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World Review of Science, Technology and Sust. Development, Vol. 10, Nos. 1/2/3, 2013 129
Copyright © 2013 Inderscience Enterprises Ltd.
Factors affecting the performance, energy
consumption, and carbon footprint for ultra low
temperature freezers: case study at the National
Institutes of Health
Leo Angelo M. Gumapas*
US Public Health Service Commissioned Corps,
Office of Research Facilities,
Division of Environmental Protection,
National Institutes of Health,
Room 2W64, 9000 Rockville Pike Building 13,
Bethesda, MD 20892, USA
E-mail: leoangelo.gumapas@nih.gov
*Corresponding author
Glenn Simons
Office of Research Services,
Division of Scientific Equipment and Instrumentation Services,
National Institutes of Health,
Room 3W51, 9000 Rockville Pike Building 13,
Bethesda, MD 20892, USA
E-mail: glenn.simons2@nih.gov
Abstract: The National Institutes of Health (NIH) recognises an opportunity to
significantly reduce the energy consumption and its carbon footprint from plug
load equipment can be realised by managing –86°C ultra low temperature
(ULT) freezers. Energy meters were installed on ULT freezers operating in
actual laboratory conditions to determine how their energy consumption is
influenced by various factors. Ambient temperature, freezer condition, age,
capacity, and set point temperature were the factors that were examined. Based
on the study, ultra low temperature freezers operated efficiently when they are:
well maintained, operating in ambient temperatures less than 25°C, less than
ten years old, are operating at a set point higher than –80°C, and have an
internal capacity greater than 23 ft
3
. The results of the case study are presented
and discussed. Freezer performance was assessed to determine how ambient
temperature and the freezer condition influenced the freezer’s ability to reach
set point temperature. The results of the study indicate a freezer that is not
maintained and operating in ambient temperatures above 32°C produce cabinet
temperatures 12.5°C warmer than the desired set point temperature.
Keywords: ultra low temperature freezer; energy consumption; performance;
carbon footprint; plug load.
Reference to this paper should be made as follows: Gumapas, L.A.M. and
Simons, G. (2013) ‘Factors affecting the performance, energy consumption,
and carbon footprint for ultra low temperature freezers: case study at the
National Institutes of Health’, World Review of Science, Technology and
Sustainable Development, Vol. 10, Nos. 1/2/3, pp.129–141.
130 L.A.M. Gumapas and G. Simons
Biographical notes: Leo Angelo M. Gumapas is a Commissioned Officer in
the United States Public Health Service and the Greenhouse Gas (GHG)
Program Manager for the National Institutes of Health (NIH). He is responsible
for developing NIH’s GHG inventory and developing strategies to reduce
NIH’s carbon footprint. He received his Bachelor of Science in Chemical
Engineering and Master of Science in Environmental Engineering from
Clemson University, and he is a licensed Professional Environmental Engineer.
He worked previously as a Consultant in Kennesaw, GA helping industries
comply with the Clean Air Act and as a State Regulator in Columbia, SC
enforcing the Clean Air Act.
Glenn Simons is a Biomedical Engineering Technician with over 33 years of
field experience, and he is currently employed by the National Institutes of
Health (NIH) in the Division of Scientific Equipment Services (DSEIS). He has
worked previously for 13 years as a Senior Biomedical Field Engineer for
Packard Instrument Company, Senior Nuclear and Spectroscopy Engineer for
16 years, and Biomedical Engineer for four years for GE Healthcare in the
Clinical Diagnostic Imaging Division. He is also the past owner and CEO of
STech Solutions, LLC, which is an independent servicing organisation in the
Washington DC Metro Area.
1 Background
A mechanical refrigeration system is designed to move heat from one location to another.
The basic components for a refrigeration system involve a compressor, cabinet to store
the perishable product, evaporator, condenser, and refrigerant. The refrigeration cycle
begins when the compressed refrigerant liquid passes through the evaporator, where it
flash-expands into a vapour and absorbs heat from the cabinet. The compressor moves
refrigerant through the system and compresses the refrigerant to a high-pressure vapour,
which then proceeds to the condenser. In the condenser, the high-pressure vapour
dissipates its heat to the ambient environment and transforms to a compressed refrigerant
liquid. The cycle continues until the desired set point temperature is achieved within the
cabinet.
A ultra low temperature (ULT) freezer typically operates between –56°C and –86°C,
but it must be able to operate within the range of –70°C and –80°C (CBEA, 2012). A
single refrigerant does not have physical properties to cover the wide temperature range
between the ambient room temperature and ULT range. In order to achieve these low
temperatures, a ULT freezer requires two refrigeration circuits with each system
requiring individual compressors and refrigerants with different boiling points for
absorption and dissipation of heat. Both refrigeration circuits use a cascade technique,
where the refrigeration circuits operate in high stage and low stage configuration. In the
cascade process, the low stage compressor removes heat from products located in the
cabinet. The heat from stored products is absorbed by the refrigerant gas in the
evaporator tubing wrapped around the cabinet. The heat is transferred to an interstage
heat exchanger where it is passed to high stage system and ultimately released to the
ambient environment into the room air that is circulated through the condenser fins
(Laporte and Mistry, 2007).
Factors affecting the performance, energy consumption, and carbon footprint 131
2 Introduction
The energy consumption attributed to equipment loads (plug load) can account for more
than 50% of the energy use in a laboratory space [Enermodel Engineering, Inc. and
NREL, (2003), p.15]. A lab grade freezer consumes approximately 20 kilowatt hours
(kWh) per day of electricity, which is about as much energy as an average family
household uses per day. There is a significant opportunity to reduce the energy
consumption and carbon footprint from plug load equipment, which can be realised by
efficiently managing ULT freezers. Questions have been raised regarding the influence of
ambient temperature, maintenance, age, internal temperature, and set point on the wide
plethora of ULT freezers operating under the various conditions. These concerns have
prompted this case study to get a better understanding of these factors.
3 Methods
ULT freezer energy consumption was measured in National Institutes of Health (NIH)
laboratory conditions using the electronic educational devices watts up? pro watt metre,
power analyser, electricity metre (watts up? pro metre). The watts up? pro metre was set
to measure the watts, volts, amps, watt hours, cost, duty cycle, power cycle, line
frequency, and volt amps with a one-minute resolution over a 24 hour period. The duty
cycle is the percentage of time the ULT freezer is ‘on’. In order to accurately assess the
duty cycle, the duty cycle threshold, which is the minimum level a ULT freezer is
considered ‘on’, is set at 100 watts for a 115 volt freezer and 180 watts for a 208 to 230
volt freezer. The costs are based on the average fiscal year 2011 electricity consumption
rate on the NIH main campus in Bethesda, MD at $0.11 per kWh.
EL USB W LCD+ ambient temperature RH/temperature data logger (ambient
temperature probe) is used to measure the ambient temperature the ULT freezer is
operating in with a ten-second resolution over a 24-hour period. The ambient temperature
probe is positioned on the condenser air intake because the air blowing over the
condenser is the air that moves the heat from high stage refrigerant to the ambient
environment.
EL USB TC LCD thermocouple data logger with K type thermocouple (cabinet
temperature probe) is used to measure the internal temperature of the ULT freezer. The K
type thermocouple is positioned in the same position as the ULT freezer’s thermocouple.
Cabinet temperature was measured with a ten-second resolution over a 24-hour period.
The energy consumption for ULT freezers is highly dependent on the conditions a
ULT freezer operates in. A qualitative assessment was used to assess the condition on the
ULT freezer. Each freezer was rated qualitatively for three conditions, which are spacing;
ice on the outer door gasket seals; and dust on the filter/condenser fins.
The ULT freezer age, capacity, and set point temperature were determined from
manufacturing or acquisition date data, vendor brochures, and ULT freezer setting
display, respectively.
The carbon footprint assessment from ULT freezer operation was computed in
accordance to the methodology B.1.1 and C.2 outlined in the Federal Greenhouse Gas
Accounting and Reporting Guidance Technical Support Document, and carbon emissions
are assessed in units of metric tons (MT) of carbon dioxide equivalent (CO
2
e) per year
(White House Council of Environmental Quality, 2010). The NIH Main Campus is
132 L.A.M. Gumapas and G. Simons
located in Bethesda, MD; therefore, the RFC East eGrid subregion output emission rate
factors were used. The carbon emissions include emissions from the transportation and
distribution of electricity.
4 Results
Sixty-four ULT freezers were evaluated in the case study. ULT freezers operated
efficiently when they are maintained; are operating in ambient temperatures less than
25°C; are less than ten years old; are operating at a set point higher than –80°C; and have
an internal capacity greater than 23 ft
3
. Maintained freezers are properly spaced; have
little or no frost on the outer door gaskets; and have no dust on the filter and condenser
fins.
4.1 Ambient temperature
According to freezer manufacturers, operating a ULT freezer in ambient temperatures
higher than 32°C (Thermo Scientific, 2011) prevents the effective heat transfer from the
high stage refrigerant to the ambient environment. Figure 1 depicts how energy
consumption and carbon footprint for maintained 17.3 ft
3
ULT freezers are influenced by
increasing ambient temperatures. A best fit curve indicates that the monthly energy
consumption increases approximately 18 kWh and releases 9.27 kilograms (kg) of CO
2
e
for every 1°C rise in ambient temperature. Each 1°C drop in ambient temperatures from
32°C lowers the energy consumption for a ULT freezer by approximately 2%, which is
in agreement with Thermo Scientific’s analysis on the Thermo Scientific TS586e
(Wisniewski, 2011).
Figure 1 Energy consumption versus ambient temperature for 17.3 ft
3
at a set point of –80°C
maintained ULT freezers (see online version for colours)
4.2 Age
Technological improvements in cold storage have resulted in more efficient operation of
ULT freezers. Advances in ULT freezer compressor design, insulation, and cabinet
design have resulted in greater efficiencies to store samples. Nevertheless, the efficiency
of ULT freezers decreases over time, due to the loosening seals, degraded refrigerants
Factors affecting the performance, energy consumption, and carbon footprint 133
and lubricants, and fatigue of mechanical systems. Figure 2 illustrates the duty cycle over
time for three different aged maintained ULT freezers at an –80°C set point operating in
an ambient temperature from 25°C to 27°C. According to the graph, the duty cycle spikes
due to the startup of the high and low stage compressors on the ULT freezer. After four to
six hours, the high and low stage compressors stabilise, and there are only slight
variations in the duty cycle. The duty cycle for a new, mid-age (five to seven years), and
old (greater than ten years) ULT freezer is approximately 55%, 70%, 100%, respectively.
As the freezer duty cycle increases there is greater risk of failure along with the increased
energy use and the compressor motors continual operation.
Figure 2 The influence of age on duty cycle for maintained ULT freezers at a set point of –80°C
operating in ambient temperatures of 25° to 27°C (see online version for colours)
Figure 3 is a graphical representation of how age influences the energy consumption for
maintained 17.3 ft
3
ULT freezers at a set point of –80°C operating in ambient
temperature ranging from 23°C to 28°C. As seen in the figure, for each year a freezer
ages, there is an increase of approximately 3% in energy consumption, and an additional
monthly release of 8.75 kg of CO
2
e.
Figure 3 Energy consumption versus age for 17.3 ft
3
maintained ULT freezers at a set point of
–80°C (see online version for colours)
4.3 Capacity
Smaller ULT freezers have a much higher energy consumption rate on a cubic foot basis
when compared to larger ULT freezers, which is depicted in the figure below. Figure 4 is
134 L.A.M. Gumapas and G. Simons
a graphical representation of a ULT freezer’s performance in kWh/day/ft
3
versus its
capacity in ft
3
. All the freezers are at an –80°C set point and operating in ambient
temperatures from 25°C to 28°C.
Figure 4 ULT freezer’s performance in kWh/day/ft
3
versus capacity in ft
3
(see online version
for colours)
Note: The ULT freezers are operating at set point of –80°C and an ambient temperature
of 25°C to 28°C.
A 3.0 ft
3
ULT freezer has the lowest energy consumption rate at 11.67 kWh/day
among all the ULT freezers measured in the study; however, in a kWh/day/ft
3
basis,
the 3.0 ft
3
freezer is six times less efficient than the best performing ULT freezer at
0.64 kWh/day/ft
3
. ULT freezers greater than 23 ft
3
are the best performing units with a
performance ranging from 0.65 to 0.80 kWh/day/ft
3
.
4.4 Set point temperature
Increasing the ULT freezer’s set point lowers the ULT freezer’s duty cycle, which in turn
lowers the ULT freezer’s energy consumption. Lowering the ULT freezer’s duty cycle
also extends the life of the ULT freezer because it decreases the frequency that the
compressor cycles on and off. Figure 5 depicts how the set point temperature influences
the energy consumption for maintained 17.3 ft
3
ULT freezers in ambient temperatures
ranging from 22°C to 26°C.
Figure 5 Energy consumption versus set point temperature for 17.3 ft
3
maintained ULT freezers
(see online version for colours)
Factors affecting the performance, energy consumption, and carbon footprint 135
Based on the data, raising the set point temperature by 5°C for a ULT freezer reduces
daily energy consumption by 3 kWh and avoids 1.54 kg CO
2
e of emissions. Based on a
University of California (UC) Davis study on set point temperature for various ULT
freezer models observed a 2 to 4 kilovolt amp-hour per day (kVAh/d) (which is
approximately 2–4 kWh per day reduction in energy consumption by raising the set point
temperature from –80°C to –70°C) (Doyle et al., 2011). Differences in the results are
attributed to how the analysis was performed between the case studies. In the NIH study,
the monthly energy consumption rate for ULT freezers with varying ages, makes, and
models are plotted against set point temperatures from –70°C to –80°C. In the UC Davis
case study, the daily energy consumption for ULT freezers were measured at –80°C and
–70°C, and the energy consumption rate data was compared to a specific ULT freezer
model. Unlike the NIH study analysis, the UC Davis study analysis eliminates the
variability with differences in age, make and model. Despite the differences in the results
between both studies, the same conclusion can be drawn: that increasing the set point
temperature for ULT freezers lowers the energy consumption.
4.5 Spacing
In order to provide proper ventilation around the ULT freezer, it is recommended to keep
at least 8″ of clear space on the top (Thermo Scientific, 2011), and a minimum of 5″ of
clear space in the rear and on both sides (Forma Scientific, Inc., 1999). Improper
ventilation around the ULT freezer can prevent the condenser fins from effectively
dissipating heat into the ambient environment from the high stage compressor, which
increases the duty cycle of the ULT freezer and can negatively affect the performance of
the ULT freezer in achieving the set point temperature.
The conditions of freezers are rated in accordance with the rating system outlined in
Table 1. Table 2 depicts the conditions of the two freezers that were evaluated to
determine the effects of spacing on the energy consumption.
Table 1 Freezer rating assessment to systematically rate the operating condition of a ULT
freezer
Spacing
requirements
met
Three out of
four spacing
requirements met
Two out
four spacing
requirements met
One out of
four spacing
requirements met
Spacing: 5 inches of
spacing around freezer
and 8″ of spacing on
top of freezer
0 1 2 3
No ice Light frost Accumulated
frost
Thick ice Ice on door
gasket seals
0 1 2 3
No dust Light dust Medium dust Thick dust Dust on filter and/or
condenser fins
0 1 2 3
Table 2 Selected ULT freezers to assess the impact of spacing on energy consumption
Manufacturer Model Age (year) Spacing Ice Dust
Thermo Forma 8516 9 0 1 1
Thermo Forma 8516 10 3 1 0
136 L.A.M. Gumapas and G. Simons
Figure 6 depicts the influence of spacing on duty cycle for a ULT freezer that meets the
recommended spacing requirements according to the manufacturer (red) and a ULT
freezer that does not meet the recommended spacing requirements, and therefore has
inadequate ventilation. According to the figure, improper ventilation results in a 4%
increase in duty cycle, which translates to an additional 85 kWh of electricity consumed
per month and the additional release 51 kg of CO
2
e per month.
Figure 6 The influence of spacing on duty cycle for maintained ULT freezers at a set point of
–80°C operating in ambient temperatures from 26° to 28°C (see online version
for colours)
4.6 Dust
Dust on the filter blocks the normal air flow through the condenser, which reduces the
ability of the ULT freezer to dissipate heat. Any air flow that bypasses the clogged filter
will result in air carrying dirt to deposit on the condenser. Dirt on the condenser prevents
the effective heat transfer from the high stage refrigerant to the ambient environment.
Table 3 depicts the conditions of two freezers that were evaluated to determine the effects
of dust buildup on the ULT freezer condenser fins and the filter.
Table 3 Selected freezer to assess the impact of dust buildup on the air filter and condenser
fins
Manufacturer Model Age (year) Spacing Ice Dust
Thermo Electron
Corporation
8604 6 1 2 3
Thermo Forma 8604 8 1 3 0
Figure 7 depicts the amp draw over time for a significantly dusty Thermo Electron
Corporation Model 8604, which is indicated in red and a dust-free Thermo Forma Model
8604, which is indicated in blue. Both freezer were operating at a set point of –80°C and
operating in ambient temperatures from 23° to 25°C. The dusty Thermo Electron
Corporation Model 8604 amperage cycles between 7.5 amps to 17.0 amps and has a duty
cycle of 99%. The dust-free Thermo Forma Model 8604 amperage cycles between
0.10 amps to 13 amps and has a duty cycle of 70%. On a monthly basis, a significantly
dusty ULT freezer consumes an additional 211 kWh of electricity and emits an additional
108 kg of CO
2
e compared to a dust-free ULT freezer.
Factors affecting the performance, energy consumption, and carbon footprint 137
Figure 7 The influence of dust accumulation on ULT freezer filters and condenser fins on
energy consumption for ULT freezers at a set point of –80°C operating in ambient
temperatures from 23° to 25°C (see online version for colours)
Table 4 depicts the conditions of two freezers that were evaluated to determine the effects
of a thick dust on the filter and no dust on the condenser fins.
Table 4 Selected freezer to assess the impact of dust buildup on the air filter and condenser
fins for Thermo Model 8604 ULT freezers
Manufacturer Model Age (year) Spacing Ice Dust
Thermo Scientific ULT2586-10HD-A41 0 0 1 0
Thermo Scientific ULT2586-10HD-A40 2 0 2 3
Figure 8 depicts the amp draw over time for a Thermo Scientific Model
ULT2586-10HD-A40 with heavy dust on the air filter only, which is indicated in red and
a new Thermo Scientific ULT2586-10HD-A41, with no visible dust, which is indicated
in blue. Both freezer were operating at a set point of –80°C and operating in ambient
temperatures from 25° to 26°C. Based on the figure, the unmaintained Thermo Scientific
Model ULT2586-10HD-A40 amperage cycles between 0.0 amps to 17.3 amps and has a
duty cycle of 87%. The maintained Thermo Scientific ULT2586-10HD-A41 amperage
cycles between 0.30 amps to 16.5 amps and has a duty cycle of 79%. On a monthly basis,
a clogged filter on a ULT freezer consumes an additional 117 kWh of electricity and
emits an additional 60 kg of CO
2
e compared to a ULT freezer with a clean filter.
Figure 8 The influence of dust accumulation on ULT freezer filter and condenser on energy use
for ULT freezers at a set point of –80°C operating in ambient temperatures from 25°C
to 26°C (see online version for colours)
138 L.A.M. Gumapas and G. Simons
4.7 Ice
Frosting occurs on any surface with a temperature that is below the dew point of freezing
air and below the freezing point of water. Frosting is observed generally on the
evaporator coils and the outer gasket seals of ULT freezers. Frost build-up can
accumulate on the door gasket seals, which can create gaps in the seals around the ULT
freezer door. These gaps will allow cold air to be lost to ambient environment while also
allowing warm air to enter into the cabinet freezer. Table 5 depicts the conditions of ULT
freezers that were evaluated to determine the effects of ice on the outer gasket doors.
Table 5 Selected freezer to assess the impact of ice buildup on the outer gasket doors on ULT
freezers
Manufacturer
Model
number
Age
Ambient
temperature
Spacing Ice Dust
Thermo Forma 8604 8 23 1 3 0
Thermo Electron
Corporation
8604 7 23 1 3 0
Forma Scientific 8516 15 23 1 2 0
Thermo Electron
Corporation
8604 6 23 1 2 0
Thermo Forma 8516 9 26 0 1 1
Thermo Scientific 904 2 23 0 0 0
Thermo Scientific 904 0 23 0 0 0
Figure 9 is a graphical depiction of the monthly energy consumption versus the ice
buildup on the freezer’s outer gasket seal. Based on the figure, there is a positive
correlation that indicates as the ice builds up on the outer gasket seal, the energy
consumption rate increases. However, on the figure, there is one point that does not fit the
trend. This point shows a decrease in energy consumption rate with an ice buildup
numerical rating of 3. In addition to the thick ice buildup on the outer gasket doors, this
particular ULT freezer has thick ice buildup on the evaporator tubing.
Figure 9 The influence of ice buildup on the outer gasket on the ULT freezer on the energy
consumption for ULT freezers at a set point of –80°C operating in ambient
temperatures from 25° to 26°C (see online version for colours)
Factors affecting the performance, energy consumption, and carbon footprint 139
Generally, as frost builds up on the evaporator coils the heat transfer rate in the ULT
freezer cabinet is decreased due to the insulating effects of ice, which results in an
increase in energy consumption. However, in this atypical case, the opposite is observed,
which can be attributed to the increase in the roughness of frost, which increases the total
surface area. This phenomenon was also observed in a study for determining the effects
of frost formation on domestic refrigerator-freezer evaporator coils (Ali and Crawford,
1992). From there an initial increase in heat transfer as frost increased was observed, but
eventually the heat transfer decreased due to the insulating effects of the frost.
4.8 Performance
In addition to energy consumption, ambient temperature, freezer condition, and age are
factors that can also influence a ULT freezer’s performance in achieving its set point
temperature. Table 6 depicts an 18 year old unmaintained ULT freezer operating with a
–80°C set point in 32°C ambient environment.
Table 6 Selected freezer to assess the impact of age, ambient temperature, and freezer
condition on the ULT freezer’s ability to achieve set point temperature
Manufacturer Model Age (year) Spacing Ice Dust
Revco ULT2186-7-D12 18 0 3 3
Before the watts up? pro metre was installed on the Revco Model ULT2186-7-D12, the
freezer was operating with a cabinet temperature of –40°C. The freezer had no filter in
place, and there was a thick layer of dust on the condenser fins. The dust was first
removed from the condenser fins using a broom, and then the watts up? meter, ambient
temperature probe and cabinet temperature probes were installed on the ULT freezer. The
ambient temperature probe indicated the Revco Model ULT2186-7-D12 was operating in
32°C ambient temperatures. Figure 10 depicts the amp draw and the internal cabinet
temperature of the ULT freezer over a 68-hour period.
Figure 10 Amp draw and internal cabinet temperature for 18 year old Revco ULT2186-7-D12
operating with –80°C set point in 32°C ambient temperature with dust covered
condenser fins and no air filter (see online version for colours)
140 L.A.M. Gumapas and G. Simons
Based on the figure, the Revco ULT2186-7-D12 operates continuously and only achieves
a cabinet temperature of –40°C never reaching its set point. After removing the dust on
the ULT freezer, the cabinet temperature does not reach –64°C until 24 hours later. When
all three probes were removed, the ULT freezer indicated the cabinet temperature was
–78°C; however, the internal temperature probe indicated the cabinet temperature was
–65.5°C.
5 Conclusions
ULT freezers consume large amounts of energy, precious research dollars, and valuable
space. The effective management of ULT freezers is imperative for ensuring freezers
perform optimally in regards to maintaining the desired set point temperature on a
consistent basis. The energy consumption for 64 ULT freezers manufactured from 1994
to 2012 was measured in ‘real world’ settings to determine the effects of ambient
temperatures, freezer settings, and condition on a ULT freezer’s performance. Based on
the study, ultra-low temperature freezers operated efficiently when they are maintained,
are operating in ambient temperatures less than 25°C, are less than ten years old, have an
internal capacity greater than 23 ft
3
, and are operating at a set point higher than –80°C.
Linear regression lines were established to correlate ambient room temperature, age,
and set point temperature to monthly energy consumption. Based on regression lines,
lowering the ambient temperature by 1°C lowers the energy consumption by 3%, raising
the set point temperature 5°C lowers the energy consumption by 14%, and each year a
freezer ages increases its energy consumption by 3%.
A 4% increase in the ULT freezer’s duty cycle was observed for a freezer that did not
meet the vendor recommended spacing requirements versus a ULT freezer that did meet
the spacing recommendations. This translated to an improperly spaced ULT freezer
consuming an additional 85 kWh per month over a properly spaced ULT freezer.
Dust accumulation on the filter results in approximately a 14% increase in energy
consumption. However, if there is dust accumulation on the filter coupled with dust
accumulation on the condenser fins, then energy consumption can increase by 25%.
In regards to frost buildup on the outer gasket seals, there is a positive correlation
with increased energy consumption with thicker frost accumulation. However, freezers
usually with thick frost on the outer gasket seals also have thick frost within the interior
cabinet. Frost accumulation initially results in an increase in heat transfer in the ULT
freezer cabinet, which is the result in an increase in heat transfer area from the roughness
of the frost. The increase heat transfer area results in a decrease in energy consumption.
Eventually, as the frost accumulates, the insulating effects of frost decrease the heat
transfer area and energy consumption increases.
Carbon emissions were computed in accordance to methodology B.1.1 and C.2
outlined in the Federal Greenhouse Gas Accounting and Reporting Guidance Technical
Support Document. Based on the accounting methodology, greenhouse gas emissions are
directly proportional to the electricity that is consumed by the ULT freezer. Lowering the
energy consumption through effective ULT freezer management will ultimately lower the
carbon footprint attributed to consumption, transmission, and distribution of electricity.
In addition to lower energy consumption and carbon emissions, effective
management of ULT freezers will assist the unit in maintaining the desired set point
temperature on a consistent basis. Based on the study, an 18 year old, unmaintained ULT
Factors affecting the performance, energy consumption, and carbon footprint 141
freezer operating in 32°C ambient temperatures cannot achieve the set point temperature,
and instead only achieves cabinet temperatures 12.5°C warmer than the desired set point.
Management of ULT freezers must address all of the factors evaluated in this case
study, including ambient air temperature, maintenance/dust accumulation, frost buildup,
ventilation space around the freezer, age of the freezer, and set point temperature settings.
Without proper management of ULT freezers, a facility is associated with an increased
legacy cost through the ULT freezer’s increased energy consumption and the sample
integrity is compromised due to the ULT freezer’s inability to maintain the desired set
point temperature.
References
Ali, D.A. and Crawford, R.R. (1992) ‘The effects of frost formation on the performance
of domestic refrigerator-freezer finned-tube evaporator coils’, February, ACRC TR-15,
available at http://www.ideals.illinois.edu/bitstream/handle/2142/9704/TR015.pdf?sequence=2
(accessed on 1 June 2012).
Commercial Building Energy Alliance (CBEA) (2012) ‘Ultra-low temperature laboratory freezer’,
19 April, Version 1.1, available at http://apps1.eere.energy.gov/buildings/publications/pdfs/
commercial_initiative/low_temperature_freezers.pdf (accessed on 3 May 2012).
Doyle, A., Getty, A. and Stewart, L. (2011) ‘Freezer challenge: are you cool enough?’,
Presentation at the CA Higher Education Sustainability Conference, 29 June 2011 to
2 July 2011, available at http://www.cahigheredusustainability.org/program/documents/
FinalGBOMRTues8FreezerWeekUCDUCSBC HESC2011_000.pdf (accessed on 20 October
2011).
Enermodel Engineering, Inc. and National Renewable Energy Laboratory (NREL) (2003)
‘Laboratories for the 21st century: energy analysis’, April, DOE/GO-102003-1694,
available at http://ateam.lbl.gov/DesignGuide/Docs/Labs21EnergyAnal_33410.pdf (accessed
on 15 December 2011).
Forma Scientific, Inc. (1999) 8500 Series Non-CFC Ultra Low Temperature Freezer Manual No
7028526 Rev. 2, 7 July.
Laporte, J. and Mistry, D. (2007) ‘Improving temperature uniformity at –86°C in a mechanically
refrigerated ultralow freezer: reducing compressor discharge temperatures and apportioning
energy management between low- and high-stage cooling circuits’, September, Vol. 25, No. 9,
pp.24–29, American Biotechnology Laboratory, available at http://us.sanyo.com/Dynamic/
customPages/docs/biomedical_BT9_07.pdf (accessed on 22 December 2011).
Thermo Scientific (2011) ‘Ultra low temperature freezers installation and operation’, August,
315673H01, available at https://static.thermoscientific.com/images/D20005~.pdf (accessed on
4 May 2012).
White House Council on Environmental Quality (2010) ‘Federal greenhouse gas accounting and
reporting guidance technical support document’, 6 October, http://www.whitehouse.gov/sites/
default/files/microsites/ceq/technical_support_document_1.pdf (accessed on 8 October 2010).
Wisniewski, K. (2011) ‘Thermo scientific TS586e eco-friendly ultra-low temperature freezer’,
2 February, available at https://www.thermo.com/eThermo/CMA/PDFs/Various/File_54917.
pdf (accessed on 11 May 2012).
