Power/Performance Bits: Sept. 17

Silicon thermoelectrics
Researchers at the University of Texas at Dallas and Texas Instruments developed a new method for thermoelectric generation that could be used with electronics to convert waste heat into reusable energy.

“In a general sense, waste heat is everywhere: the heat your car engine generates, for example,” said Mark Lee, professor and head of the Department of Physics at UT Dallas. “That heat normally dissipates. If you have a steady temperature difference — even a small one — then you can harvest some heat into electricity to run your electronics.”

“Thermoelectric generation has been expensive, both in terms of cost per device and cost per watt of energy generated,” Lee said. “The best materials are fairly exotic — they’re either rare or toxic — and they aren’t easily made compatible with basic semiconductor technology.”

In general, silicon is a poor thermoelectric material. Research a decade ago indicated that silicon performed much better as a nanowire, but making a useful thermoelectric generator from it was still difficult. In part, the nanowire is too small to be compatible with chip manufacturing processes. To address this, the team created ‘nanoblades,’ which are only 80 nanometers thick but more than eight times that in width.

The nanoblade shape loses some thermoelectric ability relative to the nanowire, noted Lee. “However, using many at once can generate about as much power as the best exotic materials, with the same area and temperature difference.”

Too much material can also hamper performance, but the nanoblade gets closer to the sweet spot, said Lee. “When you use too much silicon, the temperature differential that feeds the generation drops. Too much waste heat is used, and, as that hot-to-cold margin drops, you can’t generate as much thermoelectric power.”

This vacuum electronic probe station tests the thermoelectric circuits constructed by the researchers. A silicon wafer with thermoelectric circuits is visible in its center. (Source: University of Texas at Dallas)

“We optimized the configuration of our devices to place them among the most efficient thermoelectric generators in the world,” said Gangyi Hu, who was a doctorate student in physics at UT Dallas at the time of the study. “Because it’s silicon, it remains low-cost, easy to install, maintenance-free, long-lasting and potentially biodegradable.”

“You can live with a 40% reduction in thermoelectric ability relative to exotic materials because your cost per watt generated plummets,” Lee added. “The marginal cost is a factor of 100 lower.”

One application the team sees is for powering sensors. “Sensors go everywhere now. They can’t be constantly plugged in, so they must consume very little power,” said Mark Lee, professor and head of the Department of Physics at UT Dallas. “Without a reliable light source for photovoltaic energy, you’re left needing some kind of battery — one that shouldn’t have to be replaced.”

“We want to integrate this technology with a microprocessor, with a sensor on the same chip, with an amplifier or radio, and so on. Our work was done in the context of that full set of rules that govern everything that goes into mass-producing chips,” Lee said. “Over at Texas Instruments, that’s the difference between a technology they can use and one they can’t.”

Underwater communication
Researchers at MIT propose a way to create a battery-free communication system that uses near-zero power to transmit data from underwater sensors. The team says the system could be used to monitor sea temperatures to study climate change and track marine life over long periods.

Key to the work is the piezoelectric effect, where vibrations in certain materials generate electric charge, and backscatter, which transmits data by reflecting modulated wireless signals off a tag and back to a reader, as in RFID tags.

The Piezo-Acoustic Backscatter System uses a transmitter to send acoustic waves through water toward a piezoelectric sensor that has stored data. When the wave hits the sensor, the material vibrates and stores the resulting electrical charge. Then the sensor uses the stored energy to either reflect a wave back to a receiver or not reflect one at all. Alternating between reflection in that way corresponds to the bits in the transmitted data: For a reflected wave, the receiver decodes a 1; for no reflected wave, the receiver decodes a 0.

“Once you have a way to transmit 1s and 0s, you can send any information,” said Fadel Adib, an assistant professor in the MIT Media Lab and the Department of Electrical Engineering and Computer Science and founding director of the Signal Kinetics Research Group. “Basically, we can communicate with underwater sensors based solely on the incoming sound signals whose energy we are harvesting.”

The technique relies on the capability of piezoelectric materials to not only produce a small voltage in response to vibration but also deform when subject to voltage. When underwater, this produces a pressure wave. “That reversibility is what allows us to develop a very powerful underwater backscatter communication technology,” Adib said.

The underwater Piezo-Acoustic Backscatter System. (Source: MIT)

The system is comprised of a submerged node, a circuit board that houses a piezoelectric resonator, an energy-harvesting unit, and a microcontroller. Any type of sensor can be integrated into the node by programming the microcontroller. An acoustic projector (transmitter) and underwater listening device, called a hydrophone (receiver), are placed some distance away.

Initially, the transmitter sends an acoustic wave at the node.

When the sensor wants to send a 0 bit, the piezoelectric resonator absorbs the wave and naturally deforms, and the energy harvester stores a little charge from the resulting vibrations. The receiver then sees no reflected signal and decodes a 0.

To send a 1 bit, the node’s microcontroller uses the stored charge to send a little voltage to the piezoelectric resonator. That voltage reorients the material’s structure in a way that stops it from deforming, and instead reflects the wave. Sensing a reflected wave, the receiver decodes a 1.

The researchers tested the system underwater in an MIT pool, using it to collect water temperature and pressure measurements. The system was able to transmit 3 kilobytes per second of accurate data from two sensors simultaneously at a distance of 10 meters between sensor and receiver.

In deployment, the transmitter and receiver would need power, and could be placed on ships or buoys where batteries could be changed. The team is working to demonstrate the system at farther distances and with more sensors, and testing whether it will work for transmitting sound and low-resolution images.

Getting more from solar
Researchers at Waterloo University created an algorithm to increase the efficiency of solar photovoltaic systems and reduce the volume of power lost without changing the hardware.

“We’ve developed an algorithm to further boost the power extracted from an existing solar panel,” said Milad Farsi, a PhD candidate in Waterloo’s Department of Applied Mathematics. “Hardware in every solar panel has some nominal efficiency, but there should be some appropriate controller that can get maximum power out of solar panels.

“We do not change the hardware or require additional circuits in the solar PV system. What we developed is a better approach to controlling the hardware that already exists.”

With the new algorithm, a nonlinear optimal feedback control scheme, controllers can better handle fluctuations around a solar PV system’s maximum power point, which tends to be where potential energy is lost.

“Based on the simulations, for a small home-use solar array including 12 modules of 335W, up to 138.9 kWh/year can be saved,” said Farsi. “The savings may not seem significant for a small home-use solar system but could make a substantial difference in larger-scale ones, such as a solar farm or in an area including hundreds of thousands of local solar panels connected to the power grid.”

Farsi gave the example of the Sarnia Photovoltaic Power Plant, Canada’s largest PV plant. “If this technique is used, the savings could amount to 960,000 kWh/year, which is enough to power hundreds of households. If the saved energy were to be generated by a coal-fired plant, it would require emission of 312 tonnes of CO2 into the atmosphere.”

The researchers said that certain conditions, such as fast-charging ambient environments or situations where power is lost in the converters due to conventional control methods, the power savings could be substantially greater.