For tens of thousands of convenience stores and supermarkets in Europe, the time has come to modernize their commercial refrigeration systems. It's a chance to reconsider systems with an eye to newly available technologies and return on investment. The best condensing unit technology currently available was installed in a supermarket in Osnabrück for this purpose.
Gastronome is a 620 m² supermarket with 2 cold rooms, 4 12-meter covered, horizontal display cabinets for meat and fish and 4 other 14-meter vertical display cabinets for various fresh products.
Until October 2014, each of these units and the cold rooms ran on an independent cooling circuit with an evaporator and condensing unit in a machine room, using R404 and R22. This had the drawback – especially in the summer months – that the waste heat from the condenser of the display cabinet was discharged directly into the retail space. They are now powered by two variable-speed Optyma™ Plus INVERTER condensing units using R407F located out of the building. According to the German F-Gas Regulation, R407F may – unlike the R404A refrigerant that was used previously – also be used beyond the year 2020, because its GWP value (Global Warming Potential) is significantly below 2500.
Each of the two Danfoss Optyma™ Plus INVERTER units is set to a dedicated suction pressure, meaning that the associated cold room and display cabinets are able to operate at their optimal temperatures. Supermarket manager Viktor Jersch has noticed multiple benefits. The installation is more compact and we have low-cost operation and maintenance. New display cabinets can be added without the need to resize the refrigeration system: both condensing units can cool a larger surface area.
Description of the Installation
The second refrigerant that was used in the past, R22, is already blacklisted. One particular advantage of the installed condensing units with integrated speed control for the compressor and the condenser fan is the flexibility in terms of being able to adapt immediately to different loads. As soon as the solenoid valve closes the circuit of one of the refrigeration appliances which no longer requires cooling, the condensing unit registers this fact with reference to the falling suction pressure. The condensing unit records this information through the integrated sensor system – two suction pressure transducers for low and high pressure, and three temperature sensors. In so doing the condensing unit adjusts the speed of the compressor. In this case the speed is reduced. This causes the suction pressure to rise again, meaning that it always remains constant. The effect is the opposite if a refrigeration appliance is switched on: the suction pressure increases, the compressor runs faster (30% to 100% speed is possible) and constant suction pressure is achieved once again.
Based on electricity consumption figures for September 2014 and 2015 - which also include lighting - the supermarket has been able to save 15% on its energy bills, equating to 5000 euros per year, meaning a return on investment in 3 years, while refrigerated shelf space has increased. The variable speed of the permanent magnet motor and fan control of the Optyma™ Plus INVERTER are the contributing factors in these energy savings. It constantly and steplessly adapts the cooling capacity to the actual requirement – both with respect to the refrigeration appliance and to the ambient temperature. This also ensures that the required temperatures can be maintained in an energy-efficient fashion. In addition, one of the units is connected to a heat recovery system which simultaneously heats water for the store's needs (Figure 2).
The speed control of the Optyma™ Plus INVERTER provides a precise and stable evaporating temperature, allowing humidity and temperature management of ±0.3°C. Quality hits the spot for the supermarket manager: "The appearance of the products I display now is much better than before. Meat looks better, as the steady humidity and temperature mean it doesn't dry out. That is vital to keep my customers happy," explains Viktor Jersch.
Servicing and maintenance of the condensing units are also easier and less expensive. The number of condensing units has been reduced from 5 to 2, so servicing costs and times have been cut accordingly. Naturally Mr Jersch has his systems inspected for leaks and maintained annually. This testing now costs less, as the price is proportional to the number of units to be tested.
There is also a significant time saving when it comes to servicing the easy to clean micro channel heat exchangers (MCHE). The coil can be cleaned and decontaminated quickly and easily from the inside using one of the service doors. For this type of condenser, cleaning is performed in the direction opposite to normal airflow, as dirt adheres to the air intake and does not make it far into the microchannel, unlike finned copper tube condensers. A clean condenser coil is a prerequisite for the refrigeration system to operate properly.
Outdoor use allows for a further improvement in terms of running and servicing the condensing unit. The system's natural ventilation means that the exchangers release more efficiently the heat and are easier to service. The waste heat is released directly into the open air, rather than first being released into the retail area and only afterwards pumped outside by means of an air conditioning system. This is energy-efficient. In addition, a condensing unit placed on a flat roof is simpler and more economical to maintain than installing it in a refrigeration unit.
Lastly, the Optyma™ Plus INVERTER has 3 access doors for easy settings and servicing. (Figure 3 and 4)
Finally, the new system is much more compact and being installed outdoors allows for better ventilation (Figure 5). "The Optyma™ Plus INVERTER units run very quietly, which means they can be used outdoors and near residential areas. They very rarely run at night and rarely at full speed. You can only hear them if you stand right next to them. Up to now, I've seen an improvement on every level," explains Viktor Jersch.
"In this project, we moved one unit on the roof of the supermarket in order to test oil return in the system. Variable speed means the motor runs at low speed, so we programmed the unit to restart every 3 hours to ensure oil return in the system. The pilot projects installed in Europe have enabled us to test and prove the reliability of our new Optyma™ Plus INVERTER range," confirms Andreas Harke, accounts manager at Danfoss.
