Sensitized Thermal Cells Demonstrate Potential for Geothermal Energy Harvesting

July 30, 2019 by Scott McMahan

New energy-generation technologies are needed to meet the demands of the world's ever-rising energy consumption needs. Although renewable energy sources such as wind and solar energy have their merits, scientists from Tokyo Institute of Technology  (Tokyo Tech) point out that one permanent, and untapped energy source is geothermal energy.

Producing electricity from geothermal energy requires devices that can use the heat from the Earth's crust. Recently, a team at Tokyo Tech, led by Dr. Sachiko Matsushita, made great advances in the understanding and development of sensitized thermal cells (STCs), a type of harvester that can generate electric power at temperatures of 100℃ or less.

Several methods for converting heat into electric power already exist. So far, however, their large-scale adoption is not feasible. For example, hot-and-cold redox batteries and harvesters based on the Seebeck effect would not work if they were simply buried inside a geothermal heat source.

Dr. Matsushita's team previously reported the use of STCs as a new strategy for converting heat directly into electric power using dye-sensitized solar cells. They also replaced the dye with a semiconductor to let the system operate using heat instead of light.

An STC is a harvester with three layers sandwiched between electrodes. These layers include an electron transport layer (ETM), a semiconductor layer (germanium), and a solid electrolyte layer (copper ions).

Once thermally excited, electrons go from a low-energy state to a high-energy state in the semiconductor then get transferred naturally to the ETM.

Afterward, they exit through the electrode, go through an external circuit, pass through the counter electrode, and then reach the electrolyte. Oxidation and reduction reactions involving copper ions occur at both interfaces of the electrolyte, resulting in the transfer of low-energy electrons to the semiconductor layer so the process can begin again. This transfer completes an electric circuit.

In the diagram at the top of this article, the height of the building represents the energy state of electrons. By becoming thermally excited, electrons in the semiconductor layer rise to a high-energy state and then transfer to the electron transport layer. Then, they go through an external circuit and reach the counter electrode. Redox reactions take place in the electrolyte layer next to the counter electrode, providing the semiconductor with low-energy electrons. In spite of providing continuous heating, this process eventually stops as the different copper ions in the electrolyte relocate. However, the battery can revert this situation by opening the external circuit for a certain duration.

However, it was not apparent at that time whether such a harvester could operate as a perpetual engine or if the current would stop at some point.

After testing, the team observed that electricity did stop flowing after a certain time, and they proposed a mechanism explaining this phenomenon.

Basically, they hypothesized that the current stops because the redox reactions at the electrolyte layer stop due to the relocation of the different types of copper ions.

Most surprisingly, they discovered that the harvester can revert this condition in the presence of heat by simply opening the external circuit for some time. In other words, it can be accomplished using a simple switch.

"With such a design, heat, usually regarded as low-quality energy, would become a great renewable energy source," states Matsushita.

The team is extremely excited about their discovery because of its applicability, and potential for helping to solve the global energy crisis.

"There is no fear of radiation, no fear of expensive oil, no instability of power generation like when relying on the sun or the wind," commented Matsushita.

Further refinements to this kind of battery will be the goal of future research, with the hope of solving humanity's energy needs without harming the planet.