The efficiency of energy generation is increasingly important across all market segments. As energy costs continue to skyrocket, system efficiency is becoming an increasingly important attribute. Extracting waste heat from a system and converting some percentage of that heat into usable energy is one way in which system efficiency can be improved.
Energy harvesting, or energy scavenging, is the process whereby a portion of energy is removed, captured and stored from an existing source of unused but available energy. For example, the heat from an exhaust stack at a manufacturing facility can provide power for wireless sensors used for chemical analysis of the smoke stack effluent.
In addition to chemical analysis, the applications for wireless sensors are many and varied and include habitat monitoring, object tracking, nuclear reactor control, fire detection, and traffic monitoring, all of which have sources of waste heat for energy conversion.
The use of thermoelectric power generators (TEGs), in which a temperature difference creates an electric potential, can convert waste heat from thermal sources into usable electricity as an energy source for wireless sensor networks.
Understanding Thermoelectric Technology
The core component of a thermoelectric device is a thermocouple. A thermocouple consists of an n-type and a p-type semiconductor connected together by a metal plate. Electrical connections at the opposing ends of the p- and n-type material complete an electric circuit (left hand side of Figure 1).
Thermoelectric cooling (TEC) occurs when current is supplied, in which case the thermocouple cools on one side and heats on the other by what is known as the Peltier effect.
Thermoelectric generation (TEG) occurs when the couple is subjected to a thermal gradient (i.e., the top is hotter than the bottom), in which case the device generates current, converting heat into electrical power by what is known as the Seebeck effect.
A thermoelectric module is made from arrays of thermocouples connected in series to create a larger, active surface area, as shown in the right hand side of Figure 1. If heat is flowing between the top and bottom of the module (forming a temperature gradient) a voltage will be produced and hence an electric current will flow.
Figure 1. Thermal-to-electric conversion with thermoelectrics
Thermoelectric Conversion Efficiency
A key parameter in thermoelectric power generation is the conversion efficiency of the device, 
, given by:
1. 
Where Th is the hot side temperature of the device; 
T is the temperature differential across the device; and ZT is the inherent thermoelectric efficiency of the device. Power conversion efficiency increases with ZT: the higher the ZT of a thermoelectric module, the higher the power conversion efficiency. Power conversion efficiency also increases with T, given by:
2. 
Where Q is the heat passing through the device; K is the thermal conductance of the device; km is the thermal conductivity of the device; Lm is the thickness of the device; and A is the area of the electrical contact to the device.
Therefore, 
T increases with Lm/A; that is, for a given contact area, A, the thicker the thermoelectric material, the larger the temperature differential that can be achieved and the higher the power conversion efficiency.
The Case for Thermoelectric Energy Scavenging
As the direct and indirect costs of energy increase, the efficiency of electrical systems is rapidly becoming a key differentiator in overall product value. One step towards clean, efficient, electrical energy is to improve overall electrical efficiency of the system by extracting waste heat from the system and converting some percentage of that heat into usable energy.
Thermoelectric power generators have long shown significant promise for such thermal energy harvesting and have to date been used extensively in military and aerospace applications for this reason.
Advantages of thermoelectric power generators for clean energy production include:
* They are solid-state – there are no moving parts;
* They contain no consumable materials;
* They have demonstrated very long operating lifetimes.
Recently, thin film thermoelectric material has been integrated into the widely accepted copper pillar bumping process used in high-volume electronic packaging to achieve microscale power generation. This new thermoelectric technology, referred to as thermal copper pillar bump – or “thermal bump” – has the potential to fundamentally change how thermoelectrics can be used in clean power energy harvesting applications.
Thermoelectric modules based on the thermal bumping technology show unprecedented promise to achieve the breakthrough required to achieve high volume, low cost implementation of thermoelectric technology in thermal energy harvesting applications. Manufacturers employing thermoelectric modules based on the thermal bumping technology have demonstrated:
* Thin film thermoelectric materials that have an inherent efficiency (ZT) that is more than 2x higher than conventionally available materials;
* A low cost, solder bumping manufacturing approach to fabricate thin film thermoelectric modules;
* Thin film thermoelectric modules that deliver power generation performance similar to conventional technolog
y but at 20x smaller size and with significantly better robustness;
Recent experiments have increased efficiency of modules by 2x with a pathway to 10% conversion efficiency. Such results immediately enable power conversion for sensors in a distributed sensor network. For example, based on the physics of the thin film thermoelectric devices, it can be established that device performance can be optimized by minimizing device contact resistivity
(
c) and maximizing material thickness (Lm). It can be shown that modest improvements in these two parameters alone results in a conversion efficiency of >5% for a single stage device at a 
T of 120°K. These improvements will bring the performance of thin film thermoelectric devices well within the range of sensor applications.
Summary
The concept of generating clean power from waste heat is alluring and gaining significant attention worldwide. However there is a direct link between thermoelectric module performance, in terms of efficiency, and the applicability of thermoelectrics in key power generation markets. Furthermore, the market for remote power is undeniably large. Sensors alone represent a TAM (total available market) of 8 billion units by 2012; while only a portion of this market will benefit from remote power, a reasonable estimate puts the ultimate potential at over 100M units per year for this application.
Energy scavenging from thermal sources using thin-film thermoelectric technology becomes an extremely attractive option for energy harvesting as an alternative energy source and as a method to improve overall system efficiency.
