Energy management controls are conservation tools that are implemented to control building heating, ventilation and air conditioning (HVAC) systems. In recent years, modern Direct Digital Control (DDC) systems, which can accurately control building HVAC systems, have become very cost-effective and they are replacing traditional manual and electromechanical energy controls in retrofitted buildings, as well as being installed in almost all new commercial and institutional buildings. Minimizing the amount of energy used requires strategic control methods for controlling the building HVAC, power and communications systems.
Deciding which energy management control mode to apply is important, regardless of the technology used. It is important to understand that these modes can be implemented using many of the available technology types. In many cases, simple on-off control is adequate and very appropriate. In other cases, the desired effect can on with modulating controls. The following are basic control modes. The accompanying diagrams will illustrate typical system performance.
The term system capacitance refers to the rate of response of a system to a stimulus. Systems with a large capacitance tend to resist change, and the effects of control are felt more slowly than with systems of smaller capacitance. Comparing the effect to a flywheel or relative mas is a good way to describe this concept. Another useful example to illustrate system capacitance is an instantaneous electric water heater (small volume of water) compared to a standard residential tank-type water heater. Upon energizing the heater elements, the water temperature in the tank unit changes much more slowly because it has more mass, and we say it has greater system capacitance.
The term gain is a control term synonymous with sensitivity and is usually an adjustable amplification value used to tune the controls. If a quicker response is desired for a small input change the gain is increased, in a stronger output reaction from the controller.
On-off Energy Management Controls
Also called two position control, this rudimentary mode is used with equipment that is either on or off. A nominal setpoint exists but is rarely actually achieved except in passing. A range of control values must be tolerated to avoid short-cycling the equipment, and temperature ranges are often fairly wide for this reason. In the case of equipment that cannot be modulated, this is often the only choice. The smoothness of control depends strongly upon the system capacitance; systems with very low capacitance can experience short cycling problems using two position control.
Floating Energy Management Controls
This is a hybrid combination of on-off control and modulating control, also called incremental control. As with on-off control, there is a control range (cut-in/cut out). However, unlike on-off control, floating control systems have the ability to maintain a mid-position of the controlled device instead of being limited to full-on or full-off. Between the cut-in and cut-out thresholds the controlled device merely holds its last position. The process variable is not actually under control within this range, and it is seen to float with the load until it crosses a threshold to get another incremental nudge in the correcting direction. This control is tighter than simple on-off control, although not as tight as true modulating control, but it is inexpensive and reliable. Equipment items from small HVAC terminal units to 1000 HP water chiller inlet vanes are controlled in this manner with good success. Note that floating control is limited to processes that change slowly, and floating control actuators are usually selected to be slow moving.
Proportional-only Control (P)
This is the basic modulating control and what most commercial pneumatic and analog electronic systems utilize. It is essentially an error-sensing device with an adjustable gain or amplification. A control output is issued to regulate a process, and the magnitude of the output is directly proportional to the size of the error. This type of control is economical and reliable. A characteristic offset (residual error) is natural with this type of controller, and the size of the offset will increase with load. This offset occurs because an error must increase (further off setpoint) before an output increase can occur.
If the proportional energy management controls are too sensitive (gain set too high) the controller’s response will be excessive, and oscillation or hunting will occur. When this occurs the controller output (and the equipment connected to it) will oscillate up-and-down, open-and-closed, etc., and the control action will not settle out.
Automatic energy management controls are useful for basic regulation and quality control of processes and environments. They can also be leveraged for energy savings through optimization. Properly applied, these systems are reliable and cost effective. Returning to the chapter intent, the stated purpose of this chapter was to focus on the application of automatic controls as a tool to achieve energy savings. The reader should review the titles of each section, reflect on the key topics taken away, and decide if the stated objective was met. It is hoped that the reader has gained insight into how automatic energy management controls can help achieve energy goals and will endeavor to put these systems to work optimizing processes and saving energy.
From: Energy Management Handbook, 7th Edition, The Fairmont Press