Abstract: This paper analyzes the composition and energy-saving methods of a cold source system. Based on a new on-demand cooling control method, and using the control module library from OLS Corporation (USA), a concise and practical energy-saving management program for cold source group control was developed. The focus is on optimizing the setting of chilled water supply temperature according to cooling demand, chiller group control management, and frequency conversion control of chilled water pumps according to cooling demand. The feasibility and stability of the program were verified through testing on an experimental platform, and it achieved excellent results in practical engineering applications. Keywords: chiller plant control, cooling on demand, energy efficiency management Abstract: This paper analyzes the composition and energy-saving methods of the chiller plant system. Applicable chiller plant control energy-saving management programs were programmed, based on a new cooling-on-demand method and the graphic function blocks of American ALC Corporation. The chilled water supply temperature reset optimization program, chiller operating number control program, chiller starting in proper order control program, chiller management program, and chilled pump frequency conversion control program are included. Feasibility and stability were tested and verified in the laboratory. Ideal results were also achieved in a practical project. Keywords: chiller plant control, cooling on demand, energy efficiency management According to statistics, building energy consumption in developed countries accounts for approximately 30%-45% of the total national energy consumption. In civil buildings, HVAC energy consumption reaches 65%, with chiller plant energy consumption being the main component of air conditioning energy consumption. To ensure the comfort requirements of buildings, the cooling source system is designed with a large cooling load margin from start to finish. This has led to an increasingly obvious contradiction between the energy supply of the cooling source and the demand for cooling load at the terminals, resulting in the cooling source system operating at low efficiency for a long time and high operating costs. Therefore, based on the current development level of BAS, improving the practical control level of the cooling source system and improving the operating efficiency of the cooling source has become an urgent problem to be solved. 1 Cooling Source System 1.1 Basic Composition of the Cooling Source System This paper takes the cooling source system of a project in Ningxia as an example to illustrate its basic process, as shown in Figure 1. The main equipment of this cooling source system includes: two 100-ton screw chillers, one 300-ton screw chiller, three 11kW cooling water pumps (two in use and one on standby), three 11kW chilled water pumps (two in use and one on standby), two 32kW cooling water pumps (one in use and one on standby), and two 32kW chilled water pumps (one in use and one on standby). Each of the 100-ton and 300-ton chiller units has two spray cooling towers, with the 300-ton cooling tower containing a fan. The chiller station is equipped with a differential pressure bypass valve between the water collector and distributor. This chiller system is a typical one-to-one system. [align=center] Figure 1: Chiller System Flowchart[/align] 1.2 Energy Saving Approaches for the Chiller System The energy consumption of the chiller system mainly consists of the power consumption of the chiller, chilled water pump, cooling water pump, and cooling tower fan. The main energy-saving approaches are as follows: ① Minimize operating time. ② Optimize the condensing and evaporating pressures of the chiller units; reducing this pressure difference requires increasing the chilled water supply temperature or decreasing the cooling water temperature. ③ When multiple chillers are operating under partial load conditions, ensure they are in their high-efficiency zone. ④ The energy consumption of the water pump accounts for approximately 15-20% of the total energy consumption of the air conditioning system; because the chiller system frequently operates under partial load, the actual water demand of the system is often less than the design value. The use of a variable flow system allows the energy consumption of the transmission to increase or decrease with the increase or decrease of the flow rate, resulting in significant energy-saving benefits. 2. On-Demand Cooling Control Principle On-demand cooling refers to energy-saving control of the equipment in the cooling source system based on the actual cooling demand of users, from the perspective of unified energy management. As shown in Figure 2, the control system uses network control technology to monitor terminal equipment, chilled water users, and the cooling source, with cooling demand and cooling time requests being transmitted in a chain. [align=center] Figure 2 On-Demand Cooling Control Block Diagram[/align] Terminal users (such as VAV boxes) calculate their cooling demand and cooling time requests based on the temperature of the control area and the opening status of the air valves, and transmit these requests to the upstream air conditioning units; the air conditioning units (AHUs) set the supply air temperature based on the downstream cooling demand, compare it with the measured value, and then control the opening of the coil water valves through a PID algorithm. The AHU's cooling demand calculation method is as follows: when the water valve opening exceeds 90%, it is recorded as 1; when the room temperature is at the warning high temperature (25.6℃), it is recorded as 2; when the room temperature is at the near high temperature (24.4-25.6℃), it is recorded as 1; the sum of these three values ​​represents the cooling demand. Cooling time is calculated by representing cooling time in a specific way, distinguishing between cooling during working hours and cooling during non-working hours, and propagating this as a network variable to the upstream cooling source system. The cooling source collects cooling demand and cooling time requests from all AHU and other users to determine the optimal start-up and shutdown of the chiller, reset the chilled water supply temperature, and adjust the booster pump speed. 3. Design and Implementation of a Cold Source Group Control Energy-Saving Management System 3.1 Framework of the Cold Source Group Control Energy-Saving Management Program Based on the control principle of on-demand cooling, this paper uses the control module library from OLS Corporation (USA) to develop a cold source group control energy-saving management program. Its main tasks are: ① To collect statistics on cooling demand and cooling time requests from chilled water users and determine whether the cold source is operational; ② To control the number of chillers in operation based on the cooling load; ③ To provide equal working opportunities to each chiller and to provide mutual backup; ④ To monitor the operating status, fault status, and startup sequence of all chillers; ⑤ To set the chilled water supply temperature according to user cooling demand; ⑥ To determine the start and stop of the chilled pumps and perform variable frequency control according to user cooling demand. The framework is shown in Figure 3. [align=center]Figure 3. Framework of Cold Source Group Control Energy-Saving Management Program[/align] 3.2 Implementation of Core Program Modules 3.2.1 Optimizing Chilled Water Supply Temperature Setting Based on Cooling Demand The cooling demands from each chilled water user are collected through the building control network and summed to form the cooling demand of the entire system. The maximum value of the cooling time request from each chilled water user is selected as the cooling time request of the entire system. In the core module STPT, the setpoint of the chilled water supply temperature is continuously optimized and adjusted according to the number of cooling demands. When the number of cooling demands = 0, the setpoint of the supply temperature is increased by 0.25℃; when the number of cooling demands > 0, the setpoint of the chilled water supply temperature is decreased according to the number of cooling demands, with a maximum decrease of 1℃ each time; under any circumstances, the setpoint of the chilled water supply temperature must not exceed the allowable range; when all chillers are shut down or the system restarts, the setpoint of the chilled water supply temperature is restored to the specified initial value. The program is shown in Figure 4. [align=center] Figure 4 Module for setting chilled water supply temperature using the cooling demand method[/align] 3.2.2 Chiller group control management The main purpose of chiller group control management is to control the number of chillers in operation according to the cooling load, so as to ensure that each chiller is in the high-efficiency working range, and at the same time monitor the working status, fault status and start-up sequence of all chillers, provide equal working opportunities for each chiller, and serve as backups for each other. It includes the following three core program modules: (1) Chiller management program module The main function of the chiller management program module is to compare and process the start-up signal sent to each chiller with the running signal fed back by the chiller control program. When the chiller loses power, fails to start, or is in maintenance status, the start-up signal to the chiller is stopped and a corresponding event notification is issued. During the initial start-up, in order to avoid the power load surge caused by the simultaneous start-up of multiple units, the start-up delay time of each unit should be staggered by 30 seconds. After the chiller fails to start, the user can restart it through the software switch. Accumulate the chiller running time, and when it exceeds the set value, issue an event notification that maintenance is required. (2) Chiller Start-up Sequence Cyclic Program Module: The main function of this module is to determine the chiller start-up sequence and control the start-up sequence cycle to ensure that each chiller has an equal opportunity to work. For example, if there are three chillers numbered 1, 2, and 3, the start-up sequence can be 123, 321, or 231. Managers can choose from three methods to control the chiller start-up sequence cycle: manual cycle, chiller cumulative running time, and timed cycle. They can also manually skip starting and stopping a single chiller. (3) Chiller Operation Count Control Program Module: As the core equipment of the air conditioning system, the chiller's number optimization control is key to energy saving. For example, if there are three centrifugal chiller units, each unit can only adjust its energy by adjusting the outlet guide vanes. When the system is under low load, all three units will work in the inefficient range, resulting in energy waste. Therefore, the chiller operation count control program module adopts the load control method. Its basic idea is to install temperature sensors on the chilled water supply and return mains and flow sensors on the return mains to calculate the real-time cooling load. The calculation formula is Q=C×G×（T3-T1） where: Q is the cooling load, C is the specific heat coefficient of water; M is the total pipe flow rate, and T1 and T3 are the supply and return water temperatures, respectively. When the real-time cooling load exceeds 85% of the unit's rated cooling capacity and the chilled water supply temperature exceeds 12℃, the corresponding load phase operation condition is activated, and the number of operating chillers is increased in stages to match the cooling capacity with the load. When the corresponding load phase operation condition disappears, the number of operating chillers is reduced accordingly. It is important to note that to ensure stable system operation, a smoothing program must be used, limiting the load change rate to no more than 3KW/5 seconds to prevent unnecessary start-ups and shutdowns of chillers caused by sudden load changes. The time interval for increasing or decreasing the number of operating chillers should be at least 10 minutes to avoid frequent chiller starts. Management personnel can override and lock the number of operating chillers; if a chiller fails to start, the system will automatically start other chillers. The program is shown in Figure 5. [align=center] Figure 5 Control Program for the Number of Chillers in Operation[/align] 3.2.3 Frequency Conversion Control of Chilled Water Pumps Based on Cooling Demand The chiller and chilled water pump have a one-to-one relationship. The chiller group control management program determines the start and stop of the water pumps based on the start and stop status of the chillers. At the same time, frequency conversion control is performed on the running water pumps based on the cooling demand from users and the statistics of cooling time to achieve energy saving. When the start command is received from the chiller group control management program and the cooling demand is >3, the chilled water pump is put into operation. The setpoint optimization module is used to set the water pump speed. At the initial start, the water pump speed is set to 50%; when the cooling demand = 0, the water pump speed is reduced by 5%; when the cooling demand > 1, the water pump speed is increased according to the cooling demand, with each increase not exceeding 10%; the program uses a smooth increase/decrease module to limit the rate of change of water pump speed to no more than 1% per second to ensure stable operation. The program is shown in Figure 6. [align=center]Figure 6 Chilled Water Pump On-Demand Speed ​​Control Program[/align] 4. Conclusion This paper, starting from the concept of unified energy management and based on the new method of on-demand cooling control, developed the above-mentioned concise energy-saving management program for cold source group control. Appropriate use of modules such as "time delay," "smooth increase/decrease," "upper and lower limit protection," and "minimum signal hold time" ensures stable system operation and control. Simultaneously, some empirical values ​​are introduced, giving the control program practical application value. The developed control program was simulated and debugged using the WebCTRL/ALC system experimental platform, verifying its stability. In 2004, the above-mentioned program modules were applied to an intelligent project in Ningxia, implementing energy-saving control for three chiller units. The control system was able to load and unload in a timely manner, keeping the chiller outlet water temperature within a very small range of the setpoint, demonstrating excellent control accuracy and achieving the expected functional requirements. It also provided valuable first-hand data for the design of cold source group control. The author's innovation is that a new method for energy-saving management of cold source group control based on on-demand cooling is proposed, and a simple and practical core program module is compiled using building automation configuration software. It has achieved good control effect in actual engineering applications. References [1] Thomas B. Hartman. Global Optimization Strategies for High-Performance Controls[J]. ASHRAE Transactions, 1995 [2] Shi Jiannuo. Analysis of typical HVAC control algorithms in the United States[J]. Intelligent Building Technology and Application. China Construction Press, 2001, 10 [3] Xing Lijuan, Yang Shizhong. Energy-saving measures for central air conditioning system[J]. Microcomputer Information. 2006, 10-1: 63-65