Use
Digital Electronic Control of
Lighting & HVAC Systems for
Energy & Demand Savings
Tremendous
advances in computer technology are reflected in the sophistication
and falling costs of Direct Digital Control (DDC) systems for
buildings. DDC systems are now affordable for all but the smallest
and simplest of buildings, and allow much finer control and energy
savings than pneumatic controls. Besides flexible control of lighting
and HVAC systems, DDC can also integrate fire and intruder alarms,
security and access systems and local and wide area computer networks.
Even in retrofit
situations with existing pneumatic controls, it is usually worth
examining the use of zone or central digital controllers. Continued
use of pneumatic actuators and sensors is sometimes cost-effective,
but requires periodic compressor maintenance and more ongoing
attention to calibration and adjustment.
Central DDC
systems are typically too expensive for buildings with 10 or fewer
HVAC or lighting zones, unless they collect renewable energy using
photovoltaic arrays. However, even in these smaller buildings,
digital controls and equipment should be specified that:
- offer optimal
start and stop controls for HVAC plant equipment.
- provide
occupant-accessible local overrides for temporary off-schedule
use.
- have BACNet-compatible
sensors, actuators and controllers.
Often DDC
zone controllers are suitable and economic for small applications
with fewer than 16 inputs and outputs.
For larger
and more complex buildings, central controllers or centrally supervised
zone controllers connected with a network offer the most flexibility
and potential for energy savings. These systems should:
- provide
a graphic interface for building operators that shows:
- floor plans
of all areas of the building, locating all controlled equipment.
- schematic
drawings of each controlled device, with on/off status, current
sensor readings, setpoints, operator positions, control constants,
and a link that shows the operating control program code.
- graphic
trend logs of inputs, outputs and control points selected by
the operator.
- graphs
and tables of building energy and demand history for all fuel
types.
- have BACNet-compatible
sensors, actuators, controllers and communications.
- use optimal
start and temperature-predictor programs for cooling and heating
plant equipment.
- offer provide
storage for at least 12 months of hourly trend-logging of flows,
temperatures, pressures and other important data for all plant
equipment.
- secure
dial-up modem access for remote monitoring, supervision and
trouble-shooting by building service companies contracted by
the owner
A good sample
specification and protocol for control point names and equipment
control algorithms can be found at the British Columbia Buildings
Corporation website (see Further Information).
DDC systems
capabilities should be considered when designing mechanical and
electrical systems. Often, savings can result from using control
software to supplement equipment limitations. For example, the
traditional rule-of-thumb has been to select control valves with
linear process output / control input relationships. Intelligent
DDC programming can automatically compensate for non-linear valves,
reducing costs. Similarly, programs that use the fast dynamic
response of DDC systems can optimally control pump and fan flows,
so pressure drops are lower under non-peak loads. This saves energy,
and can often reduce the number and size of motors, especially
when variable-speed motor controllers are used.
Where natural
ventilation and cooling strategies are supplemented by mechanical
systems, their control must be carefully thought through. Sensors
for windows and air inlets and outlets should signal their opening
by occupants, and lockout or reduce mechanically supplied cooling
and ventilation. Automatic systems intended to close windows and
air openings must ensure that occupants are not endangered.
Good design
documentation for each controlled device should include schematic
drawings, listings of control points, expected operating readings
and acceptable range, and sequences of operation. Field review
and commissioning should check to ensure compliance with documented
design intentions, and any changes made should be recorded for
future reference by building operators.
Cautions
- Good communications
and documentation are essential to a trouble-free control system.
- Sensors,
actuators and controllers must be carefully commissioned.
- Building
operators must be trained in control system use.
