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Why cold stores remain cool in summer

How a cold store is cooled

Cold stores and deep-freeze stores are technically complex systems, although, strictly speaking, they are only gigantic refrigerators. The purpose of the cold store operation is to cover the constantly growing demand for frozen food and to provide the necessary cold to refrigerate the goods during transportation and storage while minimising energy consumption.

The success story of frozen food continues: more and more consumers in Germany are buying frozen meat, fish, pizza, baked goods or vegetables. In the past decades, the consumption of frozen good has increased steadily - in 2016, the per capita consumption in Germany amounted to a total of 45.2 kilograms according to the statistics portal Statista. There is no end in sight for this boom since the advantages of frozen products are very highly valued.

Many consumers are usually not aware of the logistical fine tuning which is required before salmon, salami pizza or buttered vegetables end up on their plates. The compliance with the cold chain is the basic requirement to ensure the quality of the food. That means that food must not be warmer than -18 degrees Celsius at all points of the cold chain whether they are stored or transported.

This is why refrigeration units have to ensure the required maximum temperature of -18°C for all plants and means of transport. Technically, there are numerous possibilities to generate "cold", however, closed refrigeration processes have become established.

This is how the cooling cycle works

Cold stores and deep-freeze stores are particularly important within the supply chain. According to VDKL (German Cold Stores and Logistics Association), the size is generally between 45,000 and 75,000 cubic metres. The new refrigeration logistics centre of the Nagel-Group in Schweitenkirchen even has a size of 85,000 cubic metres. Modern cold stores have developed into real logistics centres. However, their core competence remained the same: they provide refrigerated food. Behind this service is a high-tech process which is working like a clockwork. Deep-freeze stores are particularly energy-intensive systems. A closed cycle ensures that the ambient temperature complies with the requirements for temperature-controlled food. Although the principle is complex, it does not differ in principle from the functioning of a common refrigerator.

The refrigeration unit is the core element. Simply said, it removes the heat from the room and ensures a constantly low temperature. The principle works as follows: the "heat" is extracted from the rcold storage room by means of a so-called evaporator or air cooler. A liquid refrigerant is initially added to the evaporator. The heat in the cold storage room then ensures that the refrigerant evaporates at a low temperature level. The heat "disappears" into the refrigerant. The refrigerant, which is now gaseous, will then be aspirated by a compressor and, thus, brought to a higher pressure level so that it can be liquefied again at ambient temperature in the condenser.

After the liquefaction, the pressure of the refrigerant is released on the evaporation pressure level by means of a valve. The process starts again: the liquid evaporates, is condensed and liquefied. By this process, the heat will be permanently removed from the cold storage room.

Refrigerants need specific features

Due to their special use the refrigerants must have specific features. They must evaporate already at very low temperature and in case of ambient temperature be able to get liquid again. Refrigerants should not only possess the required chemical characteristics, but also satisfy the high standards of safety and environment protection. They should neither be toxic nor flammable.

Therefore, ammonia (NH3) is being used in most major cold and deep-freeze stores. It is a natural refrigerant and owns excellent environmental properties with zero ozone depletion or global warming potential. With regard to the aspects of energy efficiency it is outstanding, in addition available in unlimited quantities and cheap.

However, the energy balance is not only determined by the cooling process. It is just as important to keep the heat load as low as possible. Before a cold store is being created, all heat sources are being identified and then systematically minimised. Besides environmental influences, such as sun light or wind, heat is generated among others through opening of doors and gates, transportation or lighting as well as by new goods delivered.

Heat sources must be reduced

Walls, ceilings and floors provide contact surface for heat that emerges from the outside. All factors like sunlight, surface, ambient temperature or wind influence on the building envelope. In order to keep the influence of these factors as low as possible, an optimal insulation is required. Besides heat conductivity and thickness of insulation, the degree of reflection of the surface also influences the heat load that emerges from the outside. Consequently, the sunlight for instance can be reflected from a potential bright and reflective surface.

Moreover, the cold and deep-freeze stores which are used by logistics companies have the particularity that the high frequency of goods handling will result in additional heat sources. The plenty openings and goods movements can have an impact on the energy consumption. Therefore, increased vigilance is required with regard to gates and doors. For example, in Schweitenkirchen the Nagel-Group has used 111 particularly suitable insulated docks, which shall minimise the energy losses in the handling of chilled goods.

Energy efficiency is continually improved

Because of the fact that each door opening consumes energy, a permanent optimisation is being carried out. The installation of long sluices with high-speed doors or the installation of strip curtains reduces the heat load and increases the energy efficiency. The goal is to seal the interface between warm and cold in the most optimal way in order to preserve resources. The great art consists of harmonising logistical needs with the requirements regarding the topic energy efficiency. While from a logistical point of view an unimpeded delivery and return transport is targeted, it is best that doors and gates must be closed from an energetic point of view.

Anyhow, the topic energy efficiency has become indispensable also for cold stores. Consequently, an intensive heat recovery is now self-evident. Then, not only from ecological but also from economical reasons it makes sense to use the waste heat as by-product of the cooling process. Among others, the waste heat can be used for heating offices or for warm water treatment.

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