The new requirements for reducing CO2 emissions due to refrigerant and electricity consumption are leading retailers to consider other alternatives to traditional centralised systems. One of the preferred options is the use of plug-in and semi plug-in units, and these are indeed being increasingly used worldwide in different types of supermarkets, especially medium-sized stores.
The first proposals for self-contained cabinets, first with HFC refrigerants such as R-404A and later with HFOs such as R-448A, brought numerous advantages in terms of decreasing CO2 emissions. These included a reduced refrigerant charge and less refrigerant leaks when compared against centralised systems with long copper pipes.
Now a further step forward has been made, with the adaptation of this solution for use with natural refrigerants. In particular, R-744 (CO2) and R-290 (propane) are the refrigerants that are mostly offered in self-contained units.
What are the differences between plug-in and semi plug-in units?
Plug-in units have air-cooled condensers, whereas semi plug-in units have water-cooled condensing units. The advantage of using water-cooled condensing units is that the heat of condensation is carried away by the water loop, thus avoiding an increase in temperature inside the supermarket, and indeed this can even be used for space heating and domestic hot water production, bringing benefits for both the unit and the entire building. On the contrary, air-cooled plug-in cabinets are more flexible, due to the absence of plastic pipe connections, however they dissipate the heat into the supermarket. This means an extra cost for air-conditioning in summer, with some savings on heating in winter.
In both cases, plug-in and semi plug-in, installation is fast because the cabinets are usually “ready to use”, with only the plastic pipe connections needed in the field in the case of semi plug-ins. Additionally, the critical components such as the compressor are already contained in the cabinet, making it easier to install and commission. This also makes their use more flexible, allowing the cabinets to be moved around inside the supermarket at any time.
Reduced maintenance and consequently lower costs is another key feature of both plug-in and semi plug-in units. It is also important to note that these self-contained solutions free up the space typically occupied by the compressor racks in a traditional system, with the advantage of having a larger sales area and avoiding the noise and vibrations usually produced in the equipment rooms. Moreover, the roof of the supermarket has an improved aesthetic appearance, with no condensing units, something that is especially important if the shop is located in a historic city centre.
As regards direct CO2 emissions, the reasons why self-contained units have a lower impact on the atmosphere than centralised systems are quite obvious. On the one hand, no having long copper pipes and compressor racks means a significantly lower refrigerant charge. On the other hand, the fact that there is no welding in the field and the units are factory-tested substantially limits refrigerant leaks.
Continuous modulation for improved performance
The many advantages of plug-in and semi plug-in units for commercial refrigeration are further enhanced if these are equipped with modulating components combined with advanced control and monitoring systems. This increases efficiency and reduces indirect CO2 emissions, among other benefits.
Modulating components such as DC compressors and electronic expansion valves managed by advanced electronic controllers allow the units to always operate in the best conditions. On the one hand, DC technology means more stable parameters and increased reliability with minimum ON/OFF cycles. On the other hand, electronic expansion valves ensure optimal superheat control and synergy with envelope control. These modulating components are optimised when working with advanced control and monitoring systems, which allow full continuous control of the units with advanced algorithms, making it much easier to prevent failures.
By using advanced technology, the end result is maximum energy efficiency at part loads as well as optimum food temperature and humidity control. Furthermore, the fact that each cabinet is independent of the others gives another significant thermodynamic advantage over traditional systems: cabinets do not have to work at low evaporation temperature due to demand from the most critical cabinet, but rather the evaporation temperature is optimised for each individual cabinet.
How can the units be adapted for use with natural refrigerants?
The unique characteristics of natural refrigerants, such as the high working pressures of CO2 and the flammability of propane, mean some adaptations are required when proposing a system.
CO2 is a refrigerant that is well accepted on the market, with optimal heat transfer, and technologies that use this gas are constantly being improved. Lately, efforts have been focused on increasing efficiency in warm climates: the low critical temperature of CO2 (31.1°C) means that the system works in transcritical mode when that temperature is reached, and this has to be taken into account when designing the system.
The first proposal for CO2 consists of single circuits for both medium and low temperature cabinets. A chiller allows low temperature cabinets to work in subcritical mode, while the medium temperature cabinets work in transcritical mode. The chiller starts when the ambient temperature is higher than 20-25 °C, ensuring subcritical mode for low temperature cabinets.
The second proposal for CO2 involves double-stage circuits for each cabinet. The high stage works at high temperature (transcritical), in order to keep the low stage working in subcritical mode.
These two CO2 proposals are suitable for semi plug-in units with a water loop system.
Propane is a highly-efficient refrigerant that operates at standard working pressures. However, its high flammability requires precautions to be taken in the design of the system and compliance with specific requirements for flammable refrigerants defined by the standards. Currently, the charge limit of 150 grams for commercial refrigerating appliances specified by IEC 60335-2-89 makes widespread use more difficult. The update to the standard that will probably be approved by the end of this year, increasing the charge limit of propane to 500 grams among other modifications, will simplify the use of propane in commercial refrigeration.
For smaller loads, or when the limit is increased to 500 grams, propane plug-in and semi plug-in units can be made with a conventional single circuit, for both medium and low temperature cabinets. Compliance of the components with IEC 60079 (standard for explosive atmospheres) will facilitate approval in accordance with IEC 60335-2-89.
For higher loads, it is possible to design a multi-circuit system with a common evaporator and condenser. The typical configuration is currently three circuits of 150 grams each.
With all of these options, end users can decide which of the advantages best fit their own needs. The use of natural refrigerants for self-contained cabinets demonstrate once again that high pressures, low efficiency in warm climates, flammability and reduced refrigerant charge are not an issue, if managed correctly.
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