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- 6.2.1.2 R-744
- 6.2.1.3 HFC refrigerants
- 6.2.1.4 Cryogenic systems
- 6.2.1.5 Eutectic systems
- Vessels
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6.2.1.1 HydrocarbonsWhile hydrocarbons offer superior thermodynamic properties which lead to high energy efficiencies, the possible leakage of the refrigerant inside the box and other failure scenarios present a safety challenge, since these refrigerants are flammable. Due to the lack of specific safety standards applicable to transport refrigeration, the design usually relies on safety standards for adjacent applications, such as ISO 5149 (ISO, 2014a) or EN 378 (2008). For example, according to the scope, ISO 5149 (ISO, 2014a) covers transport refrigeration but the detailed safe design and operation for all the mentioned applications are not described (König, 2013a). Most standards rely on the concept of “lower flammability limit” combined with a maximum charge and safety factor. In a traditional concept of a transport refrigeration unit, even assuming a significant charge reduction compared to today’s typical charge levels, leakage inside the box would inevitably lead to concentrations above the flammability limit. The use of higher amounts of refrigerant charge over the thresholds established by the existing standards would require the adoption of provisions covering safe system design and safe operations. These higher amounts can be permitted only when special measures are taken to limit the refrigerant concentrations in case of leakage and the presence of sources of ignition. To justify such special measures, a risk analysis shall be performed to include all foreseeable uses (König, 2013a). All this may increase complexity and may introduce an additional risk that manufacturers must consider. Measures can include ventilation and shut-off valves, combined with refrigerant detectors and alarm systems. Ventilation may be realized using explosion-proof fans, sensors, etc. Or, as alternative, the flammable refrigerant maybe isolated in a separate circuit and placed outside of the cargo box. A secondary fluid can be used to provide cooling inside the cargo box. The above measures either do not eliminate completely the flammability risk, or make the system very complex, or increase power consumption. Refrigeration units with direct expansion using HC-290 and HC-1270 have been tested to date in a small number of trucks in the UK and Germany. One European manufacturer has developed a road vehicle system operating on HC-1270, the safety issues being addressed by having a jointless evaporator and installing a detection system. However, this unit is not commercially available. One intermodal container manufacturer (MCI, 2014) announced a possible market introduction of a refrigeration system with HC’s in 2018-19 subject to the boundaries of future legislation. In summary, specific research on the use of flammable refrigerants in transport refrigeration should continue but (as it is the case today) their use would most likely require the implementation of safety measures that could increase the complexity of the system and/or, in case of indirect system, could penalize their efficiency. 6.2.1.2 R-744R-744 is a promising alternative to HFC refrigerants and its widespread use in transport refrigeration will only come when systems using R-744 can exhibit efficiencies comparable to the efficiencies that are obtained at present from systems using HFC refrigerants. R-744 is undergoing a renaissance and the technology and learning curve in transport refrigeration is at an early stage. All major manufacturers have displayed concepts of units with R-744 at trade shows or have demonstrated an interest in R-744 solutions through research publications. One manufacturer has been testing an intermodal container unit with customers across the globe, and is now offering the unit. Since 2013, a supermarket chain in the UK has been conducting a trial with an urban distribution trailer fitted with a modified version of the intermodal container system. The annual energy consumption and the efficiency depends on the ambient air temperature and the cargo box air temperature (refer to appendix A 4.1 for details). Systems using R-744 are more efficient than systems using HFC refrigerants under low-medium ambient temperature conditions. At the same ambient temperature, systems using R-744 are more efficient for chilled cargo and less efficient for frozen cargo than systems using HFC refrigerants. Literature mainly published for commercial refrigeration and mobile air conditioning application (see Table 6-1) show “crossover temperatures” (= ambient temperatures below which R-744 is more efficient) ranging between 21C and 40C. Data for transport refrigeration is limited. Table 6-1: Crossover temperature with regard to energy consumption/efficiency
Figure 6-1 shows a comparison between the R-744 reefer container unit and the incumbent HFC-134a unit (with comparable technology level) in terms of efficiency. Data was provided from a large manufacturer. Based on typical total cost of ownership of models used in the industry, approx. 15% of the time is estimated to be spent in full load conditions, moderate - high ambient (an ambient temperature of 38 C is normally used to represent this condition), while an estimate 85% of the time is spent at medium-low ambient, part load (an ambient temperature of 25 C). The results show a disadvantage of R-744 at the 38 C full load point, but an advantage at the 25 C ambient point, with chilled cargo and in part load conditions. 
Figure 6-1: Container refrigeration, comparison between HFC-134a and R-744 for typical test conditions. Note 1: The two systems have different optimization logic (the R-744 system is mostly optimized for part load conditions, not full load conditions). Note 2: At -18C box temperature, the R-744 system has tighter air temperature control than the HFC-134a one.
Changes to refrigerant classification have been proposed to mitigate the flammability concern of the medium and low-GWP HFC refrigerants. For example, ASHRAE Standard 34 (2013) (since 2010 edition) and ISO 817 (ISO, 2014b) recognize subclass 2L (= 2 low) refrigerants that are defined as class 2 refrigerants with a maximum burning velocity of =10 cm/s. The most recent revision of standard ISO 5149 (ISO, 2014a) recognizes the fact that A2L and A3 refrigerants need to be treated differently, and proposes more relaxed charge limits for A2L refrigerant when compared with A3. Still, even the more relaxed A2L limits appear not to be compatible with the charge levels / volumes typical of transport refrigeration applications. In addition, for transport refrigeration applications, due to the flammable nature of refrigerants classified as A2L, A2 and A3, a risk assessment should be carried out to ensure compliance with occupational health and safety requirements and other regional standards and requirements. A calculation assuming a release of 4 kg HFC-1234yf in a 25 m3 trailer box (80% loaded by cargo) yields concentrations that are at best comparable to the refrigerant concentration limit (RCL) of the ASHRAE Standard 34 (2013), and above the lower flammability limit, using the recommended safety factor of 4. The opening of the box and the consequent leakage of oil or other contaminants in presence of a source of ignition or self-ignition conditions could create an ignition risk. On the other hand, in personal cars, a sudden leakage in case of collision is usually associated with the collapse of the cabin and the consequent ventilation through the broken windows; therefore it is considered a lower and acceptable safety risk for mobile air conditioning systems. A side-by-side “drop in” comparison of three state-of-the-art low-GWP 2L flammable fluids (GWP ranging from 200 to 400) was conducted as part of AHRI Alternate Refrigerant Evaluation Program (Kopecka, 2013). The evaluation showed that although these 2L flammable fluids were not suitable for retrofit in the field (drop in), they did show potential for increased efficiency and capacity with respect to R-404A. System manufacturers are also testing low-GWP HFCs to assess the impact of other concerns such as stability, risk of clogging valves and expansion devices and other aspects. Recent studies identified various non-flammable blends that offer different combination of GWP (ranging between 1300 and 2200), ease of retrofit, properties, performance and consistency with R-404A. These blends have important benefits: they are non-flammable (A1), and in some cases they can be retrofitted in the field. The main limitation is the GWP: significantly lower than R-404A, but still higher than so-called natural or unsaturated HFC (HFO) refrigerants. The list includes and is not limited to R-407A, R-407F, R-448A, R-449A and R-452A (Mota-Babiloni, 2014, Minor, 2014). In this group of non-flammable blends, R-448A and R-449A stand out as lower GWP (both approx. 1400) alternatives to R-404A, but have higher discharge temperatures, requiring system changes such as liquid injection in order to achieve comparable capacity, efficiency and reliability. This makes them especially suitable for new systems using liquid injection devices. On the other hand, R-452A has the GWP of approx. 2100 and it stands out as easiest direct drop, allowing it to achieve equivalent performance as R-404A without additional system changes. Overall, it is likely that each of these blends will have a role to play in the general move towards more environmentally friendly alternatives. While in transport refrigeration the majority of the attention is currently focused on R-404A due to the higher GWP level, several non-flammable, lower GWP blends have also been developed as alternative / retrofit to HFC-134a. Their GWP is in the 600-700 range, approximately 50% lower than HFC-134a. 6.2.1.4 Cryogenic systemsLiquid nitrogen or carbon dioxide open cooling systems entail low noise levels and low emissions of air pollutant due to the elimination of diesel powered refrigeration. This can bring benefits at the local scale, in addition to lower greenhouse gas (GHG) emissions in the use phase, and therefore of benefit for heavily populated areas. However, the energy required to produce the liquefied gases is substantial and therefore, if fossil fuels are employed during production, GHG emission would rise proportionally. Other options for reducing GHG emissions might be using alternative energy sources in production of liquefied gases such as off-peak nuclear or wind power or, in the case of nitrogen, ensuring that it is a by-product of oxygen production. Technologies currently under development include systems for recovering mechanical energy from the expansion of liquid nitrogen, leading to a potentially low-GWP nitrogen economy. Use of cryogenics systems will continue to be determined by the availability of liquefied gas charging stations (see a parallel with liquefied natural gas, or LNG, in some regions). Where available, cryogenic systems can provide an alternative to traditional diesel powered trucks and trailers on a case by case basis.
Eutectic systems generally require the use of a refrigerant to “freeze” the eutectic. Some applications use an external system to freeze the eutectic, and the vehicle can be regarded as “refrigerant free”. The external system has a different safety, space, etc. requirement and allows for deployment of R-744 or flammable refrigerants that may otherwise have constraints for application in vehicles. Technology in eutectics is also moving ahead, and some manufacturers are developing new “phase change” materials that offer high capacity/weight ratio.
HFC-134a has been the most often used alternative to HCFC-22 in new vessels for provision refrigeration. New ships may also use R-407C or R-407F for air-conditioning going forward, as it is becoming widespread in other sectors. R-744 in cascade with R-717 is used for freezing down to -50 °C (mainly with fish as the cargo). Some shipyards utilize HCFC-22 for small fishing vessels in Article 5 countries, but not for international commercial vessels. Large industrial systems can use R-717 or R-744 where it makes sense. R-717 systems are limited to ships that do not carry passengers but professional crew only (due to toxicity consideration), and ships with a relatively high refrigeration capacity. This makes it suitable for large fishing vessels. R-744 has been used as a refrigerant with own compressors (second stage in a cascade) or a secondary coolant being circulated by pumps. When used in the second stage, it can cover the temperature range between -10 and -50 °C. HFCs or R-717 is used in the first stage. Further, R-744 Refrigerated Sea Water (RSW) systems have been developed and may be used for replacing HCFC-22 systems to be taken out of operation. Again, these systems are limited to the fishing industry. Cruise ships use chillers mostly with HFC-134a and it is foreseen that they will continue with this as long as possible due to the safety of the refrigerant hence the passengers. In 2014, a large manufacturer launched a 50 Hz water-cooled centrifugal chiller that utilizes HCFC- 1233zd(E) as an alternate technology to HFC-134a. For commercial vessels and for the off-shore business there is a potential for HC-290 to be a substitute for HCFCs and HFCs as most of the fleet already have Zone 1 or 2 onboard. HC-290 can be used for air conditioning as well as for provision plants, but most systems will be indirect secondary systems using a heat transfer fluid. The classification companies have to approve a given refrigerant before it can be used onboard ships. The procedure of approval takes time and costs money and therefore only refrigerants requested by the owners are approved. This comes on top of the normal procedures on land. This also slows down the process of changing from one refrigerant to another.
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