Microsoft Word - Thermoelectric Generators

Thermoelectric GeneratorsThermoelectric Generators

Jeffrey M. WeisseJeffrey M. Weisse

October 24, 2010October 24, 2010

Submitted as coursework

for Physics 240, Stanford

University, Fall 2010University, Fall 2010

The increasing price of oil,

concerts about climate, and the

depletion of our natural

resources have drawn much

attention to renewable energy

technology. In 2007 the world

consumed roughly 500

quadrillion BTUs of energy

and is expected to increase at

1.4% per year. [1] About 90%

of this energy was generated

through fossil fuel combustion

with a typical efficiency of

30-40%. The remaining

60-70% of the energy was lost

to the environment via

automotive exhaust, industrial processes, and more. [2] It is highly desired to use the wasted heat to improve the

overall efficiency of energy conversion. The energy of the wasted heat can be collected and transformed to

electricity through a thermoelectric device.

What is a Thermoelectric?What is a Thermoelectric?

Thermoelectrics are solid state heat engines with materials properties that enable them to convert waste heat

into electricity. Their fundamental principle is based on charge carriers: electrons in N-type materials and holes

in P-type materials have the ability to move freely through metals and semiconductors. In the presence of a

temperature gradient, charge carriers diffuse from hot to cold until an equilibrium is reached between the

diffusion potential and the electrostatic repulsion potential, resulting in a buildup of charge carriers known as

the Seebeck effect. [3] Typical thermoelectric devices consist of alternating P and N-type semiconductors

connected electrically in series and thermally in parallel (Fig. 1). This allows the holes and electrons to flow in

opposite directions forming an electric current for power generation. [4]

Fig. 1: Configuration of a Thermoelectric Module. The efficiency of a thermoelectric device is closely related to the semiconductor's material properties. Ideally, a

good thermoelectric device should behave as a "phonon glass" minimizing the thermal conductivity to maintain

a high temperature gradient, but also as an "electric crystal" maximizing the electrical conductivity and Seebeck

coefficient [3]. The thermoelectric effectiveness is often described by its figure of merit, ZT, which is a

dimensionless unit depending on the Seebeck coefficient (α), absolute temperature (T), electrical conductivity (σ), and thermal conductivity (κ): [3]

In order for a thermoelectric device to be competitive with current power generation methods it must possess at ZT great than 3. However, over past five decades the room temperature ZT of materials with our best available

technology has only slightly increased from 0.6 to about 1.0. [5] Materials exist with "phonon glass" or "electric

crystal" properties, however obtaining both of these simultaneously is a challenge. The issues arise that the

Seebeck coefficient, and thermal and electric conductivity properties are all effected by flow of electrons which

conducts both heat and charge. Simply increasing the electrical conductivity simultaneously decreases the

Seebeck coefficient and increases the thermal conductivity, limiting the potential improvement in ZT for bulk

materials. The best performing materials to optimize ZT tend to be heavily doped semiconductors. [4] State-of-the-art thermoelectric research is investigating new materials to independently tailor these properties. Theoretical ZT improvements exist in decoupling the Seebeck coefficient from the electrical conductivity using engineered heterostructures and independently reducing the thermal conductivity in high atomic weight and nanostructures materials.

[3,5] Reports show there has been as much as a 100-fold decrease in thermal

Fig. 2: Thermoelectric efficiency as a function of ZT and source temperature compared to competitive heat

engines. [7] conductivity in silicon nanowires, thereby opening the door to significant improvements in ZT. [2,6] Despite all

these efforts, there have been only three reports of producing a ZT greater than 2 including the best ZT roughly

3.5 at 575 °K. Transition from these laboratory results to actually devices does not seem to be likely anytime in

the near future. [7]

Thermoelectric MarketThermoelectric Market

Unfortunately thermoelectric efficiency (electrical power produced over waste heat in) is currently only about

1/6 of the maximum Carnot efficiency. As shown in Fig. 2, a comparison of thermoelectric efficiencies as a

function of ZT and operating temperature are compared to several common heat engines. For thermoelectrics to