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DIRECT DIGITAL CONTROL ENERGY MANAGEMENT SYSTEM
Summary:
Direct digital control (DDC) refers to the application of microprocessor technology to building environmental controls. With DDC it is possible to control heating and cooling functions with software that takes into account a wide range of variables, thereby achieving greater efficiency.

 

Applicable Regulations
None.

 

Overview of Procedure
DDC logs electronic temperature measurements and compares them to the desired settings (setpoints), calculates an appropriate response if the two are not equal, and sends out new signals to correct for any difference. Start/stop control and alarm reporting are also incorporated into DDC.

At the University of Michigan (U-M), DDC is most often used to manage fan systems, controlling the supply and return fan speeds and fan discharge temperature (which may involve four or more DDC sequences for the various dampers and heating/cooling coils). Some systems require humidity and/or exhaust fan control, as well as energy recovery (using the available heating/cooling from the exhaust air to heat/cool the outside air that is being drawn into the building by the supply fans).

DDC also controls most chillers, some outside and inside lights, most hot water and chilled water pumps, and some cooling tower fans.

 

Waste Minimization Procedure
Several consoles are tied in to campus-wide networks from which program changes can be made by Energy Management Services to alter schedules and modify control operation. The changes are then sent out to the remote computer panels that control the building systems. The panels usually control without the aid of the host computer, which acts as a file server and alarm reporting/archiving device. When global information is required for control (such as outside air temperature/humidity), the host computer passes the information to all locations on campus to ensure that all receive the same information.

Currently there are more than 260 remote computer panels connected to seven networks serving 88 buildings with more than 7,400 sensors/control output "points" (and more than 5,000 additional software points for control adjustments), all linked to two host file servers in the Utilities Department. The number of remote computer panels has more than tripled in the last seven years.

 

Known Limitations
The loss of personal control over temperature settings within a building or work area may not be well received by all employees.

 

Safety & Health Precautions/Personal Protective Equipment
None known.

 

Benefits
There are many benefits to using DDC. First, it is possible to develop historical records on the operating characteristics of a building: identifying trends which can lead to better performance. The DDC system also allows for comprehensive alarm management in the event of a mechanical system malfunction.

DDC saves time by eliminating the need to change various time clocks for holidays and schedule changes.

In addition, DDC has replaced pneumatic controllers that tend to drift out of calibration and suffer from "offset" (i.e., the pneumatic controllers don't have the ability to maintain the temperature at setpoint under changing load conditions).

 

Disadvantages
The only potential disadvantage to DDC is that it requires additional training to install and troubleshoot properly when switching from pneumatic controls to DDC.

 

Project Related Costs
It is very difficult to determine the savings for the entire U-M Ann Arbor Campus from use of DDC because many projects throughout campus include other design changes that also save energy. However, one building on campus can be used as an example of the cost savings from DDC alone because the only change was the elimination of pneumatic controls and the installation of DDC. It is important to understand that the savings potential in other buildings could not be inferred unless the buildings had similar building characteristics. In this particular building, there was a project cost of $403,368 with $250,000 per year savings as a result.

Other design changes that save energy, in addition to DDC, include: more energy efficient motors, chillers, pumps, and basic changes to fan system design. Furthermore, there have been efforts to educate building occupants about the need to turn off lights, keep fume hoods closed when not in use, etc. Breaking each item down into individual associated cost savings is nearly impossible. However, since fiscal year 1988, the combined projects have resulted in estimated savings of $12.9 million.

 

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