Thermoelectric materials—materials that exploit a phenomenon in which the application of heat to combinations of certain metals induces an electric current—are emerging as potentially important  systems for increasing fuel efficiency and decreasing emissions...and maybe even as a wildcard solution for propulsion in the future.

Thomas Seebeck, a German physicist, first noticed the thermoelectric effect in 1821 when he discovered that a voltage was developed in a loop containing two dissimilar metals, provided the two junctions were maintained at different temperatures.

Some devices have emerged over the years to exploit that property, but have in general been hampered by low efficiency and high cost. The efficiency of thermoelectric (TE) materials is represented by a non-dimensional figure-of-merit: Z. (Z for any given TE material is calculated (not measured) using a combination of the Seebeck coefficient of the material, the electrical conductivity of the material and the total thermal conductivity of the material.)

There are two ways to increase the efficiency of a thermoelectric generator: one is to increase the difference in temperature (delta T or ΔT); the other is to increase the figure-of merit, Z. To put it another way: for a given ΔT, the higher the Z, the more electrical output you achieve.

Up until a few years ago, the Z of thermoelectric materials (ZT) had been hovering around 1. Breakthroughs in nanotechnology and materials science are pushing that dramatically higher, however, with some materials now achieving a ZT of between 3 and 4.

One of the more obvious automotive application for thermoelectrics (TE) is recuperating waste engine heat.

The diagram at the right (Click to enlarge) depicts the energy split in a gasoline-powered internal combustion engine. Of the 100% energy available from combustion, only about 25% actually gets applied to moving the car or running the accessories. (Diesel engines and lean combustion gasoline engines fare somewhat better: 35% of the energy flows to mobility and accessories.) Successfully harnessing waste heat from vehicle exhaust would be equivalent to developing a vehicle with 50% or higher overall efficiency, according to the DOE.

There are a number of companies looking into TE-based waste heat recovery such as Hi-Z, Amerigon, BASF, Caterpillar, GM and BMW. The goal is to use that excess heat to create electricity that would then support many of the existing vehicle electrical systems, and to deliver sufficient power to be able to replace other mechanical systems with electrical systems.

Research by Caterpillar presented at a DOE Thermoelectrics workshop early this year projected that a thermoelectric waste heat generator used in place of the standard alternator could reduce fuel consumption by 13% while the powertrain delivered the same propulsion.

Finding ways to reduce fuel consumption is increasingly a critical business issue for the trucking industry. Some 1% of all US transportation fuel is burned up by diesel truck idling at a cost to the trucking industry of more than $1B annually. Add in increasingly stringent emissions regulations and you have a pretty good impetus to discover a solution.

Hi-Z is currently working on and/or testing a 1 kW TE Generator (TEG) for heavy duty trucks,  a 300 watt generator for a light trucks and a 200w generator for a hybrid truck.

As ZT increases past 10, the opportunity for the replacement of an internal combustion engine with a TE-powered motor emerges. In that scenario, which is also being considered (albeit somewhat quietly) by the Department of Energy, TE generators could replace hydrogen fuel cells as the power source of the future.

That said, there is a tremendous amount of discovery and work to be done to achieve anything close to that level of TE efficiency. What makes it even theoretically possible is the ongoing advancements in nanotech engineering to create custom materials.

In the meantime, while there are benefits to short- to medium-term applications of thermoelectric materials, there are also barriers.

For waste heat regeneration, however, those barriers should be nothing some market demand and production scaling can’t handle. There are already thermoelectrics present in some high-end vehicles, although of a more trivial nature: seat temperature control and cup warming/cooling.  The hypothetical and disruptive application of TE—replacing engines—is much further off, and is dependent upon ongoing nanotech advances in material science. Definitely worth the work, though.

Resources:

• DOE’s Launch of High Efficiency Thermoelectrics Projects

• Status of the Application of Thermoelectric Technology in Vehicles

• The Effects of an Exhaust Thermoelectric Generator on a GM Sierra Pick-up Truck

• Automotive Applications for Thermoelectric Technology