More than half of the energy consumed by the world goes to waste, mostly in the form of excessive heat. Be it in the big industries like steel plants, glass factories, power plants or our day-to-day personal use like the car, personal computer, mobile phones, etc., more than 60% of the input energy is emitted as wasted heat energy.
High Temperature Conversion
Researchers and scientists all over the world are extensively working on methods to use waste heat energy at high temperatures. Most of these efforts have been concentrated on thermo-electric devices that use solid materials to produce electricity. This approach has limitations as these materials are scant and it works only in very high temperatures and gradients. There are other methods also in different stages of development like Co-generation/ CHP (combined heat and power), Thermo-ionic chip emission, Thermo-photovoltaics, Domain wall method, use of semi-conductors in thermo-electric devices, etc.
Low Temperature Conversion
Most of these approaches have been using wasted heat at high temperature. But now scientists are turning their attention to excessive heat at low temperatures, which is almost 30% of the total heat wasted. Here, in this post, we will take a look at the latest methods of low-temperature waste-heat conversion into electricity developed by scientists from MIT and Stanford.
Thermo-galvanic Effect
Thermogalvanic effect states that the voltage of a rechargeable battery is inversely proportional to temperature. Based on this, scientists have developed a technology that uses wasted heat at low temperatures (less than 100oC) to charge batteries at lower voltages.
Electro-Chemical Process
In this process, temperature of the uncharged battery is first increased using waste heat. Then, voltage is applied to it for charging. You know from a previous post that batteries charge more efficiently at higher than room temperature. When the battery gets fully charged, it is allowed to cool. The battery is discharged at a cooler temperature. Due to the thermo-galvanic effect, voltage increases as temperature decreases. Thus, when the battery has cooled, it actually delivers more electricity than was required to charge it. This difference in energy or extra electricity is derived from the waste heat that was added to the system.
Scope
Targeted towards harvesting wasted excessive heat at temperatures below 100oC, this technology has immense potential as almost one third of the energy consumed ends up as low grade heat. The technology can be applied at an industrial scale, since power plants, steel making units and refineries release huge amounts of such heat.
Limitations
The technology has a brilliant concept but lots of progress is to be made before it can become a viable option. The measured efficiency is just 5.7% and power density is also quite low. Moreover, the charging and discharging time is also quite high right now. This needs to be decreased substantially.
Nevertheless, the scientists need to be applauded for proposing a very promising technology in recycling wasted energy into usable form of energy.
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