Researchers make breakthrough in silicon nanowires that convert waste heat into electricity
Energy now lost as heat during the production of electricity could be harnessed through the use of silicon 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. The far-ranging potential applications of this technology include (bio)hydrogen fuel cell-powered cars, personal power-jackets that could use heat from the human body to recharge electronic devices and highly efficient, decentralised and miniaturised bioenergy power systems.
The thermoelectric nanowires are especially relevant to bioenergy in that biomass is the only common form of renewable energy that is used as a carbonaceous feedstock in thermal power systems (contrary to wind, hydropower and photovoltaics, which don't generate heat). Converting biomass into electricity while using the large amount of heat released in a useful way, is one of the most cost-effective and energy efficient ways to use the resource - much more so than using it for liquid biofuels. But cogeneration (combined heat and power) and district heating systems are relatively costly or require extensive infrastructures. The new nanowires would allow a new form of efficient waste-heat recovery and could miniaturize decentralised bioenergy systems, making them particularly suitable for the developing world.
As in fossil fuel based thermal power plants, biomass plants are currently only about 30–40 per cent efficient and lose more heat to the environment than is converted into electricity. Capturing this low-grade waste heat directly as electricity could make such power systems far more efficient and make a huge difference in the fight against climate change.
Berkeley researchers Arun Majumdar and Peidong Yang describe their findings in the January 10, 2008 edition of the journal Nature, entitled "Enhanced Thermoelectric Performance of Rough Silicon Nanowires". Majumdar, who was recently appointed director of Berkeley Lab's Environmental Energy Technologies Division (EETD) and is a member of the Materials Sciences Division, is an expert on energy conversion and nanoscale science and engineering. Yang is a leading nanoscience authority with Berkeley Lab's Materials Sciences Division and with the UC Berkeley Chemistry Department.
The rough surfaces are definitely playing a role in reducing the thermal conductivity of the silicon nanowires by a hundredfold, but at this time the researchers don't fully understand the physics, said Majumdar. They cannot say exactly why it works, but they definitely the technique's results.
Nearly all of the world’s electrical power, approximately 10 trillion Watts, is generated by heat engines powered by either fossil fuels, nuclear energy or biomass: giant gas or steam-powered turbines that convert heat to mechanical energy, which is then converted to electricity. Much of this heat, however, is not converted but is instead released into the environment - approximately 15 trillion Watts. If even a small fraction of this lost heat could be converted to electricity, its impact on the energy situation would be enormous.
energy :: sustainability :: biomass :: bioenergy :: biofuels :: cogeneration :: waste-heat recovery :: efficiency :: thermoelectric :: nanowires :: nanotechnology ::
In recent years, ZT values of one or more have been achieved in thin films and nanostructures made from the semiconductor bismuth telluride and its alloys, but such materials are expensive, difficult to work with, and do not lend themselves to large-scale energy conversions.
When scaled up, thermoelectric modules could eventually be used in co-generating power with gas or steam turbines fueled by nuclear or carbonaceous feedstocks, including biomass.
The Berkeley Lab researchers will be studying the physics behind this phenomenon to better understand and possibly manipulate it for even further improvements. They will also concentrate on the design and fabrication of thermoelectric modules based on silicon nanowire arrays. Berkeley Lab’s Technology Transfer Department is now seeking industrial partners to further develop and commercialize this technology.
This research was funded by the U.S. Department of Energy's Office of Basic Energy Science, through the Division of Materials Sciences and Engineering.
Figure 1 (click to enlarge): Rough silicon nanowires synthesized by Berkeley Lab researchers demonstrated high performance thermoelectric properties even at room temperature when connected between two suspended heating pads. In this illustration, one pad serves as the heat source (pink), the other as the sensor.
Figure 2 (click to enlarge): (a) is a cross-sectional scanning electron microscope image of an array of rough silicon nanowires with an inset showing a typical wafer chip of these wires. Figure (b) is a transmission electron microscope image of a segment of one of these wires in which the surface roughness can be clearly seen. The inset shows that the wire is single crystalline all along its length.
References:
Allon I. Hochbaum, Renkun Chen, Raul Diaz Delgado, Wenjie Liang, Erik C. Garnett, Mark Najarian, Arun Majumdar and Peidong Yang1, "Enhanced thermoelectric performance of rough silicon nanowires", Nature 451, 163-167 (10 January 2008) | doi:10.1038/nature06381
Lawrence Berkeley National Laboratory: Feeling the Heat: Berkeley Researchers Make Thermoelectric Breakthrough in Silicon Nanowires - January 9, 2008.
The thermoelectric nanowires are especially relevant to bioenergy in that biomass is the only common form of renewable energy that is used as a carbonaceous feedstock in thermal power systems (contrary to wind, hydropower and photovoltaics, which don't generate heat). Converting biomass into electricity while using the large amount of heat released in a useful way, is one of the most cost-effective and energy efficient ways to use the resource - much more so than using it for liquid biofuels. But cogeneration (combined heat and power) and district heating systems are relatively costly or require extensive infrastructures. The new nanowires would allow a new form of efficient waste-heat recovery and could miniaturize decentralised bioenergy systems, making them particularly suitable for the developing world.
As in fossil fuel based thermal power plants, biomass plants are currently only about 30–40 per cent efficient and lose more heat to the environment than is converted into electricity. Capturing this low-grade waste heat directly as electricity could make such power systems far more efficient and make a huge difference in the fight against climate change.
Berkeley researchers Arun Majumdar and Peidong Yang describe their findings in the January 10, 2008 edition of the journal Nature, entitled "Enhanced Thermoelectric Performance of Rough Silicon Nanowires". Majumdar, who was recently appointed director of Berkeley Lab's Environmental Energy Technologies Division (EETD) and is a member of the Materials Sciences Division, is an expert on energy conversion and nanoscale science and engineering. Yang is a leading nanoscience authority with Berkeley Lab's Materials Sciences Division and with the UC Berkeley Chemistry Department.
We’ve shown that it’s possible to achieve a large enhancement of thermoelectric energy efficiency at room temperature in rough silicon nanowires that have been processed by wafer-scale electrochemical synthesis. - Peidong Yang, principal investigatorThe study describes a unique "electroless etching" method by which arrays of silicon nanowires are synthesized in an aqueous solution on the surfaces of wafers that can measure dozens of square inches in area. The technique involves the galvanic displacement of silicon through the reduction of silver ions on a wafer’s surface. Unlike other synthesis techniques, which yield smooth-surfaced nanowires, this electroless etching method produces arrays of vertically aligned silicon nanowires that feature exceptionally rough surfaces. The roughness is believed to be critical to the surprisingly high thermoelectric efficiency of the silicon nanowires.
The rough surfaces are definitely playing a role in reducing the thermal conductivity of the silicon nanowires by a hundredfold, but at this time the researchers don't fully understand the physics, said Majumdar. They cannot say exactly why it works, but they definitely the technique's results.
Nearly all of the world’s electrical power, approximately 10 trillion Watts, is generated by heat engines powered by either fossil fuels, nuclear energy or biomass: giant gas or steam-powered turbines that convert heat to mechanical energy, which is then converted to electricity. Much of this heat, however, is not converted but is instead released into the environment - approximately 15 trillion Watts. If even a small fraction of this lost heat could be converted to electricity, its impact on the energy situation would be enormous.
Thermoelectric materials, which have the ability to convert heat into electricity, potentially could be used to capture much of the low-grade waste heat now being lost and convert it into electricity. This would result in massive savings on fuel and carbon dioxide emissions. The same devices can also be used as refrigerators and air conditioners, and because these devices can be miniaturized, it could make heating and cooling much more localized and efficient. - Arun Majumdar, principal investigatorHowever the on-going challenge for scientists and engineers has been to make thermoelectric materials that are efficient enough to be practical. The goal is a value of 1.0 or more for a performance measurement called the “thermoelectric figure of merit” or ZT, which combines the electric and thermal conductivities of a material with its capacity to generate electricity from heat. Because these parameters are generally interdependent, attaining this goal has proven extremely difficult:
energy :: sustainability :: biomass :: bioenergy :: biofuels :: cogeneration :: waste-heat recovery :: efficiency :: thermoelectric :: nanowires :: nanotechnology ::
In recent years, ZT values of one or more have been achieved in thin films and nanostructures made from the semiconductor bismuth telluride and its alloys, but such materials are expensive, difficult to work with, and do not lend themselves to large-scale energy conversions.
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. 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. - Peidong Yang, principal investigatorThe ability to dip a wafer into solution and grow on its surface a forest of vertically aligned nanowires that are consistent in size opens the door to the creation of thermoelectric modules which could be used in a wide variety of situations. For example, such modules could convert the heat from automotive exhaust into supplemental power for a Freedom CAR-type vehicle, or provide the electricity a conventional vehicle needs to run its radio, air conditioner, power windows, etc. You can siphon electrical power from just about any situation in which heat is being given off, heat that is currently being wasted, Majumdar said. 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.
When scaled up, thermoelectric modules could eventually be used in co-generating power with gas or steam turbines fueled by nuclear or carbonaceous feedstocks, including biomass.
The Berkeley Lab researchers will be studying the physics behind this phenomenon to better understand and possibly manipulate it for even further improvements. They will also concentrate on the design and fabrication of thermoelectric modules based on silicon nanowire arrays. Berkeley Lab’s Technology Transfer Department is now seeking industrial partners to further develop and commercialize this technology.
This research was funded by the U.S. Department of Energy's Office of Basic Energy Science, through the Division of Materials Sciences and Engineering.
Figure 1 (click to enlarge): Rough silicon nanowires synthesized by Berkeley Lab researchers demonstrated high performance thermoelectric properties even at room temperature when connected between two suspended heating pads. In this illustration, one pad serves as the heat source (pink), the other as the sensor.
Figure 2 (click to enlarge): (a) is a cross-sectional scanning electron microscope image of an array of rough silicon nanowires with an inset showing a typical wafer chip of these wires. Figure (b) is a transmission electron microscope image of a segment of one of these wires in which the surface roughness can be clearly seen. The inset shows that the wire is single crystalline all along its length.
References:
Allon I. Hochbaum, Renkun Chen, Raul Diaz Delgado, Wenjie Liang, Erik C. Garnett, Mark Najarian, Arun Majumdar and Peidong Yang1, "Enhanced thermoelectric performance of rough silicon nanowires", Nature 451, 163-167 (10 January 2008) | doi:10.1038/nature06381
Lawrence Berkeley National Laboratory: Feeling the Heat: Berkeley Researchers Make Thermoelectric Breakthrough in Silicon Nanowires - January 9, 2008.
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