By David Nemir, Jan Beck, Ed Rubio, and Manuel Alvarado—TXL Group, Inc.
Solid-state thermoelectric (TE) generation is a heat to electricity conversion technology that requires no moving parts. It is unique because it can be applied to both small- and large-scale waste heat harvesting, and it can produce electrical energy from relatively small temperature gradients at relatively low operating temperatures. In fact, in many settings, thermoelectric generation represents the only means to capture value from a small ΔT temperature gradient. Thermoelectric phenomena arise out of the intercoupled electrical and thermal currents in a material. A two-element thermoelectric generator is depicted in Figure 1. It is constructed by connecting an n-type thermo-element and a p-type thermo-element in electrical series, with both elements in thermal parallel between a heat source and a heat sink. The electrical series connection is made by attaching conductors as shown. Ideally, these series connection elements will be both good thermal conductors and good electrical conductors.
The key components of a thermoelectric device are the n-type and p-type thermo-elements, which are the active portions of the device that do the actual conversion of heat energy to electrical energy. Thermoelectric devices are generally formed by connecting many pairs (couples) of n- and p-type thermoelectric elements in electrical series and in thermal parallel. The open circuit voltage that is generated in a given thermo-element is proportional to the temperature difference, ΔT, across that thermo-element. The constant of proportionality is the Seebeck coefficient or thermo-power, S, yielding an equation for generated voltage, ΔV, of
Besides the thermo-power, S, two other materials parameters of interest when analyzing a thermoelectric material are the electrical conductivity, σ, and the thermal conductivity, λ, which are important when analyzing losses in a thermoelectric device. The three key material properties governing thermoelectric performance are often lumped into a single thermoelectric figure of merit Z:
As the parameters σ, S, and λ are temperature-dependent, Z will also be a function of temperature. Often the thermoelectric efficiency of a device.is characterized by the dimensionless figure of merit ZT, where T is the temperature in Kelvin. Currently, the standard material for high ZT at room temperature is undoped bismuth telluride (Bi2Te3), with a ZT at 300 K of approximately 0.6. A review of typical state-of-the-art ZT values is shown in Figure 2.
Figure 2. ZT values for SOA p-type (right) and n-type (left) thermoelectric materials. Download Design and Discovey
Thermoelectric generators generate electrical power from the heat energy flux that passes through the thermo-elements. Heat energy is delivered by the heat source, passes through the thermo-element, and is then carried away by the heat sink. Critical components of any thermoelectric device are the heat delivery to one side and heat removal from the other. So, it is a natural idea to add a thermoelectric generation capability to a device (or application) that is already set up to optimize heat delivery to one side and heat removal from the other. Such devices may be called boilers, condensers, recuperators, and radiators, but all fall under the generic name “heat exchangers.”
As a generic description, heat exchangers move heat energy flux from one fluid to another through an intermediate surface. That surface can be a plate, a tube, or a wall. Commercial application proposals have targeted the application of a thermoelectric generation structure to the wall of a heat exchanger, with a particular focus on applications in which the heat exchanger is already needed and for which electric generation can occur as a bonus to the job of heat exchange. Adding thermoelectric generation to a heat exchanger may not be practical as a retrofit. It will have an impact on heat transfer and may require larger surface areas to accomplish a given heat transfer job.
1. J.-P. Fleurial, Download Design and Discovey, Jet Propulsion Laboratory/California Institute of Technology, 1993.