## 2 Research Methods  
### Test Stand  
The technological challenge was to develop and design prototype external bodies for refrigeration and freezing modules. The role of the body was to provide protection of the refrigeration system elements from the influence of external factors, as well as from their destruction. The work has been carried out in the following steps: selection of an insulator, sealant, and lining material for the inside of the box; the construction of the cabinet bodies, taking into account individual components (their location); execution of the bodies (filled with a specific insulator) with the required physical properties. The development of the systems for cooling and freezing modules, as well as the execution and verification of their operation took place in the following stages: first, the cooling systems (including condensate drainage) with automation and control and the electrical installations for the cooling and freezing modules were designed and executed; next, their operation was verified in model conditions and in real environmental conditions. Prototypes of the bodies of modules (cooling and freezing) of the MainBox device were built of stainless steel with space inside for cooling and other necessary installations, including measuring instrumentation and operating the customer's access system to a given box (Figure 1). Cooling and electrical installations with measuring and control instrumentation were located in the upper part of MainBox body. The module consisted of 5 chambers (boxes) for food storage. Each box was opened at the front with a separate door. device located above the chambers and was cooled in the evaporator. The heat removed from the air was disposed of in a condenser cooled by external air. 3. Due to the fact that, in the chambers, a temperature of +5 \({}^{\circ}\)C had to be ensured and the whole system will operate at outdoor temperatures from \(-\)20 \({}^{\circ}\)C to +35 \({}^{\circ}\)C, an additional electric heater was used: - An additional electric heater on the evaporator with a power of 400 W, which, in combination with the standard defrosting heater with a power of 230 W, ensured heating of the chambers at negative outside temperatures. Both heaters were controlled by a thermostat located in the electrical box based on the temperature reading of the air returning to the refrigeration unit. - Temperature control in individual boxes was correlated with the return air temperature to the chiller, because the air returned to the chiller through a common return duct. This was averaged over all boxes. Based on this temperature reading, the module's local controller turned the chiller on and off. Temperature sensors have been installed at all air outlets from the boxes to prevent a temperature rise in individual boxes. If the return air temperature from any box exceeded the upper setpoint, then the master controller forced the cooling of the entire module regardless of the temperature reading in the common return duct. Once the temperature was reduced to the setpoint, the master controller removed the forced chiller operation signal and the unit returned to local controller operation based on the return air temperature. - Adequate circulation of cooled air was ensured by the appropriate amount of cooled air flowing into individual boxes to obtain the same temperature in each of the return air streams with an accuracy of \(\pm\)1 \({}^{\circ}\)C. - In order to eliminate the risk of the door freezing to the casing due to the appearance of water or frost on its edge, an appropriate heating installation was installed in the place of door contact. 4. The refrigeration unit was filled with the natural refrigerant (propane--R290), due to its following advantages over typical synthetic refrigerants such as R134a, R452a, and R404a: - GWP--greenhouse effect potential for propane is 3, where for R134a it is = 1430, for R452a = 2141, and for R404a = 3922.