Exploitation of waste heat for electrical energy

Solid-state devices that convert heat to power have a reputation for being inefficient. New materials may eventually change the equation.

Leland Teschler • Chief Editor

It is estimated that 61% of the energy consumed in the United States is lost as heat. It’s no wonder, then, that there is interest in finding ways to recoup some of these losses and convert the waste heat into something useful. One of the increasingly studied methods concerns thermoelectric materials, materials that generate electricity from a heat differential.

The classic example of a thermoelectric generator is a Peltier module. Although typically used as semiconductor cooling devices, Peltier modules can also function as generators. Here, one side of the device is heated to a higher temperature than the other side. Due to the Seebeck effect, a voltage difference builds up between the two sides.

But several issues limit the scenarios in which Peltier thermogenerators can make sense. The first is that the typical efficiency of TEGs is only about 5-8%. Modern devices use highly doped semiconductors based on bismuth telluride (Bi2You3), lead telluride (PbTe), calcium manganese oxide (Ca2min3O8), or their combinations depending on the temperature. Many of these materials can be expensive. Finally, it usually takes a high temperature for a Peltier module to generate a lot of electricity. Alloys based on bismuth and antimony, tellurium or selenium are considered low temperature thermoelectrics but like to see temperatures above 300°F. Thermoelectrics based on lead alloys withstand temperatures up to about 1000°F, and silicon-germanium thermoelectrics are for temperatures up to about 1800°F. Therefore, Peltier thermogenerators tend to be used only for low power remote applications.

However, there is great interest in designing thermoelectric devices that can operate at lower temperatures and convert heat into electricity more efficiently. Research is progressing in two main areas: materials that generate electricity at lower temperatures and the structures of devices that convert infrared radiation emitted by hot bodies into electric current.

To understand the direction of this work, it is useful to know the figure of merit used for thermoelectric materials, often given by ZT=S2σT/κ. Here, S is Seebeck coefficient, σ is electrical conductivity, T is working temperature and κ is thermal conductivity. Researchers from Beihang University in China say improving the ZT is difficult because making improvements in one of the parameters tends to cause one or more of the others to go in the wrong direction. Therefore, many of the ZT enhancement strategies so far only work in a narrow range of temperatures.

figure of merit of thermoelectric materials

Researchers at Beihang University in China evaluate potential thermoelectric materials based on a figure of merit called ZT. Most narrow bandgap materials perform well only over a narrow temperature range. Some wide bandgap materials have a much wider thermoelectric range. To handle a wide temperature range, thermoelectrics can also use multiple materials with narrow band gaps. The most promising wide bandgap materials are characterized by layered crystals that have low symmetry. Click on the image to enlarge.

A limiting factor in thermoelectric performance is the band gap, i.e. the discrete electron energies of the thermoelectric material. The forbidden band is given by Es=2eSmaximumT where e is the unit load, Smaximum is the maximum Seebeck coefficient, and T is the temperature corresponding to Smaximum. The Seebeck coefficient essentially measures the voltage produced with a temperature gradient (S=ΔV/ΔT).

To obtain a thermoelectric material that operates over a range of several hundred degrees, the usual approach is to use several materials that all have narrow band gaps or one material with a wide band gap. There are practical issues with material shifts in thermoelectrics that use multiple narrow bandgap materials, so the most typical approach currently is to use wide bandgap materials such as tin selenide (SnSe ) whose bandgap energy is about 0.86 eV. Beihang researchers report seeing a thermoelectric effect in SnSe that covers the range of 80-980ºF.

However, materials with wide band gaps also have another problem that can limit their usefulness as thermoelectrics: they tend to have low carrier densities, i.e. too few charge carriers available to support a large flow of electric current. The approach used to solve the problem is to configure the orientation of the SnSe crystalline material in a layered way that makes more carriers available.

Researchers from Beihang University say they have used this approach to discover several promising thermoelectric materials, including BiCuSeO, BiSbSe3K2Bi8Se13and sb2Yes2You6. But they warn that it can be difficult to turn a material with a high ZT value into a commercial device, especially one that can operate at high temperatures. One problem: the resistivity of the material used for the electrical contact can increase over time, especially in the presence of high temperatures.

Harvest thermal light

Objects at a given temperature emit heat depending on their surface temperature. The sun, for example, has a surface temperature of 6050°C. Photovoltaic cells convert this radiant energy into electricity.

Of course, most terrestrial sources are much cooler than the sun. According to Wien’s law, as the temperature of a blackbody source drops, the wavelength at its peak power increases such that source temperatures between 100 and 400 °C have a spectrum in the thermal infrared range (wavelengths from 7 to 12 μm). It is estimated that more than 95% of waste heat generated in the United States is below 400°C (752°F).

The problem is that ordinary photovoltaic cells do not efficiently convert this type of light into electricity. A photovoltaic cell is essentially a pn diode where the collected photons create what is essentially a reverse current for the diode. But the ability of a photovoltaic cell to generate electricity depends on the forbidden band of its material; the photovoltaic effect does not occur if the energy of the absorbed light is lower than the bandgap energy of the silicon photodiode (typical). Silicon at room temperature has a bandgap energy of 1.12 eV and a cut-off wavelength of 1.1 μm.

To create a photodiode that can better detect wavelengths of mid-range infrared light, one approach is to straighten the IR using a special type of high-speed diode structure called a junction diode. tunnel. Rather than creating charge carriers from photons, as with ordinary photovoltaic cells, it tunnels light waves in a manner analogous to how high-speed diodes straighten radio waves.

A tunnel diode is characterized by heavy doping at a point where the Fermi level of the diode’s P-type material is below the valence band, and the Fermi level of its N-type material is above the valence band. above the conduction band. The quantum mechanics of this configuration is complicated, but the point of this structure is that it creates current flow via quantum tunneling through the PN junction. (It also has a region in its IV characteristics where it shows negative resistance: as the voltage increases, the current through the tunnel junction diode decreases.)

Most of the heat-to-power devices created so far work best at temperatures above 1000°C. Creating devices that perform well at temperatures below this level is difficult. One reason is that there are fewer photons to work with than at higher temperature extremes.

Nevertheless, promising developments are underway in low-temperature thermovoltaic devices. A device recently created at Sandia National Labs is called a bipolar MOS tunnel junction diode. The device uses an optical grating to couple light into a small area (3-4 nm) SiO2 barrier that results in a concentrated electromagnetic field that drives electron photon-assisted tunneling from a p-type doped silicon to the n-type silicon portion.

sandia device

Sandia researchers created this device based on tunnel diodes to convert infrared radiation into electricity. The conversion mechanism is light-facilitated electron tunneling and a grating placed above the semiconductor that channels light into a thin silica barrier between the doped silicon and an aluminum grating. The concentrated light causes the charge carriers to move from P-type silicon to N-type silicon, generating a rectifying current. The multiple tunnel diodes form a charge pumping mechanism that moves electrons from P-type wells to N-type wells. Click on the image to enlarge.

The Sandia device uses what is called photon-assisted tunneling where a photon is absorbed in an occupied state near the Fermi level of the metal gate, followed by field-enhanced tunneling in an unoccupied state of the silicon . The result is a small direct photocurrent. A similar time-reversed process occurs in the semiconductor which causes a back current in the metal gate. The overall direct current is due to the difference between these two currents, which comes from the difference in the effective mass of the electrons in the metal and in the semiconductor.

Sandia researchers designed a special circuit structure to use photon-assisted tunneling. They use an interdigital bipolar PN junction network under the tunneling gate electrode which acts as a charge pump moving electrons from the P-type region to the N-type well.

The amount of energy generated by this experimental device is small. Researchers say they saw a peak power density of 27 μW/cm2 for heat sources of 250 and 400°C and 61 μW/cm2 for 350°C. The open circuit voltages produced are of the order of a few millivolts. The researchers also say they can adjust the temperature at which the device is most effective by changing the thicknesses of the semiconductor and metal layers involved.

The energy conversion efficiency of this configuration is modest. The researchers say it’s 0.4%, but there are ways to improve it by using slightly different gate dielectrics and heat-gathering designs. Additionally, the experimental devices were fabricated on a CMOS platform, which could eventually allow scaling up for mass production. DW

Filed Under: Product Design & Development, Electronics • Electrical, Energy Management + Recovery

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