Although currently used renewable energy sources such as wind and solar energy are surpassing analyst expectations, geothermal energy is still considered the poor stepchild of the sector.
Generating electricity from geothermal energy requires devices that are able to make use of the heat within the Earth’s crust. Recently, a team of scientists at Tokyo Tech, led by Dr. Sachiko Matsushita, made a step forward in the understanding and development of sensitized thermal cells (STCs), a type of battery that can generate electric power at 100 ℃ or less.
Several methods for converting heat into electric power exist, however, large-scale application is not feasible. For example, hot-and-cold redox batteries and devices based on the Seebeck effect cannot simply be buried inside a heat source to leverage their energy generation.
Dr. Matsushita’s team has previously reported the use of STCs as a new method for converting heat directly into electric power using dye-sensitized solar cells. They also replaced the dye with a semiconductor to allow the system to operate using heat instead of light. Figure 1 illustratively represents the STC, a battery that consists of three layers sandwiched between electrodes: an electron transport layer (ETM), a semiconductor layer (germanium), and a solid electrolyte layer (copper ions). In short, electrons go from a low-energy state to a high-energy state in the semiconductor by becoming thermally excited and then get transferred naturally to the ETM. Afterwards, they leave 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 take place at both interfaces of the electrolyte, resulting in low-energy electrons being transferred to the semiconductor layer so that the process can begin anew, thus completing an electric circuit.
However, it was not clear at that time whether such a battery could be used as a perpetual engine or if the current would stop at some point. After testing, the team observed that electricity indeed stopped flowing after a certain time and proposed a mechanism explaining this phenomenon. Basically, current stops because the redox reactions at the electrolyte layer stop owing to the relocation of the different types of copper ions. Most importantly, and also surprisingly, they found out that the battery can revert this situation itself in the presence of heat by simply opening the external circuit for some time; in other words, by 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 excited about its discovery because of its applicability, eco-friendliness, and potential for helping 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,” said Matsushita. Further refinements to this type of battery will be the aim of future research.