# Energy Optimization in Hotels: Strategies for Efficiency in Hot Water Systems
## 3 Materials and Methods
### Case Study: Hotel Facility  
The facility under study consists of two water chillers arranged in parallel with a cooling capacity of 363 and 505 kW, respectively, using the refrigerant R134a as the working substance. The chillers contain air-cooled condensers, which are equipped with constant-speed fans.  
Each chiller operates with a refrigeration circuit that includes expansion valves and screw compressors. The installation is designed to recover the condensation heat generated in both chillers through a heat exchanger, which is connected in series with the condenser of each unit.  
The heat recovery is integrated into a system for the production of domestic hot water (DHW), which consists of two circuits interconnected through a plate heat exchanger. A general schematic of the system is shown in Figure 2, which illustrates the layout and operation of the system.  
The heat recovered from the chillers is transferred to the primary hot water circuit, where it reaches temperatures of up to 65 \({}^{\circ}\)C under nominal operating conditions. However, when the system operates at low cooling loads, an backup heater is required to raise the temperature of the primary circuit to 60 \({}^{\circ}\)C. This ensures that the water sent to consumers reaches the established value of 55 \({}^{\circ}\)C, in accordance with the standards required for hotels in Cuba.  
The most critical situation occurs during the winter months, when chiller activity decreases. During this period, the auxiliary heater must remain on most of the time, since the heat generated by the chillers is not enough to meet the thermal demand of hot water. It is important to note that the high tourist season in Cuba coincides with these winter months, which creates a scenario in which high occupancy, maximum demand for hot water, and low ambient temperatures are combined. These variables have a significant impact on the operation and efficiency of the domestic hot water production system.  
The backup system consists of a direct-fired heater, model Pegasus-F2-102, with a useful power of 99 kW and a water flow rate of 1.25 kg/s. The heater operates with liquefied petroleum gas (LPG), reaching temperatures between 60 and 80 \({}^{\circ}\)C and an efficiency of 85%. However, the hotel lacks a control strategy for the auxiliary heater, resulting in its activation only during peak hot water demand. During the rest of the time, the heater remains off, causing a decrease in temperature and generating instability in the system. In addition, LPG consumption is not recorded separately for the cooking services and the domestic hot water system. Figure 4 illustrates the average monthly LPG consumption, with November to March being the months with the highest demand.  
The current operation of the system presents deficiencies that affect both the stability of the required temperatures and the efficiency of the heat exchange processes in the production of hot water. Although the current operating strategy allows energy savings through a limited use of the auxiliary heater, it is not the most adequate to maintain a constant hot water supply temperature.  
Therefore, it is essential to study the variables that influence the behavior of temperatures in order to propose a strategy that guarantees stability in the hot water supply and, at the same time, minimizes fuel consumption in the auxiliary heater. It is suggested to implement a new strategy that adjusts the water mass flow ratios in the hot water circuits and optimizes the use of the backup system during peak consumption times.