Page 226 - Alternative Energy Systems in Building Design

202 ENERGY CONSERVATION

and outdoor temperature. This allows the energy-management system to make complex
decisions about compressor, indoor fan, economizer, and gas valve/burner operation.

Energy recovery at the end of a cooling or heating call Some new and more
expensive heating and air-conditioning systems are furnished with a time-delay relay
(TDR) that keeps the indoor fan operating 30–60 seconds at the end of a heating or
cooling call to salvage the residual hot or cold energy stored in the mechanical system.
The American Refrigeration Institute (ARI) has recognized this technique for improving
energy efficiency, and it has reported an additional 15–25 percent improvement in the
standard energy efficiency rating (SEER) of the equipment with this control.
The energy-management system performs this function for equipment that does not
have a TDR and performs more than the simple TDR function. In air-conditioning, the
system monitors the temperature of the supply air and keeps the indoor fan operating
until the Btu output has dropped to 25 percent of normal. This is done by monitoring
the temperature drop across the indoor coil (return-air temperature versus supply-air
temperature). At the 25 percent level, the Btu output from the system is about equal to
the power required to operate the indoor fan to recover the Btus.

Intelligent indoor fan operation Many commercial users operate their indoor fan
continuously without regard to the energy being consumed by the fan or the effect on
cooling or heating call time. A fan that is operating continuously actually can be heating
the indoor environment when the thermostat is in the cooling mode. This may be caused
by high return-air temperatures, economizers stuck in the open position, poorly insu-
lated return air ducts, or makeup air vents that are set improperly.
The energy-management system can reduce this wasted energy by operating the indoor
fan in five different modes: continuously on, automatic, pulsed, timed, and continuous
on/off controlled. The user can select one or more modes of operation that do not conflict.

Energy conservation by measured efficiency Energy-management systems
generally allow the consumer to measure both operating times and efficiency. This is
accomplished by monitoring the amount of time the system operates in cooling or
heating and monitoring the cooling- or heating-degree days for the period, which
provides the consumer with a relative measure of system efficiency.

Energy conservation by monitoring efficiency and predictive maintenance
In most applications, a consumer does not repair or maintain an HVAC system until it
has failed or its performance approaches failure, resulting in a severe negative impact on
overall efficiency. The energy-management system continuously monitors HVAC system
performance and will generate and report an alarm condition if minimum performance
parameters are not maintained. The system also allows routine maintenance and repairs
to be tracked over a 12-month period. After each service of the HVAC system, the repairs
and service performed can be entered into the energy-management system and be dis-
played at any time. It should be noted that preventive maintenance is the best way to pre-
vent degradation of system efficiency.