SYSTEM DESIGN
A successful application of thermoelectric technology requires a systems perspective. There are many factors that impact performance.   Generally speaking, there are four key elements in a system as depicted below.  These are the heat energy source/sink, the thermoelectric module, the electrical load and, optionally, a power conversion element that matches electrical energy from the module to meet the load requirements.


THE SOURCE AND SINK
A thermoelectric generator (TEG) converts a portion of the energy in a heat energy FLUX into electricity.  That word "flux" has been highlighted to emphasize that thermoelectric generation operates from a thermal current as energy moves from a hot source to a cooler sink.  The source and sink have equal importance -- you need them both.   If the source is limited in its ability to deliver heat energy, it will not be able to maintain a high temperature on the hot side of the module. The module will then tend to equilibrate between the hot and cold sides and the reduced temperature gradient across the module will result in less heat flowing through the thermoelectric module and, hence a reduction in electric generation.  By the same token, if the sink is limited in its ability to absorb heat energy, the module temperature will increase, there will be less of a temperature gradient between the hot side and cold side of the module, and this will cause a reduction in electric generation.

THERMAL PARASITICS

From the standpoint of electric generation, it is desirable to obtain the highest possible temperature difference across the thermoelements in the module.  To accomplish this, it is necessary to reduce the thermal resistance between source and module and between sink and module.  These thermal resistances result in temperature drops that reduce the delta T across the module.  There are three heat transport mechanisms, namely conductive, convective and radiative.   In most thermoelectric applications, radiative transfer is generally less important, although it can be enhance by coating radiative surfaces with a high emissivity coating. 

It is important to note that while electric generation for a given module is always increased for an increased temperature gradient, this is not true from the standpoint of heat to electric conversion efficiency. That is, the extraction of the most electrical power from the heat energy flux between two thermal reservoirs. For that objective, the thermal resistance across the module should match to total parasitic thermal resistances in the system. This yields a temperature gradient across the active thermoelements that is exactly half of the temperature difference between the two thermal reservoirs.


CONDUCTIVE HEAT TRANSPORT
TXL Group's thermoelectric modules use 25 mil white alumina sheets on the top and bottom to provide mechanical support for the thermoelements.   These thin sheets have low thermal conductivity and are machined to be both flat and smooth. However, on a microscopic level, all surfaces are rough, so when a thermoelectric module is placed against a seemingly flat surface, it is actually not in full contact as shown below.   Where there is no direct contact, there will be a gap. If the gap is air, this represents a significant impediment to heat flow because air is a poor thermal conductor.   One solution is to use a wetting agent to fill the gaps.   It can be water, oil or thermal grease, all of which have a lower thermal resistance than air.   A thin layer allows gaps to be filled, as depicted below left.   It is important to only use a thin layer of a wetting agent.   This is because such agents, while good thermal conductors, are not ideal.   A thick layer (below right) can actually add to the thermal resistance and yield a reduced performance.



CONVECTIVE HEAT TRANSPORT
When the heat is delivered and/or removed from the thermoelectric module via a fluid, convective heat transport becomes important.   When a solid is immersed in a fluid having a different temperature, heat energy moves conductively into or out of the fluid through boundary layers that form over the surface of the solid.  There is often a significant thermal resistance at these boundary layers, so heat transfer can be greatly enhanced by forcing fluid flow past the surface, thereby breaking up the boundary layers and replenishing/removing heat energy.   This is called forced convection.   Often fins can be added to enhance this heat transport by providing low thermal resistive paths for the heat through solid and presenting more surface area to the fluid.

THE THERMOELECTRIC MODULE

A thermoelectric module is built from the connection of n-type and p-type thermoelectric elements that are connected in electrical series and thermal parallel as depicted below.

The electrical connections at the top and bottom of the thermoelements are metallic conductors, generally plated copper.   These connections do not directly participation in generation --- generation occurs in the thermoelements.   In fact, these conductors can contribute a loss component from two aspects.



First there is a loss due to Joule heating in the conductors, the so-called I2 R loss.   Second, there is a temperature drop across the conductor that will detract from the delta T across the thermoelement, reducing the generation potential.   Likewise the geometry of the thermoelectric pellets impacts the loss component. If the pellet is long in the dimension that heat flows, then more of the temperature drop will occur across it, resulting in higher temperatures. But this comes at a cost of an overall higher thermal resistance which reduces power generation.   A summary of some of these trade-offs and the underlying theory can be found in the following white paper on systems design:

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POWER CONDITIONING
The voltage output from a thermoelectric module will be a function of the temperature across the module.   If the temperature varies, so will the voltage. Many electrical loads impose requirements for voltage magnitude and regulation.   For this reason, it is often desirable to use a power conditioning module that goes between the TEG and the electrical load.   Power conversion can serve many functions.   In its simplest form, it might consist of a single diode rectifier that allows the TEG to charge a battery whenever there is sufficient generated power, without discharging that battery during periods of little or no generation.   A power conditioning module can also serve to implement an impedance match of the TEG to the load to allow maximum power transfer. A power conditioning module could also be selected to implement voltage regulation, converting the TEG voltage to a higher or lower setpoint voltage.   Some issues related to power conversion are described in the following white paper on impedance matching:

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