Breakthrough in Small-Scale Energy Harvesting via NT States

A research team led by Professor Toshimasa Fujisawa of the Institute of Science Tokyo (Science Tokyo) has now created a small energy-harvesting device that uses a naturally occurring non-thermal state (the NT state) in a Tomonaga-Luttinger (TL) liquid and compares its ability to convert heat into electricity to that of a state near thermal equilibrium (the QT state). The findings were published in Communications Physics.

Touching the back of a laptop typically feels warm. This is because some of the energy consumed for processing and transmission is lost to the outside as heat. However, even this “waste heat” includes a significant amount of useful energy. Energy harvesting refers to technologies that turn waste heat into energy that can then be reused.

Traditional energy-harvesting methods have evolved within the framework of classical thermodynamics. In classical thermodynamics, a heat source is usually thought to be in thermal equilibrium, a stable condition in which temperature becomes uniform, and heat flow is negligible. However, when waste heat approaches thermal equilibrium, the quantity of energy that can be reused decreases, reducing the amount that can be retrieved as electricity.

As a result, researchers have concentrated on non-thermal states, which are unusual quantum states that do not achieve thermal equilibrium. Non-thermal states have been realized in a variety of methods, such as in atoms with laser-controlled temperature distributions or in coherent atomic ensembles. In many situations, however, producing these non-thermal states necessitates extremely fine control, making practical applications to energy recovery difficult.

In recent years, a potential contender has gained popularity: the Tomonaga-Luttinger (TL) liquid. A TL liquid is a unique condition in which electrons are confined to a narrow channel and flow together, significantly affecting one another. Rather than acting independently, the electrons flow in a coordinated way like a liquid, hence the name.

Electronic energy in TL liquids does not easily relax into thermal equilibrium, and non-thermal states can be maintained spontaneously. This has raised hopes that TL liquids would be effective for energy collecting, although it remains uncertain if they are genuinely beneficial for thermoelectric conversion.

A Little Help from Entropy

The study team has now presented the world’s first unambiguous experimental proof on this topic.

The findings revealed that, when the same amount of heat was applied, the voltage generated in the NT condition was around two to three times more than that in the QT state. The team also confirmed that the NT state has consistently superior heat-to-electricity conversion efficiency.

The key to understanding why the NT state is favorable is in how electrical energy is distributed. The analysis found that in the NT state, electrons have a distribution in which high-energy and low-energy populations coexist while retaining disorder (entropy). In other words, rather than relaxing uniformly as in thermal equilibrium, a large number of high-energy electrons remain, making it simpler to extract electrical energy.

Potential Applications

This accomplishment represents a significant advancement in technology that transforms waste heat into power. Potential applications include large-scale exhaust heat recovery in factories and data centers, self-powered operation of small electronic devices, energy-saving solutions in extremely low-temperature situations, and extensions to other quantum and integrable systems.

Output power can also be enhanced by improving the design of energy filters that extract just the “high-energy side” of the non-thermal state. More broadly, this approach is predicted to apply to additional quantum systems as well as a variety of material systems that do not easily relax into thermal equilibrium.

We have conducted experiments based on the belief that naturally emerging ‘non-thermal states’ in the quantum world can achieve high thermal efficiency, but proving this was more difficult than we expected. We feel that the day is truly approaching when heat that is currently lost all around us will become useful again through the power of quantum effects.

Toshimasa Fujisawa, Professor, Department of Physics, School of Science, Science Tokyo