The exhaust energy content of Atkinson-cycle engines, which are used in many hybrid powertrains, is lower than conventional IC engines due to the cycle’s inherently higher thermal efficiency. One of the goals of the study was to investigate what the expected output of a TE generator could be for such an engine.

Principles of Thermoelectric Generation. Thermoelectric (TE) devices produce a voltage potential as a byproduct of a difference in temperature; that potential can then be used to drive an electrical current. Such devices can recover some of the energy embedded in waste heat, such as that produced by the exhaust gas of an IC engine.

(At present, commercial devices which recapture energy from exhaust gas in production vehicles, such as the heat exchanger used in the 2010 Toyota Prius to transport heat from the engine’s exhaust system to its coolant system, typically move some of the heat to a different part of the powertrain; they do not use TE material, and do not convert waste heat to a different type of energy.)

TE devices are dependent on a process called the Seebeck effect: when two dissimilar electrical conductors are joined at two junctions that are maintained at different temperatures from one another, a voltage is produced which can be used to enable current to flow through a resistance. Semiconductors—one p-type and one n-type semiconductor—can be used at each junction in lieu of dissimilar metal conductors to produce the thermoelectric effect. The effect is reversible; a current can also be used to change the temperature of the device’s junctions, if so desired.

The thermoelectric efficiency of a TE device is derived by multiplying the square of its Seebeck coefficient by the thermoelectric circuit’s electrical conductivity, then dividing the result by the thermal conductivity of the device. For an exhaust-mounted device, a systematic calculation of efficiency must also include the net effect of additional exhaust backpressure caused by the device, as well as energy expended in delivering the electrical current to its load.

“The design objective,” state the authors of the presentation, “is to produce the maximum power possible with the least amount of thermoelectric material (cost constraint) and [exhaust] backpressure (performance constraint).” A secondary design objective was a fast thermal response to ensure operation and “reasonable [TE] generation” soon after engine start-up.

A literature review of the subject was conducted, which included a recent study that estimated a 21% loss of a typical IC engine’s fuel energy as exhaust heat at ¼ load and 1500 rpm, rising to 44% at 100% load and 4500 rpm, with a drive cycle average of about 33%. Previous work had indicated a potential of between 1% and 3% improvement in a vehicle’s fuel economy, using thermoelectric devices.

Device and System Configurations. Thermoelectric generators designed to generate energy from engine exhaust gas typically comprise channels lined with TE material and mounted in the exhaust system parallel to the direction of exhaust flow. Coolant flows to the TE modules from an external source to control the cold side of the TE generator; exhaust gas drives the hot side. Assuming constant flow rates and temperatures for both exhaust gas and device coolant, TE performance constraints are dictated by channel surface area—width, length, and number of channels—and the thickness of the TE layers.

Thermoelectric generators can be designed in either direct or indirect configurations. Direct TE generators use exhaust gas to heat the TE material, as described above, while indirect TE generators employ a heat exchanger that uses a working fluid to transfer heat from the exhaust gas to the TE material. Automotive applications generally favor direct TE generators because of their relative simplicity, and Ford’s modeling study was focused on that configuration.

Experiment and Results. The design team used GT-Power engine cycle simulation software to model TE generators with different physical dimensions, so that design parameters for optimum thermoelectric and engine performance, as well as influence on exhaust back-pressure and noise, could be determined.

To reduce the complexity of the problem at this point in the design process, some secondary factors were not modeled, including:

• Contact resistance between solid to solid interfaces
• Heat paths other than those to or from TE material
• Parasitic heat loss from device to surroundings

The modeling also assumed a steady 100 ºC temperature at all cold junctions. Initial computer modeling exercises will often simplify an experiment by ignoring secondary physical processes to reduce noise in the data. As experiment results stabilize, the secondary processes can be added later, either through more complex modeling or through the construction of prototypes.

Variations in TE material thickness, number of channels, channel width, and length were modeled, and channel width was found to be the most sensitive factor contributing to the device’s heat transfer coefficient. Power generation also tended to increase as TE material thickness increased, eventually peaking and then dropping as the material exceeded its optimum thickness.

Steady-state power inputs, outputs, and thermoelectric (TE) conversion, modeled by Ford engineers as part of a preliminary investigation into TE power generation. Adapted from Hussain et al. Click to enlarge.

Using an assumed exhaust gas mass flow rate of 56.5 kg/hr and an exhaust gas temperature of 638 ºC (1180 ºF), which corresponds with the time-averaged values for the 2.5L Escape hybrid SUV engine with the Atkinson-cycle during the EPA highway test cycle, results from Design 1 (see diagram at right) projected a 325W power generation with an exhaust pressure drop of 3.21 kPa. In city driving, power generation dropped to about 52W. Thermal efficiency of the modeled TE generator at highway speeds was 5.7% with hot and cold junction temperatures of 263 ºC and 100 ºC respectively.

As exhaust gas heat is transferred to the TE generator, the gas cools, lowering exhaust backpressure downstream of the device. This partially offsets the increased backpressure caused by the restriction that the generator creates in the exhaust stream. Any net increase in backpressure, however, will increase fuel consumption and reduce benefits provided by the device.

Such a device would most likely be positioned downstream of a vehicle’s catalytic converter so that the cooling exhaust gas would not interfere with catalyst operation. Although cooled exhaust gas can be expected to reduce exhaust backpressure downstream of the TE device, this may only partially offset the increased backpressure upstream of the device. The authors of the study note that such losses, as well as losses in a DC-DC converter that would typically receive current from the device and boost that current to a usable potential, must be included in final calculations of device efficiency.

“The model predicted the potential to generate 300W to 400W under EPA highway drive cycle conditions under EPA highway drive cycle conditions”—during which time hybrid vehicles are most dependent on their IC engines—“for a 2.5L gas-electric hybrid vehicle,” conclude the authors.

Resources

• Quazi Hussain, Clay Maranville and David Brigham (2009) Thermoelectric Exhaust Heat Recovery for Hybrid Vehicles (SAE 2009-01-1327)