【正文】 ter 0–3 kW ^%Electric energy meter 360–420 V。P_ExoutP_Exin188。co222。ev222。airisproperly evaluated for the evaporator and the condenser.The exergy flow destroyed in the pressor, neglecting theheat transfer with the environment, is evaluated as:_Exdes。5222。6222。In Fig. 9 a parison in terms of the exergeticefficiency of the whole plant, when the R407C andthe R507 are used, is reported versus the frequency of thecurrent feeding the pressor。 1 2Xidi240。va222。254。4222。exin。exin。=dt ms l l ms h vhh msmsh hvhFig. 3. Membership function of the temperature difference betweenthe setpoint temperature and the real temperature of the air in thecold store.C. Aprea et al. / International Journal of Refrigeration 27 (2020) 639–648 6435. Test results and discussionSeveral experimental tests have been conducted toexplain the energy saving obtainable with the fuzzyalgorithm in parison with the classical thermostaticcontrol, that determines the on/off cycles of the pressorthat works at a nominal frequency of 50 Hz. To simulatebetter the real working conditions of the cold store, varioustypes of cooling loads have been considered. In particular, inthe experimental tests either the electric heaters or the fruitsand vegetables have been adopted as cooling load. Moreover, a further load results to be both due to the periodicopening of the cold store door and also due to the inevitableheat exchanges with outdoor air when the cold store door isclosed. In Fig. 6 a parison in terms of electric energyconsumption, measured by means of a proper electricenergy meter, between the control on–off realized by theclassical thermostat and the pressor speed continuouscontrol obtained by the fuzzy algorithm, is reported whenthe cooling load is due only to the periodic opening of thecold store door. The experimental tests have been realizedfor cold store air temperatures fixed at 25, 0 and 5 8C andfor a constant cooling load obtained opening the cold storedoor every 20 min for about 5 min with an outdoor airtemperature of about 18 8C. One can clearly observe that theenergy consumption increases when the cold store airtemperature decreases. This is due to the fact that a constantcooling load has been considered for all the cold store airtemperatures and so the time necessary to reach thetemperature of 25 8C will be greater and will determine ahigher electric consumption. Moreover, it is possible toobserve that the energy saving obtained with the algorithmwith respect to the thermostat is on an average of about 10%,even if it clearly diminishes slowly when the cold store airtemperature decreases because under this circumstance theworking time of the pressor increases.In Fig. 7 the electric energy consumption of thepressor obtainable with two control systems, related tothe R507 and R407C, is reported both for the summer andfor the winter, when the cooling load is due both to theperiodic opening of the cold store door and to the presenceof the electric heaters. In these experimental tests related tothe electric heaters it has been considered an electric powerconstant of about 200 W. It has been observed that the bestperformances are related to the R407C that allows, with acontinuous control of the pressor speed, an mediumenergy saving of about 13% in parison with thethermostatic control for both the outdoor air temperaturesconsidered. In particular, the absolute electric energyconsumption in the summer season is about 5% higherthan that of the winter season even if the energy saving inthe two seasons is practically the same. In Fig. 8 the energyconsumption related to a real cooling load, represented by200 kg of fruit and vegetables and by the periodic opening ofthe cold store door, is considered when both the fuzzycontrol and the thermostatic control are used. In addition, inthis situation the highest energy saving is obtainable bymeans of the fuzzy control with the R407C and is of about13% with respect to the one obtained by means of thethermostatic control. Selecting a value of 10 8C for theoutdoor air temperature, the results in terms of energysaving are practically the same. It is important to note thatthe fuzzy control system allows to reach the temperature ofthe air needed in the cold store and to maintain its oscillationFig. 4. Membership function of the derivative of temperaturedifference in the time.Fig. 5. Membership function of the pressor electric motorsupply current frequency.Fig. 6. Electric energy consumption for R507 using both the fuzzycontrol and the thermostatic control (cooling load ! periodicopening cold store door).C. Aprea et al. / International Journal of Refrigeration 27 (2020) 639–648644in the range ^1 8C。 thedefuzzification process permits to transform the fuzzyoutput into a defined value. The main difficulty of thefuzzy logic is connected with the necessity of a goodspecific experience in the design and the building of a fuzzycontroller. So, as for the regulating parameters someexperimental considerations have allowed us to set thecontrol variables of the reciprocating pressor speed. It isclear that the choices of the rules and membership functionsof the controller can be properly changed. However, it is tobe considered that it is certainly convenient to control fromthe energy saving point of view the pressor speedbecause it works at lower frequencies, but in this situationthe time required to obtain the setpoint temperature will beTable 1Transducers specificationsTransducer Range AccuracyCoriolis effect mass flow rate meter 0–2 kg/min ^%RTD 100 4 wires 2100 to 500 8C ^ 8CPiezoelectric absolute pressure gauge 1–10 bar。 and the derivative of this temperaturedifference with time 240。 fax: 254。 Vitesse variable。 Regulation。 received in revised form 18 December 2020。 R507Re180。 Log