Abstract: In recent years, with the development of China's construction industry, the application of central air conditioning systems has become increasingly widespread. Because central air conditioning systems consume a large amount of energy, how to save energy and improve efficiency has become an urgent problem to be solved. Based on the author's practical experience, this article introduces several commonly used energy-saving measures in detail from two aspects: cold source and heat source energy consumption and dynamic energy consumption in central air conditioning systems. Key Words: Central Air Conditioning, Energy Consumption, Energy Saving Measures Central air conditioning is an indispensable energy-consuming operating system in modern buildings. While providing people with a comfortable living and working environment, central air conditioning systems also consume a large amount of energy. According to statistics, the energy consumption of buildings in China accounts for about 30% of the total energy consumption. In buildings with central air conditioning, the energy consumption of central air conditioning accounts for about 70% of the total energy consumption, and it is showing a trend of increasing year by year. Therefore, how to efficiently utilize the energy of central air conditioning systems and save energy has become an urgent problem to be solved. The energy consumption of a normally operating central air conditioning system mainly has two aspects [1]: one is the energy consumption of cold and heat sources to supply the cooling and heating of air handling equipment; the other is the power consumption required for fans and water pumps to overcome flow resistance in order to transport air and water. The energy consumption of central air conditioning systems is affected by many factors, and there are energy-saving measures in many operating links. Therefore, central air conditioning energy saving is a comprehensive project. The following introduces several commonly used energy-saving measures in terms of cold and heat source energy consumption and power energy consumption. 1. Energy-saving measures for cold and heat source energy consumption 1.1 Temperature and humidity control It can be seen from the air handling process of the central air conditioning system that the lower the indoor temperature and the lower the relative humidity in summer, the greater the energy consumption of the system equipment; the higher the indoor temperature and the higher the relative humidity in winter, the greater the energy consumption of the system equipment, and the corresponding initial investment and operating costs also increase. Because individual comfort standards vary greatly, a relatively wide comfort zone can be established for residential central air conditioning systems. Within this comfort zone, higher temperature and humidity values ​​are used for cooling in summer and lower values ​​for heating in winter, resulting in energy savings. Changes in indoor temperature and humidity are closely related to building energy efficiency. Empirical data shows that lowering the setpoint temperature by 1°C in summer increases energy consumption by 9%, while raising it by 1°C in winter increases it by 12%. Therefore, controlling indoor temperature and humidity within the setpoint accuracy range is an effective measure for energy conservation in building central air conditioning systems. To reduce energy consumption, the baseline indoor temperature and humidity in air-conditioned rooms should be as high as possible in summer and as low as possible in winter, while still meeting production needs and human health requirements. Some homeowners blindly pursue extreme cold, drastically increasing indoor temperature and humidity design standards. This not only wastes a lot of energy but also negatively impacts comfort. The higher the temperature and humidity control accuracy of the air conditioning system, the better the comfort and the more significant the energy savings. The accuracy of signals measured at the front end of the air conditioning system directly affects the precision of the central air conditioning system's control. Therefore, the measured signals, especially analog signals such as temperature and humidity, must be as accurate as possible. Furthermore, a high-precision BAS (Battery Automation System) must be selected to control the central air conditioning. This is because the BAS uses a DDC (Direct Digital Controller) to directly control the opening of the electric water valves without the need for an intermediate regulator; in addition, the DDC contains rich computational control software, such as proportional-integral-derivative (PID) algorithms, fuzzy control algorithms, and genetic algorithms, to ensure control accuracy. 1.2 Cold Source Efficiency Control The performance index for evaluating the cooling efficiency of the cold source is the coefficient of performance (COP). The COP refers to the amount of cooling output per unit of power consumption. The COP is independent of the refrigerant properties and depends only on the temperature T0 of the object being cooled and the refrigerant temperature Tk. The higher T0 and the lower Tk, the higher the COP. Therefore, during the actual operation of the chiller in the air conditioning system, the chilled water temperature should not be too low and the cooling water temperature should not be too high; otherwise, the COP will be low, requiring more power to produce a unit of cooling output, resulting in higher power consumption and increased building energy consumption. The following measures can be taken to improve the efficiency of the cooling source: 1) Lower the cooling water temperature. The lower the cooling water temperature, the higher the coefficient of performance (COP) of the chiller. For every 1°C increase in the cooling water supply temperature, the COP of the chiller decreases by nearly 4%. Lowering the cooling water temperature requires strengthening the operation and management of the cooling tower. First, for cooling towers that are not in operation, the valves on their inlet and outlet water pipes should be closed. Otherwise, because the water temperature from the off-line cooling tower is higher, the temperature of the mixed cooling water will increase, and the COP of the chiller will decrease. Second, after a period of use, the cooling tower should be inspected and maintained in a timely manner; otherwise, the efficiency of the cooling tower will decrease, and it will not be able to adequately cool the cooling water. 2) Increase the chilled water temperature. The higher the chilled water temperature, the higher the cooling efficiency of the chiller. Increasing the chilled water supply temperature by 1°C can increase the COP of the chiller by 3%, so the chilled water temperature should not be blindly lowered during daily operation. First, do not set the chilled water set temperature of the chiller too low. Secondly, it is essential to close the water valve of the stopped chiller to prevent some chilled water from bypassing the pipeline. Otherwise, the amount of water passing through the running chiller will be reduced, causing the chilled water temperature to drop to an excessively low level by the chiller. 2. Energy-saving measures for power consumption 2.1 Control of energy consumption during transmission Power consumption mainly refers to the electrical energy consumed by the fans and pumps during system operation. From the formula for calculating the input power of the fans and pumps: (1) (W): input power; : volumetric flow rate; : head; η: efficiency From the formula (1), it can be seen that to reduce power consumption, the following three aspects can be considered: reducing the flow rate, reducing the system resistance, and improving the efficiency of the fans and pumps. In engineering practice, the following measures can be adopted: 1) Adopting a large temperature difference If the supply and return water (or supply and return air) temperature difference of the water (or air) used to transmit cold and heat energy in the system adopts a large value, then when its ratio with the original temperature difference is m, from the flow rate calculation formula, it is known that the flow rate when adopting a large temperature difference is reduced to 1/m of the original flow rate. At this point, the power required by the water pump or fan will be reduced to 1/m³ of the original. It is evident that increasing the temperature difference significantly improves energy efficiency. While meeting the requirements of central air conditioning precision, personnel comfort, and process specifications, the supply air temperature difference should be maximized. It is important to note that the temperature difference between the supply and return water should not exceed 8℃. 2) Selecting a low flow rate: Because the power consumption required by the water pump and fan is roughly proportional to the flow rate in the pipeline system, high flow rates should not be used in the design and operation to achieve energy-saving performance. Furthermore, using a low flow rate in the main pipe is beneficial to the hydraulic stability of the system. For example, changing the fan speed can change the fan's performance parameters. The fan power is cubically related to the speed, while the flow rate is linearly related to the speed. Reducing the speed to decrease the flow rate can significantly reduce energy consumption. When the flow rate is reduced by 1/3, energy consumption can be reduced by approximately 70.4%; when the flow rate is reduced by 1/2, energy consumption can be reduced by approximately 87.5%, and the fan efficiency remains essentially unchanged, allowing it to operate stably and efficiently. 3) Using high-efficiency energy-carrying media: Generally, water consumes less energy to transport cold and heat energy than air, and the diameter of water pipes used to transport the same amount of cold and heat energy is much smaller than that of air ducts, thus occupying much less building space. This is one of the main reasons for the rapid development of central air conditioning in recent years. Therefore, for centralized refrigeration systems, the chilled water prepared by the equipment in the machine room should, in principle, be transported as close as possible to the vicinity of each central air conditioning zone or the point of use, and the air should be processed by terminal non-independent central air conditioning units (such as cabinet air conditioning units, fan coil units) for local or nearby room use. 2.2 Variable Air Volume (VAV) System Control: The VAV system is an energy-saving measure to address the power consumption shortcomings of the air supply system. VAV systems can be divided into two types: one is the AHU duct system with variable air volume air conditioning unit (AHU-VAV system); the other is the FCU system with indoor fan variable air volume system (FCU-VAV system). The AHU-VAV system fixes the supply air temperature in the duct system and adjusts the supply air volume of the blower to cope with the changes in indoor air conditioning load. The FCU-VAV system fixes the chilled water supply and adds a stepless variable power controller to the indoor FCU to change the supply air volume, that is, to change the heat exchange rate of the FCU to adjust the indoor load. Both methods reduce the power consumption of the blower by adjusting the air volume, and at the same time increase the operating efficiency of the heat source machine to save the power consumption of the heat source. Therefore, energy saving can be achieved in both air supply and heat source aspects. The cooling capacity supplied to the room can be determined by the following formula: (2) Where: C is the specific heat capacity of air, KJ/(Kg·℃); ρ is the air density, Kg/m3; L is the supply air volume, m3/S; tn is the indoor temperature, ℃; ts is the supply air temperature, ℃; Q is the heat absorbed (or put into) the room, KW. If the supply air temperature is set to a constant, different Q values ​​can be obtained by changing the supply air volume L to maintain a constant room temperature. Variable air volume (VAV) control can use an air supply device that automatically adjusts the supply air volume according to changes in indoor load. When the indoor load decreases, it can keep the supply air parameters constant (no reheating required) and maintain a stable indoor temperature by automatically reducing the air volume. In this way, not only can the reheat required by the constant air volume system to raise the supply air temperature be saved, but also the power consumption of the fan and the cooling capacity of the chiller can be reduced due to the reduced air volume. According to various sources, VAV can save 30% to 50% energy compared to constant air volume in general. 2.3 Variable frequency control In most buildings, the central air conditioning is at maximum load for only a few dozen days a year. The cooling load of the central air conditioning is always in dynamic change. Various factors such as changes in the morning and evening, weather conditions, passenger flow, and activities affect the central air conditioning cooling load in real time. Generally, the cooling load fluctuates within the range of 5% to 60%, and most buildings are in this state for at least 70% of the year. However, most central air conditioning systems are designed to be driven at maximum power based on the maximum cooling load. This creates a contradiction between the actual cooling load required and the maximum power output, resulting in significant energy waste. Variable frequency drive (VFD) control can resolve this contradiction. 1) VFD control for fans and pumps: Because AC motors themselves were not speed-regulating in the past, central air conditioning systems had to rely on dampers and valves to control air and water flow, wasting a lot of electrical energy. If VFD speed control is implemented for fans and pumps, the energy wasted on dampers and valves can be saved, resulting in considerable average energy savings per pump. For fans and pumps, according to the principles of fluid mechanics, the parameters under similar operating conditions have the following relationship [2]: (3) Where: Q1, H1, N1, n1: flow rate, head, power, and speed before the speed change, respectively; Q2, H2, N2, n2: flow rate, head, power, and speed after the speed change, respectively. As can be seen from formula (3), the flow rate is proportional to the speed, the pressure is proportional to the square of the speed, and the power consumed is proportional to the cube of the speed. For variable frequency speed regulation, the speed n is basically proportional to the power supply frequency f. When the power supply frequency f decreases, the motor speed also decreases, and the required power decreases rapidly with the cube of the speed. It can be seen that the energy saving effect is very significant. Taking a fan as an example, if the required air volume is 80% of the rated air volume, the speed will also decrease to 80% of the rated speed, and the shaft power will decrease by 51.2%; when the required air volume is 50% of the rated air volume, the shaft power will decrease by 12.5%. This energy-saving effect is also very considerable. Practical experience shows that frequency conversion control of fans and pumps can save 40% to 50% of energy. 2) Frequency conversion control of chiller units: Because the compressor may run at full load for extended periods, the motor capacity must be determined based on maximum demand, resulting in a generally large design margin. In actual operation, the proportion of time spent in light-load operation is very high. Frequency conversion control is used to adjust the compressor speed, thereby controlling the cooling capacity and ensuring the chiller unit is always in the optimal (most reasonable) operating state. Frequency conversion control improves the efficiency of the air conditioner and enhances the operating effect of the chiller unit, thus achieving energy savings. The principle of a variable frequency compressor is to adjust the compressor's displacement per unit time by adjusting the compressor's speed, thereby achieving the purpose of adjusting the cooling capacity. The mathematical relationship between cooling capacity and compressor frequency is as follows: (4) Where n is the rotational speed, f is the frequency of the motor, p is the number of pole pairs of the motor, s is the slip of the motor, λ is the leakage coefficient, Ps is the suction pressure, vh is the suction volume, ni is the polytropic index, εi is the pressure ratio (exhaust/suction), δ0 is the relative pressure loss coefficient, Zs and Zd are the actual gas compressibility coefficients, and is the average energy efficiency ratio. As can be seen from formula (4), cooling capacity is directly proportional to frequency, so frequency conversion regulation can be used to control cooling capacity, thereby achieving energy saving. Some regulation methods (such as adjusting valve opening and changing blade angle) cannot reduce the operating efficiency of the motor even when the demand is small. After adopting frequency conversion speed regulation, when the demand is small, the speed of the motor can be reduced, the operating power of the motor can be reduced, thereby further achieving energy saving. Practical experience has shown that variable frequency control of chiller units can save 20% to 30% of energy. The innovation of this paper is that effective energy-saving measures are adopted for the energy consumption of cold and heat sources and power consumption of central air conditioning systems, so that the adjustment and control of the system are more accurate, the energy consumption is more reasonable, and the operating and management costs are more economical. This is a concrete manifestation of the high efficiency and high return on investment of central air conditioning systems, and it is also the goal pursued by air conditioning engineering design. References: 1 Wu Jihong, Li Zuozhou. Design and Construction of Central Air Conditioning Engineering. Beijing: Higher Education Press, 2001.-151 2 Li Qiang, Wu Jie, Wang Nianchun. Energy-saving analysis of variable frequency speed control technology in factory applications. Microcomputer Information. 2003, 19(2).-33-34