Vapor Compression Cooling (VCC) in Electronics

1
Vapor Compression Cooling (VCC) in Electronics
2
Outline

• Benefits
• Disadvantages
• History
• Basic operation
• Engineering Design
• Cold plate
• Refrigerants
• Capillary tube
• Condensation
• Product Reliability
• Current applications
• Supercomputer and Mainframe Cooling
• Current research
• Microscale VCRS

3
Benefits

• Allows cooling to below ambient temperatures,
  increasing performance, reliability, and allowing
  operation in higher temperatures.
• High COP (around 2 to 3).
• Ability to remove very large heat loads.
• Widely available compressor and fan only moving
  parts, stable, and reliable. (moran, 2001).
• Low mass flow rate of refrigerant needed.
• Ability to transport heat away from its source.
• COP up to 3 times greater than thermoelectric
  coolers or more.
• (peeples, 2001)

4
Benefits

• Sub ambient temperature operation allows CMOS
  (complementary-symmetry/metal-oxide
  semiconductor) transistors to switch on and off
  faster (peeples, 2001).
• CMOS circuits are a major class of integrated
  circuit and include microprocessors,
  microcontrollers, and other digital logic
  circuits (Wikipedia, http//en.wikipedia.org/wiki/
  CMOS) .

5
Benefits

• The following physical parameters favor low
  temperature operation carrier mobility, and
  junction leakage.
• Although at low temperatures there is an increase
  in the failure rates due to hot carriers, overall
  failure rates decrease at lower temps due to the
  overall increased characteristics mentioned
  previously (Moran,2001).
• Definitions
• Carrier mobility- velocity of charge carriers in
  a solid material with an electric field applied
  to it (Wikipedia, http//en.wikipedia.org/wiki/Ele
  ctron_mobility).
• Hot carriers- high energy carriers (Moran,2001).
• Junction leakage-undesirable conductive paths in
  certain components, for instance, in capacitors.
  Also, a pathway through which electric discharge
  may slowly take place (IEEE Standard Dictionary
  of Electrical and Electronics Terms).

6
Benefits

• Much like heat pipes, vapor compression coolers
  use the high heat of vaporizations of the working
  liquid to remove large amounts of heat.
• Therefore, low mass flow rates required.
• Advantage over a chilled liquid loop that
  requires a much higher mass flow rate.
• Much like heat pipes, VCCs use the high heat of
  vaporization of liquids to transport large
  amounts of heat, thus low amounts of liquid are
  required. However, in chilled liquid loop
  coolers, the liquid is heated but not necessarily
  evaporated.

(Cengel and Boles, 2002)
7
Disadvantages

• The cost of employing the cooling system may be
  10-20 of the cost of the entire system.
• Large space and input power.
• The disadvantages of the cooling system has to be
  weighed with the large advantages. A lot of
  times, traditional methods of cooling like air
  cooling is not feasible so a VCC is required.
• However, if the cooling requirements can be
  satisfied with traditional techniques, the
  traditional technique are the preferred cooling
  method (Moran, 2001).

8
History

• The vapor compression refrigerator was first
  proposed in 1805 and a model was constructed in
  1834.
• Vapor compression technology is well established
  but only recently used in electronics cooling.
• Kryotech was one of the first companies to employ
  the technology.
• By the late 90s companies like AMD, Sun
  Microsystems, IBM, and SYS Technologies had all
  used Vapor Compression cooling.

(Schmidt et al., 2002)
9
Basic Operation
(Peeples,2001)
10
Basic Operation

• Ideal cycle
• 1-2 The working refrigerant, a saturated vapor,
  is carried through the suction tube to the
  compressor. The compressor compresses the
  saturated vapor into a superheated vapor which is
  then passed to the condenser.
• 2-3 The heat of the hot and high pressure vapor
  is released into the environment from the
  condenser. The working gas is transformed into a
  saturated liquid.
• 3-4 The liquid is pumped through a capillary tube
  or a thermal expansion valve into the evaporator,
  dropping significantly in temperature. The
  working fluid is a saturated mixture.
• 4-1 Heat flows into the evaporator from the heat
  source. The heat vaporizes all the working liquid
  (refrigerant), at the end of this stage the vapor
  is saturated, and the process repeats.

(Peeples,2001)
11
Basic Operation

• Qout, QH denotes the heat released to the
  environment.
• Qin, QL denote heat absorbed from the
  refrigerated space.

• 2 represents the actual position of the state. 2s
  represents the position of the state if it were
  irreversible.
• In the ideal case
• 1?2 isentropic (sconst)
• 2?3 isobaric (Pconst)
• 3?4 isenthalpic (hconst)
• 4?1 isobaric ( Pconst)

(Peeples,2001)
12
Basic Operation

• The heat absorbed by the evaporator (Qin)
  released from the condenser (Qout), and actual
  and isentropic work input by the compressor can
  be determined from the equations
• The efficiency of the compressor is given by
• Where h is the enthalpy at various states, the
  subscripts s and a refer to isentropic and
  actual, and w refers to work.

(Cengel and Boles, 2002)
13
Basic Operation

• Actual cycle
• The deviation of the ideal cycle to the actual
  cylce is due to irreversibilities mainly due to
  fluid friction-causing pressure drops and heat
  transfer to the system or the environment.
• In the ideal cycle the state of the refrigerant
  is precisely know, for example, at stage 1 the
  refrigerant is a saturated vapor. However, in
  actuality the state of the refrigerant may not be
  precisely known and is usually a superheated
  vapor.

(Cengel and Boles, 2002)
14
Basic Operation

• Note that the pressure drops between stages 8-1,
  2-3,4-5 etc. cause alterations
• to the TS diagram. The process 1-2 represents a
  compression path that may
• be more desirable than the 1- 2 pathway because
  the specific volume
• of the refrigerant is lower and thus the work
  input is lower.

(Cengel and Boles, 2002)
15
Basic Operation
(Cengel and Boles, 2002)
16
Basic Operation

• The Coefficient of performance of the VCR is the
  ratio of heat into the evaporator to work put
  into the compressor. Generally, the COP is around
  2 to 3.
• The COP can be determined by finding the ratio of
  the enthalpies between the throttling valve and
  evaporator and between the evaporator and the
  compressor.

(Cengel and Boles, 2002)
17
Basic Operation

• The most efficient refrigeration cycle is that of
  a Carnot refrigerator.
• The coefficient of performance of the Carnot
  refrigerator gives the maximum performance
  between two hot and and cold temperatures.
• This value can be compared to the actual COP to
  determine how close to ideal the refrigerator is
  operating.
• Note that the Carnot cycle is a reversible cycle.
  Reversible cycles are cycles that do not generate
  any entropy due to friction, heat transfer, etc.
  For a reversible cycle

(Cengel and Boles, 2002)
18
Basic Operation

• The coefficient of performance for a Carnot cycle
  is
• Note the COP increases as the ratio of TH/TL
  decreases. Therefore, we want TH and TL to be as
  close as possible. Usually, TL is specified and
  TH can be altered. The reason for TH/ TL
  affecting COP is that for a given TL the greater
  the TH the greater the amount of work input,
  decreasing COP.

(Cengel and Boles, 2002)
19
Basic Operation

• The area enclosed in the TS diagram generally
  describes the net heat transfer and the work of
  the system. For refrigeration, work input lowers
  the COP for a given heat rejection therefore,
  altering the states of the cycle to get a smaller
  enclosed area will increase performance.

(Cengel and Boles, 2002)
20
Engineering Goals

• The design of a cost-effective and efficient VCC
  involves the following considerations
• Cold surfaces can not be allowed to collect
  condensate from the air.
• The most suitable refrigerant for the given
  application, as well as the tubing to supply and
  remove the refrigerant, must be chosen.
• Compressor and condenser design.
• The cold plate must efficiently lift heat from
  the device to be cooled.

(Peeples,2001)
21
Cold Plate/Evaporator

• The cold plate (evaporator) is a heat exchange
  device which transfers heat from the heat source
  to the working fluid.
• Cold plate design must assure efficient heat
  transfer from the device being cooled to the
  refrigerant inside the cold plate.
• The cold plate must be fabricated from thin and
  thermally conductive material to minimize thermal
  resistance while maintaining structural
  stability.
• The mating surface between the cold plate and the
  heated body must be flat and smooth to minimize
  contact resistance.

(Peeples,2001)
22
Refrigerant Fluids

• The refrigerants physical properties determine
  its evaporating temperature at a specified
  operating temperature, as well as, its capacity
  to transport heat.
• The well known refrigerants R-134a and R-404a
  (and others) perform well for cooling high power
  electronics due to their high heat transport
  characteristics, low possibility of environmental
  harm, corrosion, and risk of explosion.

(Peeples,2001)
23
Refrigerant Fluids

• When making a decision as to which refrigerant to
  use and under what pressures to operate the VCR,
  consideration of the refrigerants temperature of
  vaporization and condensation should be
  considered. If the temperature of the refrigerant
  doesnt reach the vaporization point (at the
  operating pressure) the VCR will not work
  properly. To ensure a proper heat transfer rate a
  5 to 10 degree Celsius temperature difference
  should be maintained between the evaporator and
  the condenser with the refrigerant.
• The next slide shows the P-T curves of R-404a and
  R-134a describing the refrigerants lowest
  practical operating limit.

(Peeples,2001)
24
Refrigerant Fluid Saturation Pressure and
Temperature
(Peeples,2001)
25
Capillary Tube

• The capillary tube has the function of
  transporting the working liquid from the
  condenser to the evaporator.
• The small diameter and long length of the tube
  produces a large pressure drop.
• Refrigerants chosen have a large Joule-Thomson
  coefficient, which tells us how much the
  temperature drops as the pressure drops at
  constant enthalpy.
• Since pressure drops create performance losses
  careful design must be taken so that the tube
  lengths and diameters are minimal.
• (Heydari, 2002)

26
Condensation

• Condensation, much like in TEC, is a problem
  because the surfaces of the VCR may be lower than
  the dew point.
• Since water is hazardous to electronic
  assemblies, condensation must be minimized by
  insulating surfaces from air in spot-cooling
  applications. (Peeples,2001)
• More on sealants later on in the slides.

27
Product Reliability

• Electro-mechanical systems generally have product
  life cycles called bathtub curves.
• The curve has three distinct regions
• Infant mortality- rate of failure decreases with
  time
• Normal use-rate of failure relatively constant
• Wear out- rate of failure increases with time.

(Peeples,2001)
28
Improving Performance

• In some applications the heat rejection demands
  (efficiency or amount) are higher than what can
  be handled by a vapor compression cycle running
  on a regular cycle. In these cases, modifications
  of the cycle must occur.
• Like previously mentioned, modifying the TH/TL to
  get them as as low as possible will increase
  performance but larger modifications may need to
  be made.

(Cengel and Boles, 2002)
29
Improving Performance

• An example is a cascade cycle, which performs the
  refrigeration process in two cycles that are in
  series. This is useful in situations
  (industrial), where there is a large temperature
  difference between the hot and cold side for one
  cycle to be practical.
• If the fluid used in the cascade system is the
  same, the heat exchanger can be replaced by a
  mixing chamber, known as a flash chamber which
  has better heat transfer characteristics.

(Cengel and Boles, 2002)
30
Cascade Refrigeration
(Cengel and Boles, 2002)
31
Refrigeration System w/ Flash Chamber
(Cengel and Boles, 2002)
32
Supercomputer and Mainframe Cooling

• The extremely high cooling demands of mainframes
  and supercomputers are ideal applications for
  VCRs because other cooling systems can not
  provide the necessary cooling capacity. An
  example of is IBMs G4 mainframe (shown on the
  next slide)

(Schmidt et al.,2002)
33
Supercomputer and Mainframe Cooling

(Schmidt et al.,2002)
34
Supercomputer and Mainframe Cooling

• The bulk power assembly at the top provides 250
  volts dc to the mainframe.
• Below the bulk power is the central electronic
  complex where the MCM (multichip module) is
  located. The MCM housing the 12 processors.

35
Supercomputer and Mainframe Cooling

• Below the MCM are blowers that provide air
  cooling for all of the components in the
  processors except for the processor module, which
  is cooled by refrigeration.
• Below the blowers are two modular refrigeration
  units (MRUs-the VCR) which provide cooling via
  the evaporator mounted on the processor module.
• In the bottom of the mainframe are the input/
  output (I/O)connections and two blowers. The
  blowers cool the I/O connections, as well as,
  provide the cooling for the condenser of the
  MRUs.

(Schmidt et al.,2002)
36
Multichip Module (MCM)

• The mainframes processing unit, MCM, is
  constructed as follows.
• Note the evaporator above the chips.

(Schmidt et al.,2002)
37
Modular Refrigeration Unit (MRU)

• The MRU (VCR) houses all the refrigeration
  components except the evaporator. The MRU
  contains the
• Condenser
• Thermostatic expansion valve
• DC rotary compressor

(Schmidt et al.,2002)
38
Condensation Protection

• To avoid moisture condensation on the MCM
  hardware, all the cooling hardware including the
  evaporator copper cold plate is contained in an
  airtight metal enclosure with one open face.
• 260 grams of silica gel desiccant is kept there
  to absorb any moisture leaking into the enclosure.

(Schmidt et al.,2002)
39
Condensation Protection

• The figure on the previous slide also shows a
  flat board that replaced the planar board in
  order to test the effectiveness of various
  sealants in keeping moisture out of the
  evaporator cavity.
• The results show that the
• Butyl 1 rubber sealant
• displayed the best
• sealant characteristics,
• allowing the least amount
• of humidity to enter.

(Schmidt et al.,2002)
40
Microscale VCRs

• To utilize VCRs in laptops, personal computers,
  or other cooling applications of small size, the
  VCR size must be reduced to fit within a small
  enclosure. The next section discusses progression
  in this area.
• Since miniature VCRs have high heat loads to
  transfer away, the condenser and evaporator must
  be designed such that they transfer enough heat
  to satisfy the heat removal demands.
• The most difficult part in designing a miniature
  VCR is the compressor.

41
Microscale Evaporators

• Chirac et al. (2005) suggest using an evaporator
  with microchannels through the center as an
  option to miniaturize the evaporator. The
  microchannels transfer large amounts of heat
  reducing the evaporator size needed to transfer
  the heat load from the heat source.

42
Microscale Condenser

• Chiriac et al. also describe a condenser with
  microchannels through the center. A heat sink
  surrounds the outside of the condenser while a
  fan blows air over the heat sink.

43
Refrigerant Fluids

• Heydari (2002) performed a simulation with a
  miniature VCR which included a miniature
  compressor to cool a computer system.
• The figure shows ammonia has the highest COP
  relative to the other refrigerants. However, when
  the refrigerants cost, environmental impact
  (ozone depletion and global warming potential),
  and safety issues were considered, Heydari
  concluded the optimal refrigerant to use is R134a.

44
Condenser Temperature and Performance

• Heydari found that with the computer chip
  junction temperature and heat absorbed by the
  evaporator fixed, decreasing the condenser
  temperature decreases the COP.

45
Evaporator Temperature and Performance

• Lastly, Heydari found that for a fixed junction
  temperature and the amount of heat absorbed by
  the evaporator fixed, increasing the evaporator
  temperature increases the COP but the amount of
  heat condensed decreases.

46
Refrigerant Fluids

• When it comes to the performance of refrigerants
  in miniature VCRs, Phelan et. Al. (2004)
  performed experimental analysis comparing three
  refrigerants ammonia, R-134a, and R-22 to
  determine which of the three produce the highest
  COP for a miniature VCR under various conditions.
  
• The factors tested were the evaporator and
  condenser temperatures, as well as, the
  efficiency of the compressor which was predicted
  to decrease as the size of the compressor
  decreased. The results are shown on the next
  slide.

47
Refrigerant Fluids
Phelan et. Al. (2004)
48
Refrigerant Fluids

• For each condition, ammonia has the highest COP.
• The higher COP values are due to ammonias
  greater latent heat of vaporization.
• However, due to ammonias greater adverse
  environmental and physiological effects, R-134a
  is more predominantly used.
• Ammonia is typically used only with a secondary
  loop due to its toxicity.

(Phelan et. al 2004)
49
Microscale VCRs

• Utilizing VCRs for electronics cooling
  applications has been limited by their large size
  due to the use of traditional components like
  pistons, linkages and pressure vessels.
• University of Illinois has a DARPA grant to
  develop miniature VCRs for use with cooled
  military uniforms for use in desert warfare. This
  Technology could also be used for electronics
  applications. However, the miniature compressor
  has been difficult to achieve.
• Research is ongoing on a Stirling cycle MEMS
  cooler being developed in NASA Glenn.

(Moran,2001)
50
Microscale VCRs

• The Stirling Cycle is much like the vapor
  compression cycle and is shown below.

(Moran,2001)
51
Microscale VCRs

• Using diaphragms instead of pistons, the MEMS
  Stirling cooler is fabricated with semiconductor
  processing techniques to provide a device with
  planar geometry.
• The result is a flat cold surface for extracting
  heat and an opposing flat hot surface for thermal
  dissipation. A typical device would be composed
  of numerous such cells arranged in parallel
  and/or series with all layers joined at the
  periphery of the device to hermetically seal the
  working gas.

(Moran,2001)
52
Microscale VCRs
(Moran,2001)
53
Microscale VCRs

• The expansion and compression diaphragms are the
  only moving parts.
• Expansion of the working gas directly beneath the
  expansion diaphragm in each cycle creates a cold
  top end for extracting heat, while compression at
  the other bottom end creates a hot region for
  dissipating heat.

(Moran, 2001)
54
References

• Cengel and Boles, 2002, Yunus A., Boles, Michael
  A. (2002). Thermodynamics an Engineering
  Approach. New York NY McGraw-Hill.
• Chiriac, Florea Chiriac, Victor (2005). An
  alternative Method for the Cooling of Power
  Microelectronics Using Classical Refrigeration.
  ASME/Pacific Rim Technical Conference and
  Exhibition on Integration and Packaging of MEMS,
  NEMS, and Electronic Systems Advances in
  Electronic Packaging. pp 425-430.
• Heydari, Ali. (2002). Miniature Vapor Compression
  Refrigeration System for Active Cooling of High
  Performance Computers. 8th Intersociety
  Conference on Thermal and Thermommechanical
  phenomena in Electronic Systems. pp. 371-378.
• IEEE Standard Dictionary of Electrical and
  Electronics Terms. (1973). New york
  Wiley-Interscience.
• Moran, Mathew E. (2001). Micro-Scale Avionics
  Thermal Management. 34th International Symposium
  on Microelectronics sponsored by the
  International Microelectronics and Packaging
  Society.
• Peeples, John. W.(2001). Mechanically Assisted
  Cooling for High Performance Applications.
  Advances in Electronics Packaging Procedings of
  the Joint ASME/JSME Conference on Electronics
  Packaging. Pp. 899-904.
• Phelan, Patrick Chiriac, Florea Chiriac,
  Victor (2004). Designing a Mesoscale
  Vapor-Compression Refrigerator for Cooling
  High-Power Microelectronics. Intersociety
  Conference on Thermal Phenomena, 1, pp. 218-223.
• Schmidt, Roger R., and Notohardjono, Budy D.
  (2002). High-End Server Low-Temperature Cooling.
  IBM Journal of Research and Development, 46 (
  6), pp. 739-750.
• Wikipedia the Free Encyclopedia (August 2006).
  CMOS. Retrieved August 2006. http//en.wikipedia.o
  rg/wiki/CMOS .
• Wikipedia the Free Encyclopedia (August 2006).
  Electron mobility. Retrieved August 2006.
  http//en.wikipedia.org/wiki/Electron_mobility