This paper evaluates the potential for cost savings associated with the use of a near-optimal control strategy for electrical demand limiting in hybrid chiller plants. The control strategy includes the effects of electrical and gas energy, electrical demand, and chiller maintenance costs. A demand constraint is set for each month based upon a heuristic strategy, and an energy/maintenance cost optimal strategy that attempts to satisfy the demand constraint is applied for cooling tower and chiller control at each decision interval (e.g., every five minutes) throughout the month. Evaluation of the strategy was performed using a simulation tool that predicts the utility costs associated with hybrid plants. Annual cost savings associated with employing demand-limiting control for hybrid cooling plants were evaluated for a wide range of conditions. Costs can be reduced by up to 50% compared to a strategy that is just based on minimizing energy costs. The cost savings for demand-limiting control generally increase with increasing ratio of demand to total utility costs. In addition to depending on utility rates, the ratio of demand to total costs depends on the load profile. In general, the use of hybrid cooling plants with demand-limiting control is more beneficial for buildings with shorter occupancy periods having high demand charges. Buildings with shorter occupancy have a higher ratio of peak load to integrated load.

Units: I-P