Industry leaders are applying current and emerging technologies to help customers achieve sustainable and quiet transport refrigeration
Since the late 1930s, transporting temperature sensitive goods by road and rail depended almost entirely on fossil fuels and high global warming potential (GWP) refrigerants to maintain cargo at the optimum temperature. Today, internal combustion engines have become quieter, more fuel efficient and cleaner. Nevertheless the dependence on fossil fuel and hydrofluorocarbon (HFC) refrigerants remains.
Regulations and social pressure have dramatically changed the way transporters operate in the European Union (EU). Environmental sustainability is a top objective of legislators determined to phase out high GWP refrigerants, control exhaust emissions and limit noise in densely populated areas.
For the past 15 years, the manufacturer of transport temperature control systems, Thermo King, has focused on developing a solution that would meet the future need for an alternative to the fossil fuel or HFC technologies on which the transport industry has depended for so long. In the 1930s, Thermo King pioneered transport temperature control and they have remained the leading innovator in this industry ever since.
HOW THE SYSTEM OPERATES
The company's alternative approach, which remains unique in transport refrigeration today, involves the use of recovered and commercially available liquid carbon dioxide (R744) in an indirect open-cycle system. Unlike other ‘cryogenic' approaches which spray the refrigerant directly into the load space, the Thermo King CryoTech range uses fin-and-tube evaporators as heat exchangers through which the R744 flows, absorbing heat from the load before it is vented to the outside of the vehicle.
The recovered R744 is stored under pressure in a vacuum insulated tank under the chassis of the truck or articulated trailer. It flows to the remote evaporators, one of which is installed in each compartment allowing up to three temperatures on one vehicle. An electronic expansion valve managed by the electric control module regulates the flow of liquid through each evaporator thereby varying their cooling capacity to match the demands of the load and maintain a steady temperature.
Just like conventional HFC refrigerants, the liquid R744 changes state (into a gas) as its pressure drops on leaving the expansion valve and rapidly absorbs heat energy in the process. A regulator keeps gas pressures above the critical 5.5 bar point to avoid the formation of dry ice in the evaporator. After the liquid R744 vaporizes causing the temperature to lower in the insulated box, the vapor is vented outside the box through an exhaust muffler to minimize noise. This is important with regard to the health and safety of operators and goods.
A SYSTEM DEFINED BY WHAT IT LACKS
The system is more notable for what it lacks rather than what it possesses. The truck or trailer unit is cooled with virtually no operating noise without the use of diesel engine, compressor, or HFC refrigerant.
The recovered R744 in the Thermo King CryoTech systems is obtained as a by-product from industrial processes that would otherwise have been released into the atmosphere. As such there are no new carbon dioxide emissions during operation of the CryoTech system.
OPERATING COSTS SIMILAR TO DIESEL
The cost of operating a CryoTech unit over its lifecycle is broadly comparable to that of an equivalent conventional diesel powered unit, although specific applications may favor one or the other. The initial cost of the unit is likely to be slightly higher, due to current low manufacturing volumes. This is offset by its longer service life due to its long life components and fewer "wear" items.
COOLING PERFORMANCE AND NOISE
It has been proven that CryoTech evaporators deliver significantly more cooling capacity than their diesel equivalent at both fresh and frozen box temperatures. Pull down of an empty box can be up to four times faster with R744, making it an excellent choice for distribution operations with a high number of door openings.
With more countries considering the introduction of noise limits on evening and night deliveries - following the example of the PIEK standard in the Netherlands - urban distribution operations wanting to take advantage of low traffic volumes need a vehicle that can perform at 60 dBA or less. All the CryoTech units are PIEK tested and compliant offering sound levels up to 90% lower than a standard diesel unit.
THE ENVIRONMENTAL IMPACT OF THE SYSTEM
But is the CryoTech solution actually more environmentally sustainable?
Engineers at Thermo King performed a detailed carbon footprint calculation on three equivalent systems. The CryoTech (R744) system was first compared to the latest in "conventional" technology (fossil fuel/HFC refrigerant) and to a different "alternative" approach using liquid nitrogen as a refrigerant. The study measures the environmental impact, or "carbon footprint" of each solution.
The comparison took into account significant sources of carbon dioxide emission from cradle to grave, including emissions arising from the energy required to produce the fuels and average annual operating hours. Also taken into consideration were the fuel consumption and exhaust emissions based on independent ATP test data only applicable to diesel units.
As expected, the diesel unit's carbon emissions were largely due to burning of this fossil fuel. Although the nitrogen unit consumes a similar level of fuel to the R744 unit, nitrogen itself requires about three times more energy to produce than the equivalent amount of recovered R744. The total footprint in tons of carbon dioxide over a ten year life was found to be as follows:
• Diesel unit 166 tons
• Nitrogen unit 143 tons
• CryoTech unit 46 tons
The results, while strongly in favor of the R744 solution, were not entirely surprising. CIT Ekologik AB (Engberg et al.) conducted a similar detailed Lifecycle Analysis in 2002 comparing diesel-powered units with CryoTech units. The study showed that the carbon dioxide refrigerator contributes considerably less to the environmental effects than the diesel refrigerator during refrigeration as well as heating.
THE ROAD AHEAD
The study demonstrates that the recovered R744 solution used in the CryoTech range has a carbon footprint approximately 75 percent less than a conventional diesel system and 68 percent less than a nitrogen cryogenic system. But carbon footprint alone will not make a solution commercially viable. The CryoTech range has been shown to also have a similar cost of ownership to an equivalent diesel system while substantially outperforming diesel on both noise and temperature pull down/recovery. These additional features make it ideally suited for urban distribution.
One current limitation is the availability of R744 filling stations. In the early years, there was little to no infrastructure to support the filling of CryoTech units. Since then great strides have already been made in this area.
Existing diesel fuel stations were willing to have R744 storage and dispensing stations installed so the vehicles can be refueled at the same time as the units. By November 2014, more than 47 R744 filling stations were in operation in eight European countries and the number is expected to grow in the coming years.
Thermo King has demonstrated its commitment to this technology, investing heavily in future product research and development, as well as the expansion of the filling station network.
Over the past decades, the science of transport refrigeration has advanced dramatically and the next years will no doubt bring about further innovations. The future promises to be an interesting time, as it is clear that the industry cannot continue to solely depend on traditional fuels and HFC refrigerants. Industry leaders like Thermo King are applying current and emerging technologies to help their customers achieve sustainable and quiet transport refrigeration.
Anthony Bour is product manager oc cryogenics at Thermo King
