How to cool

Thermoelectric coolers (TECs) use the Peltier effect in which a heat flux is generated at a junction of two dissimilar materials across which a DC electric current is passed. As shown in Figure 2 TEC’s are constructed with P-Type, and N-Type semiconductors that are arranged parallel in the thermal path and in series along the electrical path. When a voltage is applied across the device electrons carry heat to one side, making the other side cold. As the heat load rises the amount of current required to maintain the correct temperature also must go up since the flow of electrical current is necessary to carry the additional heat load. As a result, the efficiency of TEC devices is quite low. As with all cooling systems, the heat removed from the device must be dissipated to the ultimate heat sink, normally to ambient air with a fan. Typical coefficient of performance (cooling power divided by power draw) for a TEC device is in the range of 0.5 to 0.2, meaning that to provide 400 watts of cooling the TEC would need to draw between 800 and 2000 Watts of Power.

A primary benefit of the TEC is that it can be fabricated in a variety of configurations and made into small sizes for low power applications. It also can provide precise temperature control. Other than fans and pumps to which it is often attached, the TEC itself has no moving parts so it requires minimal service. Ideal for small space, they can be used when typical refrigeration is not more advantageous. The TEC assembly can be mounted directly onto the laser cold plate or used to chill a flowing liquid. Due to its low efficiency, thermoelectric cooling is typically only used in applications requiring less than 400 watts of cooling. For applications that require a high level of temperature stability, a systems engineer can consider a hybrid approach of using vapor compression for the bulk of the cooling power and a small TEC assembly for a high degree of regulation.

Vapor compression cycle refrigeration is a process which the physical properties of a refrigerant are exploited to realize isothermal, 2-Phase heat transfer to efficiently provide cooling. As shown in Figure 3, heat is absorbed from the source being cooled in the evaporator where a refrigerant absorbs heat as it is converted from a liquid to a gas. The compressor takes in the gas discharge from the evaporator and raises its pressure and temperature. The condenser, takes in this superheated gas and condenses it from a gas to a liquid as heat is dissipated to the outside environment. In some cases, the condenser is designed to dissipate heat to a closed loop liquid line that is part of the manufacturing facility. The expansion valve takes in the high temperature liquid refrigerant and lowers its pressure and temperature. This low pressure and temperature gas/liquid mixture, then flows into the evaporator, repeating the cycle. The components in the basic refrigeration system are the Compressor, Condenser, Expansion Valve, and Evaporator. Simply stated, refrigeration systems efficiently move heat from a cold source to a heat sink. Vapor compression refrigeration systems are very efficient, with a typical coefficient of performance between 1.0 and 2.5, meaning that for a 400-Watt heat load the power draw would typically be between 400 and 160 Watts. This is 1600 to 640 Watts less then equivalent TEC systems as shown in figure 1.

By varying the evaporator, the vapor compression cycle can be used to cool air, liquid, or cold plates. To create an air cooling system, the evaporator is a refrigerant to air heat exchanger. To cool liquid, the evaporator is a refrigerant to liquid heat exchanger with the pumped liquid being cooled by the evaporating refrigerant. A third alternative for cold plate cooled devices is to run the refrigerant directly through the cold plate. With the evaporator as the cold plate, the devices being cooled have the advantage of phase change heat transfer with minimal temperature rise. Examples of these three implementations of vapor compression cooling are shown in figure 4.4

Compressor based liquid chillers are the most widely used cooling technique for high power lasers. Compressor size can be specified for cooling power from approximately 100 Watts to greater than 10kW and can cool virtually any laser type.

In 2007 a small rotary compressor was first introduced and put into communications systems for cooling military electronics.3 The compressor shown in Figure 5 uses standard rolling piston technology that has been miniaturized and coupled with an efficient brushless DC motor. With a speed variability between 2500 and 6500 RPM the compressor can effectively load match to the desired temperature.

The small compressor enables the miniaturization of vapor compression systems into smaller form factors and provides an effective match with heat loads down to 100 Watts. These small vapor compression systems are now being developed for commercial applications including lasers, medical devices, electronics, and pharmaceuticals.4 The creation of the miniature compressor has addressed what was formerly the main barrier to using vapor compression cooling systems in a highly integrated, small form factor system.