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Converting low-grade heat into electricity

by Dr. S P Feng, Mechanical Engineering
Sep 30, 2015
Assistant Professor at HKU’s Mechanical Engineering Department, Dr. Tony Shien-Ping Feng says the “thermal-charging” electrochemical device that combines low-grade heat conversion and charge storage can be a game-changing technology.
Assistant Professor at HKU’s Mechanical Engineering Department, Dr. Tony Shien-Ping Feng says the “thermal-charging” electrochemical device that combines low-grade heat conversion and charge storage can be a game-changing technology.

 

Dr. Tony S.P. Feng and his research group in Mechanical Engineering collaborates with MIT Prof. Gang Chen’s group to investigate a “thermal charging” phenomenon in Electrical Double Layer Capacitors (EDLC), which may open up a new and cost-effective route to harness low-grade heat by converting it into electricity and directly storing the electrical charges. Their work, described in a paper published online in the journal Nano Letters on August 7, 2015, demonstrates an open circuit voltage of 80 to 300 mV by heating the supercapacitor to 65 °C via a prior electrical charge/discharge activation step. The charge generated at high temperature can be stored in the device after its returning to room temperature, thus allowing the lighting up of LEDs by connecting the “thermally charged” supercapacitors in series.

Electrical double layer capacitors (EDLCs) are charged by applying an electrical potential difference across the positive and negative electrodes (potential-dependent). In this study, the temperature-dependent characteristic of “thermally charged” voltage in EDLC are studied and possible mechanisms are discussed. The underlying mechanism is related to a thermo-electrochemical process which enhances the kinetics of Faradaic process at electrode surface (e.g. surface redox reaction of functional group, or chemical adsorption/desorption of electrolyte ions) at higher temperature.

Thermal charging of supercapacitors at low temperatures suggests potential of using electrochemical systems for converting heat into electricity. As low grade heat is abundantly available in industrial processes, environment, biological entities, solar-thermal, and geothermal energy, it is believed that further optimization and searching for new materials with high temperature coefficient, fast kinetics, and low heat capacity will lead to new development and possibly practical deployment of electrochemical “thermal-charging” device that combines low-grade heat conversion and charge storage, which can be a game-changing technology.

The lighting up of LED by connecting several thermal-charged supercapacitors in series (>2 V)
The lighting up of LED by connecting several thermal-charged supercapacitors in series (>2 V)