Sustainably Cool: ULT freezers

Laboratories have become heavily reliant upon the capability of freezers to reach exceedingly low temperatures (down to -80ºC) so that valuable biological samples can be preserved while remaining viable once thawed.  For ultra-low temperature (ULT) freezers to be effective, they must be able to recover from fluctuating temperatures that result from door-openings and sample heat loads. In doing so, they draw upon large amounts of energy, which can be costly.
Environmental impact is another critical parameter that more laboratory managers consider when making purchasing decisions. Thankfully, advances in technology and initiatives from labs and industry alike, have led not only to more efficient, greener freezers, but to an investment in the future through implementation of sustainable practices.

Compressor ups and downs

Door-opening events place a heat load burden upon the freezer every time a user accesses the stored samples. It subsequently has to work hard to recover quickly from the resulting temperature change in order to maintain thermal stability and avoid any damage to the samples. This recovery period draws upon a relatively large amount of energy as the compressor is required to engage at full power to compensate for the rise in temperature. Technological advances such as the V-drive variable speed compressor of the Thermo ScientificTM TSX ultra-low temperature freezer increases compressor speed during activities such as door openings, while decreasing compressor speed during periods of stability (when the freezer door remains closed). As demonstrated in Figure 1 below, this decreases the freezer’s energy consumption when compared to traditional single speed compressor technology.
Figure 1. ULT freezers with traditional single speed compressors (shown in gray) cycle on and off, thus increasing energy usage. V-drive variable speed compressor technology (shown in blue) increases and decreases its speed responsively, enabling energy consumption costs to be decreased.
Energy usage in conventional refrigerant ULTs can be as high as 18 kWh/day, or 6570 kWh over a year.  ULT freezers that have been designed to incorporate more advanced compressors, can reduce this figure to as low as 6.5 kWh/day at a -70°C setpoint, or 2884 kWh annually [1]. From a financial point of view, this could equate to an annual savings of more than £293 (approximately US$426) [2]. It also leads to a reduction of more than two metric tons of CO2 produced every year [2]. As such, work is being devoted towards developing better, more energy efficient compressor systems.

Greener, cooler chemicals

The choice of chemical refrigerant will also have a direct impact upon a freezer’s efficiency, along with a range of potential environmental effects. Prior to the mid-1990s, chlorofluorocarbon (CFC) refrigerants were widely used in most commercial applications. CFCs, commonly known as Freon, were initially employed as very effective refrigerants that came with the added benefits of low toxicity, low reactivity and low flammability. However, they were soon found to have a significantly destructive effect on the ozone layer. As a result, the Montreal Protocol called for the complete elimination of CFCs by the year 2000. This saw a shift towards the use of hydrofluorocarbons (HFCs) as a seemingly viable refrigerant replacement that was originally deemed to be better than CFCs. Although HFCs were shown to not deplete the ozone, they still possessed significant global warming potential (GWP).
More recently, manufacturers of ULT freezers have started to turn their attention to natural hydrocarbon refrigerants. Hydrocarbons have excellent thermodynamic properties and have proven to be very efficient refrigerants with low GWP and zero ozone depletion capacity.

Moving heat around

The heat transfer mechanism of ULT freezers is another important consideration when assessing the overall efficiency of the unit. Many ULT freezers employ a direct mechanism, whereby the liquid refrigerant is actively pushed into the evaporator. Direct cooling mechanisms operate by removing heat from the cabinet through expansion of the refrigerant in a series of tubes attached to the outside of the inner tank of the unit. This tubing system is what makes up the evaporator, and it is the evaporation following expansion of the liquid refrigerant that produces the cooling effect. The direct cooling method allows the refrigeration system to more efficiently react to door openings and entering sample loads.
Indirect mechanisms on the other hand, require the addition of an intermediate heat transfer fluid – known as the secondary refrigerant – that is cooled by the evaporator and pumped to the heat exchangers (much like a building’s air conditioning system in which cold water is distributed to the air coolers).

Meeting standards

The inherent value of samples contained within a ULT freezer makes it important for end-users to be certain that their freezers meet quality manufacturing and safety standards. Most modern freezers will typically carry a visual confirmation of this in the form of distinct marks. The most well-known of these are the CE and UL markings.
The letters CE appear on a variety of products that are traded in the European Economic Area. The CE marking is self-declared, meaning the manufacturer has checked that these products meet EU safety, health or environmental requirements and comply with EU legislation.
A UL marking means that a representative of Underwriters Laboratory (UL) has tested the product and determined that it successfully meets the UL’s requirements. This mark verifies whether a product is suitable for factory and field installation.
In addition to markings, the industry is also taking into account a product’s sustainability and environmental impact. Rating systems such as Leadership in Energy and Environmental Design (LEED) have been put in place to ensure designers make responsible and efficient use of resources when constructing new laboratory workspaces or even entire buildings. In the US, federal agencies or local governments reward or even require a LEED certification for new buildings.

Good for the environment can be good for the wallet

The need to adopt sustainable technology is something more progressive manufacturers recognise. New ULT freezer models such as the Thermo Scientific TSX Series ULTs have substantially improved energy-consumption profiles. For example, the TSX600V ULT uses less than 0.015 kWh/day per box stored – up to 50% less than other conventional refrigerant-based freezers. Data from the UK demonstrates how switching to a more advanced freezer that employs a variable speed compressor, like a TSX600V ULT, can save as much as £4,413 whilst the TSX400V can save as much as £4,318 (approximately US$6,405 and $6,268 respectively) over a 10-year period.
Select manufacturers have even gone a step further by switching to more sustainable components, as well moving to reduced or even zero waste-to-landfill facilities that also support a greater amount of recycling. Implementation of zero waste programs have already helped a great number of companies by showing them how sustainable does not have to be expensive. Thermo Fisher Scientific’s site in Asheville, NC, for example, is zero-to-waste with 93% of materials being recycled and 7% waste to energy.

More than just sample storage

Modern ULT freezers need to go beyond simple reliability, and instead provide users with devices constructed in low environmental impact facilities, with modern, sustainable components and a final product that has a low energy-consumption profile. Technological advances to compressors and refrigerants help to afford peace of mind by ensuring samples remain safe and secure. While they may not seem like monumental changes, these critical adjustments are key to bringing sustainable practices to every aspect of the lab.   

References

1. Calculated based on manufacturer published energy consumption data as of 2/26/2015 and energy cost assumption of 0.12 euros / kWh. Energy consumption is based on manufacturer published energy consumption data as of 2/26/2015. Data on file. Thermo Scientific TSU600V data with high-performance mode.
2. Calculated using data from the US Energy Information Administration Retail Sales of Electricity to Ultimate Customers published electricity and natural gas price statistics. http://www.eia.gov/electricity. Accessed October, 2015.