Spotlight Cooling & Freezing Technologies


Currently used refrigerants in laboratory freezers and/or coolers are mostly of the fluorocarbon type. These refrigerants do contribute, when emitted to the atmosphere, to the greenhouse effect. Therefore, these chemical refrigerants are included in the Kyoto Protocol that is linked to the United Nations Framework Convention on Climate Change. It is clear that there is pressure on the use or emission of these refrigerants, which means that limitations can be expected in future. This may pose problems for existing freezers at the end of life or during servicing. As an alternative it is possible to apply hydrocarbon refrigerants, which are generally excellent refrigerants with favourable properties and contribute very little to global warming. If well designed, in many cases an energy saving can be expected which reduces the environmental


impact further, by reducing the CO2 emission related to the electricity generation needed to drive the product. The drawback however, is that hydrocarbon refrigerants are flammable and adequate measures are needed to design safe products. Experience in other application areas, such as domestic and commercial refrigeration, has shown that this is well feasible.


Advantages Of Using Hydrocarbon, Green, Refrigerants In Laboratory Coolers / Freezers


INTRODUCTION


Laboratory freezers and/or coolers generally apply a vapour compression technique to generate a cold environment for storage of various goods. The basic circuit behind this cooling process always contains a compressor, condenser and evaporator (see Figure 1). In this circuit heat is released from the condenser to the ambient and heat is absorbed by the evaporator, which is typically placed inside the cabinet to be cooled. The compressor is needed to circulate a refrigerant and to raise the refrigerant pressure from the evaporator to the condenser level.


the application, which leads to a reduction in the global warming contribution related to the product. As the refrigerant used can also have a direct contribution to the global warming when emitted at the end of life (or due to a leakage) it is possible to add these two effects in the so called TEWI factor (total equivalent warming impact). This issue is addressed in section 5. Finally the conclusions will summarise the steps needed to design a safe product using HCs and the environmental advantages associated with this choice of refrigerant.


REFRIGERANT SELECTION, HISTORY AND FUTURE PROSPECTS


Figure 1. Schematic representation of a vapour compression cooling system.


“The lessons which can be learned by the earlier phase out of CFCs is that equipment operating with such refrigerants can in general still be used through its lifetime if no servicing of the refrigeration circuit itself is needed”


Depending on the application, variations to the system described above may exist. For example, to achieve very low temperatures, it is possible to arrange two vapour compression systems in such a way that the condenser of the low temperature circuit delivers its heat to the evaporator of the high temperature circuit (this is a so called cascade system). Different refrigerants are typically used in each of the two circuits due to the very different temperature conditions in the circuits. It is also possible to run low temperature systems with a single compressor and use a mixture of refrigerants in a so-called auto-cascade system arrangement. Such system requires a number of internal heat exchangers.


In any case the selection of a proper refrigerant is based on a number of design criteria. However, since the mid eighties of last century, also a number of environmental criteria’s have been added to this selection process. This will be described in the second section of this paper where both ozone depletion created by the earlier refrigerants and greenhouse effects of replacement refrigerants is discussed. Currently there is legislation under development that will also restrict the use of refrigerants with a substantial global warming impact.


During recent years the use of hydrocarbons as refrigerant has gained interests. Such refrigerants have actually been in use in domestic refrigeration systems as far back as 1920- 1930, where they got replaced with non-flammable, but ozone-depleting refrigerants. Being faced today with environmental problems of the current refrigerants and having possibilities to design, construct and maintain, safe systems applying flammable refrigerants, the interest in this solution is renewed. The properties and use of hydrocarbons will shortly be discussed in section 3.


Author Details:


M. Janssen & J. Wijnstekers Re/genT BV, Lagedijk 22, 5705BZ Helmond, The Netherlands martien.janssen@re-gent.nl


As the main concern of applying hydrocarbons relates to the safety aspects, it is evident that products applying this technology must be carefully designed. To assist this process, to date relevant safety standards have been developed, or are in progress in certain application areas. In general complying with a safety standard addressing the use of flammable refrigerants will minimise the risks associated with the production, use and destruction of the product. Section 4 addresses the relevant standards and its application to a typical laboratory freezer, resulting in a safe system using natural refrigerants.


Apart from the flammability, hydrocarbon refrigerants can be classified as excellent refrigerants, due to favourable thermodynamic properties and heat transfer characteristics. This often leads to a reduction in energy consumption of


Until the end of the eighties, generally chloro-fluorocarbon (CFC) refrigerants were employed in laboratory coolers. Due to the discovery of their damaging impact on the ozone- layer, this class of refrigerants became controlled under the UN Montreal Protocol and was effectively banned in the following decade. The replacement refrigerants are primarily of the hydro-fluorocarbon (HFC) type, where HFC- 134a, R-404A and R508B are known examples . These refrigerants have in common that they have a relatively strong global warming potential (GWP), which is generally expressed as a ratio number to the generally known greenhouse gas CO2. For example, HFC-134a has a GWP of 1430, R-404A of 3900 and R-508B a value as high as 13000 [1]. Though these effects are smaller or of the same size compared to the refrigerants being replaced, this is more and more seen as a problem. As a result this set of greenhouse gasses is included in the so-called baskets of the UN Kyoto-protocol; hence their emission is subject to control measures.


This situation has led globally already to regulatory initiatives to reduce the emissions of these substances by banning their use. Examples are that Denmark and Austria have restricted the use of HFCs in certain applications. On a European level, the so called F-gas regulation [3] with its link to MAC directives, bans the use of HFCs with a GWP above 150 in automotive air conditioning for new car types as of 1/1/2011, next to imposing measures on the containment and servicing of HFC containing appliances.


As the Kyoto-protocol deals in principle with emissions only, rather than prohibiting the use, it has been brought forward by various countries (among which the USA) to include HFCs, despite their zero ozone depletion potential, under the Montreal Protocol and to include a phase out scenario of these refrigerants. Though not active yet, this does clearly illustrate that the use of these refrigerants is under increasing pressure.


The lessons which can be learned by the earlier phase out of CFCs is that equipment operating with such refrigerants can in general still be used through its lifetime if no servicing of the refrigeration circuit itself is needed. Changes in legislation may however impact the serviceability of a product. As an example, using new HCFC-22 in existing installations, such as air conditioning systems, is forbidden since 1/1/2010 in Europe, while it well known that there is not enough recycled refrigerant available to service the sector. This forces system owners to costly early replacements or retrofitting their systems to new refrigerants (which are often HFCs at this moment in time).


RELEVANT HYDROCARBON REFRIGERANTS


As a response to the global warming contribution of HFC refrigerants, another class of refrigerants has come into the picture, the hydrocarbons (HCs), which only have a very low GWP. Though generally having quite good properties for refrigeration systems, their main draw back is their flammability. In the applications were only a limited amount is needed, this may be handled by adequate measures. Under the pressure of especially environmental organisations such as Greenpeace, the use of hydrocarbons became popular in domestic refrigeration and has now


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