LOW-TEMPERATIRE DELIVERY BODIES.
THE COLD CAR “ECO2 REFRIGERATED BODY”
The report relates the interesting ( and surprising) results of the common study that was carried out by Iarp and Cold Car, in collaboration with Prof. Petter Neksa from Sintef , on the low-temperature delivery bodies that use two-stage transcritical cycleoperating on R744. Herein we reported the different steps that led to results of performance and efficiency (COP) that are considerably better than the current HFC-R507A systems.
The solutions regarding new-conception eutectic plates and fast-coupling systems for CO2 are an issue of great interest.
The system.
Unlike transport vehicles, the vehicles intended for the distribution of perishable foodstuffs at a low temperature (picture 1), perform a daily duty cycle that is characterized by a high number of door openings. As a consequence specific solutions are required whether they be structural solutions ( they are often equipped with a high number of side doors, picture 2) or thermodynamic(picture 3). Actually the traditional systems operating with ventilated evaporator cannot ensure sufficient performance because of frost clogging of the evaporator that is due to the continuous air inlets inside the body during the numerous door openings. Consequently, the optimal solution lays in refrigerating plants with cold energy accumulation into eutectic plates or beams (picture 4) and natural (not forced) circulation of the air.
The feeding of the system is exclusively electrical (picture 5) and the refrigerating unit mainly works for the freezing of the liquid when the vehicle is parked into the warehouse, by storing most of the refrigerating energy under the form of latent heat that gets released even during the service time.
Split systems have recently been developed in which the motor-condensing unit is separated (picture 6), while the throttling valve and the evaporator (inside the eutectic plates) are on board of the vehicle. The circuit system provides for the use of the same refrigerating gas for both the high and the low pressure sectors that are connected together by means of flat face couplings (picture 7), that ensure a low level of loss (0,00002 gr) at the moment of the connecting and of the disconnecting but also during the connection and the disconnection.
This last solution has undeniable advantages:
-reduction of the vehicle’s unladen weight and consequent increase of the payload (of 120 Kg and over)
-possibility of service on the motor-condensing unit (high efficiency and reliability) without stopping the vehicle.
-possibility to realize multiple power refrigerating units in order to feed more vehicles in parallel, and saving of money and operations ( higher efficiency).
The idea.
The initial idea to use R 744 came out from the requirement that, in this kind of split systems, refrigerants leakages - even if very small – can be considered to have a negative impact on the environment and as a consequence of that some Countries were introducing restrictive rules to these systems using HFC as refrigerant.
The attention drew on natural gas having zero ozone depletion potential (ODP), and a limited global warming potential (GWP).
Once the hydrocarbons were rejected ( HC ) for obvious reasons of safety in the expected quantities for each installation (over 3 Kg), the CO2, R744 left.
The goal of the program was to check that the efficiency of the system (COP) with the R744 be at least the same as that of the HFC, in order to obtain a lower TEWI value.
A feasibility study by prof. Neksa ( Sintef), showed that in a two-stage cycle (picture 8) the efficiency of the system, that gets measured using the COP (Coefficient of Performance) parameter, would be equal or superior to that of the systems using HFC (R 404A or R 507). As a consequence, it came out that both parameters that enable to obtain the TEWI (COP and GWP) were favorable to the use of the R744.
Indeed, on the theoretical point of view, the R 744 already presents interesting characteristics at the required evaporation temperatures (-45°C). Moreover, in that specific case, the two-stage solution does not seem to be particularly prejudicial as regards the expected costs, as applications having such levels of power already require the use of compressors of at least two-cylinders (picture 9).
An other important consideration in favor of the R744 comes from the fact that the market of such applications, and more particularly Cold Car’s customers, stands at the international levels and also concerns the developing areas in which the R744 gas can be easily found at low prices and in small packaging (since the gas that is used in bars for the production of sparkling drinks can be used) with the required purity level.
Issues.
If we analyze the cycle on a graph featuring pressures (P) – enthalpies (H) (fig.8), some patterns immediately come out:
- If the delivery pressures are not controlled in a suitable way, then they will reach values that are close to 120 bars at the pull-down stage.
- The pressures of the whole system, when it is not working, can reach values that are higher tan 70 bars..
- The crown of the bell-curve in the graph as in subject is slightly below 30°C, as consequence there is no possibility of condensation over such a value.
In practice, such a situation presents two types of problems:
-Retrieval of ad hoc components or adaptation of others in a way to be suitable for such a use.
-the safety of the system at pressure levels about 7 times higher than those of the previous systems using HFC.
The high-pressure installation.
Pressures of gas-phase fluids that are superior to 100 bars and that can occur in the high-pressure part of the installation, require special design of the parts, particularly where the safety problems are more important.
The piping in general does not get seriously affected as the high density of the corresponding fluid generates low mass volumes and as a consequence, pro rata low volumetric capacities. From the P-H graph (picture 8), in the part where pressure is high, the mass volume is only 0.020 m3/Kg. Moreover, obviously the possible drops of pressure during the cycle do not have important consequences as it could be the case with other fluids. As a consequence, in the case as in subject, it is possible to use low-section tubes having a diameter of only 6-8 mm and a thickness of 1 mm, i.e. that can be easily found on the market.
Other elements such as the Gas Cooler (picture 10) that replaces the “old” condenser, were modified in a substantial way. Cold Car chose not to use the “micro channel”- type gas cooler as extremely small spacing would require a particular attention as regards cleaning. However elements deriving from traditional condensers, manufactured with lower diameter tubes (outer diameter of 7.2 mm) and higher thickness were chosen.
The expansion valve.
The expansion system with back pressure valve required a substantial work of setting-up that is mainly due to the unavailability of such valves at a low price and that can expand from 60-100 bars to 7-10 bars ( - 48°C / - 40°C). The chosen solutions using valves with control in the high-pressure area, limited undesired peaks at the pull-down stage, during which the delivery pressure remains under 100 bars.
The low-pressure installation. Eutectic accumulators.
As regards the program of Cold Car, the most compelling part of the project was concentrated on eutectic accumulators (picture 12), as the safety issues are obvious.
Actually, while the expansion pressure does not exceed 10 bars during the running, the pressures are worth about 50-60 bars when the unit is stopped and the evaporator is at ambient temperature.
In the traditional plates(picture 13), the evaporator is made of a coil, placed inside the plate and directly in contact with the eutectic solution. The current technology also requires welding between the different crop ends of the coil, In the case of CO2, this represent an unacceptable risk: actually if gas leaks from the evaporator, the whole plate will be affected by the pressure increase. The total load of the system is sufficient to fill the whole inner volume that, when the tank is deformated, reaches 150 dm3, with a hypothetical pressure of 50-60 bars. This does not occur because the eutectic plate simply bursts when lower values are reached, with risks and important damages.
Consequently, Cold Car developed special plates with external evaporator (picture 14), in contact with the area of the plate through its particular surface (patent pending). Any leakage from the plate gets dispersed in the ambientand does not enable to raise pressure into the plate. The particular shape of the contact surface of the plate enables to maintain an important exchange value. As a consequence the system performance do not get reduced but increased.
The coupling of the split systems.
Cold Car had already developed very reliable and functional solutions of split systems for R507A. The acquired experience on hundreds of such applications has enabled to develop a specific split system for CO2 (picture 15), which most considerable particularity stands in the connection coupling (picture 16), equipped with new flat faces coupligs, that were dimensioned for the involved pressures and mainly for the return temperatures that are definitely lower (the system works down to -50°C).
The results.
The pre-defined objective was to obtain a COP that would be at least the same as that of the systems with traditional refrigerating gas (R404 and R507) in the average operative conditions of the system, that were set at ambient temperatures of 27°C ( the working of eutectic system units mainly occurs at night, as it works for the distribution cycle during the day) and to freeze a solution having a eutectic point of freezing of -33°C .
The efficiency comparison with the R507A systems was carried out in the most simple and reliable way, by measuring the freezing time of the solution of the two systems , that was exactly of the same amount ( 124Kg), and maintaining the same electrical input power.
As you can see on the graphs (pictures 17 and 18) the freezing time considerably goes down, going from 6h 33’(R507A) to 4h 40’(R744), that means an increase of 31.5% of the COP of the System.
Conclusions.
The obtained results clearly prove how the R744 represents a valid replacement to the HFC in this application and has important economic returns on operations thanks to a considerable energy saving.