Power/Performance Bits: Dec. 23

GaN building blocks

A team of engineers from Cornell University, the University of Notre Dame, and the semiconductor company IQE created gallium nitride (GaN) power diodes capable of serving as the building blocks for future GaN power switches.

In spite of having many desirable features as a material, GaN is notorious for its defects and reliability issues. So the team zeroed in on devices based on GaN with record-low defect concentrations to probe GaN’s ultimate performance limits for power electronics.

“Our engineering goal is to develop inexpensive, reliable, high-efficiency switches to condition electricity — from where it’s generated to where it’s consumed within electric power systems — to replace generations-old, bulky, and inefficient technologies,” said Zongyang Hu, a postdoc at Cornell University. “GaN-based power devices are enabling technologies to achieve this goal.” For the work, the team examined high-voltage p-n junction diodes.

A state-of-the-art design of GaN p-n junction diodes that has resulted in near-unity ideality factor, avalanche breakdown capability, and record-breaking power performance. Insets show a GaN p-n diode fabricated on a high-quality bulk GaN substrate and light emission from the junction under forward bias. (Source: Zongyang Hu)

To describe how much the device’s current-voltage characteristics deviate from the ideal case in a defect-free semiconductor system, the team used a “diode ideality factor.” According to Hu, this is “an extremely sensitive indicator of the bulk defects, interface and surface defects, and resistance of the device.”

The work is the first report of GaN p-n diodes with near-ideal performance in all aspects simultaneously: a unity ideality factor, avalanche breakdown voltage, and about a two-fold improvement in device figure-of-merits over previous records.

One big surprise for the team came in the form of unexpectedly low differential-on-resistance of the GaN diode. “It’s as if the body of the entire p-n diode is transparent to the current flow without resistance,” said Hu. “We believe this is due to high-level injection of minority carriers and their long lifetime, and are exploring it further.”

Powered by urine

A pair of socks embedded with miniaturized microbial fuel cells (MFCs) and fuelled with urine pumped by the wearer’s footsteps powered a wireless transmitter to send a signal to a PC. The experiment, a collaboration between the University of the West of England and the University of Bristol, is the first self-sufficient system powered by a wearable energy generator based on microbial fuel cell technology.

Microbial fuel cells use bacteria to generate electricity from waste fluids. They tap into the biochemical energy used for microbial growth and convert it directly into electricity. This technology can use any form of organic waste and turn it into useful energy, potentially making it valuable green technology.

Soft MFCs embedded within a pair of socks were supplied with fresh urine, circulated by the human operator walking. Normally, continuous-flow MFCs would rely on a mains powered pump to circulate the urine over the microbial fuel cells, but this experiment relied solely on human activity. The manual pump was based on a simple fish circulatory system and the action of walking caused the urine to pass over the MFCs and generate energy. Soft tubes, placed under the heels, ensured frequent fluid push-pull by walking. The wearable MFC system successfully ran a wireless transmission board, which was able to send a message every two minutes to the PC-controlled receiver module.

Schematic drawing and a photo of the developed wearable generator. (Source: Ioannis Ieropoulos/UWE Bristol)

Professor Ioannis Ieropoulos, of the Bristol BioEnergy Centre, said, “Having already powered a mobile phone with MFCs using urine as fuel, we wanted to see if we could replicate this success in wearable technology. We also wanted the system to be entirely self-sufficient, running only on human power – using urine as fuel and the action of the foot as the pump.”

“This work opens up possibilities of using waste for powering portable and wearable electronics. For example, recent research shows it should be possible to develop a system based on wearable MFC technology to transmit a person’s coordinates in an emergency situation. At the same time this would indicate proof of life since the device will only work if the operator’s urine fuels the MFCs.”

Solar power all night

Researchers at Purdue University are proposing a new concept aimed at not only generating electricity with solar energy but also producing and storing hydrogen from superheated water for round-the-clock power production.

Their idea, which they call hydricity, uses solar concentrators to focus sunlight, producing high temperatures and superheating water – in this case from 1,000 to 1,300 degrees Celsius – to operate a series of electricity-generating steam turbines and reactors for splitting water into hydrogen and oxygen. The hydrogen would be stored for use overnight to superheat water and run the steam turbines, or it could be used for other applications, producing zero greenhouse-gas emissions.

“Traditionally electricity production and hydrogen production have been studied in isolation, and what we have done is synergistically integrate these processes while also improving them,” said Rakesh Agrawal, a professor of chemical engineering at Purdue.

The combined process is more efficient than the standalone processes, say the study’s members. The system has been simulated using models, but there has been no experimental component to the research.

“The overall sun-to-electricity efficiency of the hydricity process, averaged over a 24-hour cycle, is shown to approach 35 percent, which is nearly the efficiency attained by using the best photovoltaic cells along with batteries,” according to chemical engineering doctoral student Emre Gençer. “In comparison, our proposed process stores energy thermo-chemically more efficiently than conventional energy-storage systems, the coproduced hydrogen has alternate uses in the transportation-chemical-petrochemical industries, and unlike batteries, the stored energy does not discharge over time and the storage medium does not degrade with repeated uses.”