Our industrial processes generate plenty of low-grade heat – energy that is often lost and never put to valuable use.

What if you could use those extra emissions to power electronics?

The Direct Thermal Charging Cell (Image Credit: Feng)

This is a Direct Thermal Charging Cell. Its creator, Dr. Tony Shien-Ping Feng of the University of Hong Kong (HKU), sees the 1.5-cm-square, 1-mm-thick device someday finding a place on HVAC systems, electrochromic windows, and even the human body.

The bendable “DTCC” converts heat to electricity better than traditional thermal processes, according to the technology's inventor.

“DTCC yields a conversion efficiency of over 3.5%, surpassing all existing thermo-electrochemical and thermo-electric systems, which is either too costly or complicated, or too low in efficiency for everyday applications,” said Dr. Feng .

(In the above video: With the help of 10 DTCCs, waste heat from the heated pipe is covered into electricity.)

Today's heat-to-electricity conversion techniques have mainly relied on thermoelectric semiconductors – devices that turn a temperature difference into a DC power source. Thermoelectric semiconductors, however, require external electricity to create such a thermal gradient.

Hong Kong University's DTCC, by contrast, uses asymmetric electrodes: a graphene oxide/platinum (GO/Pt) cathode and a polyaniline (PANI) anode in an iron redox electrolyte.

Instead of initiating a thermal gradient, the DTCC generates voltage through a process known as isothermal heating. Under heat, graphene continuously discharges by oxidizing the PANI anode and reducing Iron III (Fe3+) to Iron II (Fe2+) on the cathode side.

The isothermal heating’s energy conversion works during the entire charge and discharge process, and the system can be self-regenerated when cooled down. Its efficiency supports a variety of possibilities, according to Dr. Feng, like recycling an HVAC system's compressor heat into electricity; offering a power source for phones and electronics in the wilderness; and supporting health-monitoring wearable devices.

“The newly designed DTCC is a game-changing electrochemical technology which can open new horizons for applications to convert low-grade heat to electricity efficiently,” Dr. Feng told Tech Briefs.

Dr. Feng’s team was selected as one of the 16 finalists to compete in the Hello Tomorrow Regional Summit 2019, a competition held in Singapore on November 7, to help start-ups to adapt their research for real-world commercial uses.

In an edited interview with Tech Briefs below, Dr. Feng explains why the DTCC qualifies as one of tomorrow's game-changing technologies.

Tech Briefs: Describe the problem of wasted heat that the DTCC is designed to address.

Dr. Tony Shien-Ping Feng: Ubiquitous low-grade heat energy (<100 °C) is usually wasted without use, which could be valuable for converting it into electricity. Conversion is however still a great challenge because converting low-grade heat to electricity is inefficient due to the low temperature differential and the distributed nature of the heat sources. The performance and cost of currently available heat-to-electricity converters operating in a low-grade heat regime do not merit widespread adoption.

"As the power required for sensing electronics continues to decrease, the conversion of body heat energy into electricity for powering wearable/attachable sensors is becoming possible." — Dr. Feng

Tech Briefs: Why is the DTCC such a breakthrough technology?

Dr. Feng: This is the first demonstration of heat-to-electricity conversion undergoing isothermal heating and chemical regeneration, which revolutionizes the design of thermoelectrochemical cells; it is fundamentally different from the state-of-the-art systems with power generation coupled to temperature differential.

The newly designed DTCC is a game-changing electrochemical technology which can open new horizons for applications to convert low-grade heat to electricity efficiently. It is a simple system with the basic unit sized only 1.5 sq. cm and thickness 1 to 1.5 mm. The cell is bendable, stackable, and low cost.​

Tech Briefs: There’s a photo of you with the charging cell attached to your arm. Can you describe what’s happening there, and what application is being demonstrated?

Dr. Feng: The DTCC is charged by body heat. The voltage of DTCC increases after being attached to the arm skin. With the increasing popularity of wearable technology, this system may one day harness body heat to power wearable electronic devices or medical devices for monitoring body health conditions like blood sugar levels and blood pressure.

(In the above video, DTCC can harness body heat to power wearable electronic devices or medical devices for monitoring body health conditions.)

Tech Briefs: What’s next for you and your research team regarding this work?

Dr. Feng: Further research will focus on body-heat harvesting by DTCC. The conversion of body heat to electricity is a formidable challenge, due to the low temperature differential between skin temperature and ambient temperature. The human body consumes ~2000 kcal per day (= 100 W) to maintain a skin temperature of approximately 32°C, which equates to a theoretical maximum power of ~5 W present in released body heat. As the power required for sensing electronics continues to decrease, the conversion of body heat energy into electricity for powering wearable/attachable sensors is becoming possible.

Unlike other temperature-gradient technologies that operate at low temperature differentials, DTCCs are unique in their potential for enabling body-heat harvesting due to their low cost, flexibility, stackability, light weight, and ability to operate isothermally in a continuous thermal charge/electrical discharge process. Further research is needed to realize the full potential of DTCCs for body heat-powered technology, with particular efforts required to enable operation at skin temperature with sufficient power density and long cycle times for powering sensing electronics.

What uses do you see for this technology? Share your comments and questions below.