How should the group control strategy for multiple chillers be designed to maximize energy savings?
I am an HVAC expert. The group control strategy design for multiple chillers is a core element of energy savings in large central air-conditioning systems and directly affects the system's annual operating energy efficiency and operating costs. According to Section 8.5 of GB 50736-2012 "Design Code for Heating Ventilation and Air Conditioning of Civil Buildings" and the requirements of GB/T 17981-2007 "Economic Operation of Air Conditioning Systems," the group control strategy must be optimally designed by combining dynamic load changes, equipment characteristics, and hydraulic conditions. The following analysis covers strategy principles, specific methods, and case studies:
**1. Core Strategy: Chiller Sequencing Control and Staging Logic Based on Load Prediction**
- **Chiller Sequencing Control**: The number of operating chillers should preferably be determined based on the ratio of the real-time cooling load to the rated cooling capacity of a single chiller, while avoiding frequent start-stop cycles. According to Article 8.5.3 of GB 50736, it is recommended to adopt a "first-on, first-off" or "rotational operation" mode to balance the operating time of each chiller. For example, when the load rate is below 40%, one chiller should be operated first, and variable-frequency water pumps should be used to regulate flow; when the load rate exceeds 80% and persists for more than 15 minutes, the next chiller should then be started.
- **Staging Logic**: A dual criterion of "load rate + leaving water temperature" is adopted. For example, when the average load rate of operating chillers is >90% and the leaving water temperature is >1℃ above the setpoint, the standby chiller should be started; when the load rate is <50% and the leaving water temperature is 0.5℃ below the setpoint, one chiller should be stopped. A time delay (e.g., 5-10 minutes) must be set to prevent false operation.
**2. Optimization Strategy: Combined Scheduling of Variable-Frequency and Fixed-Frequency Chillers**
- For multiple variable-frequency chillers, group control should prioritize matching the operating frequency of the variable-frequency chillers to load changes. According to Article 4.3.2 of GB/T 17981, variable-frequency chillers are more efficient under partial load; therefore, it is recommended to prioritize variable-frequency chillers in the 40%-70% load range and use fixed-frequency chillers as base load or peak supplementation. For example, in a hotel project, during low-load periods at night, only one variable-frequency chiller is operated, with its frequency adjusted to 45 Hz to meet demand, avoiding frequent start-stop cycles of fixed-frequency chillers.
**3. Hydraulic Balance and Cooling Tower Linkage Control**
- Group control must simultaneously regulate chilled water pumps, cooling water pumps, and cooling tower fans. According to Article 8.5.5 of GB 50736, the cooling water leaving temperature should preferably be controlled at 28-32℃, and cooling tower fans should be variable-frequency controlled based on the wet-bulb temperature. For example, when the cooling water leaving temperature is below 25℃, some cooling tower fans may be shut down to reduce energy consumption. At the same time, the variable-frequency control of chilled water pumps must be linked with the chillers to ensure the minimum flow requirement of the chiller evaporator (e.g., 0.3 m/s flow velocity).
**4. Practical Engineering Case**
- A commercial complex project adopted 4 centrifugal chillers (2 variable-frequency + 2 fixed-frequency). The original fixed-frequency group control strategy caused frequent start-stop cycles of chillers during low-load periods, with annual energy consumption as high as 12 million kWh. After optimization, a load prediction model was introduced (based on outdoor temperature, occupant density, etc.), with the following rules set:
- Low load (<40%): Only 1 variable-frequency chiller operates, and the chilled water pump is adjusted to 35 Hz.
- Medium load (40%-70%): 2 variable-frequency chillers operate in parallel, and cooling tower fans are frequency-controlled according to the wet-bulb temperature.
- High load (>70%): 1 fixed-frequency chiller is started as supplementation, and the variable-frequency chillers are reduced to 60 Hz.
- Results: Annual energy consumption was reduced to 9.6 million kWh, an energy-saving rate of 20%, and the number of chiller start-stop cycles was reduced by 80%.
**Summary**: The key to energy savings in group control of multiple chillers lies in dynamic load prediction, reasonable combination of variable-frequency and fixed-frequency chillers, linkage of hydraulic and cooling tower control, and precise setting of staging logic. It is recommended to adopt BMS system integration in the design and to calibrate sensor data regularly. The above are reference opinions. The actual project conditions still need to be comprehensively evaluated and implemented in combination with specific operating conditions, site conditions, and other factors.