The supermarket REWE in Dortmund-Hörde is banking on a particular kind of freshness: not just in the food it sells, but also for the air we all breathe. Instead of chemical refrigerants, the supermarket’s refrigeration appliances are now using only natural CO2 (R744) as a refrigerant. This ensures a significant reduction of greenhouse gas emissions: the refrigeration system reduces harmful emissions by 110 tonnes each year. The operators have additionally integrated an air-conditioning system and a heat pump to heat the entire complex, comprising approximately 1950 m² of retail space and a number of adjoining areas.
The system supplies the refrigerated display cases and lockers, the deep freezers, the heating system, the hot water system and the air conditioning system for the retail space. The design concept is the first of its kind in Germany. In fact, there are currently only four such systems installed worldwide. Schrezenmaier Kältetechnik implemented the concept by installing a CO2 refrigeration system with components from Danfoss.
A refrigeration system of this type must comply with the highest safety standards. For instance, gas alarms had to be installed in the machine room and in the cold storage room to warn of any asphyxiation hazards. Another aspect that the operator had to take into consideration during planning was the system’s high operating pressure. CO2 condenses at a higher pressure than conventional refrigerants. The pipework and components must therefore be able to withstand pressures of up to 120 bar. Conventional copper pipes are not approved for such pressures, meaning that the system requires pressure-resistant stainless steel pipes on the high-pressure side.
In refrigeration systems without speed control, the compressor is constantly running at full capacity, and its only operating states are on or off, regardless of the cooling capacity actually required. An expansion valve always tries to fill the evaporator optimally by design; such compressor cycling brings about a change in evaporating pressure, thereby causing the system to oscillate. The evaporator is never properly filled, and operates inefficiently. The consequence is suboptimal refrigerant cooling capacity. Accurate control of the compressor using a frequency converter to adjust the speed increases the system’s stability and performance, as the evaporator is filled optimally at all times. The evaporating pressure does not change as a result of such control, meaning that it is able to harness the refrigerant’s cooling capacity in the most efficient manner possible. This is why REWE supermarket installed a Danfoss VLT® Refrigeration Drive FC 103 to control the compressor that serves the refrigerated display cases, and an additional FC 103 unit to control the deep-freeze area.
The FC 103 is specially designed for the control of refrigeration equipment drives. Data on commonly used refrigerants are preloaded in the converter. The user simply enters the evaporating pressure temperature setpoint using the control panel. It is also possible to configure a user-defined refrigerant. In addition, thanks to automatic setpoint toggle between daytime and nighttime operation, the VLT® Refrigeration Drive can significantly reduce energy consumption, as lower speeds are required at night.
The AK-PC 781 compressor rack controller ensures efficient heat recovery. The main function of the controller is to control the compressor and the condenser in such a way that they operate under the most energy-efficient pressure conditions possible at all times. The voltage signals issued by the pressure transmitter enable exact control of both suction pressure and condensing pressure. The controller is equipped with oil control, a heat recovery function as well as CO2 gas pressure control. The compressor rack controller controls the output of up to eight compressors. In addition, it can handle safety monitoring of high pressure, low pressure and pressurized pipe temperature. In addition to capacity regulation, the controllers can transmit signals to other controllers about predefined operating conditions, e.g. the forced closure of expansion valves, as well as alarm signals and alarm messages.
The major benefit of this series of controllers is that they are easy to expand along with the plant. Modules of the same type can be assembled in the compressor rack controller as needed, allowing the operator to optimally tailor the system to his needs. Up to four external displays can be additionally connected, with and without operating keys. They can display various measuring values, such as suction pressure, condenser pressure or heat exchanger temperature.
