Application of a Variable Speed Compressor to a Residential No-Frost Freezer

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Application of a Variable Speed Compressor to a
Residential No-Frost Freezer
John Dieckmann, Member, TIAX LLC, Detlef
Westphalen, Member, TIAX LLC, William Murphy,
TIAX LLC, Paul Sikir, Member, Sub-Zero Freezer
Company, Christopher Rieger, Sub-Zero Freezer
Company

• Seminar 41 January 27, 2004

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Continuously variable capacity modulation has
significant advantages over on-off capacity
control in many refrigeration and air
conditioning applications.
Continuously Variable Capacity Modulation
General Discussion

• Energy efficiency
• Maximize HX utilization, minimize temperature
  lift
• Flow rate cubed fan power law
• Temperature control
• Humidity control
• Noise

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Maximum capacity vs. design load and part load.
Continuously Variable Capacity Modulation
General Discussion

• Refrigeration and air conditioning systems need
  to have enough capacity to maintain the desired
  temperature at a worst-case, design load
  operating condition
• For example, residential air conditioning systems
  face a worst-case load when the outdoor
  temperature and humidity and insolation are high
  and indoor heat generation levels (from people,
  lights, appliances, etc.) are high. Often
  conditions are much more moderate and less
  capacity is required
• Home refrigerators face maximum loads when door
  openings are frequent, warm items have been
  placed in the interior, and when the indoor
  temperature is high. When the refrigerator is
  left undisturbed for an extended period of time,
  the cooling load is much less

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The energy efficiency benefits of continuously
variable capacity modulation are attributable to
three main factors.
Continuously Variable Capacity Modulation
General Discussion
Reduced Temperature Lift
Reduced Air Moving Power
On
On
Off
Variable
90oF
Air moving power into a fixed system flow rate
cubed
Ambient - DOE energy test procedure
Freezer temperature - DOE energy test procedure
5oF
Time
In addition, continuous capacity modulation
eliminates losses associated with on-off cycling.
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Energy efficient caveats home refrigeration
scale.
Continuously Variable Capacity Modulation
General Discussion

• The method used to continuously modulate capacity
  must be efficient
• Throttle valves, like a suction pressure
  regulator are very inefficient
• Hot gas bypass is very inefficient
• Variable speed compressor operation can be a
  highly efficient means of capacity modulation
• Variable speed compressor - energy pluses and
  minuses
• Losses in electronic drive
• ECM motor efficiency gt induction motor efficiency
  for fractional horsepower motors
• Compressor speed range
• Refrigerant flow control
• Fan efficiency and speed modulation efficiency

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High compressor efficiency must be maintained at
part load.
Continuously Variable Capacity Modulation
General Discussion
Inadequate Lubrication
The turndown range should be wide enough to allow
steady state operation at DOE test conditions.
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Cost-effectiveness of variable speed drive in
home refrigerators and freezers.
Continuously Variable Capacity Modulation
General Discussion

• There are many options for reducing the energy
  consumption of a home refrigerator
• Thicker foam insulated walls
• Increased thermal resistance of door perimeter
  thermal break and door gasket
• More efficient evaporator fan
• More efficient condenser fan
• Vacuum panel insulation
• Variable speed compressor
• A basic market issue remains - if the efficiency
  advantage and energy cost savings are not
  recognized by the buyer, no market pull
• With home refrigerators, annual electric energy
  cost savings 10 - 20/year
• Appliance stores display many brands, sizes,
  colors, features no room for premium efficiency
• Energy savings are significant on a national
  basis, hence standards

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More energy efficiency caveats - component
availability.
Continuously Variable Capacity Modulation
General Discussion

• GRAPH Global Production of Variable speed
  Refrigerant Compressors for Home Refrigerators

• ECM fans
• 2-speed ECM fans
• Variable speed ECM fans
• Small thermostatic expansion valves

Data?? Source???
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NAECA - Energy efficiency standards and the
energy test procedure (10 CFR 430 etc).
Continuously Variable Capacity Modulation
Application to 700 TF

• Many (18) product classes (top mount, side by
  side, with through the door ice dispenser, manual
  defrost, automatic defrost, freezers, etc.)
• Efficiency requirement for each product class is
  expressed in terms of maximum allowable annual
  energy consumption (as determined by the DOE
  energy test procedure 10 CFR 430 subpt B, App A1
  and B1) vs. interior volume Emax a(AV) b
• The current requirements (in effect since July,
  2001) for the upright freezer with automatic
  defrost product class (Class 9) are particularly
  stringent
• Emax 12.43 AV 326.1 (AV in cubic feet)
• For freezer, AV 1.73 x actual interior volume
• For the 700 TF, AV 1.73 x 15.31 26.49 cubic
  feet
• For the 700 TF, maximum annual energy under the
  current standard is 655 kWh/year (1.795 kWh/day),
  17 less than under the previous standard (in
  effect from 1/1/93 through 6/30/01

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The efficiency standard for Class 9, upright
freezer with automatic defrost is plotted here.
Continuously Variable Capacity Modulation
Application to 700 TF
Previous Standard 1/1/93
Current Standard in Effect Since 7/1/01
700 TF
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The 700 TF (tall freezer) is a Euro-styled
upright freezer designed to be built into kitchen
cabinets.
Continuously Variable Capacity Modulation
General Discussion
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The 700 TF (tall freezer) is a Euro-styled
upright freezer designed to be built into kitchen
cabinets.
Continuously Variable Capacity Modulation
700 TF Product Description

• Thin walls to maximize internal volume
• Upper half accessible by opening door
• Lower half has two pull-out drawers
• Significant door/drawer perimeter requiring
  gaskets and thermal breaks

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The basic factors influencing refrigerator/freezer
energy consumption.
Continuously Variable Capacity Modulation
700 TF
Electric Energy Input
Input to Defrost Heater
Input to Fan
Evaporator Fan
Input to Antisweat Heaters
Heat Leak into Cabinet
Heat Leak into Cabinet
0oF
90oF

• Refrigeration System COP
• Compressor COP
• Condenser Fan Energy
• Evaporator condenser D Ts

Total Thermal Load to Refrigeration System
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The cabinet heat load was measured by the
reverse heat leak method.
Continuously Variable Capacity Modulation
700 TF Cabinet Heat Leak
Watts to Defrost
Vari ac
Watts to Fan
90oF
0oF

• Set up in a 0oF chamber
• Electric energy input is easily measured
• Wattage is adjusted until cabinet steady-state
  interior temperature is 90oF
• Average temperature of the insulation is
  approximately the same as it would be at DOE test
  conditions

The measured cabinet heat leak of the 700 TF was
420 Btu/hr.
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There are a range of options for reducing energy
use.
Continuously Variable Capacity Modulation
700 TF Energy Design Option
Option for Reducing Energy Use
700 TF Pre-July 2001
Thicker walls, conventional foam insulation
Not compatible with overall design goals
Good evaporator fan motor (PSC)
Best evaporator fan motor (ECM or equivalent)
Good condenser fan motor
Best condenser fan motor
Large evaporator (low DT)
Large condenser (low DT)
High EER compressor
Demand defrost/adaptive defrost
Improved door/drawer thermal break
Refrigerant waste heat for antisweat heaters
Vacuum panel insulation
Variable speed compressor
Relatively few options that havent already been
used were available to reduce energy use by 17
to meet July, 2001 standards level.
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The key component is the variable speed
compressors.
Continuously Variable Capacity Modulation
Implementation in 700 TF Compressor

• Variable speed compressors were nominally
  available from approximately five compressor
  manufacturers
• Full capacity EERs varied from 6 Btu/watt-hr to
  low 5s
• Speed turndown ranges varied from 2 to 1 to 2 1/2
  to 1
• Final selection for production was strongly
  influenced by strength of manufacturers
  commitment to supply compressors reliably

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Other important components needed to implement
variable speed.
Continuously Variable Capacity Modulation
Implementation in 700 TF

• Speed control for temperature control
• 700 TF was already electronically controlled
  (microprocessor based), with thermistors used for
  temperature sensing, display, and control
• The existing microprocessor had ample under used
  capacity to implement a set of speed control
  algorithms, new software was needed
• Control board hardware modifications were needed
  to provide the speed control signal to the
  compressor drive electronics
• Evaporator fan - ideally variable speed, but only
  commercially realistic alternatives were single
  speed
• Expansion device - capillary was found to be
  adequate
• Evaporator and condenser sizes were maintained at
  previous sizes (might be some potential to
  cost-optimize by down-sizing)

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Performance Test Data
Continuously Variable Capacity Modulation
General Discussion
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Conclusions/Observations/Acknowledgements
Continuously Variable Capacity Modulation
General Discussion

• Capacity modulation in a home refrigerator or
  freezer can reduce the annual energy consumption
  by 15 to 25 (as measured by the DOE Test
  Procedure)
• The costs of variable speed compressors and the
  electronic, microprocessor-based control system
  needed to control the speed have been decreasing,
  increasing the potential for cost effective
  applications.
• Beyond energy savings, advantages include quiet
  part load operation, better, steadier temperature
  control and better food preservation
• Acknowledgement Subzero and the Technology and
  Innovation Group of Arthur D. Little (since spun
  off as TIAX LLC) collaborated on this project