Why is nitrogen not used in cooling?
Liquid nitrogen instead of refrigerant
KKA: Dr. Schulthoff, please first describe the technical functional principle of a system with nitrogen as the cooling medium.
Dr. Schulthoff: For a good ten years I have been dealing with the question of how to create rooms, e.g. B. Cold stores - regardless of size - can cool safely and efficiently without conventional compressor technology. This involves organic and pharmaceutical products that have to be stored in a temperature range between +4 and -40 ° C. The solution is actually obvious and very simple, and I am amazed that it has not yet been implemented. You only need five components:
a tank filled with liquid nitrogen (LN2),
a vacuum-insulated pipeline system,
Heat exchanger and
a pipeline system to discharge the gaseous nitrogen into the atmosphere.
The schematic representation (see next page) makes the principle clear.
A tank installed outside the cold store contains liquid nitrogen at a temperature of around -196 ° C, which is brought to the consumption points (heat exchanger) via a vacuum-insulated pipe system. No pumps are necessary for this, because the tank pressure ensures that the coolant flows. As soon as the nitrogen is warmed up just a little, it changes into the gaseous state of aggregation and its volume increases by a good 700 times. In front of the heat exchangers (outside the cold store) there are solenoid valves that open so that the nitrogen can get into the heat exchangers for as long as it takes to evaporate and thereby produce the necessary cold until the temperature in the cold store reaches the level that is stored Product required. The process is regulated by a controller and software. This records the temperature of the unbuffered temperature sensors installed at various points in the cold store, compares it with the specified target temperature and regulates the nitrogen flow into the heat exchangers accordingly by opening and closing the solenoid valves. Incidentally, the solenoid valves are the only moving parts in the entire cooling process that require maintenance. At the end of the process, the now gaseous nitrogen is released into the atmosphere from the heat exchanger via an exhaust pipe system and is thus used up. In the future, however, it could also be available for follow-up processes. The gaseous nitrogen has a “residual cold” that could possibly be made usable.
KKA: So it is not a refrigerant in the classic sense that circulates in a refrigeration circuit, but one should rather compare nitrogen with gasoline in a car, which is consumed?
Dr. Schulthoff: No, you can't compare that. The energy in the gasoline is used to run the car and is used up when the fuel has been burned in the engine. The nitrogen “works” in a pipeline-bound system; at no time do the product or people come into contact with it. Only when it becomes part of the atmosphere again do we breathe it in again as air, which consists of around 78% nitrogen. It doesn't get any greener!
KKA: Have you already implemented a refrigeration system with liquid nitrogen and what is your experience from operation? And if so: Who planned and built this system and who looks after it in the event of service?
Dr. Schulthoff: Yes, as part of the construction of a new blood donation service, we implemented the refrigeration part of the processing center for storing blood in a fully automated high-bay warehouse. You have to know that a blood donation is separated into the two components "red cells / erythrocytes" and "plasma" during processing; each component is about 45% by volume. The red cells require a storage temperature of +4 ° C and the plasma, depending on whether it is to be used for industrial or human purposes, -20 or -40 ° C. According to the Medical Devices Act, very high requirements are placed on the storage of erythrocytes and plasma, in particular with regard to temperature stability and reliability. The blood products are stored in large cold rooms, each of which is at the temperature that the product needs. This is done by the control software, which also documents all product movements.
The system has been running to the customer's full satisfaction in all temperature ranges for over a year and, to my knowledge, is the only one in the world that uses nitrogen to cool the warehouse. During implementation, however, there were problems with the storage and retrieval machines in the deep-freeze area, which have to run 24 hours a day without any problems even under the extreme conditions. But that's not unusual for a prototype. The problems were solved in very cooperative work with the supplier of the storage and retrieval machines.
A large engineering company took over the planning of the entire new building, we did the refrigeration part and implemented it with various trades, as part of a work contract, together with the customer.
The customer mentions the extremely low maintenance costs as a significant advantage over conventional systems: only the solenoid valves, which are easily accessible outside the cold room in the pipeline system, can require maintenance work; this is done i. d. Usually preventive by the customer himself, so that there are no failures at all. In the event that other components need a service, the customer has concluded individual service contracts with their suppliers. This also applies to the system and control software.
KKA: What about investment, operating costs and logistics costs as well as the environmental balance sheet in this context?
Dr. Schulthoff: Since conventional refrigeration systems were also planned for the refrigeration part in the original planning of the blood processing center, we can say the following:
The investment costs for generating or maintaining the cold are roughly the same for compressor and LN2 technology, assuming technical redundancy in an emergency, which is certainly indispensable for the high-quality products that are stored in large cold stores. For conventional cooling, however, there are also considerable investment costs, e.g. building / room costs for accommodating the technical systems including the costs for static measures, uninterruptible power supply, emergency generators, piping systems, coolants. In addition, it can be assumed that parts or the entire investment will have to be replaced after seven to eight years. In the case we implemented, the unnecessary investments amounted to a good 1 million euros.
In terms of operating costs (LN2 versus electricity costs including maintenance), the costs of our solution are around 30% below those of conventional technology; this was also forecast by us and the planning company.
There are no logistics costs with our solution.
With regard to the environmental balance, our solution is extremely environmentally friendly, because we only need electrical power to operate the IT and control systems, the nitrogen itself is released into the air after the phase change in the heat exchanger - as already mentioned.
Of course, costs for the production of liquid nitrogen are incurred in the air partitioning systems; you would have to ask the producers. However, they are usually less interested in nitrogen than in oxygen and the noble gases that occur in the air, such as argon; Nitrogen is almost a waste product.
KKA: For which purposes and desired temperature ranges is this system technology suitable? Do you see the opportunity for widespread use beyond niche applications - also against the background of the availability of liquid nitrogen and the necessary spatial conditions?
Dr. Schulthoff: I can clearly say that we cover the temperature range from +4 to -40 ° C without any problems. After we have successfully completed the large installation in the healthcare sector that I have described under the harsh conditions of the MPG law, we see the market for our technology in all areas in which products have to be stored refrigerated, for example in the pharmaceutical and food industries. Cold stores without the fully automated high-bay control system mentioned in my example are comparatively easy to equip with our technology. We do not make spatial requirements or environmental conditions because they are not necessary. After all, the cooling is available “on demand” and does not have to be generated in a complex process involving several steps. All that is needed is a space outside the cold store for the installation of an LN2 tank.
And there is plenty of liquid nitrogen!
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