Page 221 - Alternative Energy Systems in Building Design

DEMAND-SIDE ENERGY MANAGEMENT AND CONTROL 197

times and especially when a commercial building or residence is vacant. The systems
also have the capability to control thermostats by programming holidays, vacations,
and complex work schedules in commercial applications and produce significant
energy savings. Additional capabilities also include use of programmable thermostats
that conserve energy by operating the HVAC system in their most efficient mode by
monitoring output or supply-air temperature, return-air temperature, room tempera-
ture, and outdoor temperature.

ALARMS AND PERFORMANCE MONITORING
Typical centralized energy-management systems monitor performance of the HVAC
equipment and in some instances provide a routine maintenance schedule to ensure that
the equipment continues to operate at their most efficient level. The data-acquisition
system software accumulates performance records and stores the energy consumption
and run times of each component of the HVAC system. Analysis of the data determines
normal system efficiencies and identifies any degradation or deficiencies occurring in the
system. The system software also generates alarms on a number of temperature and oper-
ational parameters for each HVAC unit. These alarms can be reported on a local and/or
remote basis using modem communication back to a central monitoring computer.

HARDWARE SYSTEM CONFIGURATION
A centralized energy-management system consists of a master control unit, HVAC
control module(s), and individual room-temperature sensor(s). The master control
units most often are equipped with paging receiver and RS232 serial interface for local
or remote operation with a personal computer.

Master control unit The master control unit of an energy-management system is
designed to replace conventional thermostats and can control from one to eight indi-
vidual and different HVAC systems through an RS232 four-wire serial data line using
existing thermostat wire. The keyboard and liquid crystal display (LCD) are used for
entering control parameters, program data, and operating modes and is used to display
the same data, as well as energy usage, demand, and energy efficiency of each system.
An optional radiofrequency (RF) paging receiver allows the system to receive com-
mands from a utility. An optional RS232 serial interface also allows communication
with an on-site PC or a remote PC via a modem. All critical program and energy data
are stored in nonvolatile memory within the unit, which ensures that the data are not
lost in case of a power failure. Programming the master control unit is based on a
tutorial process in which the unit displays information on the LCD, allowing the user
only to answer yes or no or to increase or decrease a parameter by pressing a key.

HVAC control module A control module (Fig. 5.10) is installed on each HVAC
system. This module receives data from the master control unit via an RS232 data line
by using existing thermostat wires and is updated continuously with temperature set-
points and operating instructions from the master control unit. The control module also
informs the master control unit of the status of the systems so that it can monitor alarm
conditions, energy usage, and energy efficiency. The control modules are designed to