A DDC controller is disclosed which implements a control strategy that providesfor near-optimal global set points, so that power consumption and therefore energy costsfor operating a heating and/or cooling plant can be minimized. The controller canimplement two chiller plant component models expressing chiller, chilled water pump, andair handler fan power as a function of chilled water supply/return differential temperature.The models are derived from a mathematical analysis using relations from fluid mechanicsand heat transfer under the assumption of a steady-state load condition. The analysisapplies to both constant speed and variable speed chillers, chilled water pumps, and airhandler fans. Similar models are presented for a heating plant consisting of a hot waterboiler, hot water pump, and air handler fan which relates power as a function of the hotwater supply/return differential temperature. A relatively simple technique is presented tocalculate near-optimal chilled water and hot water set point temperatures whenever a newsteady-state load occurs, in order to minimize total power consumption. From thecalculated values of near-optimal chilled water and hot water supply temperatures, a near-optimal discharge air temperature from a central air handler can be calculated for each stepin load. Although the set points are near-optimal, the technique of calculation is simpleenough to implement in a DDC controller.
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