August 04, 2008

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, <25W) or the temperature lift is small (say <10C) or the variation of the heat load is large (e.g., train passenger cabin). It is important to note that the COP of thermoelectric coolers increases significantly with decreasing the temperature lift.

• Instead of utilizing a full-fledged thermoelectric cooling system, it is possible to use a thermoelectric heat pump to improve the performance of an existing vapor compression system, so called “hybrid system.” For example, a hybrid vapor compression – thermoelectric cooler systems could use thermoelectric heat pumps to enhance the outlet subcooling of a condenser, in which thermoelectric heat pumps operate at small ΔT and high COP. Theoretical analysis predicted the cooling capacity and COP of the hybrid system could be significantly improved.

Thermoelectric heat pumps could operate at very high COP (possibly COP>6)under the condition of small temperature lift. They would provide a high COP boost to conventional refrigerating systems.

The thermoelectric subcooler is modeled as additional component that provides a given temperature lift. The simulation results are shown below
The important findings from the studies are listed below:
• A theoretical maximum improvement of 16.2% in COP can be achieved. The
corresponding increase in cooling capacity is about 20%.
• A theoretical maximum improvement of 35% in capacity can be achieved,
without change in COP.
• No increase in the size of the heat exchangers in the system.
• The economic aspects of coupling a thermoelectric device with a conventional
vapor compression system remain to be investigated.

• High Reliability: Thermoelectric coolers possess high reliability. Depending on
the conditions of application, the lifetime of thermoelectric coolers is in the range
of 100,000 to 200,000 hours

The old thermoelectric coolers from 1996

Thermoelectric coolers are solid state heat pumps used in applications where temperature stabilization, temperature cycling, or cooling below ambient are required. There are many products using thermoelectric coolers, including CCD cameras (charge coupled device), laser diodes, microprocessors, blood analyzers and portable picnic coolers.

12 questions about thermoelectric cooling

Let's look conceptually at a typical thermoelectric system designed to cool air in an enclosure (e.g., picnic box, equipment enclosure, etc.); this is probably the most common type of TE application. Here the challenge is to 'gather' heat from the inside of the box, pump it to a heat exchanger on the outside of the box, and release the collected heat into the ambient air. Usually, this is done by employing two heat sink/fan combinations in conjunction with one or more Peltier devices. The smaller of the heat sinks is used on the inside of the enclosure; cooled to a temperature below that of the air in the box, the sink picks up heat as the air circulates between the fins. In the simplest case, the Peltier device is mounted between this 'cold side' sink and a larger sink on the 'hot side' of the system. As direct current passes through the thermoelectric device, it actively pumps heat from the cold side sink to the one on the hot side. The fan on the hot side then circulates ambient air between the sink's fins to absorb some of the collected heat. Note that the heat dissipated on the hot side not only includes what is pumped from the box, but also the heat produced within the Peltier device itself (V x I).

Let's look at this in terms of real numbers. Imagine that we have to pump 25 watts from a box to bring its temperature to 3°C (37.4°F) from a 20°C (68°F) ambient. To accomplish this, we might well have to take the temperature of the cold side sink down to 0° C (32°F). Using a Peltier device which draws 4.1 amps at 10.4 V, the hot side of the system will have to dissipate the 25 watts from the thermal load plus the 42.6 watts it takes to power the TE module (for a total of 67.6 watts). Employing a hot side sink and fan with an effective thermal resistance of 0.148 C°/W (0.266F°/W), the temperature of the hot side sink will rise approximately 10°C (18°F) above ambient. It should be noted that, to achieve the 17° C drop (30.6°F) between the box temperature and ambient, we had to create a 30° C (54°F) temperature difference across the Peltier device.

More research papers from the Air Conditioning and Refrigeration Technology Institute.

More research projects of the ACRT institute.

Existing commercial thermoelectric refrigerating cooler [many other high end applications listed in the ACRT thermoelectric assessment report:

The Igloo Kool Mate 56-Quart Thermoelectric Cooler sold at Walmart for $119.73

- Plugs into 12V utility outlet in your boat or car
- 56-quart (1.9 cubic feet) capacity holds up to 72 twelve-ounce cans
- Cools to down to 44-degrees F below outside temperature

This site has already reported that there is about a billion dollars in research from the US military and the Department of Energy on thermoelectrics.

Replacing Freon in AC and refrigerators would be a big part of reducing greenhouse gases. Freon refrigerant gas was banned from vehicular air conditioning systems In the mid 1990’s to prevent Ozone Layer depletion. R134-a refrigerant gas was universally adopted as the replacement However R134-a has 1,300 times* the global warming potential of CO2 The European Union is prohibiting use of R134-a in cars for
•New models in 2011
•All new cars in 2017

The research goal is included on page 35 of the 55 page slide deck presentation by Vehicular Thermoelectrics Applications Overview
,John W. Fairbanks, Technology Development Manager-Thermoelectrics, U.S. Department of Energy – Washington, DC, Presented at the DEER 2007, Detroit, Michigan August 15, 2007

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