Cold storage is essential for many labs to store consumables and to preserve samples for a long time. However, cold storage equipment can use significant amounts of energy to run. For example:

• a new, well-maintained Ultra Low Temperature freezer (ULT) uses around the same amount of energy as an average UK household
• older, poorly-maintained ULTs can use four times as much energy

Optimising your cold storage can offer a range of benefits, including:

• lower running costs
• reduced carbon footprint
• preserve the life span of your equipment, reducing costs associated with repair or replacement
• more reliable equipment that is less likely to fail and compromise sample integrity

The figures shared on this page refer largely to ULTs as they are such energy-intense pieces of equipment, but the principles apply to all types of cold storage used in labs.

Download a poster to help share ULT best practice in your lab.

Energy-efficient use of cold storage

Increase the temperature

The biggest energy savings from ULTs can come from increasing their temperature to -70°C or higher. Increasing the temperature from -80°C to -70°C reduces energy consumption by around 25 to 30%, as well as reducing pressure on air conditioning and strain on ULTs’ duty cycle, extending their life span.

The use of -80°C became the default from the 1980s to 1990s as a result of technology improvements, leading labs to believe this would improve sample stability. However, there is little evidence that suggests that -80°C is any better for sample storage than -70°C or higher. Universities in the USA have provided evidence that -70°C or higher is safe for a variety of samples and further evidence can be found from the Freezer Challenge.

At the University of Bath, 24 ULTs have already increased their temperatures, leading to £11,000 in energy savings annually.

Reduce door opening times

A ULT will take at least ten minutes to recover its internal set point temperature for every minute the door is left open. To help keep temperatures stable, minimise the amount of time you open doors to less than 45 seconds. This will also avoid letting in warm, moist air which can cause frost.

Opening a door for 60 seconds increases internal temperatures by around 1.2°C to 4.7°C, compared to 0.2°C to 2.8°C when opening a door for 15 seconds.

ULTs with higher operating temperatures also have reduced temperature fluctuations upon door opening. For example, -80°C freezers experience increases of 5°C to 8°C, compared to maximum increases of 3.1°C for -60°C freezers.

Keep your cold storage organised

Keeping a clear inventory can help reduce freezer opening times. For freezers than are 80% full, studies have found that, without an inventory, their door opening times tripled, increasing energy use by 60%.

Make sure samples are labelled, with a thorough inventory and map posted near the freezer. An inventory can be a handwritten map, a spreadsheet-based list, or a barcoding system.

You should neatly pack and record the precise location of samples by including the facility, room, freezer, shelf, and rack where the sample is located. Racking helps to reduce the rate of warm-up when doors are opened or during a freezer failure event.

To minimise the number of freezers needed, regularly clear out unneeded samples to save space. In particular, your lab should have a process in place to make sure departing researchers clear out or designate their samples when they leave. You may also be able to consolidate materials into fewer freezers and fridges over less busy periods, such as the summer holidays, and temporarily switch off unneeded equipment.

Use appropriately sized freezers for your needs. If you cannot fill your freezer, consider downsizing or sharing with other users or another lab. When this is not possible, fill empty spaces with polystyrene boxes or plastic bottles of ice to reduce the volume of warm air that enters freezers when they are opened.

Use freezer drawers

Using drawers in your cold storage can reduce the amount of cold air that escapes when doors are open, reducing temperature rises by 65%. They can also aid freezer organisation to reduce door opening times.

Ideally, drawers should be solid rather than baskets to minimise loss of cold air and ensure loose samples cannot fall out. Replace any broken or cracked draws, which may have been cause by icing, as this can reduce energy-efficiency

Install alarm systems

Install temperature monitors and alarm systems to quickly alert you if there is a freezer failure. This will reduce the need to set freezers to the coldest temperature to provide a buffer time between freezer failure and damaging internal temperature warming. Time to warm up varies depending on temperature:

Running a freezer at -70°C compared to -80°C would not dramatically reduce buffer time in the event of failure.

Maintain your cold storage

It's important to undertake regular maintenance. Poorly maintained freezers can use up to 25% more energy. Regular maintenance will also help prolong the life of your equipment.

Defrost freezers regularly

You should clear freezers of ice and frost at least every three months. Frost can accumulate on evaporator coils and outer gasket seals, which hinders compressor activity and increases energy use. It can also create gaps in the seals, causing cold air to leak out and warm air to enter.

You can use brushes and scrapers to remove softer frost build up. Contact leaf@bath.ac.uk if you need an ice scraper.

When there's more than 2cm of ice, you might need to defrost the freezer. Switch the freezer off and move the contents to another freezer temporarily.

ULTs operate 10% more efficiently following a defrost, and studies suggest ice-free freezers can save £160 per year.

Keep freezers clean and clear

Dust and grime blocks air flow and reduces the efficiency of heat expulsion. Make sure you clean freezer filters, evaporator coils, and compressors at least every three months.

To clean an air filter, unscrew it by hand and vacuum it or rinse it with water, then place it back while still wet. Clean heat exchanger fins, behind the filter, by gently vacuuming, brushing, or running a towel along the direction of the coil, taking care not to bend metal lines.

You can save around £230 in annual energy costs by keeping freezer filters clean.

If freezers are operating inefficiently, you might need to replace them. However, in some cases, it may be better to replace or upgrade single components, such as compressors.

Maintain efficiency of old freezers

You should regularly monitor old freezers to determine their efficiency. Contact energyteam@bath.ac.uk if you suspect your freezer is operating inefficiently and you’d like to test it.

Each year of a ULT’s life typically increases its energy usage by 3%, though some ULTs will age well if maintained correctly, reducing the need to replace them.

Make energy efficient purchases

If you need to purchase cold storage, make sure you purchase as sustainably as possible.

You should compare the energy use of different models by converting the kilowatt-hour per day figures (often quoted by manufacturers) to a kilowatt per litre or watts per litre per day figure. Aim to purchase ULTs with electricity use of 13.5 watts per litre per day, or better whenever possible. Fridges and freezers that use 2.5 watts per litre per day should be used whenever possible.

Make sure there is a digital temperature display and controls rather than just a dial, such as 1 to 6. Advertised temperatures compared with actual temperatures can vary widely, as can temperature distribution within a single piece of equipment. It’s worth regularly testing to ensure the integrity of your samples and to avoid excess cooling and wasted energy.

Larger cold storage units are typically more energy- and space-efficient, but should only be acquired where they will be used close to their capacity. For example, an 85 litre ULT will use more energy per litre than a 650 litre ULT, but will still consume less energy overall. Chest freezers are generally more energy-efficient than an equivalent sized upright freezer.

Use hydrocarbon refrigerants in preference of other refrigerant types such as hydrofluorocarbons or chlorofluorocarbons. Hydrocarbon refrigerants are more efficient and have a lower environmental impact.

Wherever possible, fridge-freezers should not be purchased. Their compartments often fill up at different rates, which reduces their energy-efficiency.

Placement of cold storage

You should aim to allow for at least 20cm clear space on top and 10 to 15cm of clear space at the back and sides of your cold storage to allow for proper ventilation. If freezers do not have appropriate ventilation, the condenser fins cannot dissipate heat as effectively, which will increase their energy use by about 4%.

Where multiple freezers occupy the same space, installing panels between the freezers can help prevent expelled warm air re-entering adjacent freezers. The idea is to completely block the gaps between freezers to force the hot air they expel up towards the ceiling, rather than allowing it to drift to the intake of adjacent freezers, which would mean they'd have to work harder to cool down.

Ambient air temperature can also affect a freezer’s performance. For every 1°C rise in room temperature there is an increase in your freezer’s energy consumption of around 2%. A room temperature of 15 to 18°C is preferable.

Avoid storing freezers in hallways, as they are often not very effective at removing heat. Where natural ventilation is possible, cool air should be allowed to flow into rooms when weather permits, for example, by opening windows.

In warm weather, windows can remain open but should be combined with fans to artificially increase air speed. You can do this by placing them under windows and moving air in the direction of the prevailing wind. In hot weather, windows should be closed and air conditioning used to make sure rooms are kept cool.

You should optimise cold storage as much as possible to minimise energy use, although this will depend on how much space you have in your lab.