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Energy efficiency breakthroughs at MIT and Berkeley

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February 11, 2008

Rough silicon nanowires demonstrated high performance thermoelectric properties
 Image: Be...

Rough silicon nanowires demonstrated high performance thermoelectric properties Image: Berkely Lab

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February 12, 2008 The huge potential for utilizing the heat from the human body and other sources to generate electrical power is beginning to be realized on several fronts. Recently we encountered plans to capture and use human body heat in building design and now news of research breakthroughs at MIT and Berkeley that promise to advance the widespread application of thermoelectric power generation in our daily lives.

Berkeley researchers have made a thermoelectric breakthrough that could allow heat lost during the production of electricity to be harnessed through the use of silicon nanowires. Nanowires synthesized via a technique developed by researchers with the U.S. Department of Energy’s (DOE) Lawrence Berkeley National Laboratory (Berkeley Lab) and the University of California (UC) at Berkeley, demonstrated high performance thermoelectric properties even at room temperature when connected between two suspended heating pads.

The key to the technique is that it produces arrays of vertically aligned silicon nanowires that feature exceptionally rough surfaces - a factor believed to be critical to the high thermoelectric efficiency of the silicon nanowires.

The ability to coat a wafer with a forest of these vertically aligned nanowires could lead to wide-ranging applications from converting the heat from automotive exhaust into supplemental power to power-clothing that could use heat from the human body to recharge cell-phones and other electronic devices. The technology also has the potential to be scaled-up for use in co-generating power with gas or steam turbines, a process that could have a significant impact on the efficiency of global energy production and greenhouse gas emissions.

“You can siphon electrical power from just about any situation in which heat is being given off, heat that is currently being wasted,” said Arun Majumdar, a mechanical engineer and materials scientist with joint appointments at Berkeley Lab and UC Berkeley. “For example, if it is cold outside and you are wearing a jacket made of material embedded with thermoelectric modules, you could recharge mobile electronic devices off the heat of your body. In fact, thermoelectric generators have already been used to convert body heat to power wrist watches.”

The challenge of making the thermoelectric process efficient enough to be viable still remains. Performance of these systems are measured by what's known as a ZT value (thermoelectric figure of merit) and the goal for practical systems is for a value of 1.0 or higher. The use of thin films and nanostructures has achieved this in recent years but only when using prohibitively expensive materials.

“Bulk silicon is a poor thermoelectric material at room temperature, but by substantially reducing the thermal conductivity of our silicon nanowires without significantly reducing electrical conductivity, we have obtained ZT values of 0.60 at room temperatures in wires that were approximately 50 nanometers in diameter,” said Yang. “By reducing the diameter of the wires in combination with optimized doping and roughness control, we should be able to obtain ZT values of 1.0 or higher at room temperature.”

Majumdar and Yang are the co-authors of a paper appearing in the January 10, 2008 edition of the journal Nature, entitled “Enhanced Thermoelectric Performance of Rough Silicon Nanowires.” Also co-authoring this paper were Allon Hochbaum, Renkun Chen, Raul Diaz Delgado, Wenjie Liang, Erik Garnett and Mark Najarian.

Over at MIT researchers from and Texas Instruments have made a breakthrough in small scale energy efficiency that could also impact on the use of human body heat as an energy source. The new chip design incorporates circuits that work at a voltage level much lower than usual - most current chips operate at around one volt, the new design works at just 0.3 volts - and can be up to 10 times more energy-efficient than present technology. As well as extending the operationg duration of portable devices on a single battery charge, it is hoped that the energy-efficient microchip may be efficient enough to run implantable medical devices using ambient energy from the human body heat as its power source.

The development was more complicated than simply reducing the operating voltage, as existing microchips have been optimized to operate at the higher standard-voltage level. This means that memory and logic circuits needed to be redesigned to operate at very low power supply voltages. One of the key components of the new chip design is a high-efficiency DC-to-DC converter, which reduces the voltage to the lower level, right on the same chip. The redesigned memory and logic, along with the DC-to-DC converter, are all integrated to realize a complete system-on-a-chip solution. So far the new chip is a proof of concept with commercial applications potentially available in about five years.

The design was presented earlier this month at the International Solid-State Circuits Conference in San Francisco by Joyce Kwong, a graduate student in MIT's Department of Electrical Engineering and Computer Science (EECS).

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