All electric devices that we use continuously dump waste heat into their surroundings, the amount discarded as heat depends on how efficient the device is. However no matter how efficient the device is there is always some energy lost as heat. We have known for years how to convert heat into electricity (that’s how power plants work), but that requires a large amount of heat and the waste heat generated by our devices is too low to covert to electricity in a cost effective/efficient manner.
There are specialized semiconductors called thermoelectric materials that generate electricity when one side of the material is hotter than the other. Unfortunately for them to work well the heat difference between the two sides needs to be in the order of hundreds of degrees making them useless to convert low-grade heat to electricity. To solve this problem materials physicist Jun Zhou and colleagues at the Huazhong University of Science and Technology have come up with Thermocells that use liquids instead of solids in the space between the two sides. The liquid conducts charges from the hot side to the cold side by moving charged molecules or ions instead of electrons. This unfortunately also transfers heat from one side to the other making them less efficient over the long run. To solve that problem they spiked the ferricyanide with a positively charged organic compound called guanidinium that reduces the thermal conductivity of the solution making it over 5 times more efficient than the previous versions.
Zhou and colleagues started with a small thermocell: a domino-size chamber with electrodes on the top and bottom. The bottom electrode sat on a hot plate and the top electrode abutted a cooler, maintaining a 50°C temperature difference between the two electrodes. They then filled the chamber with ionically charged liquid called ferricyanide.
Past research has shown that ferricyanide ions next to a hot electrode spontaneously give up an electron, changing from one with a –4 charge, or Fe(CN)6–4, to an ferricyanide with a –3 charge, or Fe(CN)6–3. The electrons then travel through an external circuit to the cold electrode, powering small devices on the way. Once they reach the cold electrode, the electrons combine with Fe(CN)6–3 ions that diffused up from below. This regenerates Fe(CN)6–4 ions, which then diffuse back down to the hot electrode and repeat the cycle.
To reduce the heat carried by these moving ions, Zhou and his colleagues spiked their ferricyanide with a positively charged organic compound called guanidinium. At the cold electrode, guanidinium causes the cold Fe(CN)6–4 ions to crystallize into tiny solid particles. Because solid particles have lower thermal conductivity than liquids, they block some of the heat traveling from the hot to the cold electrode. Gravity then pulls these crystals to the hot electrode, where the extra heat turns the crystals back into a liquid. “This is very clever,” Liu says, as the solid particles helped maintain the temperature gradient between the two electrodes.
If we can make this more efficient and get similar energy output while reducing the cost of the cell by using more inexpensive materials in the cell then we can soon imagine a world where we can power devices using the ambient heat around us. It will also allow us to make engines/motors/gadgets etc more efficient by reducing their energy requirements.
The study was published this week in Science: Thermosensitive crystallization–boosted liquid thermocells for low-grade heat harvesting
– Suramya