Electronic Expansion Valves Versus Thermal Expansion Valves

Storage of cold and frozen food accounts for approximately 40-50% of the electricity used in supermarkets. Open vertical display cabinets are common and are also large users of electrical energy. Many refrigerating machines, for both air conditioning and refrigerating applications, use thermostatic expansion valve (TEV). TEVs are by far the most widespread expansion device, but they reveal some characteristics that can limit versatility and performance of the machines. For example this valve requires a minimum pressure drop between condensation and evaporation. This prevents possible advantages of low condenser pressure for air cooled condensers. Moreover a minimum superheating must be provided to avoid possible hunting of the valve. Some kinds of plants are more sensitive to negative aspects of TEV regulation, because of plant specifics, kind of duty or distribution of cooling load during the year. Refrigerating machinery in supermarkets is just a characteristic example. One solution to the defects of TEV is the electronic expansion valve (EEV). This electrically driven control device has been studied experimentally and theoretically in recent years and it is now widely available on the market; it controls the refrigerant flow at the evaporator by means of a pressure and a temperature sensor, both at the outlet of evaporator. The two signals are elaborated by a regulator that control, in real time mode, the opening of the valve. To evaluate the possibilities of using EEV against TEV a large supermarket located in the Tirrenic coast of Italy, not far from Pisa, in the North Italy, was retrofitted with the EEVs installed in parallel to the TEVs, in order to operate alternatively the plant with the two technologies. A simulation model was developed to compare the two technologies on an annual basis from the energetic and economic point of view under different conditions. The comparison was made for three different climates, Milano, Roma and Trapani (for which hourly Test Reference Year data are available), to investigate the behaviour of the innovative system varying the condensation conditions.