Wax may make electric cars a reality - Automotives - battery heat control - Brief Article

USA Today (Society for the Advancement of Education), August, 2002

With more than 250,000,000 polluting cars on the road, fuel prices soaring, and global warming concerns, the idea of a zero-emissions battery-powered car is certainly desirable. Hybrid cars that use both batteries and gas are available commercially, but there are currently no cars that rely solely on batteries. One of the reasons is that the large lithium-ion batteries that would be used to power electric cars generate intense heat. Found in cell phones and laptop computers, lithium-ion batteries pack a lot of power into a very small battery. Their high-energy density makes them susceptible to fire and may pose serious safety concerns. Hybrid carmakers solve this by fitting car battery cases with fans that blow excess heat away. However, fans are complex and expensive, and, if they fail, they could have a major meltdown. Researchers at the Illinois Institute of Technology, Chicago, are working on a low-tech solution to this high-tech dilemma--wax.

"When lithium-ion batteries are scaled up to the size needed to run a car, the heat they give off is very significant," notes Said Al-Hallaj, associate research professor of chemical and environmental engineering. "So much energy is released when the batteries are turned on that it becomes essential to remove this heat to keep the battery operating safe." Conventional car batteries contain water-based electrolytes, which can't burn. Lithium-ion batteries contain organic material that can vaporize and combust.

Al-Hallaj and his colleague, J.R. Selman, propose surrounding six or eight paper towel-sized lithium-ion batteries in wax. It would absorb the heat generated by the batteries, and during nights with subfreezing temperatures, the heat from the melted wax would be returned to the batteries slowly to keep them from freezing. "Its like ice in water--the ice keeps the water at a constant temperature as it melts," Al-Hallaj indicates. "Only after the last ice cube has melted does the water temperature begin to rise." By surrounding the batteries with slow-melting wax (the "ice"), the batteries would be kept at a stable temperature because the wax would absorb the excess heat as it melts in the form of latent heat. When the car is switched off, the heat in the melted wax would be slowly transferred back to the batteries, preventing them from freezing in the winter.

The challenge is to design a battery pack with an adequate amount of wax to absorb all the heat generated from the batteries during its operational period. So far, Alhallaj and Selman have worked out the details using computer modeling, and both are confident that their idea will work as expected.