‘Warming up’ ULT freezers

The massive energy savings that can be achieved by warming up the temperature during a ULT freezer's use phase makes it a crucial intervention to prioritise when attempting to reduce energy consumption of ULT storage. Raising ULT freezers from − 80 °C to − 75 °C has been shown to reduce electricity consumption by 15%, with that figure rising to 28% when a ULT is warmed 10 °C to − 70 °C [11], as well as prolonging a ULT freezer’s life by reducing the stress placed on the compressor over the course of its life cycle. Moreover, for ULT freezers that are housed in rooms with heating, ventilation, and air conditioning (HVAC) systems, the freezer requires less energy to maintain lower internal temperatures, therefore expelling less heat and, in turn, lowering HVAC requirements to maintain room set-temperature. This provides a large carbon saving for ULT storage given that the results of our own Carbon Footprinting Assessment, as well as other studies assessing the carbon impact of ULT freezer life cycles, show that the energy consumed during the use phase of a ULT freezer’s life cycle accounts for upwards of 90% of the product’s entire carbon footprint (assuming a standard UK electricity grid mix), or just under 13 tonnes of CO2e (Text E, Text F., Fig. 1 in Appendix 1, [9, 13]). To put this in perspective, the average household fridge-freezer’s electricity consumption will account for 0.89 tonnes of CO2e in the same time frame (12 years) [16].

Given the potential energy savings achieved from warming up, it has long been advocated as an important strategy for reducing carbon emissions [17,18,19,20], with advocates also pointing to the mutual financial benefit due to the money saved on energy bills. Nevertheless, a norm has developed within biobanking, and laboratory science more generally, to maintain ULT freezers at − 80 °C as default [11, 20]. This norm has become so widespread that within biobanking culture there is now a strong resistance to warming ULT freezers above this temperature. In fact, many biobank managers have raised concerns about effects on sample quality and viability at higher temperatures—concerns that some authors argue are blurred by emotional attachments to the samples themselves, with reports of biobank managers labelling them as “precious” and “irreplaceable” [15].

Concerns about sample quality are underpinned by the ‘magic number’ of cryopreservation of − 136.5 °C—a temperature known as the ‘glass transition stage of water’ [21]—at which the vast majority of metabolic activity ceases. At this temperature, sample quality and viability is perceived to be ensured for long periods of time [5], with sample quality being more ‘secure’ the closer the temperature is to − 136.5 °C. Once technical capacity permitted ULT freezers to operate at − 80 °C, this became deemed the optimum temperature—possibly also because the number corresponds to the sublimation point of ‘dry ice’ (− 78.5 °C), a fact that seems to have contributed to historical standards in cryopreservation [22, 23].

Some scholars have criticised the importance of the − 80 °C set-temperature, arguing that it merely represents a push from ULT manufacturers as they invested in evermore technological capacity as a way to sell newer models capable of reaching ever colder temperatures, rather than the fact that these lower temperatures were necessarily required for the maintenance of biosample quality [17, 19]. While these claims have not been substantiated, prior to the turn of the century, ULT freezers operated at − 70 °C without any known effect on sample quality [22, 24]. Furthermore, a growing body of evidence points toward ongoing safety and stability of biosamples stored at − 70 °C [19].

Nevertheless, despite the progressing demystification of the − 80 °C figure, biobank managers have other concerns to contend with. Research suggests that the internal temperature of ULT freezers can often fail to correlate with the display temperature [25] meaning that a − 80 °C display temperature could mean an internal temperature of several degrees higher than − 80 °C. During our workshops, participants explained that this was a cause of concern for some, as a − 70 °C display temperature might actually mean an internal temperature closer to − 65 °C.

Logistical issues also contribute to reluctance to warm up. ULT’s can often hold more than one researcher’s samples, leading to a collective reluctance, either on the side of technical staff or researchers (or both), due to the perceived risk of jeopardizing multiple research projects. Furthermore, some research projects may have started storing samples at − 80 °C and while a researcher may be willing to warm up in principle, there is a fear that it might impact the reproducibility of their experiments.

Workshop participants also raised concerns that funding bodies stipulate that ULTs used for storage in research projects must be set to − 80 °C. Furthermore, they spoke about the need to align with the regulatory requirements of the UK Human Tissue Authority (HTA), which licenses biobanks. Participants emphasised that Designated Individuals (DI) (the individual within the biobank designated to ensure regulatory compliance [26]) are uncertain about how the HTA would react should a DI choose to warm up a biobank’s ULTs, and therefore are concerned that DIs would bear the burden of negative repercussions. Finally, participants explained that those who worked at larger biobanks, which collect samples for access by a range of researchers for unspecified future purposes, may be particularly cautious about warming up freezers because of the uncertainty about the effect on the full range of (potentially currently unknown) analytes within samples that future researchers might seek to access.

Recommendations

To demystify the − 80 °C norm that currently exists in biobanking, we recommend:

stakeholders to refrain from using ‘− 80s’ as a shorthand for ULT freezers as this perpetuates the − 80 °C norm.

widespread promotion of the University of Colorado, Boulder database [19], which records instances of active and successful − 70 °C ULT set point temperature practice.

promote future research on the quality and viability of common analytes stored at − 70 °C vs. − 80 °C in order to assuage biobank stakeholder concerns.

compiling historic and future research and evidence that specifically explores the effects of − 70 °C on samples into an accessible comprehensive database/library.

intra-institutional promotion of warming up examples—workshop participants emphasised that once researchers saw that freezers within a biobank had been warmed up without any effect on sample quality and viability, this practice ‘rippled out’ and encouraged more to follow suit.

implementing warming up protocols at the start of a research cycle, when researchers can be confident that it will not have an effect on the reproducibility of their experiments.

biobank managers supporting researchers to warm up ULT freezers by referencing existing evidence of sample viability at -70 °C, as well as the potential financial and carbon-saving benefits—as one of our workshop attendees put it—“don’t mandate researchers, take them on a journey”.

backup ULT freezers, which are maintained in case of emergencies, such as freezer failure, should be maintained at higher set point temperatures than in use ULTs [11] and be filled with materials such as polystyrene or spare ULT racking to increase thermal mass inside the freezer (see ULT freezer management practices and cooling strategies for more information).

deconstructing attitudes towards sample preciousness—(unpacked further in Sample management and centralisation).

Biobank managers should be aware of discrepancies between ULT display temperatures and internal temperatures, which could be redressed by requesting extra temperature probes from manufacturers.

Biobank stakeholders should push manufacturers to address internal temperature issues and guarantee the proper functioning of new ULT models.

Biobank stakeholders should push for clarity from HTA and funding bodies regarding their stance on warming up in order to ease DI concerns. This process can begin with researchers requesting specification in the early stages of HTA applications.

ULT freezer management practices and cooling strategies

Effective ULT management best practices to help maintain biosample quality can ensure the efficient running of freezers and therefore play a vital role in decreasing freezer-associated energy use. These include regular de-icing, regular cleaning of air filters, efficient use of freezer space and capacity, maintenance of an appropriate room set point temperature, adequately spaced freezers, and limiting the number of daily door openings. They also include the location of ULT freezers: institutional building space constraints often lead to the placement of ULT freezers in unusual locations such as corridors or basements, which lack proper ventilation and/or air conditioning. This leaves them vulnerable to excess heat build-up (due to heat expelled by ULT freezers’ compressors, external weather conditions, or both) which, in turn, affects freezer efficiency [10]. As a result, most facilities will require a cooling strategy. Both passive and active solutions are possible, including the positioning of freezers so that heat is not trapped but pushed out, as well as the usage of supplementary cooling.

The importance of these practices for reducing the energy consumption of ULTs is often not stressed enough. Combined dust build-up on the filter and an obstructed door seal from a lack of de-icing can increase energy consumption by as much as 27% [10, 25]—an energy loss that approximately equates to the gains associated with warming a ULT freezer by 10 °C. In another example, poor door-opening practices (leaving doors open for longer than needed) can lead to a rise in a freezer’s internal temperature [7, 11], which forces the compressor to work harder, using more energy, and ultimately placing increased stress on the compressor over time [10]. Meanwhile, poor ULT freezer placement can lead to a 4% increase in energy consumption [25].

The stresses placed on ULT freezer’s compressors also becomes relevant when considering biobanks’ need for backup freezers. These freezers will, by definition, be mostly empty, and ULT freezers with little to no thermal mass inside will require the compressor to work harder in order to maintain set point temperature [27]. As mentioned in Sect. "‘Warming up’ ULT Freezers", backup freezers should maintain a higher set-point temperature in order to save energy and reduce compressor load, but they might also be used as decanting spaces when in use freezers are being defrosted, so that this compressor wear might be spread across a biobank’s ULT catalogue.

Recommendations

A best practice freezer management plan should be implemented to ensure all freezers are working as close to their optimal efficiency as possible and that energy savings can be secured.

Backup ULT freezers can be used as decanting spaces when fully loaded ULTs are being defrosted in order to share the load on ULT compressors across a biobank’s ULT catalogue.

Biobanks should employ a facility cooling strategy:

effective spacing between ULT freezers.

maintain a facility temperature of between 15 and 20 °C, through both passive and active cooling solutions [28, 29].

Biobank managers need to maintain and document freezer management programmes to ensure that ULT freezer replacement is due to freezer inefficiency caused by age and/or mechanical fault, rather than poor management practices.

Thorough assessment of ULT replacement strategies

Our Carbon Footprinting Assessment of ULT freezers, and other such analyses, suggest that because the energy generated by a ULT freezer’s use phase dwarfs that generated by its manufacturing phase ([9, 13, Text F, Fig. 1 in Appendix 1), within most scenarios it makes sense solely from a carbon perspective to replace ULT freezers even if they are relatively new and their energy efficiency has only dropped slightly. This is because the amount of electricity saved by replacing a relatively young freezer with a new more efficient replacement will always outweigh the amount of carbon ‘lost’ by not using the old unit for its full lifespan, despite the replacement only running at a marginally better efficiency. For example, if we take 7.5 kWh per day as an assumed ‘maximum’ efficiency for a new replacement ULT (570 L), a figure cited in manufacturer’s literature [30], if a biobank manager meters their freezers and finds that a 5-year old freezer is running at 10 kWh per day, it would make ‘carbon sense’ to replace it.

However, this distorted picture ignores two important factors. First, freezer replacement is expensive and biobanks often lack resources. Whilst replacement is often touted as a cost savings method because of savings in energy use, only in extreme cases does replacement become justified from a financial perspective (such as when a freezer is performing at two to three times the energy efficiencies that a replacement would) (Text G, Table 2 in Appendix 1). Second, as we saw above, new ULT freezers may not operate at ‘maximum’ efficiency in practice because of poor freezer management practices. Equally, older freezers may run at high efficiencies even though it is generally assumed that they decrease in efficiency as they age up to their end of ‘lifespan’ (normally placed at somewhere between 10 and 12 years). In fact, during our research, we came across freezers that maintained efficiency levels comparable to ‘new’ freezers well-over the 12-year mark (Table 3 in Appendix 1), and some workshop participants discussed freezers running efficiently upwards of 20 years, suggesting that hypothetical energy efficiency claims can be inaccurate in practice. Indeed, in the same vein as the − 80 °C debate, a pertinent question emerges regarding the origin of the 10–12 year lifespan figure, though not one we have to the time to explore here.

As such, a replacement strategy based on hypothetical maximum efficiencies of ULTs is fundamentally flawed. Instead, collecting ‘in situ’ data on ULT freezers through freezer metering is needed—this is despite the financial and time-based considerations associated with the increased workload it entails for biobank employees. Without this metering data, a replacement strategy can only be based on the limited assumption that new freezers operate at their maximum efficiency, and older freezers operate far less inefficiently, without ruling out the possibility that poor ULT management practices are contributing to energy inefficiencies.

Recommendations

Rather than a blanket policy of age-based replacement, we recommend that replacement should occur only when metering data can be collected. Replacement of functional freezer units should only be considered where there is a potential energy saving of 2.5 kWh per day available.

Where the above is not possible, emphasis should be placed on why a freezer is not performing and an understanding of a biobank site’s limitations and areas of potential improvement, before replacement is considered (also see section below).

If these conditions are met and ULT is still not performing as well as a new ULT might be expected, then replacement should be considered, targeting the oldest units first.

End-of-life practices

End-of-life (EOL) best practice for ULT freezers is a relatively unexplored research area, however, in principle, they are similar to best practices in EOL scenarios for domestic refrigeration appliances. ULT freezer removal and disposal services must perform relevant processes to minimise emissions, including ensuring that oil and refrigerants from the ULT’s compressor are drained so that gasses with high global warming potentials can be removed and degassed. This is particularly important for older ULT models that utilise hydrofluorocarbon (HFC) refrigerants, such as R-404a and R-508b, which have global warming potentials of 4728 and 13,412 times that of carbon dioxide, respectively [8]. Should large quantities of these gasses escape the ULT system due to improper disposal it could multiply the carbon footprint of the ULT’s lifecycle by many times. This point equally applies to the processing of polyurethane foam used for insulation in older ULT freezer models: older ULT freezers used HFC blowing agents in the manufacturing of polyurethane foam, such as HFC-245fa, which has a global warming potential of 962 times that of carbon dioxide [8, 31].

Beyond gas escape, waste management centres should reclaim recyclable material, such as steel, from the ULT unit, to ensure that the EOL phase has as low an impact on a ULT freezer’s overall carbon footprint as possible. Finally, if a ULT freezer is being replaced due to energy concerns, rather than a complete age-related failure, biobanks might consider recycling their ULT unit with a company that refurbishes second-hand laboratory equipment for re-use [32]. This can extend the overall lifespan of a ULT freezer by allowing a second-hand user to avoid incurring the full carbon price of a newly manufactured ULT freezer, provided the second-hand freezer runs at comparable efficiency to that of a new ULT.

Recommendations

Ensure the use of waste management services that provide explicit information on their disposal processes when disposing ULT freezers.

During tender for new units or when units are to be disposed of, biobank managers should engage manufacturers and suppliers to reclaim parts and materials.

Consider repurposing ULT freezers through laboratory equipment reclamation companies, unless freezer units have been identified as particularly inefficient, in which case they should be targeted for disposal.