# Research on Real-Time Control Strategy for HVAC Systems in University Libraries
## 2 Materials and Methods
### Energy-Saving Strategies
#### 2.1.3 Climate Change-Based Optimization of Cooling Water Return Temperature Setpoints  
The cooling water return temperature setting value is optimized according to the outdoor meteorological parameters and system operation data to improve the system operation efficiency so that the total energy consumption of the chiller and cooling tower fan is the lowest. When the HVAC cold load is constant, the amount of heat removed by the system remains unchanged. When the rotational speed of the cooling tower fan is constantly increasing, the cooling water return temperature is constantly decreasing, which can be infinitely close to the wet bulb temperature. The efficiency of the chiller is improved, and the power consumption is reduced, but the energy consumption of the cooling tower is increased. In contrast, when the cooling tower fan speed continues to decrease, the cooling water return temperature continues to increase, the efficiency of the chiller is reduced, and the energy consumption increases, but the cooling tower energy consumption decreases. Therefore, there is an optimal cooling water return temperature such that the total power consumption of the cooling machine and cooling tower fan is minimized. In general, the optimal cooling water return temperature is the set value of the cooling water return temperature of the chiller. However, with the continuous change in outdoor temperature and humidity, the optimal cooling water return temperature also constantly changes. The system structure is shown in Figure 6. The outdoor wet bulb temperature is the basis for setting the cooling water return temperature of the chiller. To maintain the cooling water return temperature and the outdoor wet bulb temperature of the approximation  
Figure 5: Basic steps of the strategy in Section 2.1.2. (the approximation changes with the change in the outdoor wet bulb temperature, usually 2-4 \({}^{\circ}\)C), when the outdoor wet bulb temperature changes, the cooling water return temperature optimization algorithm module calculates the optimal cooling water return temperature setting value and sends it to the energy-saving control unit, which controls the cooling water return temperature by controlling the starting and stopping of the cooling tower fan. Figure 7 shows a basic step diagram of the proposed strategy. 1.4 Optimal Control of Intermittent Operation of Small-Load Systems Based on Climate Characteristics  
According to the outdoor meteorological parameters and end cold demand, the refrigeration station system implements intermittent start-stop control; that is, when the chiller unit is in small-load operation, the chiller unit, cooling water pumps, and cooling tower  
Figure 6: Cooling tower/cooling unit system diagram. Figure 7: Basic steps of the strategy in Section 2.1.3.  
use intermittent start-stop control, make full use of the cold stored in the chilled water system, reducing the energy consumed by the system. Intermittent control refers to the system in accordance with certain rules for intermittent start-stop control. When the water system runs for a period of time, in accordance with the order of priority of the refrigeration unit, cooling water pumps, cooling towers, and chilled water pumps to maintain the operating state, the system stops for a period of time. Then, in accordance with the order of priority, the cooling tower, cooling water pumps, and refrigeration unit start up again. During the night or transitional season, when the HVAC load is low, the refrigeration unit usually operates under low-load-rate conditions, and the operating efficiency is low. After using intermittent control, when the refrigeration unit stops for some time, the chilled water is pumped to the end of the continuous provision of chilled water, and the chilled water temperature gradually increases. When the refrigeration unit starts again, it runs at a higher load factor, and the cooling efficiency is greatly improved.