Application of a Variable Speed Compressor to a Residential No-Frost Freezer Seminar 41 January 27,...

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

Seminar 41 January 27, 2004

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

2JD/db/armyD00154/4-03

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

Energy efficiency Maximize HX utilization, minimize temperature lift Flow rate cubed fan power law

Temperature control

Humidity control

Noise

Continuously Variable Capacity Modulation General Discussion


Maximum capacity vs. design load and part load.

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



The energy efficiency benefits of continuously variable capacity modulation are attributable to three main factors.


5oF

90oFAmbient - DOE energy test procedure

Freezer temperature - DOE energy test procedure

Time

In addition, continuous capacity modulation eliminates losses associated with on-off cycling.

Reduced Temperature Lift Reduced Air Moving Power

On OnOff

Variable

Air moving power into Air moving power into a fixed system = flow a fixed system = flow

rate cubedrate cubed


Energy efficient caveats – home refrigeration scale.

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 > induction motor efficiency for fractional horsepower

motors Compressor speed range

Refrigerant flow control

Fan efficiency and speed modulation efficiency



High compressor efficiency must be maintained at part load.


0

1

2

3

4

5

6

7

0 1000 2000 3000 4000 5000

Compressor Speed (RPM)

Co

mp

res

so

r E

ER

The turndown range should be wide enough to allow steady state operation at DOE test conditions.

Inadequate

Lubrication


Cost-effectiveness of variable speed drive in home refrigerators and freezers.

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



More energy efficiency caveats - component availability.


ECM fans

2-speed ECM fans

Variable speed ECM fans

Small thermostatic expansion valves

GRAPH Global Production of Variable speed Refrigerant Compressors for Home Refrigerators

Data?? Source???


NAECA - Energy efficiency standards and the energy test procedure (10 CFR 430 etc).

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

Continuously Variable Capacity Modulation Application to 700 TF

10

JD/db/armyD00154/4-03

The efficiency standard for Class 9, upright freezer with automatic defrost is plotted here.

0

100

200

300

400

500

600

700

800

900

0 5 10 15 20 25 30

Adjusted Volume

Ma

xim

um

An

nu

al E

ne

rgy

C

on

su

mp

tio

n, k

Wh

Continuously Variable Capacity Modulation Application to 700 TF

Previous Standard 1/1/93

Current Standard in Effect Since 7/1/01

700 TF

11


The 700 TF (“tall freezer”) is a “Euro-styled” upright freezer designed to be built into kitchen cabinets.


12


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

13


The basic factors influencing refrigerator/freezer energy consumption.

Continuously Variable Capacity Modulation 700 TF

EvaporatorFan

0oF90oF

Heat Leak into Cabinet

Electric Energy InputElectric Energy Input

Refrigeration System COP

Compressor COP

Condenser Fan Energy

Evaporator + condenser T’s

Input to Defrost Heater

Input to Fan

Input to Antisweat Heaters

Heat Leak into Cabinet

Total Thermal Load to Refrigeration

System

14


The cabinet heat load was measured by the “reverse heat leak” method.

Continuously Variable Capacity Modulation 700 TF Cabinet Heat Leak

90oF

Watts to DefrostWatts to Defrost

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

Watts to FanWatts to Fan

0oF

Vari acVari ac

The measured cabinet heat leak of the 700 TF was ~420 Btu/hr.

15


There are a range of options for reducing energy use.

Continuously Variable Capacity Modulation 700 TF Energy Design Option

Option for Reducing Energy UseOption for Reducing Energy Use 700 TF Pre-July 2001700 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 T)

Large condenser (low T)

High EER compressor

Demand defrost/adaptive defrost

Improved door/drawer thermal break

Refrigerant waste heat for antisweat heaters

Vacuum panel insulation

Relatively few options that haven’t already been used were available to reduce energy use by 17% to meet July, 2001 standards level.

Variable speed compressor

16


The key component is the variable speed compressors.

Variable speed compressors were nominally available from approximately five compressor manufacturers Full capacity EER’s varied from ~6 Btu/watt-hr to low 5’s Speed turndown ranges varied from 2 to 1 to 2 1/2 to 1

Final selection for production was strongly influenced by strength of manufacturer’s commitment to supply compressors reliably

Continuously Variable Capacity Modulation Implementation in 700 TF Compressor

17


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)

18


Performance Test Data


19


Conclusions/Observations/Acknowledgements


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

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Application of a Variable Speed Compressor to a Residential No-Frost Freezer Seminar 41 January 27,...

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