Electricity from automotive exhaust heat

Dr. Neil Canter, Contributing Editor | TLT Tech Beat March 2011

A new thermoelectric material is capable of more efficiently generating electricity.

KEY CONCEPTS
• Approximately 40% of the fuel energy produced from an internal combustion engine is lost as heat.
• A new thermoelectric material converts automotive exhaust heat to electricity and shows promise to be more energy efficient.
• The thermoelectric material is a barium-filled binary skutterudite containing cobalt and antimony.

Increasing the efficiency of automobiles and heavy-duty vehicles is becoming paramount with the rising cost of fuel. The search for ways to improve the efficiency of automobiles and heavy-duty diesel vehicles has moved beyond just focusing on engine design and changing the characteristics of the automotive lubricant.

A good deal of attention is being paid to determine if there might be ways to recover fuel energy that is currently lost as friction and heat. A previous TLT article examined a hydraulic hybrid system that was developed to take the place of a transmission for use in Class 8 refuse trucks (1). This technology captures the kinetic energy obtained during the braking process and uses it for the next acceleration.

With all of the starting and stopping that occurs in this application, this approach is ideal for saving fuel, extending brake life and reducing emissions. Fuel economy savings between 40% and 80% were attained.

A potentially larger benefit can be realized if the heat generated during combustion can be recycled (for example turned into electricity). This is an attractive objective because approximately 40% of the fuel energy produced during combustion is wasted in the exhaust gas, 30% is picked up by the engine coolant and 5% is lost as radiation and friction (2). Only 25% is available to accelerate the vehicle.

Efforts have been underway to utilize exhaust-based thermoelectric generators that can convert heat to electricity. Massoud Kaviany, professor in the department of mechanical engineering at the University of Michigan in Ann Arbor, Mich., says, “Thermoelectric generators can function in a similar fashion to a heat pump. If heat is available, they can convert it to electricity. In addition, thermoelectrics can use electricity to produce cooling in a similar fashion to an air conditioner.”

The thermoelectric effect can generate electricity through a temperature difference or use an electric potential to create a temperature difference. Kaviany indicates that the temperature of exhaust from an automobile engine can range from 400 C to 800 C. The difference between this temperature range and ambient temperature means that it should be possible to utilize a thermoelectric material to generate electricity.

Currently used thermoelectric materials exhibit conversion efficiencies of only 5%. One class of promising materials that has not been fully evaluated is known as the skutterudites. Kaviany says, “This class of crystalline materials can contain cobalt, rhodium or iridium in combination with phosphorus, arsenic and antimony. Skutterudites refers to a cubic-structure crystal (crystals have repeating, order unit cells or lattice) made of eight smaller cubes.”

A better understanding of the thermoelectric properties of skutterudites may lead to the development of more efficient thermoelectric materials. Such work has now been carried out.

BARIUM FILLER
Kaviany and his researchers at the University of Michigan’s Center for Solar and Thermal energy Conversion studied the properties of a binary skutterudite prepared from cobalt and antimony. The binary skutterudite contains a cobalt atom at the center of a body-centered cubic lattice. Octahedrally coordinated to the cobalt atom are six antimony atoms.

Kaviany says, “A binary skutterurdite of this type displays good electrical properties but does not transfer heat as poorly (good thermoelectric materials should be poor heat conductors). This skutterudite contains voids in the crystal that potentially can be filled with a third material. In this way, we believe the heat transfer properties of the skutterurdite can be reduced without affecting its electrical properties.”

Kaviany believes that filling the voids with a specific material can lead to a change in the vibrational properties of the lattice, which is an important measure of heat transfer. Heavy elements in the rare-earth family are mainly used as the filler.

Kaviany says, “We decided to work with barium, which is an element that should perform in a similar fashion to rare-earth elements. A series of theoretical calculations were done to see how the properties of the skutterurdite changed as a function of the concentration of barium and the temperature.”

The researchers used a combination of ab initio (quantum mechanics) calculations and molecular dynamics (classical mechanics) simulations. Barium forms good covalent bonds with the other atoms that cause the existing bonds to soften. In this manner, the heat transfer properties of the skutterudite decline.

The researchers predicted the phase diagram of the barium- filled cobalt antimonite as a function of barium concentration and temperature. Kaviany says, “We found that barium atoms may not be uniformly distributed in the crystal lattice, but the void are filled in an ordered fashion I Including two-phase mixtures. Two prominently ordered phases known as a gamma phase and an alpha phase are predicted.” 

Figure 1 shows the atomic structures of both phases. The structure of the barium atoms is shown in green circles, while the cobalt atoms are shown in small blue circles.


Figure 1. Two of the prominently ordered phases predicted in the barium-filled cobalt antimonite are known as a gamma phase and an alpha phase. (Courtesy of the University of Michigan)

Kaviany envisions that barium-filled skutterudite could be wrapped in a cylindrical form around the exhaust system of an automobile. The exhaust gas flow would contact the material on one side while ambient air is on the other. None of the pollutants in the exhaust stream will affect the process because they will not be able to get into the thermoelectric material.

Kaviany hopes that this type of thermoelectric material can realize an improvement in performance to between 15% and 20%. He adds, “There is a unitless parameter known as the thermoelectric figure of merit, which is an expression of the ratio between electrical conductivity and thermal conductivity. We are hoping that the barium-filled skutterudite will increase the figure of merit by 50%.”

Future work will involve preparing different skutterudites that contain germanium and tellurium in place of antimony. The researchers are hoping that better properties will be achieved.

Thermoelectric materials have the potential for extracting more energy out of the combustion process, which would make the automobile more efficient. Further information can be found in a recent publication (3) or by contacting Kaviany at kaviany@umich.edu. 

REFERENCES
1. Canter, N. (2008), “Hybrid Hydraulic Drive Reaps Fuel Savings,” TLT, 64 (8), pp. 14–15.
2. Saqr, K., Mansour, M. and Musa, M.(2008),“Thermal design of Automobile Exhaust Based Thermoelectric Generators: Objectives and Challenges,” International Journal of Automotive Technology, 9 A(2), pp. 155–160.
3. Kim, H., Kaviany, M.,Thomas, J., der Ven, A., Uher, C. and Huang, B. (2010), “Structural Order-Disorder Transitions and Phonon Conductivity of Partially Filled Skutterudites,” Physical Review Letters, 105 (26), 265901.


Neil Canter heads his own consulting company, Chemical Solutions, in Willow Grove, Pa. Ideas for Tech Beat items can be sent to him at neilcanter@comcast.net.