Laser Cooling

Keeping Cool About Your Hot Laser

The trend of higher laser output power in increasingly smaller packages has many benefits in reducing the floor space requirements in production floor, cleanroom, medical operating room, clinics, and laboratory environments.  Laser product managers are seeing similar pressure to reduce the size of the external laser chiller while still offering improved cooling and energy efficiency.  Some are considering adding the refrigeration system directly into their laser system to form an integrated compact package.  It is important to make decisions early in the concept development of the product development cycle to ease integration of advanced cooling methods such as direct refrigerant cooling.

Cooling System Choices

Table 1 lists virtually all the cooling methods used in modern laser systems.  Laser efficiency, laser temperature requirement, heat load, heat flux at the device, ambient air temperature, and facility infrastructure are determining factors in choosing a cooling system design.  For systems with low heat output and/or relatively high allowable temperature limits the above ambient systems shown in the top half of the table normally provide sufficient cooling.  For systems with more stringent temperature requirements where fans and pumped liquid loop systems are not adequate, then either vapor compression or thermoelectric systems are used to manage laser temperatures.  Both Thermoelectric (TEC) and Vapor Compression (VC) systems draw power to enhance the heat transfer from the laser to ambient air and enable cooling levels and heat fluxes not possible with passive means.

 

Cooling Temperature Requirement Method Cooling Medium Power Draw Cooling Capacity Use When
Above Ambient Free  Convection
Heat Sink – No Fans
Air None Very Limited Heat load is Low
Forced Convection
Fans blowing on heat sink
Air Very Low Very Limited Heat load is Low
Benchtop Air Cooled Liquid -closed loop pumped through radiator to cold plate Liquid Low Limited Heat load is low, & laser is liquid cooled
Liquid Cooled Liquid Loop
Facility chilled water
Liquid Low Limited Heat load is low and facility chilled water is available
At & Below Ambient Direct TEC Cooled Air High Low Cooling load is low
Efficiency not a factor
Close to ambient T required
TEC Cooled –
Liquid Loop
Liquid Medium High Cooling load is low
Efficiency not a factor
Close to ambient T required
Vapor Compression
Cooled Liquid Loop
Liquid High Medium Moderate to high cooling load
Efficiency important
Vapor Compression
Cooled Liquid Loop Liquid Cooled Condenser
Liquid Medium High Moderate to high cooling load
Efficiency important
Facility cooling water available
Vapor Compression – Direct Refrigerant Cooling Refrigerant Medium – Low High Moderate to high cooling load
Efficiency very important

 

 

Fig 1: Electrical Power Requirements

Multiple studies by the US government agencies have examined cooling technologies for efficiency and effectiveness in a variety of applications.  A 2010 government-sponsored study (DOE) compared VC systems with five competing technologies, including thermoacoustic, thermoelectric and magnetic technologies.1 The study emphasized efficiency and determined that VC systems are at least three times more efficient than all other current options.  An earlier study conducted by the Environmental Protection Agency concluded that “vapor compression refrigeration using non-CFC refrigerants is the most desirable technology of those considered for use in the five application areas: domestic, commercial, and mobile air conditioning; and commercial and domestic refrigeration).”2  The results rank vapor compression first in the technical assessment of each of the above-mentioned methods.  Vapor compression cooling uses 3 to 5 times less power than TEC’s.  A power draw comparison of TEC and VC technology is shown graphically in figure 1.

 

Thermoelectric Coolers

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.

Fig 2: Thermoelectric diagram

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 Refrigeration

Fig 3: Vapor compression cycle diagram

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.

Miniature Rotary Compressor

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.

Fig 5: Miniature compressor is 1/10th the volume and weight of conventional systems.

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.

Liquid Chillers

Regardless of the cooling technique water to air, water to water, TEC or vapor compression, liquid recirculation requires a mechanical pump, reservoir, tubing and fittings.  With the vast improvement of laser reliability, the liquid cooler is seen as the weakest link in the system.  Pump and fittings are known to fail and leaks around electronics can cause a laser failure.  Fluid levels in these systems need to be monitored and occasionally refilled due to evaporation.  In addition, it is recommended that an algaecide be used to prevent biofouling of the recirculating liquid line. Pump selection is critical to ensure proper flow rate and pressure for the intended application and prices can vary wildly from a few tens to over $1000 depending on the quality of construction.  A common thread heard from laser manufacturers is that an end user, whom may have spent $100k or more on a laser, will purchase an untested low-cost chiller to save a few hundred dollars.  When these chiller systems lose effectiveness or fail, they can cause down time for the user and warranty issues for the laser supplier.  Another common complaint from the laser users is that the chillers are unreliable and difficult to maintain.  Although the liquid chiller is the most common approach to cooling lasers, this type of feedback highlights an opportunity for improvement.

Direct Refrigerant Cooling

An exciting new development being explored by some of the leading laser manufacturers is direct expansion cooling.  Here, the refrigerant flows directly through the cold plate with the circulation being driven by the compressor.  The advantages are multifold.  This method eliminates the need for a liquid recirculating pump, fluid reservoir and associated tubing and connectors – sources of failures and leaks.  The efficiency is improved by removing the losses associated with cooling a liquid and energy savings of driving a pump (8 times less vs. TEC, 2 times less vs. liquid chiller – Figure 1.  As shown in Figure 6, the cooling capacity for direct refrigerant is 25% greater than a liquid chiller at a 25 degrees C.  The system is simplified improving reliability (MTBF >80,000 hours) and further reduces the size of the cooling system.

Three design issues need to be addressed when using direct refrigerant cooling.  These are, vibration from the compressor which may adversely affect the beam pointing stability, a reliable means of decoupling the cooling circuit from the laser for laser or cooling system maintenance without loss of the refrigerant, and attaining sufficient control of the laser temperature tolerances.  In a typical direct expansion cooling system copper tubing is rigidly connected from the compressor to the cold plate allowing vibration to transmit along these lines.  One way of addressing this is the use of flexible refrigerant lines that can decouple the compressor vibration from the laser.

For removing the cooling system from the laser, refrigerant line quick disconnects that are designed to prevent air ingress and refrigerant leakage can be used.  These are specifically designed for use in refrigerant lines and provide an easy means of disconnecting the two lines while maintaining system integrity.

Enhanced control over the laser temperature can be attained by coupling the laser controls directly to the refrigeration system controls.  In this manner, any thermal lag in the direct refrigerant system can be overcome by the control system.  This degree of controls integration is not necessary with the much larger liquid chiller systems since they often use the fluid reservoir and its innate heat capacity to maintain tight thermal control.

 

CONCLUSION

Miniature refrigeration technology is now advanced for effective application to laser systems that require active cooling with high efficiency. These systems are being custom-designed to meet original equipment manufacturer (OEM) and end user conditions producing a wide selection of laser thermal management applications. Applying direct refrigerant cooling further miniaturizes the cooling system, improves efficiency and reliability.  So don’t sweat it, there are some clever ways to chill your laser.

References:

  1. “The Prospects of Alternatives to Vapor Compression Technology for Space Cooling and Food Refrigeration Applications”, DR Brown, N Fernandez, JA Dirks, TB Stout, Department Of Energy Report No. PNNL-19259, Prepared under contract No Contract DE-AC05-76RL01830 March 2010
  2. “Alternative Technologies for Refrigeration and Air-Conditioning Applica­tions” D.C. Gauger, .N. Shapiro, and M.B. Pate, ‘=Research and Development EP/600/SR-95/066 May 1995
  3. “2U Rack Mountable Vapor Compression Cooling System for High Power Electronics” G. Deming, Electronics Cooling, May 2008.
  4. “Advances in Vapor Compression Electronics Cooling” J. Burnett Electronics Cooling, June 2014.

 

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Edited article appears in Laser Focus World Magazine, Jan 2017