Thermoelectrics and refrigerators

Members of the Quantum Simulations Group at Lawrence Livermore National Labs provide an extensive discussion of how thermoelectrics can replace freon based refrigerators when inexpensive thermoelectric materials reach a ZT of 3.

The Livermore group has begun working on simulations [modeling material processes using quantum molecular dynamics methods] for a diverse group of technological applications. For example, nanoscale materials could improve cooling technologies in military equipment and reduce the size of gamma radiation detectors being developed for homeland security.

Thermoelectric materials convert heat into electricity and vice versa. They have no moving parts and release no pollutants into the environment. A few niche markets have used them for decades to cool electrical parts or generate power. Researchers have considered using thermoelectric-based refrigerators to replace current heat-pump-based refrigerators that compress and expand a refrigerant such as Freon.

Canted nanowires grown in the [001] direction can achieve a ZT of 3.5 but require considerable doping with either phosphorus or boron. “I doubt that the wires could be doped strongly enough for this surface to work,” says Vo. “Wires grown in the [011] direction will probably be the best compromise.”

Although the low effective mass of silicon increases electrical conductivity, it also contributes to a high thermal conductivity. Thermal conductivity must be low for a thermoelectric material to be efficient. One solution is to change the material used for the wires. Vo’s simulations indicate that a SiGe combination will reduce lattice thermal conductivity by as much as five times without affecting electrical conductivity.

From the Air Conditioning and Refrigeration Technology Institute:

The final report on thermoelectric technology assessment of the ACRT institure

The energy conversion efficiency, or Coefficient of Performance (COP) of thermoelectric cooling devices, is determined by thermoelectric figure-of-merit, commonly denoted by ZT. The highest ZT to date is reported in Bi2Te3/Sb2Te3 and PbSeTe/PbTe superlattice thin films. Coolers based on such materials typically have a COP of ~2, which is lower than the COP of 3-4 vapor compression refrigerators. However, there is no known theoretical impediment to significant increases in thermoelectric energy conversion efficiency, and given a breakthrough in materials, thermoelectric technology might offer the possibility of a safe, efficient, and affordable alternative to fluorocarbon compression equipments.

The use of thermoelectric devices and systems has been limited by their relative low energy conversion efficiency. Present commercial thermoelectric devices operate at about 10% of Carnot efficiency, whereas the efficiency of a compressor-based refrigerator increases with size: a kitchen refrigerator operates at about 30% of Carnot efficiency and the largest air conditioners for big buildings operate near 90%.

Today’s thermoelectric devices are particularly useful when the efficiency is a less important issue than small size, low weight, or high reliability. For example, thermoelectric devices are suited for situations where the heat load is small (say,

About The Author