Wherever temperature-sensitive foods are involved there is a need for refrigeration. Canteens, hotels and restaurants, chemists, laboratories and medical facilities, bakeries, butchers and the fish trade make use of the advantages of low temperatures every day to slow down the development of microbial germs and retard chemical spoilage processes. To achieve this they have at their disposal a wide range of cooling equipment in different sizes and with various capacities and these are suited to practically all requirements. Refrigerators, refrigerated tables, counters, cabinets and shelves, cooling and freezing units, shock freezers and, of course, ice machines produce the right ice in the required quantity for virtually every need, be it for use in hotels and restaurants, laboratories or the food industry. Special products often require special cooling systems. Recently, for example, special refrigerated cabinets for sushi have become popular for they keep the appetizing portions fresh for longer and enable much more attractive product presentation. With the development of superfrozen fish products the need arose for high-performance cooling technology that can guarantee temperatures of about minus 60°C. Because initially such equipment was not readily available in the food sector the first interested users chose refrigeration technology from the medical sector to fulfil their purpose.
The demands users make on cooling systems and equipment are high. Refrigeration technology should not only be easy to operate, quiet and hygienic but should also be available in precise dimensions, in high-quality timeless design, and it should be profitable. If in the past it sufficed for a new fridge or freezer to be energy-saving, customers investing in new technology today also want their systems to be environmentally and climate friendly. Because more attention is in the meantime paid to the sustainable and respectful use of nature the choice of refrigerant that is used in the cooling systems plays an increasingly important role.
Changing states in a continuous circuit
Without going into too much detail, a small digression into the technology and functional principle of cooling systems might be helpful at this point. Regardless of the size and construction of a unit, the cooling principle is almost always the same. The refrigerant used circulates in a closed circuit and in the process successively assumes different aggregate states. At the start of the process the gaseous refrigerant is compressed so strongly that it becomes liquid under pressure as it releases heat. The liquid refrigerant passes through a pressure pipe into the actual cooling compartment and there the pressure is relieved via an expansion valve or a system of capillary tubes, i.e. the pressure produced by the compressor is reduced again. The liquid refrigerant evaporates during this process and, in spite of the low temperature (“evaporative cooling”) absorbs heat from the cooling compartment. Subsequently the meanwhile gaseous refrigerant flows back to the compressor and the cycle begins again. The cooling process is thus maintained externally (mechanically) by the compressor which supplies the required power so that the refrigerant can absorb the heat present in the cooling compartment and discharge it to the outside (for example at the rear of the refrigerator) at a higher temperature level.
Various refrigerants can be used in cooling systems. Choice depends, among other things, on the location of the ice machine, its integration in existing cooling systems, and environmental aspects. Ideally the refrigerant should fulfil the following requirements:
- good availability
- suitable thermodynamic properties
- high physical and chemical stability
- the lowest possible environmental impact
Conversion or replacement of refrigeration plants?
Since not all refrigerants meet these criteria equally, legislation has in recent years increasingly restricted the choice of permitted substances. After in the early 1990s halogenated hydrocarbons (CFCs) were banned internationally on account of their ozone-damaging impact, since 1 January 2015 hydrofluorocarbons (HFCs) have also been gradually withdrawn from the market in the EU. The aim of this measure is to gradually reduce the sales volume of HFCs to one-fifth of today's volume by 2030. The refrigerant R404A, which is currently most frequently used, is no longer to be permitted in new plants as from 1 January 2020. And from 1 January 2022 refrigerant R134A may no longer be marketed in commercially operated refrigerators and freezers. By 2025, stationary refrigeration plants with particularly climate-damaging F-gases must be shut down or converted so that they can use less climate-damaging refrigerants. These requirements also apply to propellants used by individual manufacturers when foaming insulating materials. There is still some time left before the required measures become effective but plans should be made in good time and due preparations made for any necessary investments. Although some refrigerants will not be allowed in the future, the number of possible alternatives is nevertheless so great that the decision for or against a particular one often requires expert advice.
The refrigerant R449A, also called Opteon XP40, is not harmful to the ozone layer, and can easily replace traditional refrigerants such as R404A, R507A, R22 or R407F in industrial and commercial cooling plants. This refrigerant is also suitable when refitting existing plants. At the same time R449A has considerably better environmental properties. With GWP-values (Global Warming Potential) of around 1,280 the impact of this refrigerant on world climate is about 67% lower than R404A and, depending on the plant technology used, energy requirements are 8 to 12% lower. With that the use of Opteon XP40 – at least according to the current state of the art – would seem possible in the longer term.
Climate protection gaining importance for refrigerants
With regard to the climate-damaging greenhouse gases, the alternative refrigerants CO2 (R744) and propane (R290) perform even better. Both refrigerants are convincing due to their low GWP values, they work reliably and are user friendly and highly economical. They protect the environment and do not damage the ozone layer of the atmosphere. Carbon dioxide is already used fairly frequently as a refrigerant in some food sectors with cooling requirements, for example supermarkets. CO2 or R744 has two important advantages. Firstly, its GWP value of 1 is extremely low (in comparison: the GWP of R449A is 1,280). And secondly, due to physical factors, the efficiency of CO2-compounds is mostly much better than that of other refrigerants. The cooling capacity of CO2 is thus higher with the same energy expenditure and the production costs are lower. CO2-cooling units are thus a particularly profitable and forward-looking technology that should be used more strongly. In spite of this, they are currently seldom seen. This is mainly because of the high technical requirements of this technology. Carbon dioxide has a high vapour pressure and all components of the refrigeration circuit must be designed for high pressures. Whilst conventional cooling units work at a maximum of 40 bar, 120 bar must be possible if CO2-gas is used. In the meantime quite a few manufacturers of cooling systems are able to cope with this challenge and offer CO2-systems but this refrigerant is still rarely used in single-stage plants. However, CO2 finds more frequent use as a low-temperature refrigerant in two-stage systems.
Propane (R290) and butane have similar properties to CO2 as refrigerants. They belong to the group of hydrocarbons (HC), which are chemical compounds consisting of the elements hydrogen and carbon and they occur naturally in high concentrations in crude oil, for example. Both propane and butane are an environmentally friendly alternative to fluorocarbons (CFC, HCFC, HFC) that are harmful to the environment. They do not attack the ozone layer, are gentle on the climate and are particularly energy-efficient, which reduces operating costs. However, it is a disadvantage that propane and butane are highly inflammable and thus entail risk of fire. Their use as a refrigerant is only possible if strict technical specifications for explosion protection are complied with. Actually, this is not a problem, because gas is also used for cooking and heating in the household. However, refrigeration technology such as refrigerators, freezers or ice machines, whose cooling circuit is under considerable pressure, requires special safeguards which still function reliably even after many years of non-stop operation. Hydrocarbons such as propane, butane or isobutane (R600A), with GWP values of around 3, offer such significant advantages that hardly any refrigeration engineer can do without these refrigerants today. Instead of using the recently introduced R134a, many manufacturers switched directly to the more environmentally friendly R600a. In domestic refrigerators and freezers, isobutane is in the meantime the most widely used hydrocarbon. More than 700 million household refrigerators currently work with R600A, and by 2020 three-quarters of all refrigerators manufactured worldwide will probably be using this refrigerant. Propane (R290) is mainly used in commercial refrigeration and freezing systems, air-conditioning systems and heat pumps.
High safety requirements for cooling plants
The transition from conventional synthetic refrigerants such as R22 which damages the ozone layer of the atmosphere to propane or other hydrocarbons is relatively easy from a technical point of view because these refrigerants work with the same cooling circuit and so are in a certain sense “compatible” with existing equipment and systems. The only difference really lies in the safety requirements which are considerably higher for hydrocarbons. Due to their potential fire hazard they are internationally subject to strict safety regulations and laws. The permissible filling quantity, for example, is limited to 150 grams per device (this value can be exceeded, however, under certain conditions in Europe). The manufacturers of hydrocarbon refrigeration systems must comply with prescribed safety regulations. Legislation demands, for example, tests in which leaks within the cooling system have to be simulated and recognized. Electric cables and components that are in the vicinity of the refrigerant flow must be adequately insulated. In order to rule out any risks the manufacturers of refrigeration units use particularly long-lasting safety valves and reliable pressure monitoring technology.
From an environmental point of view ammonia NH3 (R717) continues to play an important role as a natural refrigerant. It is mainly used in industrial cooling plants that have to be cost-effective and yet powerful. Examples of these work areas are breweries, cold stores and slaughter houses, or ice rinks. Ammonia is so to speak a classic product among the refrigerants and has been used in industrial cooling plants for more than 130 years. Ammonia is hardly inflammable and it is climate-neutral and so it does not contribute to the greenhouse effect or ozone depletion. Its half-life in the atmosphere is only about two weeks.
A disadvantage of NH3 is its toxic effect and physiological hazard since the gas can corrode the lungs upon inhalation and damage the eyes because it forms an alkaline reacting solution when it comes into contact with water. The characteristic biting smell of ammonia is, however, already perceivable at very low concentrations of 3 to 5 mg per cubic metre of air which guarantees a reliable warning effect. This warning threshold is far below the critical concentration of 1,750 mg/m³ which causes chemical burns and other health damages. A further disadvantage of ammonia is its incompatibility with non-ferrous metals which increases material expenditure considerably during the installation of large ammonia refrigeration plants. Copper pipes, galvanized parts or fittings made of bronze or brass are not suitable for this refrigerant.
Cooling and freezing with solar energy
However, the climate and environmental friendliness of a refrigeration plant does not only depend on the refrigerant used. Its sustainability is also dependent on the energy required for compressing and circulating the refrigerant. This fact revived an idea that dates back to the middle of the nineteenth century. At that time the French engineer Augustin Mouchot combined concave mirrors with collectors to evaporate water and drive steam engines with the help of concentrated sunlight. In 1866 the first usable solar steam engine was put into operation. When Mouchot directed the steam into overheated rooms the water condensed and cooled the air as a fine spray. The engineer recognized the potential of this idea and continued to work on its further development. At the Paris World Exposition in 1878 Mouchot presented an ice machine which was operated by solar-generated steam and, to the astonishment of the audience, produced the first ice block using solar energy.
Modern solar cooling systems are technically much more complex and effective but the basic idea is quite similar: the aim is to produce ice for cooling purposes with the help of sunlight. The refrigeration technology supplier Ziegra, for example, offers ice machines which can be a real alternative to conventional ice machines, especially in rural areas in tropical countries where the power grid is often incomplete and power supply unstable. The production of electricity using solar panels has the advantage over current generators that no fuel is needed for this and that it works silently. Sunlight is mostly available in abundance and with suitable batteries the solar energy can also be stored “gratis” for the dark night hours.
Ammonia, freon, halons, and hydrocarbons
What exactly are refrigerants?
Ziegra’s solar driven systems are not off-the-shelf solutions for they are developed from commercially available components according to the local concrete requirements. The manufacturer’s qualified personnel have the necessary engineering knowledge to combine all parts from the solar panel to the batteries into fully functional systems. In Senegal, Ziegra has already gained experience with solar energy generation using smaller machines producing about 375 kg of ice per day. Battery-powered ice production continues until late into the night and switches off only in the early morning. Ziegra’s solar ice machine is used by the German Association for International Cooperation (GIZ) in Senegal in a rural area without constant power supply to cool fresh fish with the produced ice. The smallest ice machine which can be combined with solar panels produces about 150 kg of ice per day.
Ziegra has identified opportunities for its solar ice machines especially in African countries where fresh fish spoils quickly if there is no adequate cooling available. In these countries, too, a consumer group is growing which is characterized by a higher quality awareness and more emphasis on hygienically clean, fresh fish products. Although the interest shown in the solar systems is considerable, willingness to buy is rather low. Many fishmongers currently still regard the use of ice for cooling as a "luxury" which leads only to unnecessary costs. Innovation-wary buyers often believe that a trader who packs his fish in ice is only trying to conceal the lack of freshness of the goods or other defects. While refrigeration manufacturers are now developing more and more fascinating products, improving the efficiency of their appliances, and making them more environmentally and climate friendly, such developments seem to overwhelm some customers. It is presumably easier to push ahead the state of the art than to change something in people's minds, in the way they think.