Thermoelectric materials generate electricity while in a temperature gradient. In order to be a good thermoelectric, materials must have the unique combination of both high electrical conductivity and low thermal conductivity: a rare set of properties for one material to hold. Nanotechnology can now be used to lower the thermal conductivity of semiconductors whose electrical properties are excellent, but manufacturing nanomaterials is not trivial.
Anything—steam, for instance—will flow from hot to cold in a temperature gradient. In a thermoelectric material, electrons do the same thing. The extent to which electrons flow from hot to cold in an applied temperature gradient is governed by the Seebeck coefficient, also known as the thermopower.
In order for a thermoelectric to establish a large voltage while in a temperature gradient, its thermal conductivity must be low. This ensures that when one side is made hot, the other side stays cold. For many decades, the only semiconductors known to have both low thermal conductivity and high power factor were bismuth telluride (Bi2Te3), lead telluride (PbTe), and silicon germanium (SiGe): three expensive compounds using rare elements.
Today, low thermal conductivity can be achieved by creating nanoscale features such as particles, wires or interfaces in bulk semiconductor materials. These nanoscale features lower the thermal conductivity of the semiconductor and do not effect their strong electrical properties.
The efficiency of thermoelectric materials is governed by their “figure of merit” z. A large z is important in creating an efficient thermoelectric generator, but it is not the only important metric.
A thermoelectric module is a circuit containing thermoelectric materials that output usable electricity. There are several types of efficient thermoelectric materials, but not all are capable of operating in a power generation circuit, or “module,” under typical waste heat recovery conditions.
A thermoelectric module for power generation must operate in a very large temperature gradient—and thus be subject to large thermally induced stresses and strains—for long periods of time. They must also be able to withstand a large number of thermal cycles, which cause mechanical fatigue. These two requirements represent some of the toughest thermal and mechanical environments that any electronic device must withstand.
Furthermore, the geometrical design of a thermoelectric module will greatly effect its efficiency. The technology that goes into the design, joining and assembly of a thermoelectric module is copious.
A thermoelectric module requires two thermoelectric materials to function: one, an n-type (negatively charged) semiconductor; the second, a p-type (positively charged) semiconductor. This is so that a continuous circuit can be made whereby current can flow and power can be produced. With only one type of thermoelectric material, a voltage would be induced but current would never flow. These two n-type and p-type semiconductors form a thermoelectric “couple,” but do not form a p-n junction. Both must have high “figure of merit” z and tightly controlled properties.
The two types of thermoelectric materials must be configured within the module such that they are electrically in series, but thermally in parallel. The module must therefore have internal wiring that accomplishes this, as well as junctions and materials that survive the harsh mechanical conditions it is subject to. A selection of materials that minimize thermal expansion coefficient mismatches—and the technologies to fabricate them and their interfaces—is of utmost importance in a thermoelectric module.
A thermoelectric power generation system takes in heat from a source such as hot exhaust, and outputs electricity using thermoelectric modules.
A thermoelectric module needs a large temperature gradient to generate electricity: something that is technically challenging to implement in real-world applications. In a power generation system, the heat for the hot side of this temperature gradient must be supplied efficiently from a heat source such as an exhaust flue. The cold side must be cooled by air, water, or another suitable medium. To supply this heating and cooling, technologies known as heat exchangers are used on both the hot and cold sides. A thermoelectric power generation system can be thought of as two heat exchangers, each of which have to move heat to (or from) the hot (or cold) side of the thermoelectric modules.
Maximizing the efficiency (or, conversely, the total power output) of a thermoelectric power generation system requires extensive engineering design. Trade-offs between total heat flow through the thermoelectric modules and maximizing the temperature gradient across them must be balanced. The design of heat exchanger technologies to accomplish this is one of the most important aspects of engineering of a thermoelectric generator.
In operation, the entirety of a thermoelectric power generator actually sits in multiple large temperature gradients. It also contains interfaces between materials at several places that require low thermal losses. The challenges of designing a reliable system that operates at very high temperatures are many. In addition, the system must not cause large pressure drops in the heating and cooling sources, another difficult engineering constraint.
A thermoelectric generator produces AC power only after the original DC power from the thermoelectric modules passes through an inverter. An integrated power electronics system is necessary to deliver AC power to the customer.
The result: electricity from otherwise wasted heat.