TECHNOLOGY UPDATE
Page 46
Page 47
If you've noticed an error in this article please click here to report it so we can fix it.
For years scientists have been working on environmentally friendly refrigerant gases. Ironically they could have bought the perfect gas for the job at any high-street camping shop...
Amongst road hauliers, reefer operators may feel they've been asked to do more than their fair share when it comes to providing clean, Green transport Back in 1987 the Montreal Protocol launched the quest for the ultimate refrigerant—one that manages to combine zero ozone depletion potential (01W) with zero global warming potential (GWP). Some good alternatives, such as R401A and R410A, have already emerged but common or garden propane (R290) could come from behind if operators can live with its flammable nature.
Efficient refrigeration was made possible by the advent of man-made refrigerants such as chlorofluorocarbons (CFCs) which are odourless, colourless, non-flammable and relatively non-toxic. They have been used since the 1930s but, once scientists implicated them in the depletion of the ozone layer and global warming, their days were numbered.
Ozone layer
CFCs have been identified ma substance that depletes the ozone layer, and a timetable has been drawn up for their replacement. Such was the international support for the protocol that the more common CFCs such as R12. R500 and R502 were banned for use in new equipment as early as 1993, and banned from production within the EU from 1995. Other FICFCs, such as R22, are due to be banned by the ELT from 1 January 2000 and could be banned from EU manufacture as soon as 2005.
These bans were inspired by CFCs' ability to deplete ozone in the upper atmosphere, but more recently the emphasis has switched to
global warming and future refrigerants will be judged on both factors. The so-called greenhouse gases only represent a tiny fraction of the total atmosphere (in the order of 0.04%), but they have been shown to make a decisive impact on climate.
Roughly 50% of the greenhouse gas in the atmosphere is carbon dioxide; CFCs only account for around 15% but they have a greater impact than carbon dioxide or methane. Toyota, for example, gave evidence to the US Senate that one charge of CFC12 lost from a car's air conditioning system makes as big a contribution to the greenhouse effect as all the CO, emitted from the same car in travelling 100,600 miles—CFC12's GWP is 7,300 times higher than that of CO,.
So when seeking eco-friendly refrigeration systems for trucks, researchers have to look for a compromise. The current system, known as Total Equivalent Warming Impact (TEWI), takes into account the indirect effect of the fuel burned in the compressor to achieve the desired temperature, and the effect of losing refrigerant through leakage. A typical TEWI measurement would relate to the amount of energy used over a 2000-hour operating year and a leakage rate of 20% per year. The actual amount of heat lost through the insulated skin is
quite small (around 1-5% of the total), so the greatest efforts are directed at reducing the GWP
Much effort has been put into finding ecofriendly alternatives to CFCs. R22 is still in production and is widely regarded as an industry benchmark.
R410A is an R22 replacement made by Allied Signal and used by the German =many Frigoblock which sells it under the trade name AZ20. It seems to be an ideal solution for transport refrigeration as it has an even lower energy consumption than R22 and achieves similar compressor discharge pressures.
Compromise
AZ20 also has a similar indirect GWP to R22. It has a somewhat poorer direct GWP but its TEWI values show that AZ20 is an excellent compromise. The high condensing pressure of AZ20 requires condensers and compressors to withstand 35bar rather than the more usual 25bar. This is not a problem with new systems but it cannot be used as a "drop-in" refrigerant for existing systems, On the other hand, AZ20's relative refrigeration capacity is up to 70% higher, while its specific energy consumption is up to 200 lower. This allows the use of smaller, lighter compressors. Alternatively, by retaining current equipment dimensions and weights, refrigeration capacity can be increased. This is important given the increasingly stringent temperature controls that are being called for.
Despite notable advances such as AZ20,
some German food retailers are looking for even more environmentally-friendly refrigerants which meet the "double zero solution" by completely eliminating ODP and GWP.
That solution could take the form of propane: a major constituent of liquid petroleum gas. Frigoblock has built an MAN-based evaluation unit for the German food retailing group REWE. The truck runs on methane, with propane as its refrigerant. Propane almost matches the high capacity of R22 with an equally low specific energy consumption, but it is flammable.
However, only 3.5-4.5kg of propane is needed to run the system, and there is no reason why it should be any more dangerous than, say, petrol, LPG, or diesel in vehicles. Frigoblock's standard safety system was modified so propane could be used without additional equipment beyond that already used for methane. To prevent any leaks in the loadspace no detachable pipe connections were used in the evaporator area: all pipes were welded or brazed.
Most transport reefer units are driven by diesel or gas-powered engines and it is generally accepted that the resulting vibration is responsible for most reefer failures. 1-,'rigoblock uses only electric drive motors. Typically a three-phase motor uses power from the truck's engine via an alternator, switching to an outside supply when parked up. The electrical installation is strictly protected to IP54 and IP6:5 standards—component surfaces are only 30% of the legal limit. Frigoblock has considerable knowledge of flame.proof design, having supplied systems for chemical transport for several years. For the REWE system all electrical components in which sparks might occur are permanently sealed with a flexible gas-tight compound. Also, a gas sensor switches off the entire refrigeration system in the event of a leak.
Propane is particularly environmentally friendly--it doesn't deplete the ozone layer or add to the greenhouse effect. It also has excellent specific energy consumption.
AZ20 and propane have been approved by the German Environmental Office for use where a MAT value of 2500 or less is required. Both these refrigerants can be used for the entire life of the refrigeration unit, and they have been given the all-clear by the German authorities for their toxicological properties, which is more than can be said for R404a or R507.
Time will tell if propane will become a viable refrigerant. It fits the environmental bill and looks viable from an engineering point of view. In the long-term Frigoblock expects to use AZ20 in freezer systems, with propane reserved for less demanding chiller systems. CI by Gibb Grace REFRIGERANT UPDATE R12 No longer produced; effectively phased out in the EC. No refrigeration equipment is manufactured with this refrigerant; its price is rising. R134a: New replacement medium-temperature refrigerant with a similar performance to R12; its price is falling as production increases. R502: No longer produced, and effectively phased out in the EC. No refrigeration equipment is manufactured with this refrigerant; its price is rising. Most units have been converted to R403B. R404A New replacement lowtemperature refrigerant with a similar performance to R502 and R403B; its price is falling as production increases. Still in production. The EC requires it to be phased out by 2015 but this could happen as early as 2005; its price is fairly stable. R410A: Marketed as AZ20, this is a new replacement low-temperature-range refrigerant with similar performance to R502 and R403B; its price is falling as production increases. Naturally occurring propane gas, commercially available, has zero ozone depletion potential, has zero global warming potential, but is flammable and requires unique system protection.