Researchers in the ERATO Saitoh Spin Quantum Rectification Project in the JST Strategic Basic Research Programs have elucidated the mechanism of the hydrodynamic power generation using spin currents(1) in micrometer-scale channels, finding that power generation efficiency improves drastically as the size of the flow is made smaller.
In a microchannel, the flow takes on a state referred to as laminar flow (2), where a micro-vortex-like liquid motion is distributed widely and smoothly throughout the channel. This leads to properties that are more suitable to miniaturization, and an increase in power generation efficiency. Group leader Mamoru Matsuo, et al., predicted the basic theory of fluid power generation using spin currents in 2017, and in this present study, the researchers experimentally demonstrate the fluid power generation phenomenon in the laminar flow region. As a result of experiments, they confirm that in the laminar flow region, energy conversion efficiency was increased by approximately 100,000 times.
The characteristics of the spin fluid power generation phenomenon in laminar flows that they elucidate in this research are that an electromotive force proportional to flow velocity can be obtained, and that conversion efficiency increases as flow size decreases. Also, whereas hydroelectric power generation (also known as fluid power generation) and magnetohydrodynamic power generation(3) require additional equipment such as turbines and coils, the phenomenon in the research requires almost no additional equipment, both inside and outside of the flow channel. Due to these characteristics, application to spintronics-based nanofluidic devices such as liquid metal flow cooling mechanisms in fast breeder reactors or semiconductor devices, as well as application to flowmeters that electrically measure micro-flows, can be hoped for.
(1) Spin current
The flow of spin angular momentum. For example, electrons have a charge (an electrical degree of freedom) and a spin angular momentum (a magnetic degree of freedom), where the flow of the former is called an electric current and the flow of the latter is called a spin current.
(2) Laminar flow
Flow within a channel is characterized primarily by flow-velocity, size and viscosity. In a low-velocity flow in a small-sized channel, viscosity dominates, and the fluid will flow regularly, and in layers, along the channel axis. This is referred to as laminar flow.
(3) Magnetohydrodynamic power generation
When a charged particle moves in a magnetic field, it is subjected to a force (Lorentz force) that is perpendicular to both the particle's direction of motion and the direction of the magnetic field. Particles with charges of the same polarity (positive or negative) are subjected to a force in the same direction, and move in one direction. As a result, electric charge accumulates at the destination of the particles' movement. Magnetohydrodynamic power generation is a power-generation method that uses the potential difference (electromotive force) generated from this accumulation.
This research was conducted under the ERATO Saitoh Spin Quantum Rectification Project of the JST Strategic Basic Research Programs. The members of the project are as follows: Research Director, Eiji Saitoh (Professor, University of Tokyo), Group leader, Sadamichi Maekawa (senior researcher at RIKEN), Group leader, Mamoru Matsuo (former deputy chief researcher at the Japan Atomic Energy Agency, currently associate professor at the University of Chinese Academy of Sciences), Vice Group leader, Hiroyuki Chudo (deputy chief researcher at the Japan Atomic Energy Agency), Research Supporter, Ryo Takahashi (former postdoctoral researcher at the Japan Atomic Energy Agency, currently assistant professor at Ochanomizu University).