Figure 1. The device consists of a TEG module sandwiched between the blue heat sink and copper plate. The heat source is applied to the bottom copper plate. The electronics package consists of a watch battery that is charged by the TEG, and a wireless temperature sensor that draws power from the battery. The electronics package is mounted on a plastic board designed to thermally isolate the electronics from the heat source.
Figure 2. Voltage of battery versus time during discharge. The battery is able to power the temperature sensor, which requires a minimum of 2 V, for around 12 hours. The red bar indicates the recharging time for the battery and is much less than the discharge time.
Although our research is fundamental in nature, we work with a number of companies to translate the results of our research to applications. For instance, we are investigating thermoelectric waste heat scavenging in the aerospace field in collaboration with Boeing, and we are also working with the Jet Propulsion Laboratory (JPL) to design better infrared thermopile detectors. Below we give a few examples of our ongoing work to translate our research to applications in energy, space exploration, and other fields.
Thermoelectric Waste Heat Scavenging in Aircraft
Modern aircraft require numerous electronic devices such as sensors, radar, and other avionics. Each of these devices requires wiring for power and data transmission, resulting in a large mass just for cabling that decreases the range of the aircraft. Additionally, the presence of many wires makes sensor failures due to wire disconnection much more likely, impacting the reliability of the aircraft. Therefore, the range and reliability of aircraft could be simultaneously improved if the electronics could be powered at their location using scavenged heat generated by the temperature difference between the plane and the ambient. While some electronics require a large amount of power, many devices require less than a watt of power and could be easily powered from scavenged heat. Thermoelectrics are ideally suited for waste heat scavenging because they possess no moving parts, are silent, and are highly reliable.
In collaboration with Boeing, we are presently investigating the feasibility of waste heat harvesting in a flight environment. We have designed a prototype device, shown in Fig. 1, that is able to scavenge hundreds of mW from temperature gradients as small as 5 K. As shown in Fig. 2, after charging a battery for only 15 minutes, the battery is able to power a wireless temperature sensor for over 12 hours. We are now designing a second generation prototype that will have greater efficiency and lower mass.
Infrared Thermopile Detectors for Planetary Science
Thermal imaging is a critical remote-sensing technique that is used to provide temperature measurements of planetary bodies. Many thermal imagers are based on thermopile detectors because they are broadband, exhibiting a flat response over a wide spectral range (0.2-200µm); lightweight because no cryogenic cooler is required; and versatile as the detectors are insensitive to substrate temperature variations. This class of instruments has successfully flown on many missions such as Pioneer 10 & 11 (Infrared Radiometer), Voyager (IRIS instrument), Viking Orbiter (IRTM), Cassini (CIRS), Mars Reconnaissance Orbiter (MCS), and Lunar Reconnaissance Orbiter (Diviner).
Thermopiles operate by absorbing electromagnetic radiation, causing the temperature of a suspended region to increase. This temperature difference is measured by a thermoelectric material that converts a temperature difference into the measured voltage. Essential to the performance of the thermopile is minimizing the thermal conductance from the suspended region to the rest of the device because this heat loss decreases the signal to noise ratio (SNR).
We are working with JPL to optimize the device design and thermoelectric material properties to achieve better SNR. Interestingly, the figure of merit for thermoelectric material properties is different than the traditional ZT figure of merit used for thermoelectric power conversion, meaning the typical rules for thermoelectric material optimization are not applicable. Our preliminary calculations suggest that significant increases in SNR are possible by implementing a device design that minimizes heat conduction losses as well as properly optimizing the TE material composition for this application.
Selective Surfaces for Solar Thermal Energy Conversion
Solar thermal energy conversion uses the energy contained in sunlight to produce a temperature difference that can be used to generate electricity, provide process heat for industry, or simply generate hot water for domestic and commercial use. While photovoltaic cells are a familiar technology to utilize solar energy, solar thermal energy conversion is an increasingly important approach for making the best use of solar energy. For instance, domestic solar hot water heaters are now used in millions of homes in China.
A critical component of a solar thermal energy conversion technology is a selective surface, which is designed to absorb the visible light in the sun’s spectrum while minimizing emission of waste heat in the form of infrared radiation. We have designed a novel, semiconductor-based selective surface that promises to be much more effective than other selective surfaces in common use today. An image of the surface is shown in Fig. 3. We have secured seed funding to translate this advance to commercial applications.
Figure 3. (a) Image of the selective surface (right side of sample) as fabricated on a silicon wafer (bare surface at left). (b) Image of the samples after deposition. The coating can be deposited over large surface areas.