Ultra-Low temperature storage
Due to the development of a COVID-19 vaccine and its special requirements transportable ultra-low temperature storage rooms are more and more in the focus for the upcoming distribution. At the moment the market is flooded with solutions so that it is increasingly difficult to find the optimal solution. With this overview we provide an overview of the important criteria. We will also present our solution which was developed in close cooperation with our partner companies Teledoor , Mirai Intex and Secon that match all these criteria. Have a look!
3D model of low-temperature storage room with cold air refrigeration machines (Mirai Intex), air distribution system and moisture extraction device inside the cold room
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Important criteria
Isolation & Specific Surface
The most important parameter for energy efficiency
The greater the outer surface and the greater the temperature differences between ambient and storage temperature, the higher is the heat input from the outside. Good insulation is therefore essential when storing at ultra-low temperatures. One possibility is to increase the wall thickness. But at a certain thickness this hardly brings any improvement and further limits the storage volume. This can be optimized by combining PU foam and integrated vacuum panels which result in high insulation values with moderate wall thicknesses.
The outer surface is significantly influenced by the geometry. Therefore it makes sense to keep the ratio of volume to surface as small as possible. The quotient of the external surface to the storage volume, which is referred to as the specific surface, is used as a reference value.
To illustrate this several individual refrigerators are compared to a large storage room. In general larger machines are more efficient than small ones. But even if many refrigerators are operated in the exact same way as one big storage room, they differ considerably in terms of energy consumption. This is due to the fact that the individual refrigerators have a significantly larger specific surface than the storage room. It can quickly add up to a factor of 10 in energy consumption.
Nature provides a fitting example for this. When looking at two different penguin species you can see from their natural habitats how drastic this influence is:
Emperor penguin
Specific surface
Energy consumption
per body volume
Lives in Antarctica
Is able to survive ambient temperature of -47°C
African penguin
Specific surface
Energy consumption
per body volume
Lives in South Africa
Needs at least 0°C ambient temperature to survive
One big storage
Specific surface
Energy consumption
per storage volume
Low, outer heat inlet
Efficient
Many, small refrigerators
Specific surface
Energy consumption
per storage volume
High, outer heat inlet
Inefficient
Refrigeration technology & regulations
Air as a refrigerant is the most suitable and safest solution
In the case of ultra-low temperature storage it is important to have a correspondingly high-performance and efficient refrigeration technology. The energy input from the environment (transmission), waste heat from electrical installations and loading and unloading procedures result in a corresponding power requirement. In addition, the moisture must be constantly removed from the cold room.
In terms of refrigeration technology it must also be ensured that it does not fall under the current regulations or is restricted by them. This currently affects the characteristic value "Global Warming Potential" (GWP) which is reduced to an average value of approx. 500 by 2030 due to the quota regulation of the European F-Gas Regulation and already prohibits refrigerants with GWP over 2500 for normal applications.
The refrigerants with high GWP, such as R23 (GWP = 14900 ) may still be used in ultra-low temperature applications as an exception. However, we strongly advise against this. For example R23 will most likely be banned in the next revision in 2021 and is currently massively affected by the shortage due to the quota regulation. New systems with this refrigerant are dubious and harmful to the environment. To make the impact on the environment tangible, let's compare the CO2 equivalent (GWP) of one kilo of R23 with the distance a car could travel until it emitted the same amount of CO2:
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In concrete terms, this means that a single kilo of R23 has as much global warming potential as the amount of CO2 that a car would emit if it traveled twice around the world.
We consciously use the natural refrigerant ambient air for cooling. This means you are future-proof and environmentally friendly . In addition, you do not need any additional safety measures as with other natural refrigerants such as ethane, ammonia and CO2.
Loading and unloading processes
Locks reduce the heat input when access-ing the cold room
Opening the access to the storage room creates a brief, large flow of air which brings a lot of warm air into the cold room. This has a drastic effect especially in low-temperature applications as the difference in density of the air between the storage room and the ambient is high due to the temperature difference. This means that a lot of cold air leaves the cold room when the door is opened. Due to the integration of a sluice which as a buffer zone is only tempered by transmission and convection from the cooling chamber the influence on the storage temperature in consequence of loading / unloading procedures remain low .
Electrical Installations
Evaporator and superstructures bring in heat
With conventional refrigeration systems an evaporator is necessary in the cold room. It is equipped with a fan and a fan ring heater and must be thermally de-iced at regular intervals. Both components and the de-icing process bring a lot of heat into the cold room. In addition, active cooling cannot be used during defrosting. With air cycle refrigeration machines an evaporator is no longer necessary, which means that these heat sources are no longer present and it can be continuously cooled. This makes the ultra-low temperature storage room with air cycle refrigeration machines more efficient. Furthermore, all electrical installations should be carefully considered and suitable for the storage temperature.
Optimal storage temperature
Speed-controlled air cycle refrigerations machines are very flexible
Since conventional refrigeration systems can only be used in a certain temperature range, the area of application of the storage room is limited. With cold air technology , any range between -40 ° C and -110 ° C can be approached without any problems, because the cold-air cooling machines are speed-controlled . In addition, these storage rooms have an air distribution system, which means that there is a homogeneous temperature distribution in the entire cold room.
