## 2 Research in Sustainable Energy Systems at DTG: Main Results and Discussion
### Energy and Economic Analysis of Direct and Indirect Evaporative Cooling
The authors considered two cases: An eight-direct expansion air conditioner plant (120 kW cooling capacity) located in a telephone control room in Bologna (Italy) [36]), and a supermarket located in a resort in the north Tirrenic coast of Italy [37]. Figure 14: Free cooling as a percentage of total cooling and ventilation loads using different technologies; increase of cooling energy savings by IEC_SS and IEC_DS technologies (IECs) with respect to DEC; increase of cooling energy savings by all evaporative cooling (ECs) technologies (DEC, IEC_SS, IEC_DS) with respect to HR. In both air conditioner [36] and refrigerating cabinet [37; 38] case studies, the research featured both an experimental and a simulation part. In the former, both electronic and thermostatic expansion valves were assembled. To compare the two systems at similar load conditions, TEV and EEV were activated alternatively on a daily basis. Furthermore, a transient simulation program allowed an annual analysis of the behaviour of the systems in three European climates to quantify the economic and energy advantages of EEV in two different operating modes. The main results obtained by the simulation models, based on the experimental data, revealed that when using EEV the consumption was substantially lower than TEV; the conditions of the outer air mainly influenced the consumption (Figure 15). Energy savings with EEV were in the range of ten per cent, depending on the external air temperature and the month of the year. In fact, the low condensation pressure allowed significant savings in winter. Furthermore, part of the savings was due to the fairly constant sort, for the high internal heat gain in the air conditioners of the telephone control room, Figure 15, or the less variable sort, for the refrigerating cabinets in the supermarket, Figure 15, of energy consumption throughout the year for the TEV technology. As a matter of fact, lower condensation pressures were allowed by EEV in systems with air-cooled condensers. Furthermore, the refrigerating capacity was also increased by Proportional-Integral-Derivative (PID) control on the superheating, which allowed the optimization of the use of the evaporator due to lower superheating. After the analysis of EEV advantages, the authors' research work focused on supermarkets [39; 40; 41; 42]. In a first approach, the impact of cycling losses was evaluated in refrigeration and air conditioning units [39]. Cycling losses reduce performance during the transient period due to the on-off control of a unit. The authors experimentally evaluated the advantages of the variable speed capacity control strategy of brushless compressors (BLDC) compared to the on-off strategy. First, the operation of a traditional chiller with TEV was tested between the \"cycling mode\" and the \"continuous mode\". The different energy efficiency ratio (EER) was due to cycling losses, which resulted in up to 13%. Lower load factors increased penalization. Installing a solenoid valve in series with the TEV allowed improved performance when the load factor fell below 30%, but with still not negligible cycling losses with low load factors. As a final step, the best solution turned out to be equipping the chiller circuit with an EEV: For all load factors, this allowed losses to be kept below 6% (Figure 16). Figure 15: (**a**) Monthly electrical energy savings; (**b**) compressors’ specific electrical power consumption concerning the low temperature circuit. The research of the authors was successively focused on the whole refrigeration plant of a supermarket. Research focused on water-loop self-contained (WLSC) refrigeration plants, due to the positive characteristic that they reduce the required amount of energy and refrigerant, therefore direct and indirect CO2 emissions, respectively [40]. In WLSC systems, the condenser and the variable speed (BLDC) compressor equip each cabinet that rejects thermal power to a cold water loop. The latter is cooled by a dry cooler or a chiller (Figure 17).