New Energy Scavenging Technique Lights the Way

I’ve always had a bit of a fascination about energy harvesting. Reclaiming microjoules of energy from the environment to convert into electrical energy to power small circuits, such as sensor clusters, seems very futuristic. It is also the definition of environmentally friendly - taking things we normally consider as waste and recycling them into something useful. There have been many attempts to get a usable energy scavenging design and try to scale it up, with no real success so far. These designs have used different types of waste, such as heat, solar power, vibration and RF energy, but other than operating the smallest of circuits, they have not proven viable for large scale commercial use.

The latest attempt at finding a viable energy scavenging solution comes from a research team run by Professor Guo Yong Xin, from the National University of Singapore (NUS), and Professor Shunsuke Fukami and his team from Japan's Tohoku University (TU). The team has worked together to develop a system that uses the WiFi in our homes as a source. The new method uses spin-torque oscillators (STOs) to capture wireless signals and convert them into energy. WiFi and other signals on the 2.4GHz radio frequency band are an obvious choice to use as sources as their use has grown exponentially and they can be found almost everywhere.

The research team used the spin-torque oscillators (STOs) to turn the signals into the energy needed to power an LED wirelessly. Spin-torque oscillators are devices that generate microwaves, and have previously been used in wireless communication systems. However, that use has been hindered due to low output power and a broad linewidth. These problems can be overcome by mutually synchronizing multiple STOs together. But, short-range magnetic couplings have spatial restrictions and long-range electrical synchronisation has a frequency responses of only a few hundred MHz. Long-range electrical synchronisation also needs dedicated current sources for the individual STOs, which make the chip implementation more complex.

As an alternative, the research team implemented an array with eight STOs connected in series which converted 2.4 GHz radio waves into a voltage, which then fed a capacitor connected to a 1.6-volt LED. After five seconds of charge, the capacitor could operate the LED for a minute after the wireless power was disconnected. In future, the researchers will try to increase the energy harvesting capability of the system by increasing the number of STOs in the array. They will also attempt to use their system to wirelessly charge other types of electronics and sensors. The research team is now looking to develop partnerships with industry to develop on-chip STOs for new applications.

Like all attempts to harness energy from the environment before, this technique has only performed at the smallest scale so far. There is a huge demand for self-powered electronics, particularly for IoT applications. It will be interesting to see if capturing our spare WiFi signals could be the answer. It is definitely worth keeping an eye on for the future.