Electrochemistry: new way to turn low-grade heat to electricity 22 May 2014

Researchers at MIT and Stanford University in the US have discovered a new way to transform low-temperature waste-heat into electricity, where temperature differences are less than 100C.

To date, most efforts have focused on thermoelectric devices – solid-state materials that can produce electricity from a temperature gradient. However, the efficiency of such devices is limited by the availability of materials.

The new approach – which is based on a phenomenon called the thermogalvanic effect – is described in a paper published in the journal Nature Communications.

Since the voltage of rechargeable batteries depends on temperature, the system combines the charging-discharging cycles of these batteries with heating and cooling, so that the discharge voltage is higher than charge voltage.

In operation, uncharged batteries are heated by waste heat, and then charged at the higher temperature. When fully charged, they are allowed to cool and, because the charging voltage is lower at high temperatures than at low, once they have cooled, the batteries can deliver more electricity than was used to charge them.

The extra energy comes from the waste heat that was added to the system, explain postdoc Yuan Yang and professor Gang Chen at MIT.

They expect their new system aim to harvest heat of less than 100C, which accounts for a large proportion of waste heat. In a demonstration with waste heat of 60 C the new system has an estimated efficiency of 5.7%.

Yang and Chen accept that for now their system has a much lower power density than can be achieved with thermoelectrics. It will also require further R&D to improve reliability over a long periods of use and to improve the speed of battery charging and discharging.

Brian Tinham

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