In the large retail sector, the focus is increasingly on the optimisation, management and monitoring of the systems installed. In particular, the running costs of refrigeration systems account for over 50% of total energy consumption, as highlighted by many studies on supermarket management. Where and how to save, how to optimise, how to control and manage the loads are consequently important issues for both the industry and end consumers. As will be shown, current technology for the large retail sector that can ensure substantial energy saving, as well as environmental compatibility (direct and indirect) is industrially available, both in terms of accessibility and reliability and, above all, return on investment.
Optimisation of the refrigerant circuit is based on the assumption of being able to operate at maximum energy efficiency, in the different environmental conditions and loads that act on the system throughout the year, but also on the same day: take, for example, the differences in load inside the store between day and night. The heart of the system that provides cooling for foods is the compressor rack, which also consumes most of the power in the installation. The power input of a compressor is proportional to the difference between the suction and discharge pressure. Consequently, if this pressure difference can be reduced, immediate energy savings are possible, along with an increase in the efficiency of the thermodynamic cycle for the same results. Enabling the two floating condensing and suction pressure control functions it is possible to bring energy saving of 20% or higher when used with proportional electronic expansion valves on the refrigeration units. Floating condensing pressure control can be easily enabled by using an outside air temperature probe. The actual condensing pressure set point, therefore, will vary according to the environmental conditions, drawing advantage from temperature differences between day and night. The floating suction algorithm, based on how efficiently the showcases and cold rooms are operating, allows dynamic shifting of the suction pressure set point. The supervisor, acquiring all the information required from the showcases and cold rooms, manages the change in the compressor rack suction pressure set point according to the parameters set for every refrigeration unit, ensuring, at all times, stable and safe operation of the installation. The underlying and now consolidated concept is the search for the best operating conditions based on the conditions outside (temperature) or inside the installation (critical units and refrigerant charge).
As mentioned, these optimisation algorithms are most effective when proportional electronic valves are used. Indeed, these allow a wider operating range (pressure difference) than thermostatic valves, as well as constant and efficient optimisation of the evaporators on the refrigeration units, irrespective of the operating conditions, even when compared to PWM expansion valves. PWM expansion valves, in fact, adjust the flow of refrigerant by opening or closing an opening of fixed dimensions, while mechanical thermostatic valves are always sized for the most critical operating conditions, that is, in summer. Consequently, when the normal operating conditions change, PWM expansion valves cannot ensure constant and efficient optimisation of the evaporators. Vice-versa, stepper proportional valves adjust flow by continuous variation of the flow-rate.
The use of electronic valves and control of the condensing and suction pressures, as demonstrated in several cases in recent years, bring savings in the order of 20%. In particular, in one of the most recent installation at Conad Adriatico in Sulmona, management of the heating system was also implemented, thus maximising integration between condensing boilers and heat pumps. The flexibility and integration of the system also makes it possible to trace and log energy consumption for heating and cooling. Nonetheless, to minimise installation and retrofit costs, these algorithms can also be applied, with good results, to mechanical thermostatic valves, as shown by experiences with Tesco (UK), with proven energy savings of 7%. This result led to the retrofit of over 300 existing Tesco stores to implement the above optimisations. Another possible algorithm for managing the operation of compressor racks, which saves energy by optimising energy consumption and ensuring greater system stability, is the DSS algorithm (Double System Synchronization). This works by exchanging information between the various compressor racks installed, so as to avoid simultaneous peaks when starting compressors, and at the same time synchronises operation of the systems through continuous interactions to predict refrigerating demand and where necessary start the compressors in advance. To decrease the power consumption of compressor and fan motors, inverters can be installed (inverters or VFD): above all in winter, when the compressor rack is oversized, inverters allow greater stability in the condensing and suction pressure, avoiding continuous compressor ON/OFF cycles that affect the stability and correct operation of the entire installation, as well as increasing wear on the mechanisms that make up the compressor. The demand for heat recovery is also growing, used to increase the efficiency of the entire system (refrigeration, heating / air-conditioning). The heat from the gas discharged by the compressors can be used to heat or pre-heat the water supplied to the boiler, thus retrieving thermal energy that would otherwise be lost (wasted).
ENERGY CONTROL AND MANAGEMENT
The work and commitment that goes into system optimisation would nonetheless be useless if the results reached in the design and installation of the system were not maintained and monitored over time. The PlantVisorPRO supervisor system allows to check and control system energy consumption and performance, from showcases to compressor racks, up to the air-conditioning and heating systems, as mentioned earlier. The KPI module (Key Performance Indicator) integrated into the supervisor makes it possible to analyse the entire installation and identify any critical points, taking action where necessary. Given the significant quantity and quality of data and information that can be recorded, statistical analysis of temperature and energy consumption is both useful and easy to carry out. Consumption can be controlled quite simply using the Energy module available in PlantVisorPRO, which acquires data directly from energy meters. As a result, the instant, daily, weekly and monthly power consumption can be displayed, divided into areas and groups, for the entire store. These modules thus ensure control and guarantee performance.
System optimisation and best performance mean energy saving regardless of the strategy implemented to decrease power consumption. The strategic management of power consumption must indeed retain the optimisation and functioning of the technological systems used, while reducing energy requirements (increasing the efficiency of the system) or distributing energy usage over time so as to exploit the cheapest rates (consumption strategies). These optimisation and management methods can be easily developed and implemented in new systems, as electronic controllers and the related control software come with such logic already integrated. For example, the MPXPRO controller for showcases, with or without built-in electronic valve driver, features an algorithm for modulating the operation of anti-sweat heaters. This algorithm allows all display cases where condensate may form on the surface to modulate the power input of the heaters used to heat the glass (above dewpoint). Without such function, the heaters operate 24 hours a day, irrespective of the environmental operating conditions. For maximum precision, this function requires a glass temperature sensor on each display case and a temperature and humidity sensor to calculate the dewpoint. To reduce installation costs, the algorithm however allows the glass temperature to be estimated based on the temperature inside the case and other data shared by the entire system. Consequently, the ambient temperature and humidity values (serial probe) for the calculation of the dewpoint can be shared by the controllers. These two functions mean savings in the installation of probes and wiring. The algorithm, based on the difference between the latter two temperatures, modulates the operation of the heaters, bringing, as verified in specific experiences both in Italy and abroad, energy saving of over 35%. This result is not surprising when considering the fact that the showcases are designed to operate in different countries in Europe and around the world, that is, in a wide variety of climatic conditions.
WIRELESS: SIMPLIFYING ENERGY SAVING SOLUTIONS FOR SYSTEM RETROFITS
Even when renovating and restyling existing stores, many of these algorithms can be used as described previously, and indeed wireless solutions are available to save installation costs and minimise wiring, depending on the desired results. Carel has patented a wireless solution that, using ZigBeeTM Mesh technology, self-organising and regenerative, with a series of repeaters located in the focal points of the store, creates a layout that can exchange data and information with the supervisor. This system responds to three particularly interesting market requirements: – in existing systems, to enable logging and display of the refrigeration unit values only via a wireless link (logging for HACCP requirements). Very fast installation thanks to operation without mains power supply. – again for existing systems, however also enabling management of the refrigeration units (and consequently wiring some control units on the showcases), for the acquisition of the data required to implement the optimisation algorithms described above (floating evaporation and condensing pressure and anti-sweat heaters) – in new systems, to connect and supervise units (not only refrigerators) in points that are hard and expensive to reach with wired solutions. In one recent experience, in collaboration with Epta, a system for monitoring and logging the temperature of refrigeration units was installed in the store in just one day, without having to replace the electronic controllers and without requiring major renovation work and laying serial communication cables. The PlantVisorPRO supervisor can monitor and print the HACCP data for each individual refrigerating showcase and create daily/monthly reports in PDF format.
CO2 AND EXPERIENCES
Carel, as well as searching for solutions that optimise system efficiency, is constantly looking for new technologies and solutions to reduce the direct and indirect greenhouse effect. In this regard, one important aspect involves the optimisation and management of systems that use carbon dioxide as the refrigerant. The greenhouse effect is contributed to directly by leaks of refrigerant into the environment, and indirectly due to the production of electricity for the operation of the system throughout its working life. Over 50% of the greenhouse effect is due to direct sources , hence the importance of finding natural fluids to replace the current HFCs while ensuring the same energy efficiency in the installation, so as to avoid increasing the indirect effect. One of the simplest and best known solutions used on the market involves the use of CO2 in cascading, allowing operation in subcritical conditions without any problems of availability of materials resistant to high pressure. Conventional cascading solutions are now practically market standards for those who wish to implement the CO2 solution with a good compromise. Other solutions proposed on the market to minimise HFC usage include one designed by Crea SpA, which uses CO2 as the cooling carrier fluid and limits the HFC refrigerant charge to the local units. The solution proposed combines a chiller operation on R404a, with pressure control, which condenses the carbon dioxide pumped to the medium and low temperature units, as illustrated in the diagram. The same Case Study also presented a comparison between the consumed by this system and others currently available in the market. The conclusion was that the solution proposed should also contribute to reducing the indirect greenhouse effect . A similar solution to the one described above has been adopted at the new Bitzer laboratory and training centre in Brazil. At this laboratory, Bitzer has installed the best technology available on the market. In the laboratory, as well as a solution that uses CO2 in subcritical operating conditions, there is also a dual technology system that uses electronic or mechanical thermostatic valves installed in parallel upstream of the evaporators. By installing energy meters and integrating these into the supervisor, energy consumption can be compared between the two systems.
The use of CO2 in transcritical operating conditions nullifies the direct greenhouse effect. This however involves completely different management of the thermodynamic cycle, in the attempt to ensure maximum optimisation and avoid increasing the indirect greenhouse effect.
Carel has worked on two transcritical installations: – The first transcritical installation in Australia, supplying the controllers for the compressors, for all the refrigeration units and the electronic valves in the cold rooms; – The second in England, a project managed by Space Engineering for Tesco, where Carel developed software for complete management and optimisation of the installation, integrating it:
o Optimum gas cooler pressure management
o Heat recovery
o Pressure control valves
o Oil cooling
The transcritical solution is taking hold in northern Europe, given the lower average annual temperatures (some working days in transcritical operating conditions): this means less time needed to manage the transcritical cycle, which currently is not as energy efficient as subcritical operating conditions. On the other hand, increased knowledge of the properties of this fluid simplify operation and routine maintenance.The information shown here demonstrates that there are existing installations which are already saving energy through consolidated optimisation strategies. Carel continues to invest in research and the optimisation of the entire control system, anticipating new technologies, testing eco-sustainable solutions that reduce system running costs without neglecting the practical aspects: return on investment, usability in the sector, reliability. The results achieved and the effective market response will encourage us to keep moving in this direction.