These Batteries Recharge With Waste Heat

A new system developed by a team from Stanford and MIT takes excess heat and turns it into electricity.

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Jose-Luis Olivares/MIT

As a country, more than half of what we spend on energy is going to waste, according to a report by the Lawrence Livermore National Laboratory

One of the primary culprits is heat. Factories, such as steel mills, give off a tremendous amount of energy in the form of heat— but that heat almost always escapes into the atmosphere, where it can’t do much good.

But a team of scientists working cooperatively between the Massachusetts Institute of Technology and Stanford University have developed a new type of battery that can help harness heat exhaust and funnel it back into the grid, taking advantage of a lesser-known principle called the thermogalvanic effect.

Until now, the majority of the research surrounding the conversion of waste heat has focused on thermoelectric power. Thermoelectric generators, for instance, have been growing in popularity over the last several years. The systems move electrons from the hot side of a conductive material, such as metal, to the cool side; once there, electrons can be converted to a current to power devices or charge a battery. The generators are used to power things like radio and telemetry systems on gas pipelines, as backup power sources for unmanned research sites, and even as the renewable power source on the Mars Curiosity rover.

The system is so well-known and well-researched that it’s already being used in consumer-facing products, including the popular BioLite CampStove.

But, according to Yi Cui, an associate professor at Stanford who helped lead development on the new battery, thermoelectric generators can't adequately harvest energy from large plants and factories that don’t run as hot as, say, a campfire.

The waste heat coming off a steel mill, for example, isn’t hot enough (or the battery can’t be cooled down enough) for a thermoelectric reaction to work.

Working closely with a team at MIT led by Gang Chen, a researcher with a deep background in thermoelectrics, Cui instead developed a battery designed specifically with so-called “low-grade” heat in mind.

The new concept is centered around a fairly standard water-based battery with a positive and negative electrode. The team placed an empty battery in an area with a lot of waste heat and then began to charge it. Once the battery was fully charged, they cooled it to room temperature, at which point it was discharged— and the cooled battery can discharge more energy than was put into it.

That’s the thermogalvanic phenomenon at work.

“A change in temperature causes a change in free energy, and the wattage changes a lot,” Cui says. In effect, the battery is taking on energy from the waste heat—otherwise wasted energy that could be fed back into the grid.

The batteries, unlike thermoelectric systems, cannot currently go entirely off-grid, since they require a direct current to charge. The idea is, though, that you’ll need to draw less power from the grid to do it.

The team is still experimenting with how quickly it can heat and cool the batteries and how many times a cell can be cycled before it’s spent. In the lab, it takes a couple hours for the battery to complete one charge-discharge cycle. The team hasn’t pushed any single cell through more than 50 cycles. 

Right now, we don't have a clear sense of how much power a system like Cui's can produce. Cui eventually envisions a circuit of several cells that can be installed in a factory. As one cell’s temperature rises from exposure to waste heat, another moves into the cooling cycle. 

“Half of them are charging in a high temperature, and half of them are discharging in a low temperature,” he says.

At the moment, the primary target is factory-produced waste heat, but Cui feels the system could be applied elsewhere in the future. The team may also experiment with other battery materials that might allow the thermogalvanic effect to be applied to higher heat levels, such as those produced by a fireplace or an oven. 

At a time when energy-harvesting systems are already becoming commonplace overseas, systems like Cui’s could prove invaluable to exploring new realms of energy in the United States. Within the next few years, heat from the London Underground will be used to warm about 1,400 homes. And much of Denmark’s energy comes from waste heat.

With inventions like this, we could start to catch up.

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