A DDC controller is disclosed which implements a control strategy that provides for near-optimal global set points, so that power consumption and therefore energy costs for operating a heating and/or cooling plant can be minimized. The controller can implement two chiller plant component models expressing chiller, chilled water pump, and air 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 mechanics and heat transfer under the assumption of a steady-state load condition. The analysis applies to both constant speed and variable speed chillers, chilled water pumps, and air handler fans. Similar models are presented for a heating plant consisting of a hot water boiler, hot water pump, and air handler fan which relates power as a function of the hot water supply/return differential temperature. A relatively simple technique is presented to calculate near-optimal chilled water and hot water set point temperatures whenever a new steady-state load occurs, in order to minimize total power consumption. From the calculated 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 step in load. Although the set points are near-optimal, the technique of calculation is simple enough to implement in a DDC controller. IMAGE
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