often getting released as heat. For example, when we burn fuel in the automobile engines, only about 30 per cent of the total energy content is used to drive the vehicle. Similarly, when coal is burnt to boil water in large turbines of power plants, or even atom is split in nuclear reactors, large amounts of energy is wasted in form of heat.

This waste heat can be tapped, and further converted into electrical energy, but this process too is highly inefficient. Typically, not more than 15 to 20 per cent of the waste heat can be utilised. The underlying physical phenomenon, known as the thermo-electric effect, is rather simple and known to scientists for more than 200 years. If two ends of an electrically-conducting material, like a metal, are maintained at different temperatures, current flows from the hot end towards the cold one.

This phenomenon makes it possible, for example, to generate an electric current in a metallic attachment that is connected to the exhaust pipe of an automobile engine, or a generator, or any other source of waste heat. The end connected to the heat source would be hotter than the other end, a current would be induced, which can be tapped by connecting the cooler end to a battery or some other device.

The problem in facilitating this transfer of energy, however, is the fact that most materials that conduct electricity, also happen to be good conductors of heat. That means that there would not be any significant temperature difference between the two ends of the material for very long.

Traditionally, thermo-electric effect has been demonstrated — and utilised — by using two different metals joined together, and by mechanically maintaining two different temperatures at the ends. But such materials do not offer efficient or economical solutions. The quest has been to find a material that is a good conductor of electricity but a bad conductor of heat.

Two years ago, Biswas and his colleagues had synthesised a compound of Silver, Copper and Tellurium, called Silver Copper Telluride, that showed this property. A crystalline solid, it had free electrons that helped in conduction of electricity but its lattices were rather inflexible, and vibrated quite slowly thereby inhibiting the propagation of heat. Before this, Biswas had also been working on Lead Telluride, another material with this property. In fact, lead is a common ingredient in many materials that show this property. But because lead has very adverse environmental impacts, it was important to look for lead-free materials.

The Silver Copper Telluride offered lesser efficiency than Lead Telluride in the conversion of heat into electrical energy, but it was considered an important breakthrough because it was environmentally benign. The continuous search for a better material has led Biswas and his colleagues to Silver Antimony Telluride which is more efficient at energy conversion.

“There are two factors that facilitate the thermo-electric effect in any material – electrical conductivity and heat conductivity. Electrical conductivity has to be high, while heat conductivity must be low. What we had been able to do with Silver Copper Telluride was to keep its heat conductivity at a very low level. But we could not do anything about the electrical conductivity,” said Biswas.

“With Silver Antimony Telluride, we have taken one step further. In fact, it is quite a big leap. We have devised a concept called atomic ordering. We have doped the material with a dopant that brings in a very high degree of order in its atoms. And, we are able to control both the electrical conductivity, as well as heat conductivity of the material. We can then optimise the results. In addition, this concept of atomic ordering can actually be applied to a variety of other materials as well, to exploit the benefits of thermo-electric effect. So, that is a novelty too,” he said.

“The Silver Antimony Telluride significantly outperforms the Silver Copper Telluride. So this is quite exciting right now,” he said.

The material has been found to have an efficiency of 14 per cent, though a prototype device has so far been able to achieve 10 per cent efficiency. But that already is more than any other known material and can turn out to be extremely useful in utilising at least some of the energy that goes entirely waste.

In fact, Tata Steel has already shown interest in the material and is collaborating with Biswas and his team to develop this material for deployment in its steel manufacturing units where a huge amount of waste heat is generated.

“There are a variety of potential applications. It is of course quite useful in large industrial set-ups. And, we are waiting for more industry partners to check this out. But there are other places where it can be deployed. We are currently working on an international projects that is exploring the possibility of tapping the heat from traditional cooking chulhas to run some electrical appliances in rural households,” Biswas said.

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