Difference between UPPC® and Traditional Chiller Plant Control
The current market offers various types of control systems namely for the chilled plant efficiency, among which a significant majority of such systems should be classified as traditional chiller plant control. The primary distinction between these traditional controls and the UPPC® system is that the former does not have the global and comprehensive energy-efficient control software. In other words, the traditional plant control systems mainly adopt the direct digital controllers (DDC) as hardware and on/off or PID (proportional-integral-derivative) control loop as their basic control logic, along with VFDs to pumps as implementation measures. In summary, such traditional systems modulate running speeds of chilled or cooling water pumps, and cooling towers and chillers with on/off logic.
As illustrated in the diagram to the right (Figure of Typical Schematic Diagram of a Traditional Chiller Plant Control System), traditional plant control systems generally assign one controller for a number or group of equipment. Each controller is installed with a simple default PID or an on/off control logic programmed by the control supplier, executing isolated, closed-loop control for that particular piece/group of equipment. The upper level computer only receives data uploaded from DDC, but rarely reverses the signal chain by issuing control commands to the DDC. This is due to the upper computer’s lack of global control algorithm software. Figuratively speaking, it lacks a “brain”.
The traditional control logic generally functions as follows: activate/deactivate number of chillers according to supply/return water temperature; conduct PID control over chilled water pumps according to differential pressure between the supply and return water; turn on/off condenser water pumps and cooling towers in a dedicated manner to the number of activated chillers. Such strategies perform no trade-off diagnosis for energy consumed by different groups of equipment, nor do they attempt to balance energy-use interactions between chillers, cooling water pumps, and cooling towers, and between chillers and chilled water pumps.
UPPC®, on the other hand, deploys optimized control that operates with a “brain”--- the control software based on optimization algorithm. This software aims to achieve the minimum energy consumption for the whole chiller plant, conducts synchronized calculations on operating variables measured from equipment in plants, and then sets new control variables for equipment by sending optimization results to PLCs (programmable logic controller). In such way, the UPPC® control system enables two-way communication between system components. Signal transmission between the upper central computer and the PLCs is two-way interactive --- the upper level computer receives signals from PLCs and issues control commands to the PLCs. Table 1 summarizes the unique and innovative features and characteristics of the UPPC® system compared to those of a traditional control system.
