HVAC Control in the New Millennium.pdf - HVAC.Amickracing

Energy and Power Management, Distributed Control Trendstimes. Electronic timers or clocks use semiconductor timer chips andswitches to perform the same functions. Popular chips like the 555 and556 can perform these functions and cost less than 50 cents.Applications range from pool heaters to advertising signs andoutdoor display lighting. They have also been adapted to control electricenergy use and to limit demand. The timer is set to turn a load off andon for brief time intervals during each demand period. This has theoverall effect of limiting energy consumption and reducing demand.The control is limited to those devices which can be cycled continuously.Electronic Demand LimitingOther types of electronic demand limiting devices can be used tomonitor and measure the actual demand and provide control actions tolimit the operation of attached devices when the measured demandreaches a preset value. These devices need to receive two signals fromthe utility meter, the kilowatt hour (kWh) or demand pulse, which indicatesthe units of electrical energy consumed and a timing pulse, whichindicates the end of one demand pulse and the start of the next one.The demand pulse is used to increment an electronic counter. Asthese counts accumulate within a demand period, they are compared toa target value which corresponds to the maximum desired demand.When the value accumulated in the counter exceeds the target value, theattached load is switched off.If the demand still exceeds the demand target, a second load isturned off. This is done until all available loads are turned off or untilthe demand no longer exceeds the target.Once it is turned off, the load stays off until the end of the demandperiod. A timing or reset pulse from the meter restores the counter tozero and turns the loads back on again. Some load shedders use a demandtarget that is not fixed but increases at a steady rate. Other devicesallow the off-on setpoints to be adjusted independently for individualloads.Computer-based SystemsUsing computer technology in the regulation of electrical energyallows a wide range of control options. The computer’s memory and©2001 by The Fairmont Press, Inc. All rights reserved.