Thermo Scientific ULT Freezers: Superior Thermal Performance

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1 Thermo Scientific ULT Freezers: Superior Thermal Performance Abstract In addition to being able to reach and maintain temperatures as low as -86 C, today s ultra-low temperature (ULT) laboratory freezers should also consider energy efficiency. Such units must not only ensure that the internal environment is accurately maintained at the set temperature while the door is closed, but also that the temperature is stabilized as quickly as possible following inventory access, or when new samples (often at ambient temperature or above) are added. Power failure is also a concern and ULTs need to delay temperature increases as long as possible if one occurs. As a result, modern ULT freezers must combine robust refrigeration systems and advanced insulation, with electronic controls to ensure that valuable samples are stored safely and securely over long periods of time. In this study, the performance of two different ULT freezers from the Thermo Scientific Forma and freezer lines was assessed, along with that of three competitor products, in a series of controlled tests by a third party thermal evaluation laboratory. Both Thermo Scientific ULT freezers performed comparably and were consistently better than all three competitors in demonstrating higher heat removal capacity and superior insulation properties while offering lower energy consumption. Introduction Storing samples such as cells, reagents, antibodies and enzymes at or below -80 C significantly reduces their inherent biological activity, ensuring that they will be viable once thawed, even during prolonged storage. It is critical therefore, that these ULT freezers not only provide the perfect environment for such long-term storage, but that this is maintained following inventory access and, to the extent possible, power failure. Therefore, ULTs are designed with high quality compressor-based refrigeration systems, high density insulation and door seals, as well as carefully developed interior designs (individual compartments and racking systems). Furthermore, there is also a drive towards reducing energy consumption. To investigate how well two Thermo Scientific ULTs meet the underlying key performance characteristics required, a series of tests were conducted that measured temperature maintenance under increased heat-load and power f a i l ure conditions, as well as the energy consumption during standard operation. Materials 1. Thermo Scientific Forma 906, ULT freezer 2., ULT freezer 3. Haier DW-86L626 freezer* 4. So-Low U /60Hz freezer* 5. Sanyo MDF-U73V, 60Hz freezer* * This order is not significant and bears no relation to the order in which the results are presented.

2 Methods Each of the five ULTs under investigation was set-up in the same way: Sixteen thermocouples were mounted inside the cabinet for temperature measurements (Figure 1). The locations of the thermocouple sensors were the same for all ULTs except for thermocouple 16 (T16), which was mounted within 1 of the temperature probe. The location of the T16, therefore, is dependent on where the manufacturer positioned their temperature probe. All shelves were removed and no samples were stored during the performance testing. BTU reserve capacity The average BTU (British thermal unit) reserve capacity is a measure of a freezer s ability to maintain a cold temperature across the entire cabinet in the presence of a heat load. Accordingly, a heating coil was placed in the bottom center of the freezer and encased in an enclosure to allow the heat to escape into the cabinet in a uniform manner. The freezers were set to bottom out temperature, which is the lowest temperature the freezer can maintain. An electrical current was then applied to the coil to produce a 40W heat load within the freezers and the resultant temperature increase was measured. This was repeated, sequentially, for 60W, 90W and 120W heat loads. Warm-up time The time taken for an empty ULT to warm from -80 C to -50 C during a simulated power outage was measured, based on an average cabinet temperature. Energy usage The energy consumption of each freezer during a normal cycling at a -80 C set point was measured in kilowatt hours. Results This study clearly indicates that across all the thermal parameters measured, the two Thermo Scientific T1 Sensor placement inside cabinet T11 T6 T2 T12 T13 T11 ULT freezers performed comparably and consistently outperformed the three competitor products in these performance tests. BTU reserve capacity Figure 2 shows each freezer s performance with increasing heat loads. The bottom out temperatures range from approximately -88 to -83 C depending on the manufacturer and are displayed in the graphs as the 0W control. Both the Thermo Scientific units showed a similar average BTU reserve capacity and superior performance against the competitor units. Furthermore, as the heat load increased, the gap between the Thermo Scientific units and the poorest performing competitor increased greatly. Figure 2a reflects the average of all 16 thermocouples, where as figure 2b details the results from T16 only (the one positioned next to each ULTs temperature probe). Temperatures measured by the T16 probe may not necessarily be the average temperature of the cabinet and can vary from the average by more than 5 C. T3 T1 T9 T7 T5 T6 T15 T4 T2 T10 T8 T14 T12 Figure 1 Locations of the 16 thermocouples (T1-T16). NB: T16 was positioned within 1 of the ULTs temperature probe, so varied with each unit.

3 -60 BTU reserve average of T Temperature [C] Figure 2a BTU reserve capacity, average of T1-T16 0[w] (BOT) 40[w] 60[w] 90[w] 120[w] -60 BTU reserve measured at of T16 (probe location) -65 Temperature [C] Figure 2b BTU reserve capacity measured at T16 (probe location) only 0[w] (BOT) 40[w] 60[w] 90[w] 120[w]

4 Warm-up time Figure 3 demonstrates the time it took for an empty ULT to warm from -80 C to -50 C during a simulated power outage and is based on an average cabinet temperature. During prolonged power outages, freezers will warm over time, but the length of time to warm is based on the cabinet s insulation and door seal properties. Fully-loaded or partially loaded freezers will warm more slowly than an empty freezer, but this test provides a baseline for this parameter. The two Thermo Scientific freezers took a significantly increased length of time to warm to -50 C compared to competitor freezers, taking nearly twice as long (90%) as the worst performing competitor. Energy usage Figure 4 shows the energy consumption, in kilowatt hours, of each freezer during its normal cycling at a -80 C set point. Lower energy consumption results in lower operating costs over time. Again, both Thermo Scientific units used significantly less energy, providing greater energy efficiency than the three competitor products. This can lead to significant savings in energy use over the lifetime of a freezer. For example, a 10% lower energy usage can be expected to save over $900 during a 10 year period based on US DOE energy cost averages of 10.4 cents/ kilowatt hour ( e5_6_a.html). Conclusion Sample integrity during long term storage at ultralow temperatures is of paramount importance and requires equipment designed to cope with specific needs, including high heat removal capacity, slow warm-up during power-failure, as well as energy efficiency. The Thermo Scientific Forma 906 and maintain a lower temperature than similar units across increasing heat loads, illustrating their outstanding heat removal capacity. The data suggests that the Thermo Scientific freezers can better accept new samples without affecting storage conditions, i.e., the instruments may return to their set temperatures more quickly following inventory access. The two freezers also demonstrate superior insulation properties, taking significantly longer to warm up than competitors under simulated power failure conditions. In addition to providing superior storage conditions, the Thermo Scientific units also showed that they use less energy to achieve this, offering long term energy and cost efficiencies.

5 Warm-up time (average time T1-T16 takes to warm from -80ºC to -50ºC) Warm-up time [min] Figure 3 Warm-up time (average time T1-T16 take to warm from -80 C to -50 C) Energy usage at -80ºC set point during normal cycling Energy consumption [kw-h] Figure 4 Energy usage at -80 C set point during normal cycling